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

[Robert Ham]

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


[0] http://www.ebuyer.com/product/111661
[1] http://www.hobby-hour.com/electronics/computer_microphone.php
[2] http://www.m-audio.com/images/global/manuals/070208_Delta1010_UG_EN01.pdf
[3] http://www.tkk.fi/Misc/Electronics/circuits/micamp.html

 

[Bob Masta]

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?

Correct, you just remove C3 and the dynamic input.

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


[0] http://www.ebuyer.com/product/111661
[1] http://www.hobby-hour.com/electronics/computer_microphone.php
[2] http://www.m-audio.com/images/global/manuals/070208_Delta1010_UG_EN01.pdf
[3] http://www.tkk.fi/Misc/Electronics/circuits/micamp.html

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
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!

 

[tempus fugit]

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'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):

http://focus.ti.com/docs/prod/folders/print/tl074.html

 

[Brendan Gillatt]

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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


[0] http://www.ebuyer.com/product/111661
[1] http://www.hobby-hour.com/electronics/computer_microphone.php
[2] http://www.m-audio.com/images/global/manuals/070208_Delta1010_UG_EN01.pdf
[3] http://www.tkk.fi/Misc/Electronics/circuits/micamp.html

Take a look at
http://www.epanorama.net/circuits/microphone_powering.html
if you want some more reading material

- --
Brendan Gillatt
brendan {at} brendangillatt {dot} co {dot} uk
http://www.brendangillatt.co.uk
PGP Key: http://pgp.mit.edu:11371/pks/lookup?op=get&search=0xBACD7433
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