Re: Looking for sensitive audio RMS meter

Liam Perkins

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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