Decoupling cap hum

toufu · 8th January 2011, 11:50 PM

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.

trobbins · 9th January 2011, 12:27 AM

Do you have a photo or two?

toufu · 9th January 2011, 12:37 AM

Here you go:

Miniwatt · 9th January 2011, 01:03 AM

You could turn the pcb 180 degrees. Or swap the tranny with one of the ps pcb's?

toufu · 9th January 2011, 03:44 AM

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.

trobbins · 9th January 2011, 05:50 AM

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.

singa · 06:01 AM

Quote:

Originally Posted by toufu

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.

SY · 9th January 2011, 12:10 PM

Desolder the caps and reverse them. You want outer foil to ground.

8th January 2011, 11:50 PM	#1
toufu diyAudio Member Join Date: Jun 2008 Location: San Francisco	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.

9th January 2011, 12:10 PM	#8
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Desolder the caps and reverse them. You want outer foil to ground. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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9th January 2011, 12:27 AM	#2
trobbins diyAudio Member Join Date: Feb 2009 Location: Melbourne, Oz	Do you have a photo or two?

9th January 2011, 01:03 AM	#4
Miniwatt diyAudio Member Join Date: Nov 2004 Location: Den Haag	You could turn the pcb 180 degrees. Or swap the tranny with one of the ps pcb's?

9th January 2011, 03:44 AM	#5
toufu diyAudio Member Join Date: Jun 2008 Location: San Francisco	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.

9th January 2011, 05:50 AM	#6
trobbins diyAudio Member Join Date: Feb 2009 Location: Melbourne, Oz	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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