GC amp picking up unrequested radio frequency - diyAudio
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Old 11th September 2008, 09:02 PM   #1
paean is offline paean  Canada
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Default GC amp picking up unrequested radio frequency

I've just finished putting together my first "system". My music is played through a Squeezebox, RCA out to a GC amp, out my Raw Acoustics HT2 speakers.

The RCA cables from the SB to the GC is pretty hefty. The speaker wire from the GC to the speakers is 16g flat wire. I'm using banana plugs at each end.


So here's my problem. There is a slight hum when the whole system is connected. Its not overly bothersome while playing music, but it definately bothers me when the music is off. When I unplug the RCA cables from the SB, I notice that the cables pick up a radio station, which mix into the background of the hum.

The GC has a wood case, and not a metal one to sheild it from interference. Could this be the problem?


Thanks in advance for any suggestions or recommendations!
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Old 11th September 2008, 09:16 PM   #2
wboyd is offline wboyd  United States
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A few quick questions....

1. How is the gainclone grounded - do you have a seperate grounds for power and signal....or star grounding?

2. Have you tried different cables from the SB to the gainclone?

3. is the gainclone a kit (like chipamp.com/audiosector) or is it basic diy?

Some pics might be helpful...

Wayne
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Old 11th September 2008, 11:22 PM   #3
paean is offline paean  Canada
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I'm not the original builder of this amp, so I'm not going to pretend I know how he put all this together, but those are LM3875 chips.

Upon photographing this, I realized one circuit board has a cap that looks like it overheated. That might help explain this away if only one channel had this problem, but I can pick up the radio on both speakers.

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Old 11th September 2008, 11:24 PM   #4
paean is offline paean  Canada
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And no, I don't have another pair of RCAs. Granted, they are new and I haven't tested them elsewhere.
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Old 12th September 2008, 09:19 AM   #5
Ted205 is offline Ted205  United Kingdom
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The hum will be poor grounding/ground loop.

The power supply looks non-existent. I can't see any smoothing capacitors or any real bypass capacitors anywhere.

My guess is its oscillating too
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Old 12th September 2008, 02:06 PM   #6
wboyd is offline wboyd  United States
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I have to agree with Ted205...looks like a grounding issue.

Another point which Ted205 brings up is the power supply issue...

Can you tell us the capacitance/voltage of the caps on the rails of the chip (on the boards)? Also, I would add some bigger caps at the bridges - depending on the size of the caps on the boards.

There are a couple of tests that you can run at this point.

1. Check the voltage coming from the power supply - should be anywhere from 25-31(?) vdc.

2. Touch the chips after about 20-30 minutes of using it....are they hot?

3. Play something complex...see if either channel cuts in and out.

I am no expert on these things at all, but I have built a few and have had some issues myself - and this is where I came to solve them.

Wayne
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Old 12th September 2008, 02:08 PM   #7
wboyd is offline wboyd  United States
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One more comment...it is a very nice looking gainclone....hefty trannys, big heatsink, etc..I hope that you get your issues resolved soon.
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Old 12th September 2008, 07:12 PM   #8
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Quote:
Originally posted by Ted205
My guess is its oscillating too
Definitely the reason for the blown Zobel capacitor. That should be replaced as soon as possible.

The hum might come from an inductive coupling to the transformers magnetic field. You can test that easily by moving the transformers further away from the heatsink or turning them by 90 degrees.
The small electrolytics on top of the rectifiers might also pick up hum easily from the transformers and couple it into the DC side.

A metal (shielding) case 'may' help against the radio reception. However a flaw in the PCB layout may as well be the reason. On the other hand, if you disconnect cables, the amp should always be off, so where's the problem?

Did you try to connect the audio ground to the screw on the heatsink, where your PE is connected?
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