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Circuit to connect "Soundblaster" electret microphone to line-input

Hi there,

I have three cheap PC "Soundblaster" electret microphones[0] that I need connect to a multi-channel sound card. The problem is that the microphones expect a +5V DC bias from their connection with the sound card[1]. The multi-channel sound card only has line-level inputs and doesn't provide power for electret microphones[2]. I need to build three circuits to a) provide the necessary voltage to the microphones, and b) amplify the microphone signal.

I have found a circuit that appears to cover both issues[3]. However, I need some clarification on a couple of things. Firstly, I will not need the dynamic microphone input. Can I simply remove the dynamic mic terminals and capacitor C3 from the circuit, or is there more to it than that?

What I'd like to do is build a single box with three circuits on a single piece of copper-track board and have it supplied by a single wall-wart power supply. I'm a little unsure how to adjust the circuit to accommodate this. How can I supply three such circuits from a single power supply? From what I recall of my physics lessons, I can either run them in parallel or serial, the consequences of which are either a tripling of the power supply voltage (from 9V to 18V) or a tripling, or possibly 1/3ing of.. something else.. amps? I forget. Any pointers would be appreciated.

Thanks,

Robert Ham

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Reply to
Robert Ham
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Correct, you just remove C3 and the dynamic input.

Build your 3 circuits such that they all share the same common ground. If you want 3 separate power switches, connect all 3 circuits together at the 9V point above their switches. If you want a single switch for all, you can connect them just below a single switch. You should keep all 3 LEDs unless you want to redesign the bias circuit. You don't need both C1 and C2. This series connection is a method to make a non-polarized capacitor out of two polarized electrolytic types, but here the input on the mic side should always be more positive, thanks to the electret bias through R1. You can thus reduce the single-cap value from 10 uFd to 5 (actually 4.7) uFd to keep the same effective capacitance.

Note that '4k7' means '4.7k' and '120R' = 120 ohms. You don't need this exact transistor part number; try any small-signal NPN like a 2n3904.

As the circuit author notes, this is not a low-distortion circuit... you could do better with an op-amp design, or even a better single-transistor design. But if it is for conferences instead of concerts, it should be fine.

Best regards,

Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis

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Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!

Reply to
Bob Masta

I've got that soundcard too. The software allows you to use faders to select the input level over a fairly wide range. I hooked up an electret mic to the line in just to see what kind of signal level I could get, and it was typically in the -30 to 25 db range. If that'll do the job for you, you might consider dropping the preamp idea altogether, particularly since the one you linked to has a distortion figre of 2-3% (that's pretty damn high). You could also make one based around an opamp, which wouldn't really be any more difficult and would sound WAY better. You can get opamps 4 to a package, so 1 16 pin chip would do the trick, plus a few resistors and caps....

Here's just 1 of many (and there are better out there too):

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tempus fugit
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Brendan Gillatt

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