Ground loop only when both channels connected

[Nuuk]

I've got a
MASS 500 studio amplifier that has a hum when I connect both channels to a source. If I connect only one channel, it is silent.

I am using ICs with a phono plug on one end and XLR on the amp end. I've tried with both the ground and screen pins connected, and without.

The amp has a single transformer and then each channel is separate with its own rectifier bridge etc.

Can any one suggest how I may get of of the hum please?

[BWRX]

The best way to connect and unbalanced source to a balanced input is to connect signal to pin 2 (non inverting input), ground to pin 3 (inverting input), and the cable's shield (if any) to pin 1. If there's still hum you can try connecting the inverting input to ground at the amplifier.

Do you have any other mains outlets available? You could try plugging your amp or source into a different outlet. Whether or not this will help depends on how the outlets are wired.

[AndrewT]

Hi,
can you identify the audio ground for each of the power amp channels?

Are they both connected together?

Are they connected to the safety earth?

Is there a ground lift switch in this commercial amplifier? Don't lift it!!!!!!!

250W into 4r0 is just +39% power (-1.6dbV) into half load resistance. This amp may be safe with a 4ohm load but it is obviously not designed to sound good on a 4ohm load. It may not be suitable for 4 to 8ohm either.
I would recommend that it only be used as a 180W into 8ohm only.

[Nuuk]

Thanks Brian. The XLR plugs are wired as you suggest. The cables have a central conductor and an outer braid. So the braid is connected to pins 1 and 3 although I have tried it on pin 3 only.

If I plug the amp into a seaparate ring mai I still get the hum which is significantly louder on the RH channel.


Andrew, the audio grounds )as taken from the back of the XLR input sockets) on each channel are connected but not connected to safety earth. I cannot detect a ground lift switch.

[AndrewT]

Hi Nuuk,
is this amp double insulated with the double square symbol?

the XLR common connection is almost certainly causing the hum when the second channel interconnect is inserted.
There is a ground loop from source to XLR to XLR back to source through the second cable.

Try Bob Cordell's mod using a resistor in each feed from the unbalanced XLR connections to input of the PCB.

Alternatively, IF THE AMP IS DOUBLE INSULATED.
Break the connection between the two audio grounds and float each audio ground from the other channel and from chassis.

[Nuuk]

is this amp double insulated with the double square symbol?

No it is not - at least there is no symbol anywhere on the case.

Try Bob Cordell's mod using a resistor in each feed from the unbalanced XLR connections to input of the PCB.

Is that a resistor in series with the signal connector or all three pins?

Could I put the resiator(s) in the XLR plug as it would be a lot easier. The input connectors are mounted on the PCBs so breaking the connection would not be easy.

And what value resistors? I'v elooked on Bob Cordell's site but can't find a reference to this issue.

[AndrewT]

Hi,
it's in Cordell's thread PSU discussion.
Low value R in the return connections, but if your's are direct to PCB this may be a write off as well.
http://www.diyaudio.com/forums/showt...28#post1082528

Maybe they could work in the cable/connector?

I have never tried this method.

Since it is not double insulated how have they complied with "all exposed conductive parts must be safety earthed"?

[Nuuk]

Thanks again Andrew. I will try a 4R7 in the XLR plug later and see what happens.

[Nuuk]

With the resistor between the shield/cold conductor of the IC and pins 1-3 of the XLR plug, the hum is slightly louder but no longer varies according to the pot between source and amp.

Or is the resistor meant to go between pis 1 and 3?

[AndrewT]

post9 and a pot appears!
Is this a passive pre that has been added or was it there all along?