Amp (A30) humming when XLR plugged in - diyAudio
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Old 9th January 2005, 03:32 AM   #1
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Default Amp (A30) humming when XLR plugged in

I just built an Aleph 30 and I have humming in the right channel. When the XLR cable is plugged into the amp and the other side is plugged into by BOSOZ there's humming. When it's disconnected from either side (amp or preamp side) the hum is almost gone.

XLR cable shield is connected to chassis at the jacks and it is not connected to circuit ground.

I also get hum when the RCA jack is plugged in. But not when it's unplugged.

Circuit ground from both channels meet at the terminal of one of the PS caps. This point goes to chassis ground through a CL-60. Transformer secondary 0V goes to the PS cap terminal too.

I don't think it's a ground loop through the AC jacks as I have tried unplugging the power cord from the BOSOZ and the hum is still there.

I don't get why the hum is only on one channel. I've checked the soldering to make sure all solder joints are secure.
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Old 9th January 2005, 09:38 AM   #2
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just a few questions: one channel humming and the other not although they are exactly the same?

2. did you try changing the channels at the BOSOZ to see if the hum stays on the right side?

3. Is music coming out of the amp although it hums?

If the answer is yes (3x) then you must have a bad/wrong connection in the right channel. Wich input topologie did you use (the old 10k or the new 47kOhm)?

een ooievaar is geen konijn want zijn oren zijn te klein!
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Old 9th January 2005, 07:33 PM   #3
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Here are my answers...

1. Actually, the hum sounds more like a buzz. It exists in both channels but is slightly louder on the right channel. They can both be heard with the ear about 1m away from the driver.

Buzzing gets louder then cables plugged in.

The two channels are exactly the same.

2. Changing the channels on the BOSOZ does not affect it in any way.

3. Music plays through the amp.

I'm using the input topology from the Aleph 30 service manual which is using 10K. I did not know about the new topology.

Don't know if this will help but here's a picture of the amp with the cover plates and heatsinks removed. The perfboard with the amp circuits goes on the top layer.
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File Type: jpg inside2.jpg (97.0 KB, 301 views)
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Old 9th January 2005, 07:35 PM   #4
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Here's another picture which shows the heatsinks attached and the amp circuit board in place.
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File Type: jpg withheatsinks.jpg (86.8 KB, 307 views)
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Old 20th January 2005, 03:23 PM   #5
beedlo is offline beedlo  Canada
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I built a Aleph 3 once with a similar problem except I did not have XLR connectors. One channel would buzz, while the other channel would be fine. The solution was simple, although I did not entirely understood what had happen. The problem was 'fixed' when the input wire for the offending channel went directly across in the middle of my caps, (they were beside the caps at first). I predict that the other channel was fine because the input wire was much shorter, and away from the caps (since both boards are mirrors, one board has a much longer input wire).

Click the image to open in full size.
Yes, it looks absolutedly messy inside. Result of an inexperience highschooler

I have a unrelated question though; how hot does your amp get?
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Old 20th January 2005, 04:27 PM   #6
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I've decided to redo the boards. I'm making PCBs of BrianGT's Aleph gerbers. If that doesn't help, I'll move the transformer out of the enclosure.

I'm going away this weekend so I won't be able to get to it for a while.

My amp gets to about 55C on both heatsinks measured with an infrared temperature gun.
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