Date   

Re: Looking for sensitive audio RMS meter

 

On Sat, May 15, 2021 at 7:47 AM Jean-Paul <jonpaul@...> wrote:

The measurement issue is well adressed by others.

But the objective of your work is not clear to mme.

In case you are building or designing a digital audio device or studio, suggest to see the fine work of the late Bruce Jackson, of Apogee, who coined "Marketing Bits" also the many AES papers on dynamic range, noise, etc.

Especially the classic Steve Harris papers in 1970s...1980s (Ex Cirrus logic) and the late and great Julian DUNN at Cambridge.

You will discover that the claims of 20, 24 and even 30 bit ADC and DACs are often optimistic or illusory, and that the true noise level and dynamic range of a microphone, or digital device is never as wide as advertising or theory predicts.

Finally, even a perfect mic will be limited by studio room ambient noise levels, especially LF rumble.

Bon chance


Jon
Thanks a lot for the profuse references.

Regarding my objective.

In my audio amplification chain I have an active analog crossover
which seems to be the source of noise. So for that reason I am looking
for a good way to measure and quantify that noise. The noise is so
high that it is audible in speaker outputs. I will be taking care of
that.

I am aware of marketing bits. But we are not dealing with them here -
there's an audible noise source that I am trying to eliminate. I agree
that just looking at "number of bits" while many of them are drowned
in noise is a fool's errand.

Liam,
thanks for the links

I think it's Julian Dunn, not John Dunn. I think I ran across mentions
of Julian Dunn in the past, he's the jitter guy. I think he
popularised and figured out a lot of the ways in which jitter will
make audio worse.

He wrote the digital measurement book for AP.
http://www.collinsaudio.com/Prosound_Workshop/Julian%20Dunn_Jitter.pdf


Tek 577 Step Gen & Volt Div knobs skirt pictures needed

Rogerio O
 

Hi Folks,
I am in the process of trying to bring a 577/177 back to life.
The CRT seems to be working and so is the step generator.
There is a problem on the vertical circuit so it can only display horizontal traces now, but since there are no "unobtainable" parts on this circuit I think I will eventually find out the problem and fix it.
Unfortunately all the skirts of the 577 and the 177 are severely damaged so I would like to replace them.
It seems it would be impossible to find the original Tek knobs so I will have to improvise.
My approach will be to print an image of the skirt on a transparent film in a photo printer and then sandwich it with plastic films in a hot lamination machine.
However in order to do this, I will need good photos of the knobs.
I was able to find one of the 177 Vertical knob on this forum, but I had no luck with the 577 Step Generator knob and the Horizontal Volts/Div.
Can anyone on this forum take a good picture (top view) of this two knobs and post the file here?
Thank you very much in advance for your cooperation.
Regards,
Roger


Reparing a dead FG504

um-gs@...
 

The following lists my findings and maybe some hints for others fighting with the tricky (but ingenious) construction of the Tek 504 while trying to repair it.

The unit was dead - no lights, no output. Obviusly someone had it plugded in/out under power...

Measuring the supply voltages showed some volts on the - 25V line, the others were zero.
Fuses 1616 (+25V) and 1715 (-15V) were blown.
Since the 25V lines depend on the 15 V - and both share an overload crowbar switch - circuit isolation had to be the next step.
Internal connections to the +-25V auxiliary power supply board were removed.
Fuses were replaced.
The Main board only (with the +-15V pwr suplly) was powered via an extension cable from a TM500 mainframe.
This showed both 15V supplies being operative.
Externally supplying +-25V (via the testpoints on the main board) showed a high current flowing at +25V.
Measuring the +-25 V supplies (on the testpoints on the main board) against ground with an ohm meter showed a short circuit on the +25V.
After checking several suspects (all power transistors and diodes connected to +25V) without success, finally the typical culprit was found:
a tantal capacitor (C978) between +25V and ground - it did not show any outer signs of damage.
Removing the tantal brought the complete circuit back to life.
After replacing it with an identical new type - it blew again next time at power on!
Obviously the initial voltage after power up (and to much current) do this damage.
Replacing the tantal with a 50V rated type fixed the problem.
When checking the functions everything worked except the square wave function on the main output.
Again - the tricky Tek circuit is generating the square wave out of the triangle via kind of a feedback switch around U375.
Following the signals showed it to arrive at the base of Q450 which was obviosly killed by the preceeding power supply problems.
As this SPS6927 is "unobtanium" I was lucky to find the information in another post that it it is equivalent to the MPSH81 - which is stil available at below 1€/pce at ebay Germany.
Now all works well.

Just one last comment on modern vs. good old analog Scope technology:
I did all checks for the repair with my small digital tek TBS1152 - working fine.
But for the calibration - step 15 requires you to do an amplitude flatness check of the swept output and monitor the envelope curve.
That proved to be impossible with the (damned) digital thing - but no problem at all with my nice 547/1A4 combination with stable external trigger.

Bests, Gordian


Re: Looking for sensitive audio RMS meter

Liam Perkins <hifi@...>
 

I found some of Steve Harris' work here:

http://www.madsteveharris.co.uk/papers1.htm

A Google search for John Dunn here:

https://tinyurl.com/576erdpc

turned up a different John Dunn who, amoung others, wrote, "The Cunning of
Unreason" available here:

https://b-ok.cc/book/1124834/875c6c

that while I expect is also about noise, political noise, isn't quite what
we're about here.

"Matt" has been mentioned a few times but without a last name searching is
pointless even though his work sounds both solid and interesting; any
follow-up would be appreciated.

On Fri, May 14, 2021 at 11:47 PM Jean-Paul <jonpaul@...> wrote:

The measurement issue is well adressed by others.

But the objective of your work is not clear to mme.

In case you are building or designing a digital audio device or studio,
suggest to see the fine work of the late Bruce Jackson, of Apogee, who
coined "Marketing Bits" also the many AES papers on dynamic range, noise,
etc.

Especially the classic Steve Harris papers in 1970s...1980s (Ex Cirrus
logic) and the late and great Julian DUNN at Cambridge.

You will discover that the claims of 20, 24 and even 30 bit ADC and DACs
are often optimistic or illusory, and that the true noise level and dynamic
range of a microphone, or digital device is never as wide as advertising or
theory predicts.

Finally, even a perfect mic will be limited by studio room ambient noise
levels, especially LF rumble.

Bon chance


Jon







Re: Converting a rack to a bench

Andy Warner
 

I recently swapped the tube from a donor 5000, to fix my 577.

I would be quite happy to send you the carcass of the 5000, for the cost of
shipping. I can disassemble to reduce weight/size. Then this 5000 would
have helped two on this list. Let me know off-list if you would like to
explore this (andyw [at] pobox [dotcom].)

On Sat, May 15, 2021 at 01:43 Dave Peterson via groups.io <davidpinsf=
yahoo.com@groups.io> wrote:

That's great Tim. Thanks.

Found the instructions pdf that includes all necessary parts.

