# Why won't my pre-amp circuit enable my condenser microphone to work with my boombox?

I recently bought a boombox for my mother to play and record cassette tapes. I didn't realize at the time that the boom box did not have a built in microphone. I then bought a condenser microphone for her and plugged it into the audio in jack. It failed to work. I tested the microphone on my computer and I tested the audio in jack with my cell phone. Both worked. I thought the problem may lie in the strength of the signal from the unpowered microphone. So I built a pre-amp circuit modeled after the one in the old lab manual "A First Lab in Cuicuits and Electronics" by Yannis Tsividis.

simulate this circuit – Schematic created using CircuitLab

I'm an amateur when it comes to electronics, where am I going wrong?

• Condenser microphones require power. Does the microphone require phantom power, or does it have the option to use a battery if phantom power isn't available? Also, you need a bias resistor from the + input to ground, perhaps 10k ohms.
– qrk
Jan 11, 2023 at 3:17
• The mic that needs to be powered is called an electret mic, not a condenser mic. When the preamp is designed correctly, what will prevent the mic from hearing the speaker and causing acoustical feedback howling? Jan 12, 2023 at 2:47

The "condenser microphone" is a simple electret condenser that is powered by the computer. All it needs is power. Most condenser capsules are specified to work for something like 2V to 10V with a maximum current of 0.5mA. If you take just one 9V battery, put 10kohm from its plus pole to the mic and connect that with your boom box aux input (and minus pole of battery to the ground of mic and aux input), you are likely fine. To even out noise, put a tantal capacitor of a few uF and at least 10V (and get its polarity right) across the battery. If you don't think that the boombox likes that kind of DC offset on its aux input, put another such capacitor between mic+battery and the aux input.

If you think the mic contains other circuitry than just an electret condenser capsule that might not stand for 9V, you might rather try with 3V and 2.2kohm: that's typical for laptop inputs and will be in the expected ballpark for the mic.

Larger resistors make for larger signal but there's a limit to how low the voltage across the capsule may get before things become too non-linear.

I would have thought that the description had been clear enough, but here you are:

• Thanks, I'll give it a try in the morning. I'm an amateur when it comes to these things and I had no idea that microphones required power. Do you mind drawing a diagram on how I should connect the battery to the microphone?
– Nil
Jan 11, 2023 at 3:45
• Sorry for the late update. I had caught covid that weekend and had been preoccupied since. But I tried a 3 volt battery and 2.2kohm resister and that did the trick. Thanks a million
– Nil
Feb 5, 2023 at 5:11

The boombox does not have a microphone input. It has an auxiliary line input, which does not provide power for an electret microphone.

Your computer has a mic input which does provide power to the electret mic.

The circuit you drew is just an amplifier and it will also not give any power to the microphone.