Cmoy ground problem?
Sep 10, 2009 at 12:59 AM Thread Starter Post #1 of 5

Batistiini

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Hi

Im building my first cmoy amp and just tested it.
It sounds pretty good but i have one major problem.
When i unplug my cmoy from source(computer) my headphones start to scream very loudly and when i plug cmoy back screaming stops immediately.
Then i noticed that it only happens when ground of the cmoy doesnt touch sources ground. Im using 24v regulated wall power supply. I tried with 9v battery but it still screams but at lower volume.
I will post some pics tomorrow. I hope you can help.

Sorry for my bad english
biggrin.gif
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Part list:
2 1000uF power caps
2 4,7k resistors
2 4,7k gain resistors
1 opa2134pa
2 10k resistors
2 0,47uF caps

pic1
pic2
pic3
pic4

edit: posted some pics
 
Sep 11, 2009 at 1:00 PM Post #2 of 5

the_equalizer

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Quote:

Originally Posted by Batistiini /img/forum/go_quote.gif
Hi

Im building my first cmoy amp and just tested it.
It sounds pretty good but i have one major problem.
When i unplug my cmoy from source(computer) my headphones start to scream very loudly and when i plug cmoy back screaming stops immediately.
Then i noticed that it only happens when ground of the cmoy doesnt touch sources ground. Im using 24v regulated wall power supply. I tried with 9v battery but it still screams but at lower volume.
I will post some pics tomorrow. I hope you can help.

<snip>



Hi Batistiini:

It sounds absolutely normal to me. Does it 'scream' when you unplug both cables from your source (not only the ground)? I'm betting it doesn't.

You see, ground is the reference against which all (or most) voltages in a circuit 'work'. If you remove that reference, you have no way of knowing if the signal in the hot cable is 1 milivolt or 1 megavolt (because you have no point against which to compare it, e.g. ground), so crazy things start happening.

In short, removing the input ground is an abnormal situation and you can expect most amplifiers to misbehave.

cheers!
 
Sep 11, 2009 at 4:59 PM Post #3 of 5

Batistiini

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Quote:

Originally Posted by the_equalizer /img/forum/go_quote.gif
Does it 'scream' when you unplug both cables from your source (not only the ground)?


Yes it screams even if i unplug whole cable. But its not loud anymore, but still noticeable. Maybe it helped when i installed cmoy to box and added jacks. Dunno. Im just noob and this is my first audio project.
smile_phones.gif
 
Sep 11, 2009 at 10:38 PM Post #4 of 5

hiker101

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Hi Batistiini,

Your parts list is incomplete- there don't appear to be enough resistors. Can you refer us to a picture of your schematic? It certainly looks like you have a standard CMOY from your photos.

It does sound like there is something screwy going on with your ground. If the inputs are disconnected AND you are running on batteries, then your CMOY should be very quiet. Disconnecting the source should cause both amp inputs to be pulled to virtual ground- they are connected via resistors- this should cause no output to the headphones (my CMOY is totally silent.)

Tangent's web site has a nice page on tracking down noise in an amplifier. How to Build the CMoy Pocket Amplifier


If you have a meter, you could do a bunch of sanity checks.
1) Amp off. Check that the resistance to ground for each of the chip inputs is what you expect. On the tangent CMOY, the + input has 100K to ground, and the minus has 1K. Yours should be similar.
1a) Check that all points in your schematic that are claimed to be ground really have zero resistance to ground on your board.

2) Amp on. Check voltages. Rails to ground. Chip supplies. Chip inputs(these should be at ground).

I'm not a big fan of making such large loops with the leads of the tombstoned resistors, but I would be surprised if they were causing the oscillations you are seeing.

Good luck.
 
Sep 12, 2009 at 2:12 AM Post #5 of 5

the_equalizer

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Hmm.. the 2 'missing resistors' apparently do show up in his pics though... What value are those other 2 resistors? They should be 100k as stated by hiker101

cheers!
 

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