Dave


On Friday, May 14, 2021, 11:28:02 PM PDT, Tim Phillips <
timexucl@...> wrote:

Dave;
Hakan (Zenith) has the info on his site, in the 'Kits & Mods' section.
The trickiest part is installing the bolts that hold the two halves
together - the front bolts must be low-profile to clear the plug-ins,
and the rear ones are hard to reach and tighten. They are long hex-head
bolts, but ordinary steel bolts will do.
I used a flexible-shaft nut driver. You will also need different
cover-panels, of course. Maybe swap with someone going the other way ?
Where are you situated?
Also, be VERY careful re-installing the big 10-way ribbon - one pin out can
fry all your hard work ! (Go on, ask me !!)
Tim


On Sat, 15 May 2021 at 03:07, Dave Peterson via groups.io <davidpinsf=
yahoo.com@groups.io> wrote:

I'm looking through my 5440 instruction manual and looking at the
instrument conversion/rack mounting instructions. It says field
conversion
kits are available.

Well this is from the 70's, and I'm sure Tektronix is no longer providing
these kits. I'm also guessing equipment was generally sold in bench
cabinets, and what the manual is referring to is a rack conversion kit.
Was
there ever such a thing as a bench conversion kit?

I just bought a rack configured 5111A. Are there kits out in the world to
convert a rack configuration to a bench cabinet?

I suspect it's just a matter of finding a gutted 5000 series scope.

Dave















--
Andy


Re: Where 3ACPx(x) and other crts originally used

Jean-Paul
 

Bonjour à tous

I recall buying these CRTs in OEM mil boxes, ~$5 on Courtland St in NYC.

I think P11 phosphor is highly actinic, for photo recording, high writing rate.

the application was in defense, nuclear and mil.

Enjoy

Jon


Re: Where 3ACPx(x) and other crts originally used

Colin Herbert
 

Hi,
You are asking a question which is opposite to that usually asked; "I have a "XXX" scope which is missing its CRT, can anyone tell me what it used"? I'm afraid that Peter Keller's book makes no mention of the 3ACP11 but I would guess it was made by Dumont and might have been used in of Dumont's scopes.

There were a couple of pages of addenda to the above-mentioned book which were posted on this forum by Dennis Tillman, IIRC. Some of the references therein might be of use - or not.

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Mark Vincent
Sent: 11 May 2021 22:58
To: TekScopes@groups.io
Subject: [TekScopes] Where 3ACPx(x) and other crts originally used

I have three NOX 3ACP11s and some other scope type crts that I cannot find
where they were originally used in. I have looked online for makes, model
numbers, etc. for several years. I cannot find anything. I do NOT mean the
modern clocks using crts. A site to get the specs is frank.pocnet.net.
Other crts with 1-5 and a few others larger than 5 as the first JEDEC crt
numbers would be nice to know what they were originally used in. Dumont,
Sylvania, E.T.C., etc were some makers of these types. I know other
countries had their own numbers. Some had flying leads instead of a base.
Some have a ring on the neck with pins facing the same way as the socket at
the end instead of the neck pins. The two I asked about last year, with
photos, would be nice to know what they were originally in. Some numbers
are multigun types.

It would be nice to build a scope with these numbers or find an original
and fully restore it. I do not know the maximum frequency the one I
mentioned above would go.The length of the wires to the plates and
capacitance would be the main factor for ft.

Would some of the "unusual" numbers be too hard to drive in the X and Y
direction, spot size too great, innards not being as tight in tolerance,
etc. to be in commercial or lab grade that Tek., H/P, etc did not decide to
use these other numbers? The ones I am asking about are not the ones for
high frequencies, say above 100mc. I know a 3WPx(x) is used in the 310/A
and 315 and few early models used ones like 5CPx(x), 5XPx(x).

I may be asking things that are obvious to some.

Mark


Re: Converting a rack to a bench

Dave Peterson
 

That's great Tim. Thanks.

Found the instructions pdf that includes all necessary parts.

Dave

On Friday, May 14, 2021, 11:28:02 PM PDT, Tim Phillips <timexucl@...> wrote:

Dave;
Hakan (Zenith) has the info on his site, in the 'Kits & Mods' section.
The trickiest part is installing the bolts that hold the two halves
together - the front bolts must be low-profile to clear the plug-ins,
and the rear ones are hard to reach and tighten. They are long hex-head
bolts, but ordinary steel bolts will do.
I used a flexible-shaft nut driver. You will also need different
cover-panels, of course. Maybe swap with someone going the other way ?
Where are you situated?
Also, be VERY careful re-installing the big 10-way ribbon - one pin out can
fry all your hard work ! (Go on, ask me !!)
Tim


On Sat, 15 May 2021 at 03:07, Dave Peterson via groups.io <davidpinsf=
yahoo.com@groups.io> wrote:

I'm looking through my 5440 instruction manual and looking at the
instrument conversion/rack mounting instructions. It says field conversion
kits are available.

Well this is from the 70's, and I'm sure Tektronix is no longer providing
these kits. I'm also guessing equipment was generally sold in bench
cabinets, and what the manual is referring to is a rack conversion kit. Was
there ever such a thing as a bench conversion kit?

I just bought a rack configured 5111A. Are there kits out in the world to
convert a rack configuration to a bench cabinet?

I suspect it's just a matter of finding a gutted 5000 series scope.

Dave






Re: Converting a rack to a bench

Tim Phillips
 

Dave;
Hakan (Zenith) has the info on his site, in the 'Kits & Mods' section.
The trickiest part is installing the bolts that hold the two halves
together - the front bolts must be low-profile to clear the plug-ins,
and the rear ones are hard to reach and tighten. They are long hex-head
bolts, but ordinary steel bolts will do.
I used a flexible-shaft nut driver. You will also need different
cover-panels, of course. Maybe swap with someone going the other way ?
Where are you situated?
Also, be VERY careful re-installing the big 10-way ribbon - one pin out can
fry all your hard work ! (Go on, ask me !!)
Tim


On Sat, 15 May 2021 at 03:07, Dave Peterson via groups.io <davidpinsf=
yahoo.com@groups.io> wrote:

I'm looking through my 5440 instruction manual and looking at the
instrument conversion/rack mounting instructions. It says field conversion
kits are available.

Well this is from the 70's, and I'm sure Tektronix is no longer providing
these kits. I'm also guessing equipment was generally sold in bench
cabinets, and what the manual is referring to is a rack conversion kit. Was
there ever such a thing as a bench conversion kit?

I just bought a rack configured 5111A. Are there kits out in the world to
convert a rack configuration to a bench cabinet?

I suspect it's just a matter of finding a gutted 5000 series scope.

Dave






Re: Looking for sensitive audio RMS meter

Jean-Paul
 

The measurement issue is well adressed by others.

But the objective of your work is not clear to mme.

In case you are building or designing a digital audio device or studio, suggest to see the fine work of the late Bruce Jackson, of Apogee, who coined "Marketing Bits" also the many AES papers on dynamic range, noise, etc.

Especially the classic Steve Harris papers in 1970s...1980s (Ex Cirrus logic) and the late and great Julian DUNN at Cambridge.

