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Old 3rd October 2007, 05:23 AM   #1
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Join Date: Oct 2006
Default chokes, caps, and long distance relationships

I am not in a long distance relationship, and neither are my chokes and caps in my Aikido 6SN7 preamp.

The PSU is 125-0-125 into 1N4007 bridge; 0.68uF cap; 10H; 65uF; 10H; 65uF to give 312VDC with around 20mA load.

This design is very, very similar to Bruce Anderson's original configuration on his Aikido. However, his PSU was mounted separately to the preamp and the PSU cmponents were mounted differently. He had no hum, I do (although it is very quiet).

My PSU is mounted on either side of a 20 gauge steel plate - trafo and chokes on the upper surface mounted at 90 degrees to each other and separated by at least a 1/2 inch. My caps are mounted onto the underside. They're hot-glued to the plate. This means that the caps are located next to the iron-ware with only 20 gauge steel between them.

Should my caps be in such a close relationship to my iron-ware? Would this cause hum rather than "harmony"? Many amps with large caps, place the caps vertically above the chassis, where in conjunction with trafos and chokes become reminiscent or a city skyline. Besides looking industrial, is there a real, tangible, hum-reducing benefit to this type of mounting? I don't recall reading in Morgan Jones' book about problems associated with cap/choke placement, only trafo-trafo and choke-choke placements.

Thanks,
Charlie
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Old 3rd October 2007, 02:52 PM   #2
Sheldon is offline Sheldon  United States
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Join Date: Dec 2003
Location: San Diego
Is the hum line frequency or is it higher (double)? Knowing that will help narrow the search. Here are a couple of general references regarding guitar amps, but the princiiples apply:

http://www.lynx.net/~jc/hum.html

http://paulrubyamps.com/info.html
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Old 3rd October 2007, 04:05 PM   #3
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Join Date: Oct 2006
Sheldon,

You know, it is really hard to tell whether it is 60Hz or 120Hz, especially now as the hum is so quiet. However, it sounds closest to a 60Hz squarewave. I suppose that this means that I have a ground-loop issue, which will be real hard to track down.

However, when I add a 20K bleeder across the final cap, the level of hum drops slightly!

You know, I need to make a schematic of my Aikido and post it for people to look at. That may help significantly.

Charlie
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Old 3rd October 2007, 04:54 PM   #4
Sheldon is offline Sheldon  United States
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Join Date: Dec 2003
Location: San Diego
Download RMAA or something similar and look at the output. No need to guess. With a screen shot, the smart folks here might be able to better guide you.

Have you played at all with the noise cancelling caps in the Aikido circuit? I'm guessing ground or noise pick up issues, so this may be a lower probability, but easy to parallel in a little more capacitance top or bottom and see if that changes anything.

Sheldon
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