Measuring Noise Levels (S/N)

[Paul Schreiber]

There really are only about 3 instruments made capable of accurately making these measurements. The most widely used (by far) is an Audio Precision Model 1. It is a true 18bit (100dB) device. This generates all of the SNR plots you see in magazines like Stereophile. There is a new Model 2 out that is 22bit (128db). The downside is that these are $10K boxes. I'm trying to find a used Model 1 for $4500.

For 16bit measurements (say 80dB) stuff, there is a Stanford Research FFT analyzer for $1000 that does a nice job. There is an old HP DSP-based box (can't remember the number offhand) that you can get used for around $1300 that is like the SR box, few more bells.

The gist of the problem is that if you are over 35 years old, your high frequency rolloff in your ears is about 14KHz and so you are correct about not hearing anything. Also, it will greatly depend on your amp&speakers if it is audible.

Paul Schreiber
Synthesis Technology

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From: 	Bob Zimmer[SMTP:bzimmer at voicenet.com]
Sent: 	Friday, March 21, 1997 8:57 AM
To: 	synth-diy at horus.sara.nl
Subject: 	Measuring Noise Levels (S/N)

It's been a cold winter in my basement and I'm starting to get back into
building now that it is warming up here on the East Coast!

I've been experimenting with some VCA circuits using various types of
op-amps.  What I've found is that I really can't tell much of a difference
in noise levels between say a TL071 and a NE5532.

If I'm trying to match the performance (or at least get close to) of a
CEM3330 VCA chip (100 db S/N), how can I measure just what the noise floor is???

Also, I've seen headroom measurements on VCA's vary between 55 db and 80 db.
I'm assuming that this is measuring the voltage difference between the
strongest signal and dead quiet in what will pass through the VCA. What is
an appropriate target for a first rate VCA and how does one calculate this?

I'd appreciate any advice you folks can offer!


  >=== Bob Zimmer -- Philadelphia PA ===<
  >=== bzimmer at voicenet.com          ===<