You will discover that the claims of 20, 24 and even 30 bit ADC and DACs are often optimistic or illusory, and that the true noise level and dynamic range of a microphone, or digital device is never as wide as advertising or theory predicts.

Finally, even a perfect mic will be limited by studio room ambient noise levels, especially LF rumble.

Bon chance


Jon


Re: Looking for Tek T922 HV transformer part number 120-0996-00

Michael W. Lynch
 

I have one in a parts machine. I tried to send you a PM and never got a response. Check your spam folder. If you still need one, contact me off list at:

mlynch003 at yahoo dot com

--
Michael Lynch
Dardanelle, AR


Re: Looking for sensitive audio RMS meter

 

Sorry, who's Matt? There isn't a person named Matt in this thread...

On Sat, May 15, 2021 at 1:33 AM Keith <coolblueglow@...> wrote:

And I can vouch for Matt’s design skills and the output quality. He’s a
real old school mil-spec curmudgeon. The tube preamps he built work
flawlessly and sound absolutely awesome! I still have it and still use it
all the time it on major recording sessions.

BTW, Matt - I rescued those SS preamps you mentioned - the ones you built
for the duh-mixer. I saved them when they were wrecking out the old
building... I have them now. 🤓






Re: Looking for sensitive audio RMS meter

 

On Sat, May 15, 2021 at 1:16 AM Ted Rook <rooknrol@...> wrote:

On 14 May 2021 at 2:26, cheater cheater wrote:

Thanks again Ted. I didn't know you worked at A&H.
When I was there we were a very small operation, in the 1970s maybe a dozen people total. I
was a live sound technician (roadie) which in those far off simple days qualified me for the
test department (which was a new development, business was good) where I learned to drive
a scope a voltmeter and a signal generator. Later I contributed to development of new
designs for live and recording mixers evaluating prototypes designed out of house, then later
still developing new prototypes from the ground up inhouse. I worked in London at two
locations and in Brighton, by which time we had a factory in Cornwall that is still the company
HQ.
What a great story. What do you think were the most interesting
circuits you designed there?

I've had a few more questions about the meter as well, if that's OK.

You say the TM3B is single-ended, not balanced. What does that
actually mean for a device that's battery powered and essentially
floating? I could connect the Bryston 10B's XLR output - the hot and
cold pins - to the input of the TM3B. Isn't that balanced?
Yes, the meter itself is floating however once connections are made to scope/listening device
at the meter output terminals grounding via the meter probe common enters the picture.


My PMC / Bryston 10B only has symmetrical balanced XLR outputs. If I
wanted to measure the noise, what should I measure: between hot and
cold, gnd and hot, gnd and cold?
Hot and cold. The voltage between ground and one of the phases alone is half amplitude,
-6dB.


You mention using a small listening device for listening from the
TM3B's output. That's a really good idea. I shall use a small desktop
monitor. But maybe I should use a battery powered speaker, that should
prevent any ground loops. What do you think?
Yes that would work for the listening device but leaves the scope ground as a source of
grounding.
I was wondering why you mention scope ground. Are you saying I should
use the TM3B and hook up a scope and a listening device to its output,
both to the same port?

First find out how it behaves 'out of the box'. A good brand like Bryston probably takes care to
eliminate contamination with ripple and hum at the design phase.
That's the plan, thanks!


Converting a rack to a bench

Dave Peterson
 

I'm looking through my 5440 instruction manual and looking at the instrument conversion/rack mounting instructions. It says field conversion kits are available.

Well this is from the 70's, and I'm sure Tektronix is no longer providing these kits. I'm also guessing equipment was generally sold in bench cabinets, and what the manual is referring to is a rack conversion kit. Was there ever such a thing as a bench conversion kit?

I just bought a rack configured 5111A. Are there kits out in the world to convert a rack configuration to a bench cabinet?

I suspect it's just a matter of finding a gutted 5000 series scope.

Dave


Re: Looking for sensitive audio RMS meter

Brad Thompson
 

Ted wrote on 5/14/2021 7:32 PM:

<snip>

Same story, if Bryston did their job right and there are no AC transformers nearby extra
shielding may be superfluous. As you probably know exclusion of magnetic fields due to AC
line current requires ferrous shielding. My Bryston 4B has a very fine metal case however it
mostly aluminum.

Hello--

You're no doubt familiar with older-vintage CRT displays' ability to radiate
stray magnetic fields. For newer group members who have never had to deal
with CRTs, magnetic coupling between two side-by-side CRTs produces
some interesting undulating and distorted displays.
Suppressing inadvertent and undesired radiation poses some interesting challenges:

https://*en.wikipedia.org*/*wiki*/*Tempest*_(*codename*)

73--

Brad AA1IP


Re: Looking for sensitive audio RMS meter

Keith
 

And I can vouch for Matt’s design skills and the output quality. He’s a real old school mil-spec curmudgeon. The tube preamps he built work flawlessly and sound absolutely awesome! I still have it and still use it all the time it on major recording sessions.

BTW, Matt - I rescued those SS preamps you mentioned - the ones you built for the duh-mixer. I saved them when they were wrecking out the old building... I have them now. 🤓


Re: Looking for sensitive audio RMS meter

Ted Rook
 

On 14 May 2021 at 2:26, cheater cheater wrote:

Thanks again Ted. I didn't know you worked at A&H. I haven't owned any
of their products but they are really good quality and I'm often
jealous of friends who have them. If you don't mind me asking, what
was your role at the company? It's fascinating to hear people talk
about what they did at famous companies like that.
When I was there we were a very small operation, in the 1970s maybe a dozen people total. I
was a live sound technician (roadie) which in those far off simple days qualified me for the
test department (which was a new development, business was good) where I learned to drive
a scope a voltmeter and a signal generator. Later I contributed to development of new
designs for live and recording mixers evaluating prototypes designed out of house, then later
still developing new prototypes from the ground up inhouse. I worked in London at two
locations and in Brighton, by which time we had a factory in Cornwall that is still the company
HQ.


I've had a few more questions about the meter as well, if that's OK.

You say the TM3B is single-ended, not balanced. What does that
actually mean for a device that's battery powered and essentially
floating? I could connect the Bryston 10B's XLR output - the hot and
cold pins - to the input of the TM3B. Isn't that balanced?
Yes, the meter itself is floating however once connections are made to scope/listening device
at the meter output terminals grounding via the meter probe common enters the picture.


My PMC / Bryston 10B only has symmetrical balanced XLR outputs. If I
wanted to measure the noise, what should I measure: between hot and
cold, gnd and hot, gnd and cold?
Hot and cold. The voltage between ground and one of the phases alone is half amplitude,
-6dB.


You mention using a small listening device for listening from the
TM3B's output. That's a really good idea. I shall use a small desktop
monitor. But maybe I should use a battery powered speaker, that should
prevent any ground loops. What do you think?
Yes that would work for the listening device but leaves the scope ground as a source of
grounding.

