DIY Audio Projects Forum

Cmoy Amp help!!
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Author:  Lidders [ 25 May 2017, 14:09 ]
Post subject:  Cmoy Amp help!!

I've built my 4th cmoy amp and still i'm unsuccessful. The first ever one worked until the day I tried to move it to a smaller tin and then it never worked again. The next two only worked in one ear and this last one works but is distorted, really badly. However, if I turn it over I noticed that if I placed my thumb over pins 5-8 (this was a fluke!) it plays beautifully. Until I remove my thumb of course! So I guess I ground issue but I can't see why. Please have a look and tell me where I've gone wrong. Photos to follow.

Author:  Peter W. [ 26 May 2017, 08:26 ]
Post subject:  Re: Cmoy Amp help!!

Your thumb is acting as a high-level resistor and a 'gimmick' capacitor.
Try putting a 10-meg resistor across pins 5 & 8.
Try touching a low-value resistor to either pin 5 or 8 while holding the other end.
There is a lot of 'slop' in the stranded wire connections. Try dressing them to be a clean connection.
Try reducing the voltage. Start at around 6V, if possible.
Try reducing the input level. If you are using a headphone output from your source, you are likely overloading it severely. Test it with an adjustable line-level output - say from a CD player or similar.

These answers are fairly generic as the information give is fairly generic.

Best of luck with it.

Author:  Lidders [ 26 May 2017, 09:28 ]
Post subject:  Re: Cmoy Amp help!!

Thank you so much. I'll try the above and see how i get on. Apologies for the generic info, i'm still a newbie to all this.

Author:  Lidders [ 27 May 2017, 01:52 ]
Post subject:  Re: Cmoy Amp help!!

Peter, thanks for your advice. I only had a 1m resistor to hand as the highest value and this worked a treat. Actually seemingly giving it more of bass sound too. Cheers!

I have no idea why this works or why I needed a resistor across pins 5-8 but i'm pleased it works!

Author:  Peter W. [ 30 May 2017, 07:43 ]
Post subject:  Re: Cmoy Amp help!!

Very good!

Every so often, check to see if the new resistor gets warm, or not. It really should not, but do check.

Otherwise, enjoy your efforts! It is always more satisfying to enjoy something you made over something you purchased already made.

Author:  mwhouston [ 31 May 2017, 19:46 ]
Post subject:  Re: Cmoy Amp help!!

Be careful you are not mistaking signal earth for battery negative or vice versa. In other words the resistive network you make to get the +/- (differential) voltage, the centre of that is signal earth. Your inputs and outputs are referenced to this not the negative of the battery. Also is you have a filter cap (usually 220uf 16V) in the wrong way you only get half the the PS voltage at the chip and it is out of balance.

Look at you cct. brd. and reverse engineer the build to draw a complete schematic with voltages. I have built a lot of these and over the years have made and fixed a few mistakes. It sounds like you have it wired wrong or a bad solder joint or a solder splash.

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