Simple 20 dollar mic preamp.

Friend of mine needs a mic pre and I’ve been buying a bunch of the jaycar $5.00 1k/8 ohm signal transformers so I thought I’d throw something together using one of these and a transistor.
A Jewel!
I was in Dominion Rd and the antique place we all know and love because of it’s absolute abundance had the usual boxes out front full of cheapies and I happened to have a look through the old watch and ring and necklace boxes… and found one or two metal ones. The above, after some work to remove all the sticky crud inside and out, made quite an interesting enclosure thats speaks volume as metaphor!
As you can see it’s a very simple schematic. Given a 600 ohm microphone on the 8 ohm winding the impedance ratio becomes 600/70k which is good for hitting the base of the transistor. Some tweaking of values might get even better S/N and bandwidth but it pretty darn good with the guesstimates I used. The transistor is a BC546 which was on the bench and I didn’t check the hfe but anywhere fro 200-500 will work. The hardest part was the switching for the battery. If a plugpack is used it switches out the battery but even that won’t allow the circuit to power up until the signal output is plugged in. That means batteries won’t drain unless you leave it plugged into something… Complicated explaining it and even more complicated wiring it up.
I’m going to make some in some pie tin enclosures I’ve got, which really are tin coated steel, and I’ve even thought about well made big wooden and alloy boxes with a choice old knob… containing a point to point wiring job and then seeing if I can sell them on trade me for a silly price. I like the idea of really neat looking enclosures with a kind of vintage aesthetic… and really simple circuits!

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