When trying to lower noise, what are the options? Here's a few things
that I think of:

1. use a cable that goes directly from the XLR output of the 10B to
the BNC input of the TM3B. Make a cable myself using one of those well
shielded cables like here:

https://gothamcables.com/en/gothamcables/balancedanalog

they have cables that are shielded 5 times... whether that's necessary
is a question, but it's more interesting to ask whether it'll make
things worse in some surprising way. Either way the shielding is super
thick on those cables, even the ones that are just shielded 1 or 2
times.
Special shielding precautions are probably unnecessary unless you are working close to
powerful sources of RF like transmitters or arc welders, in which case give up noise testing! I
used ready-made BNC to banana cords with croc-clip probes for signal and shield. At the
equipment insert an open female XLR shell onto the output pins and hook up the croc-clips to
the terminals of interest, often pins 2 and 3. Should experimentation be needed to find the
appropriate balanced/unbalanced hookup things are easily varied just by reclipping.

Some active balanced output circuits will give a signal 6dB low unless both phases are connected.
Some circuits require grounding the unused phase as a way to trigger automatic internal 6dB gain increase that compensates for unbalanced connection using only a single signal phase.
If you get a noise result that differs from the published figure by precisely 6dB it might be wise to suspect a bal/unbal hookup quirk and investigate.


2. Cover everything in aluminum foil which has then been connected to
the Bryston 10B's ground via an alligator clip
Try it without and see if EMI (RF) is a problem before seeking solutions that may not be
needed. Foil may keep out some airborn EMI (RF) but won't do anything for induced AC line
frequency contamination or power supply ripple.


3. Possibly put everything in a metal box. Bear in mind the Bryston
10B already comes in a thick metal box... so not sure what would be
helping here.
Same story, if Bryston did their job right and there are no AC transformers nearby extra
shielding may be superfluous. As you probably know exclusion of magnetic fields due to AC
line current requires ferrous shielding. My Bryston 4B has a very fine metal case however it
mostly aluminum.


4. Don't run the 10B off mains, instead run it off a clean sinewave
generator that's battery backed. For example, a true-sinewave UPS
that's had its output filtered via some chokes (to get rid of DAC
zipper noise). (I do know that "stepped sinewave" UPSes are terrible).
First find out how it behaves 'out of the box'. A good brand like Bryston probably takes care to
eliminate contamination with ripple and hum at the design phase.


One of the common way-points in measuring maximum signal within preamps equalizers and
line level gear in general that is useful to memorize is the clipping level of an opamp based
linear amplifier with symmetrical +/-15V DC supplies, maximum output at clipping is close to
+22dBv, just a little less than 10V rms.
That's interesting. This decibel calculator helped me figure out
quickly where those numbers came from:

http://www.sengpielaudio.com/calculator-db-volt.htm

The Bryston has +/- 24 volt rails, and discrete op amps running off of
them. Which means it has 4.4 decibels more output at clipping than a
+/- 15V system. Makes sense. Thanks. Good to remember that stuff.
Appreciate you pointing this out!

I will read the AP booklet. Thanks. Good idea on starting with simpler
measurements and on calibrating using the HP.

On Fri, May 14, 2021 at 1:41 AM Ted Rook <rooknrol@...> wrote:


On 13 May 2021 at 18:31, cheater cheater wrote:

Hi Ted, thanks a lot. Great ebay find by the way. It's on its way to
me now. Thanks for sharing your experience with someone learning like
I am!
You are welcome.

I have interspersed some replies...........


I've had a few questions, if that's ok:

1. Is it possible to measure dBV readings with this meter? I know it
doesn't have the markings for dBV, only dBm, but is it just a case of
taking the reading, adding some sort of fixed offset, and now I have
the number in dBV? I know there's a simple relationship between dBu
and dBV but I think the relationship between dBm and dBV is on a case
by case basis. I know the meter has high input impedance - so it's not
really going to load the output of the DUT like a real 600 Ohm input
meter would. But it provides a dBm "into 600 Ohm" measurement. I guess
that's just simulated?
This is a voltmeter having a high input impedance and it is calibrated as an rms voltmeter
with 0dB being 0.775V rms which is 0dBv (0dB little v). When the circuit under test is loaded
by 600ohms then the 0dB reading is also 0dBm (which is 1mw in 600ohms). There are
probably few applications in audio that require dBm these days so what matters is the 0dB =
0dBv calibration = 0.775V rms.
0dBV (0dB big V) is as you probably know 1Vrms. The difference between 0dBv and 0dBV is
2.2dB. When using the meter I worked most of the time in dBv and on the few occasions that
dBV was needed I made the simple arithmetic conversion 0dBV = +2.2dBv.


2. The device I'd like to measure right now is the Bryston 10B. I am
not sure if they use rms noise or NRI. Thanks for teaching me about
the distinction. The specifications are here:

http://old.bryston.com/PDF/Manuals/300001[10B].pdf

go to page 4, it's in the middle. It just says "Noise: > -100 dB unweighted".

Its output impedance is 100 Ohms. I see the meter itself has 2MOhm
input impedance. Does this mean it'll be perfectly able to measure the
noise at its lowest settings?
Yes, though in this case probably on the 80 or 90dB range because although they don't state
it the -100dB noise is probably referred to maximum signal output, at the onset of clipping,
which might be +20dBv for example, this means the -100dB noise floor is at -80dBv.

When making a published noise specification for a noise specification to be valid a number of
data points are needed; input condition; voltage (and type of rectifier); reference level;
bandwidth.

input condition may be open circuit (often a poor choice and unrepresentative of real world);
short circuit (a better choice but may be also unrepresentative of real world); source
impedance specified, 200 ohms fitted across the input terminals for example.

the voltage choices are rms or peak rectification

the reference level should be stated unambiguiously XdB is not enough, XdB with reference
to what? some reference level relative to clipping? clipping level itself? at what frequency?
The choices dBv, dBm and dBV are all usable for the reference; 1kHz is traditional.


bandwidth requires two statements, the filter bandwidth itself -3dB at 10Hz and 10kHz for
example; then the measurement may have been made using some kind of weighting network
which should be described, or if none then the statement unweighted should be made.

here is an example of a well written noise specification:

output noise with input shorted: -75dBv rms 20Hz - 20kHz -3dB unweighted

note that the use of dBv as the voltage description means the reference level is included, it is
0dBv 0.775V rms


Does the measurement of RMS noise at the
lowest settings get affected by the DUT's output impedance?
No


I'd also like to measure the noise of DAC outputs. I think someone
mentioned that those should be measured differently, but I'm not
really sure how that is supposed to work.
Digital audio does some things differently, it is outside my experience.
There is a little booklet published by Audio Precision that might provide an explanation. The
booklet seems to be available for download at the AP website, it might be helpful

https://www.ap.com/technical-library/?search_type=tech-library&search=audio+measurement
+handbook&tech_library_category[]=white-papers-books&load_page=1

If you go over to the AP website checkout their models Portable One and Two, these were
made in the 1990s into the 2000s and are standalone, no operating system to run. The One
seems common on ebay.com, how they age I don't know.


