well.... if it were me, here's how I'd do it:
first off, it looks like those mics want to see a low-Z preamp. The unit he's using now has a 1k input impedance. It's not unusual for mics to want low-Z, that's generally how mixing board preamps are configured. The classic Mackie VLZ series has 2.5k input Z on all its mic channels, for example. This means that, while a bipolar supply opamp circuit is fine, you won't want one that's optimized for guitar (with a very HIGH input Z). You'll want to design an opamp circuit that has a similarly low input Z. Try something like this:
Attachment:
inverting amplifier.PNG
for this circuit, gain is determined by this equation: Vout = Vin x (1 + (R2/R1))
the input Z is basically the value of R1.
So, to have a input Z of 1k, just make R1 = 1k. Then choose a value for R2 that will set the gain as desired. If you want to get a little fancier, R2 could be a pot that would serve as a gain control. You'd just want to make sure that at whatever the pot's maximum resistance is, you won't hit the "rails" of the power supply. Of course, the only way to know this for sure would be to know what the maximum input voltage will be. If you don't have a digital storage oscilloscope to test the mics with, this might be tough to figure out! A safe bet would be to assume that, because this is an electret mic we're talking about, max output will be quite small. At a guess, I'd bet no higher than 1V peak. That means that, if you're using a bipolar power supply (like the +9/-9 from a roadrage board), you could safely have a gain as high as 10 without clipping. At any rate, I'd start with a 10k resistor in R2 and adjust to taste.
Now, since we've got a stereo pair of mics, simply make two of these (you could use both sides of a dual opamp), and attach the "hot" wire from each mic to the inputs.
At this point, I'd probably just tack on a passive tone control and see what you get. Maybe not the big muff one, since the midrange scoop might not work so well with a concertina! I'd follow that passive tone control with another pair of opamp circuits. The first would be a unity gain summing amplifier (both outputs of the preamps go thru matching resistors, then connect to a single opamp + input, and the - input is connected directly to the output. The second would be an adjustable gain amplifier like the the first ones, to make up for any losses from the passive tone control.
So now, we're talking a quad opamp (i.e. TL074), six resistors (one would be a pot for a volume control in the R2 position of the last amp stage), plus whatever the tone circuit requires. OH! and you'll need caps at the inputs to keep DC from leaking around where you don't want it. The finished thing might look something like this: (but I forgot the output cap!)
_________________
Muad'zin wrote:
I want Pterodactyl sounds dammit, not a nice little analog sustain.
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