we need help

[PGA]

Koz,

Forgive the intrusion of an unknowledgeable idiot - but is this circuit called a microphone pre-amp? If so, I know the name of the chap in the UK who builds them and can supply contact details - but not in open forum. He prides himself on their being totally noise-free.

[steve]

Koz,

Forgive the intrusion of an unknowledgeable idiot - but is this circuit called a microphone pre-amp? If so, I know the name of the chap in the UK who builds them and can supply contact details - but not in open forum. He prides himself on their being totally noise-free.

Not quite. The circuit described does not "amplify" the signal (a mic pre-amp does), it just supplies the power that the computer mic requires.
As Koz wrote, the problem for anyone wanting to market these battery packs is that they are competing in price with the likes of these: https://www.google.com/search?q=logitec ... e&tbm=shop (and these sound surprisingly good for the price). By the time that you add up the cost of parts (XLR plug $3, metal project box $3, PP3 battery $2, mini-jack socket $1, ...) it's hardly worth the effort (but fun and educational to do, and useful if you already have a couple of good computer headset mics and a mixing desk).

[kozikowski]

He prides himself on their being totally noise-free.

"Low Noise" perhaps. "Totally noise free" is impossible without exotic conditions like liquid nitrogen baths, etc. Not likely.

Electret microphone elements by themselves produce a sound signal with all the robust, crushing power of butterfly wings. Just to get the signal to go down the cable requires a way to stiffen the signal and give it a good push. The best way to do that requires a transistor with power gain, but no volume gain. That's what the 9 volts (or 5 volts from the computer) does. It gives the transistor something to chew on while it's working.

OK, now you have this amazingly tiny signal successfully inside the computer or mixer. That's when the Mic Amp does its magic and boosts the volume of the signal high enough for the sound card or mixer to use it. All amplifiers of this type have noise and the juggling act is to make the noise as low as possible and the signal as high as possible. This is the exact point where many sound cards fall apart. Just being inside the electrically noisy computer is strike one for quality work. Then the sound card has to work with the same 5 volts that the rest of the noisy computer is using. Strike two. Top Quality microphone amplifiers use 30 volts, two fifteen volt supplies.

And it goes on. The best way to create a very low noise mic preamp is to tailor it to the microphone. That's why most commercial microphones say "150 ohm nominal output impedance." Without getting too deep into this, that is designed to exactly match the connection to most top quality microphone amplifiers resulting in the very lowest possible noise and best music quality.

None of this has anything to do with computer microphones. They do not match anything and the mic preamp inside the computer is lucky to get out of bed in the morning, much less amplify your performance with low distortion good volume and low noise. That's why the first step in good quality work is immediately divorce everything analog from the computer. External digitizers, external mic amps. better microphones, and good quality sound mixers. Then you hit the computer, hopefully with a signal already digitized which makes most computer noise and distortion irrelevant.

Koz

[PGA]

That's why the first step in good quality work is immediately divorce everything analog from the computer.

That's precisely how I work: music ripped to WAV from CD (under the terms of a copyright clearance licence), everything else (voice-over and location ambience) recorded to WAV using my Zoom H4 (giving me the copyright); then import the lot into Audacity and "slice and dice"!