I assume given its ranges, this meter can also be used to measure the
noise output of power amps (when nothing is playing through them).
yes, all voltages, noise and signal, can be measured.

One of the common way-points in measuring maximum signal within preamps equalizers and
line level gear in general that is useful to memorize is the clipping level of an opamp based
linear amplifier with symmetrical +/-15V DC supplies, maximum output at clipping is close to
+22dBv, just a little less than 10V rms.


3. Why the Levell specifically, and not some other device? Are there
other such good devices you would recommend?
The Levell has some advantages being a straightforward analogue instrument there is no
operating system to support unlike PC based machines. Being a small dedicated instrument
running off battery it is portable and inexpensive. I believe it is suited to audio, where
absolute precision is often less important than ease of use (short learning curve) and
robustness, and being single-ended (unbalanced) is often not a problem.

I purchased a TM3B unit myself ten years ago via ebay, the internal harness for the 9V
battery is an obsolete terminal type, I substituted the generic 9V 6LR61 battery harness by
snipping the wires, modern alkaline batteries seem to last about as long as the big old zinc
9V batteries originally specified.
For many years the TM3B was part of my test bench in the small lab developing prototypes in
the early days of Allen + Heath the mixer company.
One of the things you are going to encounter while attempting precision noise measurement
is the prevalence of interference from magnetic and electromagnetic sources, chiefly nearby
power supply transformers, power supply switching devices, lighting power supplies, AM and
FM wireless radiation, and anything else that is transmitting. When the wanted signal is a few
microvolts it should be presumed that the noise voltmeter display is erroneous because of
contamination with hum ripple and RF until inspection of the voltmeter output on a scope
reveal clean noise and only clean noise. Listening to the noise is also a useful check to
make. The TM3B output terminal allows connection to a scope and/or listening device and is
20dB below the meter indication. In my experience decontaminating the signal was the
skilled part of noise (and distortion) measurement.

Let's hope the unit arrives safely and is working. They take about one minute to power-up so
don't be alarmed when you don't see a display immediately after switching on. To evaluate
performance I suggest some simple sine wave measurements across the range of
frequencies and voltages before digging into the realm of noise. Perhaps you will be able to
evaluate the absolute accuracy of the Levell using the HP?

Good luck

Thanks a lot!

BTW, I compared all the meters mentioned in this thread. This is what
I came up with (see link below). Some of the numbers might be a bit
off, so double check - but it looks like the Levell is best
inexpensive meter for my use due to low self noise, good range, and
high input impedance. I'd love to hear comments.

https://imgur.com/gallery/bmdNN01

I'm also getting a 3457A (from Liam, thanks!) which will be very
useful and has a much
higher resolution - but might not always be the most practical.


On Thu, May 13, 2021 at 2:16 AM Ted Rook <rooknrol@...> wrote:

Audio Precision make some very good dedicated audio instruments which will probably serve
your purpose.

At a different and simpler level of sophistication is the Levell TM3B audio microvoltmeter,
they were made in the UK in the 1970s-90s. This is a dedicated audio microvoltmeter ideally
suited to measurement of signals and noise between about 10V rms and 10uV rms, the FSD
of the -100dB range is 15uV. Bandwidth can be set at 10k or 100k. The unit requires a 9V
battery so introduces no ground loops or noise. there is one for sale on ebay.co.uk right now,
search for Levell TM3B.

In seeking to measure in the range below -100dBu you are probably going further than you
need towards complexity and expense. It is probable that the equipment you have noticed
quoting noise performance of -110 and -120dB may be not rms noise voltage specifications
but rather a type of noise measurement used for high quality microphone preamps known as
'noise referred to input' that for a nominal 200 ohm microphone source at room temperature
has a theoretical best value of about -128dB NRI. This is calculated by measuring the gain of
the device, measuring its output noise at that gain with a 200 ohm resistor at the input
teminals, and adding the gain to the noise floor, so 60dB gain and -67dBu noise floor gives
-127dB NRI. Notice that the measurement of noise is in the range of millivolts rms, a very
different proposition to 130dBu rms noise.

It is quite likely you can find out most of what you need to know using the Levell audio
microvoltmeter.

Ted




On 12 May 2021 at 1:20, cheater cheater wrote:

Hi Liam,
thanks, I appreciate your extensive experience in this. I'm sure I'll
have a lot of questions that you'll be able to also answer along the
way. I'm setting up a mastering studio (as a hobby) and one of the
things I'd like to do is to be able to measure the noise floor of my
monitoring chain. Some elements claim to be -100 dBV noise, but I
don't necessarily trust that, so I'd like to measure that. Some other
circuits are claiming to be -120 dBV to -110 dBV self-noise. But also
for the future, I'd like to be able to measure other circuits. I'll be
replacing parts of my monitoring chain in the near future as well and
that'll require some noise measurement as well (among other things,
but low noise measurement is one of the hardest things). For the
future I'd like to be able to use the set up I learn about in this
process in designing analog audio circuits. I don't think I'll be
creating ultra low noise mic amplifiers, but I'll be creating analog
audio electronics none the less.

Thanks

On Tue, May 11, 2021 at 8:32 PM Liam Perkins <hifi@...> wrote:

OK, look: what you want to do is not. easy. and short of something
modern like a Keithley nanovoltmeter there's essentially nothing off the
shelf that will get you there, ballpark but not there.

I spent 15 years measuring vacuum tube equivalent input noise and know
exactly what I'm on about. See this:

https://www.pearl-hifi.com/03_Prod_Serv/Cryo/Cryo_Intro.html

I measured 1,000s upon 1,000s of the very best of the legendary NOS
parts for people who then went on to sell them for 100s of dollars. I
provided a 13 month sliding scale warranty and during that time never
needed to replace more than a mere handful of parts because anything that
made it thru what I put parts thru was a good part.

I recommended Jim Williams work and that of Geller labs.
The Williams LT app notes you want are nos. 124 and 159 and Geller Labs you
can find on the WayBack about 2013 and the J-can article is here:

http://physicsopenlab.org/wp-content/uploads/2016/10/JCan-NV-article.pdf

Further, at the bottom of this page on my site, here:


https://www.pearl-hifi.com/06_Lit_Archive/07_Misc_Downloads/Misc_Downloads.html

see nos. 100, 103 and 105 as goldmine info on electrolytic caps and a
little known NIST paper from the days when it was NBS on a clever way to
use two-channel FFT to correlate the noise floor of the lowest noise amps
you can build down about 20dB; takes all day to run 10K averages but it
gets you there.

I spent hours on the restoration of that doc and the included refs.
That same method is well known in low noise metrology and Google on that
topic will keep you out the bar for at least the next month wading thru it
all.

Ralph Morrison is someone whose many, many works you need to know
backwards. I have about 6 of his titles in hardcopy, one of which I I
photocopied 30 years ago and had hardbound into a proper book. I also have
about 6 more in indexed PDF I'll provide free for the asking.

