Good Example Balanced Input Circuit? (Ground Loop Isolation)

[lumanauw]

Are all of the supplies driven by the same IC? If not, are the ICs sync'd together

I agree with Perry. You should consider this possibility too. If those 3 SMPS are not sync'ed, there will be beating frequency among them (the difference of frequencies appear at the loudspeaker). Old Phoenix Gold amps (MS250 generation) advice syncing if we use several amps (=several smps')
If you use SG3525 or SG3524, there are osc-out and sync pins.
I think if you have found the core of your problem, your PXA701 should work fine in your system without DC-DC (normal usage).

[lumanauw]

Or this. In some multichannel car amps, the input RCA shield (gold plated) is not directly connected to system ground, but they are connected via 47ohm resistors.
Could you try putting 47ohm resistors in your RCA's input before they hit common amp ground? For 8 input, you will need 8pcs of 47ohm resistor.

[MartyM]

"Are all of the supplies driven by the same IC? If not, are the ICs sync'd together?"

Sorry I did not mean to overlook replying to this. No, I didn't originally design in that ability on the PCBs but I can modify anything I need, and will be sure to add that to my "list."

However I guess my question is, what affect would that have on the ground loop issue? I am assuming NONE but however it would be relevant for the noise floor & noise behavior of the amplifiers, correct?

Also, are the comments here leaning towards having me convert the inputs to balanced? (At least for a test trial?)

Thanks guys

Attached is a photo of the current amp...top cover removed for testing...quite a mess now! (looks ok with top on)

[Perry Babin]

Using the transformers or the circuit I posted would have given you balanced inputs.

Did you try disabling 2 of the supplies to see if the amp worked properly with 2 inputs plugged in?

You showed 3 supplies on the block diagram but there are 4 transformers in the amp. What's the 4th being used for?

[MartyM]

lumanauw, I did try that in the past but no problem giving it a shot again (although after your last post [previous] about that idea it didn't work for me).

Perry, the 4th SMPS wasn't present when I started this thread. What I did was after I discovered that noise was reduced when only one RCA pair per SMPS/amp module set was used I built & added a 4th, hence my posting above. I moved the center channel from SMPS #3 to be shared with the subwoofer channel.

I realize 4 SMPSs is absurd but consider it temporary until I can get the amps to stop oscillating & popping fuses when GLIs are used or 'some alternative.'

I'll go later and try running only one and removing the RCAs from all others-if I don't, problem will still be there, whether or not they're powered.

Thanks again everybody. Can't believe you were up so late last night posting on DIYAUDIO.com!