Built a CMOY, noticing hum
Apr 18, 2012 at 4:58 AM Thread Starter Post #1 of 13

HairyMerkin

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I've got a hum when my CMOY is silent.
 
I'm fairly certain it's a ground thing because I can eliminate it entirely if I touch the 3.5mm jack and the outside of the case.
 
nb. I have a cable that ends in a gold connector (metal) and that's what I'm touching.
 

It's a ground loop right?
 
I've tried it with a cord that ends in a plastic connector but it's still the same.
 
Apr 21, 2012 at 5:49 PM Post #3 of 13

wdahm519

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Need some more information.  Did you use insulated input and output connectors or is the ground shielding of the in/out connectors touching the metal case?  
 
Its usually not a good idea to ground the input and output to the same ground.  Try an insulated output connector.
 
Apr 21, 2012 at 8:16 PM Post #4 of 13

DefQon

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Background hums are GND and power related issues.
 
Sounds like you might have a GND loop here or if your Cmoy is using 9v batteries, one of the batteries are on low power. If your using a power supply via transformer, you need a regulated PSU.
 
Apr 22, 2012 at 1:24 AM Post #6 of 13

wdahm519

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Apr 22, 2012 at 1:35 AM Post #7 of 13

HairyMerkin

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Quote:
 
Quote:
The battery wasn't flat
The PSU is regulated
 
 
http://parts.digikey.com/1/parts/103376-conn-rca-jack-r-a-black-pcb-rcj-041.html are the two coaxial SPDIF connectors

 
Erm... those are not S/PDIF connectors.  Those are just standard mono RCA jacks.  I think that's what you meant though.  Also, those are not the connectors I'm talking about.  I'm talking about the 3.5mm stereo audio jacks on the CMoy.  
 
 

 
 
Isn't that what S/PDIF connectors are? RCA jacks for coaxial S/PDIF, and toslink for optical?
 
How do I tell if they are insulated on the cmoy?
 
They look identical to these; http://au.mouser.com/ProductDetail/ADAM-TECH/ASJ-6-4A/?qs=ogqIPVdloe%252bUizPi9ZU1ImlMhigygBB9RCjPw0SWK4E%3d
 
Apr 22, 2012 at 1:56 AM Post #8 of 13

wdahm519

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Quote:
 
 
 
Isn't that what S/PDIF connectors are? RCA jacks for coaxial S/PDIF, and toslink for optical?
 
How do I tell if they are insulated on the cmoy?
 
They look identical to these; http://au.mouser.com/ProductDetail/ADAM-TECH/ASJ-6-4A/?qs=ogqIPVdloe%252bUizPi9ZU1ImlMhigygBB9RCjPw0SWK4E%3d

 
Uhh, sure.  If you want to think about it that way.  But S/PDIF is Sony/Phillips DIGITAL Interface Format, and you're passing analog.  So, it doesn't really make sense to talk about S/PDIF.  Its an RCA, and more people will understand what you're talking about if you say that.
 
Well, if the ground shield is tied to ground on the 3.5mm jack and is touching the metal of the case then its unshielded.  You'd need a multimeter to figure this out.  But, its probably your problem.  When you touch the input/output, you become ground and it silences the hum.
 
Apr 22, 2012 at 3:08 AM Post #10 of 13

wdahm519

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Check for continuity between the ground pin on the 3.5mm jack and the case, and also for continuity between ground on the board and the case.
 
It doesn't have hum when you're running the DAC?  I mean, you still have a 3.5mm connector being plugged into the CMoy.  It should be the same.  You're sure this isn't just a noisy signal problem?  Have you omitted all other possibilities other than the Cmoy itself? 
 
If not, start there.
 

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