Although I pulled them down here:

https://b-ok.cc/

it ain't exactly legal to be puttin' them up on my site for all and sundry,
nor the highly useful works of Burkhard Vogel nor Horowitz & Hill whose
"Art of Electronics" which has been a standard for decades. The 3rd edn is
also found at Z-Lib.

Now, -what- are you trying to do, exactly; because until we know we're
all just throwing sh*t at one wall or another.

Do you need HF and if so how high, are you looking at 1/f and if so how
low, do you -really- need true rms and if so, why, because HP's earlier
400-series rms-reading, average-responding AC voltmeters will get you
within about a dB if you're measuring noise.

The 400GL and the 400F provide FSDs of -80dB, I have two of each and
plans to swap out the input JFETS in the 'Fs for modern much quieter parts
from Linear Integrated Systems.

Put Matt's +60db LN gain block in front of one of those and you are
home and dry.

Liam

On Tue, May 11, 2021 at 9:44 AM cheater cheater <cheater00social@...>
wrote:

On Tue, May 11, 2021 at 3:07 PM Matt <mhofmann@...> wrote:

I have used an HP 3400A to measure the noise in circuits for several
applications since the 1970s, both for business and pleasure. I found it
useful for determining the equivalent input noise for various microphone
preamps that I had built. Typically I used a low noise solid-state preamp
on the front end of the HP 3400 with a low-pass filter on the input of the
3400A to reduce the bandwidth to the audible range. With this arrangement
I could get 60 dB of gain on the preamp, and I could measure the equivalent
input noise of the microphone preamp I was testing. I could get quite a
bit of sensitivity with this arrangement. I would set the HP 3400A to 1
mVrms and add another 60 dB of gain with the low noise preamp, resulting in
1 uVrms full-scale sensitivity on the meter.

It seems like this is the kind of scenario I should be looking at.
What LNA were you using?

Liam mentioned the J-Can and he had parts for it available. I think
this should be the way to go.

Is it possible to modify the HP 3400A to have a dBV scale?

Thanks.

I used this arrangement for solid-state microphone preamps that I was
designing and building as well as a tube based microphone preamp that later
on I built for my boss.
I have also used an FFT based spectrum analyzer program on an old laptop
PC that was useful in identifying the noise floor of these preamps.
I bought the HP 3400A on eBay a number of years ago for about $50 (I
could have been a bit more).

Matt































Re: Looking for sensitive audio RMS meter

Ted Rook
 

On 14 May 2021 at 2:26, cheater cheater wrote:

Thanks again Ted. I didn't know you worked at A&H. I haven't owned any
of their products but they are really good quality and I'm often
jealous of friends who have them. If you don't mind me asking, what
was your role at the company? It's fascinating to hear people talk
about what they did at famous companies like that.
When I was there we were a very small operation, in the 1970s maybe a dozen people total. I
was a live sound technician (roadie) which in those far off simple days qualified me for the
test department (which was a new development, business was good) where I learned to drive
a scope a voltmeter and a signal generator. Later I contributed to development of new
designs for live and recording mixers evaluating prototypes designed out of house, then later
still developing new prototypes from the ground up inhouse. I worked in London at two
locations and in Brighton, by which time we had a factory in Cornwall that is still the company
HQ.


I've had a few more questions about the meter as well, if that's OK.

You say the TM3B is single-ended, not balanced. What does that
actually mean for a device that's battery powered and essentially
floating? I could connect the Bryston 10B's XLR output - the hot and
cold pins - to the input of the TM3B. Isn't that balanced?
Yes, the meter itself is floating however once connections are made to scope/listening device
at the meter output terminals grounding via the meter probe common enters the picture.


My PMC / Bryston 10B only has symmetrical balanced XLR outputs. If I
wanted to measure the noise, what should I measure: between hot and
cold, gnd and hot, gnd and cold?
Hot and cold. The voltage between ground and one of the phases alone is half amplitude,
-6dB.


You mention using a small listening device for listening from the
TM3B's output. That's a really good idea. I shall use a small desktop
monitor. But maybe I should use a battery powered speaker, that should
prevent any ground loops. What do you think?
Yes that would work for the listening device but leaves the scope ground as a source of
grounding.

When trying to lower noise, what are the options? Here's a few things
that I think of:

1. use a cable that goes directly from the XLR output of the 10B to
the BNC input of the TM3B. Make a cable myself using one of those well
shielded cables like here:

https://gothamcables.com/en/gothamcables/balancedanalog

they have cables that are shielded 5 times... whether that's necessary
is a question, but it's more interesting to ask whether it'll make
things worse in some surprising way. Either way the shielding is super
thick on those cables, even the ones that are just shielded 1 or 2
times.
Special shielding precautions are probably unnecessary unless you are working close to
powerful sources of RF like transmitters or arc welders, in which case give up noise testing! I
used ready-made BNC to banana cords with croc-clip probes for signal and shield. At the
equipment insert an open female XLR shell onto the output pins and hook up the croc-clips to
the terminals of interest, often pins 2 and 3. Should experimentation be needed to find the
appropriate balanced/unbalanced hookup things are easily varied just by reclipping.

Some active balanced output circuits will give a signal 6dB low unless both phases are connected.
Some circuits require grounding the unused phase as a way to trigger automatic internal 6dB gain increase that compensates for unbalanced connection using only a single signal phase.
If you get a noise result that differs from the published figure by precisely 6dB it might be wise to suspect a bal/unbal hookup quirk and investigate.


2. Cover everything in aluminum foil which has then been connected to
the Bryston 10B's ground via an alligator clip
Try it without and see if EMI (RF) is a problem before seeking solutions that may not be
needed. Foil may keep out some airborn EMI (RF) but won't do anything for induced AC line
frequency contamination or power supply ripple.


3. Possibly put everything in a metal box. Bear in mind the Bryston
10B already comes in a thick metal box... so not sure what would be
helping here.
Same story, if Bryston did their job right and there are no AC transformers nearby extra
shielding may be superfluous. As you probably know exclusion of magnetic fields due to AC
line current requires ferrous shielding. My Bryston 4B has a very fine metal case however it
mostly aluminum.


4. Don't run the 10B off mains, instead run it off a clean sinewave
generator that's battery backed. For example, a true-sinewave UPS
that's had its output filtered via some chokes (to get rid of DAC
zipper noise). (I do know that "stepped sinewave" UPSes are terrible).
First find out how it behaves 'out of the box'. A good brand like Bryston probably takes care to
eliminate contamination with ripple and hum at the design phase.


One of the common way-points in measuring maximum signal within preamps equalizers and
line level gear in general that is useful to memorize is the clipping level of an opamp based
linear amplifier with symmetrical +/-15V DC supplies, maximum output at clipping is close to
+22dBv, just a little less than 10V rms.
That's interesting. This decibel calculator helped me figure out
quickly where those numbers came from:

http://www.sengpielaudio.com/calculator-db-volt.htm

The Bryston has +/- 24 volt rails, and discrete op amps running off of
them. Which means it has 4.4 decibels more output at clipping than a
+/- 15V system. Makes sense. Thanks. Good to remember that stuff.
Appreciate you pointing this out!

