Month: April 2010

The little Mic Preamp works well but singers would have to do the old moving of the mic for big signals as in move the mic away when you go large and bring it in real close for whispers…. which is all well and good if the microphone is being held by someone competant in that way but can be rather nasty if sent into clipping all the time a peak goes through.

If it’s statically picking up sound pressure from a source with lots of dynamics then problems are going to occur and some form of anti logarythmic amplification is required, ie; compression.

The signal source gets louder and so the amplification goes down. One of the easiest way to acheive such things, and given I’m seriously vacant when it comes to what really can be acheived with electronics know how, is to use LED’s and LDR’s. The signal gets changed to a voltage and that lights an LED which in turn effects an LDR, which is a light dependant resistor. The LED gets bright, big signal, and the resistance through the LDR gets lower. The simplest way to do this would be to split the signal at the output of any amplifier and have the signal going out also fed through an LED and have an LDR across the volume pot so if the signal is big the LED lights and the lower resistance in the LDR across the pot brings the signal closer to ground and therefore siphons some off.
Though this is simple it is fraught with complications and would rely on having the right LED and, even harder, the right LDR which may or may not exist. So what we do is add an ability to taper those effects to our needs and this is what I’ll try to do with the schematic below.

Basically you set P1 and P2 so that low signals come through uneffected but as medium signals come in a little compression is added by LDR 2 getting lower in resistance and adding negative feedback to T3 therefore lowering it’s input signal and therefore the output signal.

But peaks still might be too great and end up cutting out the signal altogether so what you do, which seems somewhat counter intuitive, is bring in signal from P3, which up until now has been grounded, and slightly decrease the signal to LDR 2 by way of increasing the resistance of P1 and P2 thereby having less initial compression but allowing more for the peaks.

Now I’ll, well not right now, breadboard it to see whether it’ll even work.

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.

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!