Microphones vs. ear sensitivity

Cleaning the power line (1K - 220 uF, that sort of thing), may help tremendously. Batteries are noisy! Power supplies more so.

Gain distribution, not too much gain in any stage. e.g. ~20x in the transistor, same in the opamp. Never let the frontend transistor amplify flat out (grounded emitter)

As I had the pleasure of commenting before - a junkbox electret / transistor / low end opamp ckt. easily matched my hearing treshold. (when I built it -long ago- my hearing was not all that bad actually)

Hi all.

thanks for all the comments, very educational to this mainly digital guy.

I'd assumed the input impedance should match the source for maximum power transfer. Seems this does not give the best quality.

Surprised to hear batteries are noisy. I had a decent decoupler.

I plugged the preamp into my good-quality hi-fi, and found far less hum than using my TV. No surprise there. Turning up the gain, I found I was picking up radio! Sticking it in a crude faraday cage (okay, biscuit tin) didn't help either.

Sigh, more prototyping to do and more complex circuits.

JT> (1) Shunt feedback stage with a pot as the source impedance.

Hmm.. I thought "hello, that looks a bit odd"

JT> (2) CB input stage... barf.

I had to read up on why people used a transistor for unity gain, and found that it was useful for buffering signals that need a low input impedance (like 75R video) and avoiding the Miller effect.

Ban > You better use a specialized IC with your state of knowledge, Ban > then you get at least some reference circuit and a Ban > good result.

Yep, I'd be happy to buy a chip specifically designed for dynamic mic preamping. I expected they would be pretty common, but haven't seen any.

Ban > Win made a proposition for a good Mic preamp last year

Thanks, I'll look for it.

Has it been tested and the performance measured?

Indeed. Power matching was only needed when gain was expensive.

You get better results with 'voltage matching'. I.e. low source Z higher input Z.

Graham