Cmoy good with low impedance, goes mental with higher..?
Nov 19, 2008 at 5:38 AM Thread Starter Post #1 of 4

theo404

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As in the title, my cmoy works magicaly with low impedance phones, such as generic philips ear buds and my modded koss plugs, but as soon as I plug my senn HD555s in it goes mad. Distorted with massive hiss at zero/low volume (set by pot) only recovers when there is signal put through it and the pot is turned up to 3/4 travel... will then also go back to loud hiss and very little music at full travel. As I said this doesnt happen with other earbud phones, it sounds sweet from zero up to max. Doesnt matter what level I have the source set to.

Have tried putting R5 (load resistor) in at 47R, no difference...

Have tried batteries, variable dc supply (current limited) and a 12V wall wart, all give the same result.

Have tried with and without input caps.... same result.

Stumpted....

Any input welcomed...

Thanks,
Theo
 
Nov 19, 2008 at 11:15 AM Post #3 of 4

NelsonVandal

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What opamps do you use? Is it the true CMOY with OPA2132 and resistors as railsplitter? What volume pot do you use? Show us some pictures. Have you hooked everything up the right way?

It sounds like your amp is oscillating. If you use OPA2132, this is unusual, and you should look for bad solder joints or some other error. Maybe your pot is connectded faulty. Is it connected to ground like it should? Maybe the capacitors are too far from the opamps power pins. You could try to install 0.1 uF ceramic capacitors from positive to negative rail and from rails to ground as close to the opamp as possible. If that doesn't help you could try to add a Zobel network at the output (a resistor and a capacitor in series from output to ground, something like 50R and 50nF).
 
Nov 19, 2008 at 7:24 PM Post #4 of 4

theo404

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It is indeed a cmoy with resistors to create the split supply, however I am using BB OPA637s... a picky opamp.

Thanks a lot for your advice, after shortening the feedback loop and the power supply lines (which improved things slightly) I tried adding a zobel network, bingo! Fixed.

Will probably add decoupling caps when I can find some...

Thanks again!
 

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