50¢ mic preamp

Some time ago I was trying to rebuild after losing my recording studio to a fire, I got some unreasonably good microphones (for the price - I think they were about €10 each!) but had no money and just a few components to try to get the signal from these mics into my laptop. They were supposed to operate OK from the 3V coming out of the laptop for capacitor mics, but didn't do well on that - so I decided to put together something to boost the signal, send 48V phantom power into the mics, and give me the option of using them both with sterio or one with a balanced input going into the laptop that could get be recombined internally for reduction of the noise in the laptops own circuits. it needed to get the signal up to line level so I could turn off the laptops terrible built-in amps, and provide them with proper 48V power.

I had previously experimented with doing mic preamps with op-amps and had come to the conclusion that op-amps are just too noisy to use in high gain, low input situations like this, So I came up with a design based on bipolar transistors, based on the principles of one I found online. I had a bunch of BC546/BC556's hanging about that cost about 5¢ each and can handle the 48V power, the design called for 5 of them in total - 4 carrying signal and one helping with power supply smoothing for the 48V phantom and amplifier. other than that it was just a few resistors and about 10 2¢ 10uf RS-basics capacitors with 100nf SMT ceramic capacitors to reduce their inductance to carry higher frequencies better, and a little bit of stripboard.

And it worked. Really F-ing well. Low noise, even into my laptop's mic input, clear highs and solid lows too. Not as good as the USB soundcard with XLR inputs I got later on, but for something that cost so little it did so well, and the most expensive part was the box! (2€ electrical box from local diy store). The voltage booster I used for the 48V was about €1.60 as well, but the preamp itself was a total of about 50¢.

If anyone is interested I'll post schematics, photo's and soundclips.