Hum on LM1875T - I can not get rid of it

[mertol]

I have read the similar posts and I am not exactly sue about the fix.

I bought the pcb's (pictures attached) through ebay and used them in my active speakers (posted in an other thread) However I am unable to get rid of the 50 hz hum on both of the speakers.

I have tried to relocate the transformer and using all shielded cabling and connecting the ground to large metal objects like the central heating system (this helped a little though) and nothing so far cured the problem.

I have read in other thread that with an other chip amp people were successful with ripping off the ground lines and connecting them in star topology. This seems a little big harder for me to implement due to the way I mounted the pcb inside the speaker.

I sense that something is wrong around the rectifier bridge and this could be the switching noise of diodes.

Before diving into my speakers I have spare board I can try your recommendations (I don't have transformer, but I will be getting one this weekend hopefully)

PS: My active speaker is a relatively small one with a 4.5 inch woofer so the 50 hz hum is eating much of it's xmax and as I used passive line level XO the 50hz humm also hits to the directly coupled tweeter, probably killing it slowly.

I tried to attach hiüres pictures

Mertol

[mertol]

btw I didn't wire the ground of the power socket. Could this be the problem?

Mertol

[madtecchy]

Please post pitures of the amplifier and wiring. Your problem sounds a bit bigger than a grounding problem.

[mikojat]

Hai mertol, if you mean didn't conect ground for the input power supply..that is the problem! you must conect them...from your picture's, I see that is voltage input 18-Gnd-18 V, right?? If you use EL trafo's, conect Center Tape (CT) to Ground,don't leave it.

you must use isolator on the LM1875 metal tab, there is conect to negative rail (-Vcc).

[siliconray]

What did you use for grounding? the enclosure, or the metal water piple?

[mikojat]

Ahh...sorry,I miss understand.

If you mean that is power cord on the wall, I think this is not big issue. In my country, most electronics device just have main cable and neutral. In this case, my LM3886 and Aleph preamp death silent even on max vol.

[madtecchy]

Look if that humming or buzzing is having that much effect on you speaker you have a bigger problem than grounding problem . you also say the sound is comming from your tweeter too i think you have messed up bad somwhere

[mikojat]

Yes, for sure...the clear picture's maybe will help to solve your problem here..

[Michael Chua]

Did you install a heatsink for the LM1875?

[madtecchy]

Originally Posted by mikojat

Hai mertol, if you mean didn't conect ground for the input power supply..that is the problem! you must conect them...from your picture's, I see that is voltage input 18-Gnd-18 V, right?? If you use EL trafo's, conect Center Tape (CT) to Ground,don't leave it.

you must use isolator on the LM1875 metal tab, there is conect to negative rail (-Vcc).

This could be a solution to the problem but only if the heat sink is connected to the o volt . If this is the case im sure there would be magik smoke by now as the nagative supply rail would be a direct short to 0 volts . Then again if each rail is fused then there wil be a large dc offset on the output and the speaker will soon be destroyed