Pre-amplifier circuit

Thread Starter

PeterStudio

Joined Mar 6, 2021
7
Good evening.

Please advise a circuit for a preamplifier (low noise) of an audio signal from a headset (two set)?


Thank You.
 

AnalogKid

Joined Aug 1, 2013
9,351
Just to be clear - are you asking about a mono microphone preamp for an electret microphone element in a stereo headset?

ak

1615071606980.png
 

Audioguru again

Joined Oct 21, 2019
3,510
My simulation uses R3 to represent the impedance of an electret mic parallel with the 10k resistor that powers it.
The 100k resistor causes the transistor to conduct too much which causes it to clip the bottoms of waveforms.
Increasing the 100k resistor to 1.2Meg causes the gain to be too high which causes distortion even when the input level is reduced.
Adding an emitter resistor fixes it.
 

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Thread Starter

PeterStudio

Joined Mar 6, 2021
7
Good evening.

I made this circuit, but very hi level noise (without connect microphone).

Noise - constant hum and whistle (battery power).

Thank you.

Mod: added the datasheets.
 

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Audioguru again

Joined Oct 21, 2019
3,510
Hum is caused by an unshielded input wire in a high gain amplifier that is picking up 50Hz or 60Hz from your electricity on your walls.
A shielded input audio cable is needed. Messy wires all over the place and the rows of contacts on a solderless breadboard also pickup hum.
Your input opamp produces a lot of hiss. Use a low noise audio opamp instead.
The gain is very high and might cause the mic to hear the headphones and produce acoustical feedback whistling.
 

sghioto

Joined Dec 31, 2017
2,394
If you still plan on pursuing that schematic you can down size the part count as in the circuit below.
Some component values will probably need adjusting for best performance.
1615146284196.png
 

Audioguru again

Joined Oct 21, 2019
3,510
For example, which opamp?
You might not need your input opamp. Texas Instruments has a page full of low noise audio opamps.

There is no hum when connecting this headset to other industrial equipment. It's not about the cable.
It is the wires in and feeding the amplifier, not the headset that picks up hum. A wire carrying a low level signal needs a shielded audio cable to block hum pickup.
 
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