As what the title says When I connect my designed preamplifier,which is in a breadboard, unto a power amp kit (Acoustic 50W Mono) with a speaker, there is a farting sound, as if it is a pulsating DC. And if I directly connect my music or mic to the power amp the output of the speaker is fine but very weak since there is no preamp.

In addition, the farting sound is always present, meaning with or without an input of music or mic the farting sound is there.

From what I could think of right now is that it is due to the bias also going to the poweramp or too much low frequency that I'm not aware. And I need help in solving this problem.

And here is an image of the preamp. enter image description here


1 Answer 1


one source of the farting sound is low-frequency oscillation, called "motorboating" in the trade. this occurs when there is a feedback path inside the amplifier which couples the output to the input at low frequencies. The most common cause is when the different stages of the power supply are poorly decoupled, so that a power surge drawn by one stage causes the rest of the supply voltages to droop. the upstream stages of the amp then interpret the supply droop as a signal and the process continues. Check your power amp supply wiring against the schematic and make sure the series dropping resistors are the correct value, and that the filter caps are wired with the correct polarity.


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