Multichannel ground loop trouble

[gasjack]

Hi All.

I have built a six channel amp based on the 3886 chip (see picture). The amp plays just fine when only two channels are connected, but as soon as I connect two more I get buzzing and humming which I think stems from ground loop trouble.

On the picture you can see the PCB. The orange and grey wires are signal for the two channels, and the green wires are signal ground from the RCA sockets (which are not insulated from the chassis). The brown and blue wires are AC + / - and the purple is ground from the psu (toroidal).

I have tried running a thick wire from one board's ground star to the other one's and this decreased the buzzing somewhat, but not nearly enough. Would it help if I used isolated RCA sockets, or is that pointless, when the preamp uses common ground for all the outputs anyway? Any other suggestions? I am getting pretty tired of resoldering this amp...

Best regards
Rasmus

[2Bak]

I have recently build a four-channel inverted Gainclone on pcb's. I also have a little bit of humming, and even though I have to put my ears close to the woofers to hear it, I'd like to get rid of it completely!
However, in my amp the hum doesn't increase if I connect more than the two first channels.
I've also tried to connect a wire between the four channels' star grounds, and also by me this reduces hum significantly. This makes me focus on the possibility that my PSU configuration perhaps is causing the hum...
So next I'll have a look at the transformer/bridges configuration to see if something can be done here...Perhaps you should focus on this also..?
I've also noticed that a 500VA transformer makes more hum than a 300VA transformer, cause when I compare this amp with another one I made with only two channels on the same pcbs and exactly same circuit, the humming is a bit louder on the 500VA amp.

Chassis grounding also helps. I use a 100R from starground to chassis bypassed with a 0,022 uf filmcap.

/Jan

[gasjack]

Originally posted by 2Bak

I've also tried to connect a wire between the four channels' star grounds, and also by me this reduces hum significantly. This makes me focus on the possibility that my PSU configuration perhaps is causing the hum...
So next I'll have a look at the transformer/bridges configuration to see if something can be done here...Perhaps you should focus on this also..?
I've also noticed that a 500VA transformer makes more hum than a 300VA transformer, cause when I compare this amp with another one I made with only two channels on the same pcbs and exactly same circuit, the humming is a bit louder on the 500VA amp.

Chassis grounding also helps. I use a 100R from starground to chassis bypassed with a 0,022 uf filmcap.

/Jan

Hi Jan.
Thanks for the input. I am using a 500VA transformer, but when I am only using two channels there is no noise whatsoever, so I doubt the transformer is the culprit.
I feel like I have tried just about everything now and I am thinking about just settling for the two working channels to drive my main speakers, and then stick with the surround receiver for the remaining three channels.
I might give it another go tomorrow however - my pride will not allow me to quit

Rasmus