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?
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?
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.
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.
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.
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.
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.
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.
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.
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.
No it is not - at least there is no symbol anywhere on the case.
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.
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/showthread.php?postid=1082528#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"?
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/showthread.php?postid=1082528#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"?
AndrewT said:
Power comes from a single torroidal transformer feeding two separate power supplies, the idea being to minimise inter-channel modulation of the audio via the power supply rails. Because a great many studios are run by semi-professionals, the amplifier has been fitted with Neutrik Combi input sockets, which can accept both balanced and unbalanced jacks, as well as balanced XLRs, and a further pair of the same type of sockets are provided as sends, for applications where two or more amplifiers need to be chained from the same input source.
Power comes in via a standard IEC mains connector, and the speaker outputs are on chunky terminals that also accept banana plugs. The amplifier is fitted with separate, 41-step detented volume controls for the two channels, and warning LEDs show the power, thermal overload, and clip status of each channel. A couple of rack handles and a centrally-mounted mains switch complete the picture, and an electronic delay circuit mutes the speaker outputs during power-up, to prevent thumps.
Andy
Andrew, to eliminate the pots in the MASS 500 as a possible cause of the problem, I have temporarily replaced them with a simple voltage divider. And I have added a passive volume control between CDP and amp.
But the hum issue is there what ever I have before the MASS 500 so I didn't mention it.
I have connected the resistor across pins 1 and 3 now and connected the braid to pin 3. Hum is reduced and no longer affected by the passive pot setting. I'm now going to try the braid connected to pin 1.
It may be that I need some cable with two conductors and a separate screen.
But the hum issue is there what ever I have before the MASS 500 so I didn't mention it.
I have connected the resistor across pins 1 and 3 now and connected the braid to pin 3. Hum is reduced and no longer affected by the passive pot setting. I'm now going to try the braid connected to pin 1.
It may be that I need some cable with two conductors and a separate screen.
This may not help since your amp does not have double insulation but I had severe hum when connecting more than one channel in a triple mono block of DIY UcDs.
The hum disappeared when I changed the ground strategy from star grounding (all PS secondaries grounds connected to chassis and chassis connected to mains ground via 10 ohm resistor, inputs floating) to PS grounds floating, mains ground unconnected, and all three input grounds connected to chassis. Obviously you would need double insulation to safely implement the latter configuation.
Note that the UcDs have balanced input and were connected to the RCA jacks as described above.
Guy
The hum disappeared when I changed the ground strategy from star grounding (all PS secondaries grounds connected to chassis and chassis connected to mains ground via 10 ohm resistor, inputs floating) to PS grounds floating, mains ground unconnected, and all three input grounds connected to chassis. Obviously you would need double insulation to safely implement the latter configuation.
Note that the UcDs have balanced input and were connected to the RCA jacks as described above.
Guy
Thanks Guy but it would be quite a lot of work to dismantle the MASS 500 and re-arrange the grounding system.
At present I have the following arrangement.
The resistors are 4R7.
This has slightly reduced the hum. Should I now disconnect the screen from the RCA plug?
At present I have the following arrangement.
An externally hosted image should be here but it was not working when we last tested it.
The resistors are 4R7.
This has slightly reduced the hum. Should I now disconnect the screen from the RCA plug?
Hi,
I wish you had shown the RCA and added a few more labels.
is 2 the hot feed?
is 1 the cold return?
If these are your flow and return then the screen should only be connected at one end.
Is that resistors fitted to both the feed and return?
I think the loop breaking resistors are, according to Cordell, only needed in the return legs.
I wish you had shown the RCA and added a few more labels.
is 2 the hot feed?
is 1 the cold return?
If these are your flow and return then the screen should only be connected at one end.
Is that resistors fitted to both the feed and return?
I think the loop breaking resistors are, according to Cordell, only needed in the return legs.
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