what could cause low volume distortion

[jaybell]

i bought a cheap "technical Pro" 4 channel power amp. and at low volumes, it distorts terribly. as you bring the level up, it begins to clean up and run well, but at reduce the levels, it breaks up and distorts to a more and more gravely sound till eventually signal stops all together like a crappy gate.

I thought it may be that the power supply was wired to 220 and was undervolting. but i put a meter on it and found that was not the case.
but i did notice that if i try to run it in 220 mode (undervolt) the distortions cleans up a bit.

it also produces different noise depending on how many channels are connected to speakers. and runs cleaner in bridge mode.

Caps? toroidal power supply? just hopelessly cheap?
any ideas?

 

[Bigun]

bias is set wrong ? - wild guess, I'm not an expert, more dangerous and useful.

 

[wahab]

I thought it may be that the power supply was wired to 220 and was undervolting. but i put a meter on it and found that was not the case.
but i did notice that if i try to run it in 220 mode (undervolt) the distortions cleans up a bit.

QUOTE]

If the amp is wired for 230/240V and that wiring it in 220V mode
improve the sound audibly, then it means surely that the amp s
output stages quiescent current (bias) is too low.

When you wire it in 220V mode, the amp s supply voltage increase
and so do its quiecent current , leading to the improvement you
did mention..

 

[jaybell]

thanks

I will look for an adjustable bias and see if i can experiment with it
let me clear up something too. the power supply has a switch "120/220" if i switch it to 220, the fans slow down, and the output voltage on the toroidal transformer drops. but the distortion and noise seem to quiet some.

 

[jaybell]

how would i find / adjust the bias?
what am i looking for?

 

[nigelwright7557]

Classic cross over distortion.

Reset the bias as per the user manual.