Preamplifier Construction Help

[Mr Smith]

Hello all,

I am constructing a speaker system with a 3.5mm mono microphone input. I am currently using an amplifier evaluation board that has a preset gain of 6dB which is not enough for a small microphone signal. I am building a preamplifier to help with the gain situation, however I seem to be unable to get an amplified signal on the output. I tried using a lapel microphone with a 3.5mm jack and did not get an amplified signal out and I also replaced the 3.5mm jack and lapel microphone with an electret microphone and I still cannot get an amplified signal. I have the schematic attached as well as a picture of the circuit on the breadboard and the output waveform on the oscilloscope. The transistor is an NPN 2n3904. Any help would be appreciated.

Thank you!

 

[bill_a]

For an audio amp your test frequency seems a tad high.

I think you may have a wiring error as well, as the circuit simulates just fine in ltSpice, with a gain of about 33 dB at 1kHz, up to a maximum input signal of about 50mV pk. I'll let you check the wiring.

 

[Mooly]

What DC voltages are you seeing on the transistor ? A self biased circuit like this is very transistor dependant. If the collector voltage is low then you need to increase the 100k.