# transistor mic preamp

Hi I built this mic preamp circuit for use with an electret mic.

The mic I have is this PUI AOM4544 electret mic

I have plugged this mic preamp circuit to my soundcard line in. But I think the gain is so low that I cannot hear anything. I built this to use as a measurement mic. I tried measuring the output from the mic alone without the mic preamp ( supplied 3v power to mic) using a voltmeter; I measured 0.001V ac when I blow at the mic. Is this mic preamp circuit a wrong one to use ?

• You might be interested in the eevblog episode where doug ford talks about linear electret/condensor mic amp circuits (hint: the electret mic is usually a jfet itself) – PlasmaHH Sep 7 '15 at 13:53
• I think there is nothing wrong with the circuit. It can be that 3 V is not enough for the electret mic to work properly. Did you try this circuit at 9 V ? – Bimpelrekkie Sep 7 '15 at 13:54
• @PlasmaHH actually it contains a JFET since the (extremely small) charge that gets displaced in an (electret) capacitor by the sound waves is not enough to drive anything. The JFET acts like a voltage buffer for this. To work it needs some DC biasing, hence the 10 K ohm to supply. – Bimpelrekkie Sep 7 '15 at 13:56
• The Q1 collector voltage should be roughly half the supply voltage, if the transistor is properly biased. Check it. – Leon Heller Sep 7 '15 at 14:01
• <looked at datasheet> Oh, the supply of 3V should be OK, the Mic. is polarity sensitive, did you try connecting it the other way round ? What is the DC voltage across the microphone ? It should be less than 3 V. That would indicate that some biasing current is flowing, which is what you need. – Bimpelrekkie Sep 7 '15 at 14:01

That's a crappy circuit. Don't believe everything you find lying around on the internet.

For reference, here is your circuit:

The 10 kΩ pullup to power from 3 to 9 V should be good enough for most electret microphones. C1 then feeds just the AC part of the microphone voltage to the amplifier stage. That's all fine.

The problem is the poorly designed biasing for this common emitter amplifier. The transistor will be on with only a little more than the B-E drop on the collector. This leaves little room for the output to swing low.

A quick bandaid for this circuit is to add another 100 kΩ resistor from the base of Q1 to ground. That with R2 will then form a voltage divider so that the collector DC voltage should be in the 1 to 1.5 V range. That leaves some room for it to swing both ways.

Another problem with this circuit is the unpredictable gain. The gain applied to the microphone signal has a high dependency on the gain of the transistor. In good transistor circuits, everything works well with the transistor gain from some minimum specified value to infinity.

Another problem may be the rather high output impedance of this amplifier, which is about 10 kΩ. That together with the sound card input impedance forms a voltage divider. If the sound card input impedance is 600 Ω, for example, then the signal will be attenuated by over 15x when connected to the sound card.

Check that your line-in is correctly connected and that it is enabled in the sound card properties. Also check the mixer settings. Be sure you haven't plugged into the microphone input accidentally.

Use a program like Soundcard Oscilloscope to see what is really going into your soundcard. This will show you the input, and you will really notice when you speak or blow into the microphone. It also has a selector so that you can see really weak signals.

A Voltmeter usually isn't much good at detecting audio - the AC function often assumes that you will be measuring AC at 60Hz or 50Hz (depending on your local power line frequency) and doesn't really behave well when confronted with audio signals.

Once you do all of that and still don't get any signal, check the tips that @Olin Lathrop gave you in his answer.

BTW: I agree with Olin about that being a crappy circuit. You might do better to go with a better design (like this one) especially since you mention wanting to use this for meaurements.

• Thanks for suggesting a new mic preamp circuit JRE. I would like to build this to try but I don't have three 1K resistors with me now. In the meantime I built another mic preamp using a LM358 opamp.This is the circuit. It too has low gain when built with R5= 500K. In the soundcard oscilloscope Vp-p is 6 with 0.5 amplitude signal generator at 500Hz. – buzzy Sep 7 '15 at 18:51
• The link is dead. – Chupacabras Jul 5 '19 at 19:57

The circuit is not so bad. You have to measure DC voltage between collector and ground. It should measure about half of power supply voltage. If not - You can change the R2 for bigger value or lower.

The gain is predictable, it is equal of R2 divided by electret microphone output impedance (electret microphone output impedance should be around 10k in this circuit) so if the bias is good - voltage gain is:

${{100k}\over{10k}} = 10$