DIYbanter - View Single Post - Reducing hiss by changing op-amps ?

wrote:

I've just been playing around with the desk and have found that the
biggest source of noise does seem to mic preamps - If I sweep the input
trim pot next to the 20bD pad switch (sorry - can't work out where it
is on the schematic !), then I get a massive change in background hiss
- 30dB's worth !

That's not untypical.

Would be interested to know what Q101 and 102 are though. Unusual to use
npn devbices there - pnp are quieter. Too late to do anything about that
though.

You need to 'terminate' the mic input with around 200 ohms ( to simulate a
mic's source impedance ) to get a realistic idea of the noise though. It'll
be noisy as hell with open-circuit inputs.

I can easily re-scan any section of the schematics if anyone needs a
closer peek.

Later maybe.

Graham