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Old 8th January 2011, 11:50 PM   #1
toufu is offline toufu  United States
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Default Decoupling cap hum

I have 2 pairs of decoupling caps to use for my Aikido phono amp - 2 Mundorfs and 2 Ampohms. When I connect the Ampohms, everything is quiet. But I get significant hum if I connect the Mundorfs. I think the caps are picking my torroid transformer noise as moving the transformer away reduces the noise. And the ampohms are in metal cans and mundorfs are not. How can I solve this problem? I dont have too much space so moving things around is not an option. Can I wrap the Mundorfs in tin foil to insolate them? Thanks for any ideas.
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Old 9th January 2011, 12:27 AM   #2
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Old 9th January 2011, 12:37 AM   #3
toufu is offline toufu  United States
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Old 9th January 2011, 01:03 AM   #4
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You could turn the pcb 180 degrees. Or swap the tranny with one of the ps pcb's?
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Old 9th January 2011, 03:44 AM   #5
toufu is offline toufu  United States
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The board wont fit the other way. my real question is why the mundorfs hum and not the ampohms. and if i can make them act like the ampohms.
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Old 9th January 2011, 05:50 AM   #6
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Tricky question. It may be the metal can aspect, but it may also be another mechanism such as earthing - or the cans shielding the inputs?

Can you explain the powering and grounding schemes - it's difficult to make out a single point grounding scheme? Is it powering or shields coming in at the cap end. The chassis is at the opposite end. Heaters and HT (??) - are they the twisted red/black - how are they referenced?

The primary ac cabling is a mess.
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Old 9th January 2011, 05:54 AM   #7
singa is offline singa  Singapore
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Quote:
Originally Posted by toufu View Post
The board wont fit the other way. my real question is why the mundorfs hum and not the ampohms. and if i can make them act like the ampohms.
Yes they can provided you are willing to redrill but that would mess things up and the signal wires may have to be longer, what to do?

1. The easyiest way mabe is to add an alluminium plate with flanges to screw them across the transformer power supply.

2.Use shielding copper braid and sleeve the power wires and signal wires.

3.Last resort move things around.

4.As to why? Who knows but electromagnetic interference is everywhere
so it's a matter of luck or the mundorfs could be faulty?

5.Maybe recheck the Akkido pcb and other pcbs?

6. Email John Broskie?

Edit- I see the signal wires are shielded make sure they are grounded at one end only, preferbly at the rca terminal.
Reroute the twisted heater wire underneath the Akkido pcb.Is the spacer high enough?
regards.

Last edited by singa; 9th January 2011 at 06:01 AM.
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Old 9th January 2011, 12:10 PM   #8
SY is offline SY  United States
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Desolder the caps and reverse them. You want outer foil to ground.
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