Audio Preamplifier Design Project

[UniEngineer]

Hi,

I am designing an audio preamplifier with the following specifications:

Gain Stage:

Input 1: Low impedance source (1.2k) balanced input (differential i/p plus ground) with a variable gain of 0dB to 60dB

Input 2: High Impedance source (10k) unbalanced (signal plus ground) with a variable gain of 0dB to 40dB.

I have decided to use an instrumentation amplifier for the first input, I have a designed circuit but im not sure on how to work out the component values of the resistors and POTs.

Are there any rules of thumb i need to apply?

[audioguru]

I think of audio opamps with very low noise, very low distortion and bandwidth to about 100kHz.
I think of instrumentation opamps for low frequency hearbeats and DC weighting scales. Noise , distortion and bandwidth don't matter.
Did you find an instrumentation opamp that would be good for an audio preamp?

True Audio and Rane have schematics of their modern Pro Audio equipment on their websites. They use audio opamps in their preamps.

[Kevin Weddle]

Maybe you could use a noise generator to test how well it rejects noise. If you know the source of the noise in your particular application, you might incorporate a filter.

[audioguru]

Kevin,
Noise in the audio band is an audio signal. All audio amplifiers will pass it. If you filter out all noise then you filter out all audio.

The same applies to video signals.

If the noise is a low frequency rumbling noise then filter out all the bass frequencies.
If the noise is a high frequency hiss then filter out all the treble frequencies.
If the noise is midrange frequencies then filter out all the midrange frequencies.
If the noise is wideband then filter out everything or just replace or turn off the noisy opamp.

[Kevin Weddle]

I agree. Instrumentation amplifiers are used to rid large signal common mode noise from small signal single ended input information. I only know of airwave interference when it comes to common mode noise, or maybe crosstalk. I would certainly want to test my circuit to prove that my instrumentation amplifier was actually doing some good. And a filter circuit could work better in cases where the common mode signal noise was not of equal input amplitude or out of phase, and the information would not be lost by filtering.