i've been messing with a cheap ebay amp pcb. i'd rather not disclose the model for the fear of hurting the seller, and my construction is always less than perfect, so i don't want the blame shifted all to the product, but, this amp was suffering from an oscillation in one channel. (it's a stereo pcb) the channel farther away from the psu caps, as expected. the flaw shows up as a static noise in high frequency rich moments such as piano or... yeah, mostly just piano. 😛
i try everything i known until on a whim i just run my finger over the pcb, and to my surprise the noise goes away when my finger's over certain spot. so on a further whim i lay aluminium foil over where my finger was, and now the noise is gone.
what could've been the issue and how did this foil solve it?
one culprit that i can think of is my resistors being inductive.
i try everything i known until on a whim i just run my finger over the pcb, and to my surprise the noise goes away when my finger's over certain spot. so on a further whim i lay aluminium foil over where my finger was, and now the noise is gone.
what could've been the issue and how did this foil solve it?
one culprit that i can think of is my resistors being inductive.
Your finger or foil acts as a shield to block sensitive areas from picking up signal from elsewhere in the circuit.
It's almost impossible to tell without seeing the PCB or schematic, but when a circuit is this sensitive to a nearby finger it's usually a construction or design flaw. Ground loops can cause oscillation, but if foil stops the oscillation then your problem is probably with FETs oscillating or the stability compensation needs fixed.
'Inductive resistors' are almost certainly not the problem. Capacitive feedback is the most likely cause. Your finger or foil alters the stray capacitance.
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