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Old 21st March 2008, 06:50 AM   #1
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Default Noob amp wiring question...

So I just finished building Jef Larson's 2A3 Aikido amp...it looks awesome...but it hums like CRAZY! The hum is as loud as the music that comes through...
I have checked and re-checked the schematic, my guess is there is something wrong with my heater wiring or grouding? I understand that poor heater wiring placement can cause hum, but can poor routing really cause this much hum?
The 2a3 hum adjustment pots don't seem to be affecting the hum that I am hearing, except maybe making it a little louder at each extreme end of adjustment. Also, I don't have this hooked up to proper speakers yet, it is hooked up to some self amplified computer speakers, and since they are self amplified I can hear the hum (at a much lower volume level of course) while the heaters are on (before I turn on the power to the rectifier and the rest of the amp)
Perhaps I made a very noob mistake, or someone can point me in a direction for troubleshooting...
As always, any insight would be greaatly appreciated!

http://abraxasaudio.home.att.net/pro...k2a3-2-amp.bmp
http://abraxasaudio.home.att.net/pro...k2a3-2-psu.bmp
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Old 21st March 2008, 07:16 AM   #2
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First of all, why not try it with the regular speeks? This may be nothing more than overdriving those self amplified computer speeks.

Otherwise, looks like a ground loop problem, given the high gain of the up front SRPP. Do you have the DC returns going to the DC neutral of the PS, or do they ground through the chassis? Make sure your input jack is isolated from the chassis? Include a ground lift between the DC neutral and chassis ground? Made sure your third wire ground is actually grounded?
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Old 21st March 2008, 04:20 PM   #3
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Miles,

Thanks for the response.

I don't have my 8-ohm speakers yet, they are still in the mail. But I hooked up some 4-ohm speakers and the hum was still there...very much so.

All grounds are going back to the DC neutral line of the PS, which then goes to the chassis. The input jacks are isolated from the chassis, and I do not have a ground lift anywhere.

So I guess I'll start with a ground lift! And maybe star grounding....
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Old 21st March 2008, 07:33 PM   #4
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It might help to know what kind of hum you're getting. Is it 60Hz or 120Hz? They're usually caused by very different problems. It seems as though it might be 60Hz, though, from your statement that you can hear it in your active PC speakers even with only the heaters on. That's usually a sign of electromagnetic pickup from the power tranny by the OP tranny. This can be minimized if the cores of the two trannies are at 90 degrees to one another. Try rotating one of them, if you can, to find the least hum position.
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Old 21st March 2008, 08:02 PM   #5
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I actually fixed my hum problem-- I had a filter capacitor hooked up wrong....very wrong. Fixed it and the hum is gone!

However, now the 220 ohm resistor in the power supply is smoking...its called out to 15W, and i put a 25w on it, this is odd....
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Old 22nd March 2008, 03:43 PM   #6
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also fixed the overheating resistor problem! my center taps for the 2a3 filament trannies were shorting to ground by laying on the chassis...i forgot to block them off. now its making sweet music!
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