Re: Looking for sensitive audio RMS meter


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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