Recently purchased an already built pair of M125 amps and an SP9 preamp. When all hooked up I get about a 4db hum from both speakers, one about 5db, the other about 3db.
I pulled the RCA cable out of the preamp and shorted it while it was connected to the M125. and got silence. So the problem seems to be coming from the preamp and/or its interaction with the monoblocks.
For additional testing I lugged my Adcom GFA 555 II (heresy, an SS amp) in and hooked it up. I turned everything on and I got a much louder hum (probably more sensitive). Interestingly (to me at least) If I power off the preamp but leave it attached to power, the hum decreases a little, but it is still there.
If I disconnect the Adcom from the preamp or I hook my source (a computer) directly to the Adcom, dead silence.
The preamp has a 3 wire power connector. The M125 which is has the lower volume hum has the ground lug cut off (previous owner). The Adcom has a two prong power cord with one lug larger than the other.
What should I do to further troubleshoot this issue?
Or if someone knows precisely what the issue is and how to fix it this would be good too.
I pulled the RCA cable out of the preamp and shorted it while it was connected to the M125. and got silence. So the problem seems to be coming from the preamp and/or its interaction with the monoblocks.
For additional testing I lugged my Adcom GFA 555 II (heresy, an SS amp) in and hooked it up. I turned everything on and I got a much louder hum (probably more sensitive). Interestingly (to me at least) If I power off the preamp but leave it attached to power, the hum decreases a little, but it is still there.
If I disconnect the Adcom from the preamp or I hook my source (a computer) directly to the Adcom, dead silence.
The preamp has a 3 wire power connector. The M125 which is has the lower volume hum has the ground lug cut off (previous owner). The Adcom has a two prong power cord with one lug larger than the other.
What should I do to further troubleshoot this issue?
Or if someone knows precisely what the issue is and how to fix it this would be good too.
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If it were mine, I would TEMPORARILY install a ground lift adapter on the 3 wire plug and see if the hum was
gone. IF gone measure the Voltage difference between the house ground vs the 3rd wire. In either event,
remove the ground adapter. If the computer is a desktop it almost certainly has a 3 wire cord which you
should never defeat except for controlled testing. I had noise from my PC to the audio system because of
the antenna feed to the TV tuner in the PC. That was solved with a galvanic isolator in the antenna feed.
You might also look into transformer coupling the audio feed from the PC to the SP9. If that fixes the hum
but you don't like the transformer you could go to a differential amplifier between the PC and preamp to
subtract out the noise. Basically convert the preamp to balanced input and simply connect both '-' inputs
together and connect them to the PC ground. Totally safe and problem solved.
BTW my guess is the Voltage differential between grounds is quite low but the loop current might be significant.
Good luck
G²
The computer is a laptop. as part of my original testing I removed all inputs from the preamp and the hum persisted.
The actual way everything is connected is, I have a 3 wire 6' extension cord from the outlet. Plugged into this is a three way three wire splitter. The amp & preamp are both plugged into the splitter.
Based on my limited knowledge of ground loops the loop would need to be somewhere in the cables from the splitter to the equipment. Is this correct?
I've had cheap RCA cables pick up 60hz noise. Does your preamp have a ground lug for a turntable? If so you might try running a jumper from it to the shield side of one of your cable ends to see it it kills the hum. Your preamp may not have a good signal ground system. But I'd suspect your RCA cables first. It happens. And keep the preamp away from power transformers. And I wouldn't trust a computer jack or the mini-plugs cables that go into them either. Splitter jacks and extension cord jacks are trouble. Mini-plugs just suck.
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I have tried two different interconnects, both better than Radio Shack cheapO's but neither were $5,000 interconnects. I think one set was Monster the other set do not have name brands on them, but are a lot sturdier than most other interconnects I have.
I will check the grounding trick when next I play with the system.
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