I will read the AP booklet. Thanks. Good idea on starting with simpler
measurements and on calibrating using the HP.

On Fri, May 14, 2021 at 1:41 AM Ted Rook <rooknrol@...> wrote:


On 13 May 2021 at 18:31, cheater cheater wrote:

Hi Ted, thanks a lot. Great ebay find by the way. It's on its way to
me now. Thanks for sharing your experience with someone learning like
I am!
You are welcome.

I have interspersed some replies...........


I've had a few questions, if that's ok:

1. Is it possible to measure dBV readings with this meter? I know it
doesn't have the markings for dBV, only dBm, but is it just a case of
taking the reading, adding some sort of fixed offset, and now I have
the number in dBV? I know there's a simple relationship between dBu
and dBV but I think the relationship between dBm and dBV is on a case
by case basis. I know the meter has high input impedance - so it's not
really going to load the output of the DUT like a real 600 Ohm input
meter would. But it provides a dBm "into 600 Ohm" measurement. I guess
that's just simulated?
This is a voltmeter having a high input impedance and it is calibrated as an rms voltmeter
with 0dB being 0.775V rms which is 0dBv (0dB little v). When the circuit under test is loaded
by 600ohms then the 0dB reading is also 0dBm (which is 1mw in 600ohms). There are
probably few applications in audio that require dBm these days so what matters is the 0dB =
0dBv calibration = 0.775V rms.
0dBV (0dB big V) is as you probably know 1Vrms. The difference between 0dBv and 0dBV is
2.2dB. When using the meter I worked most of the time in dBv and on the few occasions that
dBV was needed I made the simple arithmetic conversion 0dBV = +2.2dBv.


2. The device I'd like to measure right now is the Bryston 10B. I am
not sure if they use rms noise or NRI. Thanks for teaching me about
the distinction. The specifications are here:

http://old.bryston.com/PDF/Manuals/300001[10B].pdf

go to page 4, it's in the middle. It just says "Noise: > -100 dB unweighted".

Its output impedance is 100 Ohms. I see the meter itself has 2MOhm
input impedance. Does this mean it'll be perfectly able to measure the
noise at its lowest settings?
Yes, though in this case probably on the 80 or 90dB range because although they don't state
it the -100dB noise is probably referred to maximum signal output, at the onset of clipping,
which might be +20dBv for example, this means the -100dB noise floor is at -80dBv.

When making a published noise specification for a noise specification to be valid a number of
data points are needed; input condition; voltage (and type of rectifier); reference level;
bandwidth.

input condition may be open circuit (often a poor choice and unrepresentative of real world);
short circuit (a better choice but may be also unrepresentative of real world); source
impedance specified, 200 ohms fitted across the input terminals for example.

the voltage choices are rms or peak rectification

the reference level should be stated unambiguiously XdB is not enough, XdB with reference
to what? some reference level relative to clipping? clipping level itself? at what frequency?
The choices dBv, dBm and dBV are all usable for the reference; 1kHz is traditional.


bandwidth requires two statements, the filter bandwidth itself -3dB at 10Hz and 10kHz for
example; then the measurement may have been made using some kind of weighting network
which should be described, or if none then the statement unweighted should be made.

here is an example of a well written noise specification:

output noise with input shorted: -75dBv rms 20Hz - 20kHz -3dB unweighted

note that the use of dBv as the voltage description means the reference level is included, it is
0dBv 0.775V rms


Does the measurement of RMS noise at the
lowest settings get affected by the DUT's output impedance?
No


I'd also like to measure the noise of DAC outputs. I think someone
mentioned that those should be measured differently, but I'm not
really sure how that is supposed to work.
Digital audio does some things differently, it is outside my experience.
There is a little booklet published by Audio Precision that might provide an explanation. The
booklet seems to be available for download at the AP website, it might be helpful

https://www.ap.com/technical-library/?search_type=tech-library&search=audio+measurement
+handbook&tech_library_category[]=white-papers-books&load_page=1

If you go over to the AP website checkout their models Portable One and Two, these were
made in the 1990s into the 2000s and are standalone, no operating system to run. The One
seems common on ebay.com, how they age I don't know.


I assume given its ranges, this meter can also be used to measure the
noise output of power amps (when nothing is playing through them).
yes, all voltages, noise and signal, can be measured.

One of the common way-points in measuring maximum signal within preamps equalizers and
line level gear in general that is useful to memorize is the clipping level of an opamp based
linear amplifier with symmetrical +/-15V DC supplies, maximum output at clipping is close to
+22dBv, just a little less than 10V rms.


3. Why the Levell specifically, and not some other device? Are there
other such good devices you would recommend?
The Levell has some advantages being a straightforward analogue instrument there is no
operating system to support unlike PC based machines. Being a small dedicated instrument
running off battery it is portable and inexpensive. I believe it is suited to audio, where
absolute precision is often less important than ease of use (short learning curve) and
robustness, and being single-ended (unbalanced) is often not a problem.

I purchased a TM3B unit myself ten years ago via ebay, the internal harness for the 9V
battery is an obsolete terminal type, I substituted the generic 9V 6LR61 battery harness by
snipping the wires, modern alkaline batteries seem to last about as long as the big old zinc
9V batteries originally specified.
For many years the TM3B was part of my test bench in the small lab developing prototypes in
the early days of Allen + Heath the mixer company.
One of the things you are going to encounter while attempting precision noise measurement
is the prevalence of interference from magnetic and electromagnetic sources, chiefly nearby
power supply transformers, power supply switching devices, lighting power supplies, AM and
FM wireless radiation, and anything else that is transmitting. When the wanted signal is a few
microvolts it should be presumed that the noise voltmeter display is erroneous because of
contamination with hum ripple and RF until inspection of the voltmeter output on a scope
reveal clean noise and only clean noise. Listening to the noise is also a useful check to
make. The TM3B output terminal allows connection to a scope and/or listening device and is
20dB below the meter indication. In my experience decontaminating the signal was the
skilled part of noise (and distortion) measurement.

Let's hope the unit arrives safely and is working. They take about one minute to power-up so
don't be alarmed when you don't see a display immediately after switching on. To evaluate
performance I suggest some simple sine wave measurements across the range of
frequencies and voltages before digging into the realm of noise. Perhaps you will be able to
evaluate the absolute accuracy of the Levell using the HP?

Good luck

Thanks a lot!

BTW, I compared all the meters mentioned in this thread. This is what
I came up with (see link below). Some of the numbers might be a bit
off, so double check - but it looks like the Levell is best
inexpensive meter for my use due to low self noise, good range, and
high input impedance. I'd love to hear comments.

https://imgur.com/gallery/bmdNN01

I'm also getting a 3457A (from Liam, thanks!) which will be very
useful and has a much
higher resolution - but might not always be the most practical.


On Thu, May 13, 2021 at 2:16 AM Ted Rook <rooknrol@...> wrote:

Audio Precision make some very good dedicated audio instruments which will probably serve
your purpose.

