Re: Looking for sensitive audio RMS meter


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!

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?

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:[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? Does the measurement of RMS noise at the
lowest settings get affected by the DUT's output impedance?

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.

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).

3. Why the Levell specifically, and not some other device? Are there
other such good devices you would recommend?

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.

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 <> 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 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


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.


On Tue, May 11, 2021 at 8:32 PM Liam Perkins <> 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:

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:

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

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

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:

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.


On Tue, May 11, 2021 at 9:44 AM cheater cheater <>

On Tue, May 11, 2021 at 3:07 PM Matt <> 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?


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).


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