At a different and simpler level of sophistication is the Levell TM3B audio microvoltmeter,
they were made in the UK in the 1970s-90s. This is a dedicated audio microvoltmeter ideally
suited to measurement of signals and noise between about 10V rms and 10uV rms, the FSD
of the -100dB range is 15uV. Bandwidth can be set at 10k or 100k. The unit requires a 9V
battery so introduces no ground loops or noise. there is one for sale on ebay.co.uk right now,
search for Levell TM3B.

In seeking to measure in the range below -100dBu you are probably going further than you
need towards complexity and expense. It is probable that the equipment you have noticed
quoting noise performance of -110 and -120dB may be not rms noise voltage specifications
but rather a type of noise measurement used for high quality microphone preamps known as
'noise referred to input' that for a nominal 200 ohm microphone source at room temperature
has a theoretical best value of about -128dB NRI. This is calculated by measuring the gain of
the device, measuring its output noise at that gain with a 200 ohm resistor at the input
teminals, and adding the gain to the noise floor, so 60dB gain and -67dBu noise floor gives
-127dB NRI. Notice that the measurement of noise is in the range of millivolts rms, a very
different proposition to 130dBu rms noise.

It is quite likely you can find out most of what you need to know using the Levell audio
microvoltmeter.

Ted




On 12 May 2021 at 1:20, cheater cheater wrote:

Hi Liam,
thanks, I appreciate your extensive experience in this. I'm sure I'll
have a lot of questions that you'll be able to also answer along the
way. I'm setting up a mastering studio (as a hobby) and one of the
things I'd like to do is to be able to measure the noise floor of my
monitoring chain. Some elements claim to be -100 dBV noise, but I
don't necessarily trust that, so I'd like to measure that. Some other
circuits are claiming to be -120 dBV to -110 dBV self-noise. But also
for the future, I'd like to be able to measure other circuits. I'll be
replacing parts of my monitoring chain in the near future as well and
that'll require some noise measurement as well (among other things,
but low noise measurement is one of the hardest things). For the
future I'd like to be able to use the set up I learn about in this
process in designing analog audio circuits. I don't think I'll be
creating ultra low noise mic amplifiers, but I'll be creating analog
audio electronics none the less.

Thanks

On Tue, May 11, 2021 at 8:32 PM Liam Perkins <hifi@...> wrote:

OK, look: what you want to do is not. easy. and short of something
modern like a Keithley nanovoltmeter there's essentially nothing off the
shelf that will get you there, ballpark but not there.

I spent 15 years measuring vacuum tube equivalent input noise and know
exactly what I'm on about. See this:

https://www.pearl-hifi.com/03_Prod_Serv/Cryo/Cryo_Intro.html

I measured 1,000s upon 1,000s of the very best of the legendary NOS
parts for people who then went on to sell them for 100s of dollars. I
provided a 13 month sliding scale warranty and during that time never
needed to replace more than a mere handful of parts because anything that
made it thru what I put parts thru was a good part.

I recommended Jim Williams work and that of Geller labs.
The Williams LT app notes you want are nos. 124 and 159 and Geller Labs you
can find on the WayBack about 2013 and the J-can article is here:

http://physicsopenlab.org/wp-content/uploads/2016/10/JCan-NV-article.pdf

Further, at the bottom of this page on my site, here:


https://www.pearl-hifi.com/06_Lit_Archive/07_Misc_Downloads/Misc_Downloads.html

see nos. 100, 103 and 105 as goldmine info on electrolytic caps and a
little known NIST paper from the days when it was NBS on a clever way to
use two-channel FFT to correlate the noise floor of the lowest noise amps
you can build down about 20dB; takes all day to run 10K averages but it
gets you there.

I spent hours on the restoration of that doc and the included refs.
That same method is well known in low noise metrology and Google on that
topic will keep you out the bar for at least the next month wading thru it
all.

Ralph Morrison is someone whose many, many works you need to know
backwards. I have about 6 of his titles in hardcopy, one of which I I
photocopied 30 years ago and had hardbound into a proper book. I also have
about 6 more in indexed PDF I'll provide free for the asking.

Although I pulled them down here:

https://b-ok.cc/

it ain't exactly legal to be puttin' them up on my site for all and sundry,
nor the highly useful works of Burkhard Vogel nor Horowitz & Hill whose
"Art of Electronics" which has been a standard for decades. The 3rd edn is
also found at Z-Lib.

Now, -what- are you trying to do, exactly; because until we know we're
all just throwing sh*t at one wall or another.

Do you need HF and if so how high, are you looking at 1/f and if so how
low, do you -really- need true rms and if so, why, because HP's earlier
400-series rms-reading, average-responding AC voltmeters will get you
within about a dB if you're measuring noise.

The 400GL and the 400F provide FSDs of -80dB, I have two of each and
plans to swap out the input JFETS in the 'Fs for modern much quieter parts
from Linear Integrated Systems.

Put Matt's +60db LN gain block in front of one of those and you are
home and dry.

Liam

On Tue, May 11, 2021 at 9:44 AM cheater cheater <cheater00social@...>
wrote:

On Tue, May 11, 2021 at 3:07 PM Matt <mhofmann@...> wrote:

I have used an HP 3400A to measure the noise in circuits for several
applications since the 1970s, both for business and pleasure. I found it
useful for determining the equivalent input noise for various microphone
preamps that I had built. Typically I used a low noise solid-state preamp
on the front end of the HP 3400 with a low-pass filter on the input of the
3400A to reduce the bandwidth to the audible range. With this arrangement
I could get 60 dB of gain on the preamp, and I could measure the equivalent
input noise of the microphone preamp I was testing. I could get quite a
bit of sensitivity with this arrangement. I would set the HP 3400A to 1
mVrms and add another 60 dB of gain with the low noise preamp, resulting in
1 uVrms full-scale sensitivity on the meter.

It seems like this is the kind of scenario I should be looking at.
What LNA were you using?

Liam mentioned the J-Can and he had parts for it available. I think
this should be the way to go.

Is it possible to modify the HP 3400A to have a dBV scale?

Thanks.

I used this arrangement for solid-state microphone preamps that I was
designing and building as well as a tube based microphone preamp that later
on I built for my boss.
I have also used an FFT based spectrum analyzer program on an old laptop
PC that was useful in identifying the noise floor of these preamps.
I bought the HP 3400A on eBay a number of years ago for about $50 (I
could have been a bit more).

Matt































Re: Looking for sensitive audio RMS meter

Harvey Parent
 

The comment ‘Smacks of Snake Oil’ can be your opinion if you wish, but


Re: 7934 troubleshooting

JJ
 

In my 7904, I purchased some ribbon cables on eBay (UXCELL Hot 40Pin Header Rainbow Color Flat Ribbon Cable Idc Wire 1.3mm DIY for F). to use as extender cables for the power supply. The wires can be stripped off to get the number of wires you need for each cable. I needed to move the power supply out the back so that I could remove the cage and get to the circuit boards while still having the PS connected to the 7904. It wasn't that difficult. Just be careful that you don't misalign the cables. And, work slowly. Stop when you get tired. Good luck.

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