Cmoy Amp Troubleshooting

[burntpotato]

Hey All -

Yesterday I assembled a cmoy amp using a schematic from the tangentsoft tutorial page (
http://tangentsoft.net/audio/cmoy-tutorial/.)

I can hear the music through it, but it sounds very tinny. I can't turn it up very far before the treble hurts my ears and the bass is barely there. Can anyone help me diagnose the problem?

I'm testing with a pair of Sennheiser HD-212's, which have very BOOMY bass, so it isn't the headphones...

Thanks and have a great weekend!
Sal

[bigmike216]

What have you used for caps? is the value of your input cap high? What about power supply caps? What opamp?

[tobias_svensk]

Yeah, post All your values.

[burntpotato]

I didn't stray from the schematic and parts recommendations on the tutorial website.

- Schematic - http://tangentsoft.net/audio/cmoy-tu...angent-sch.pdf

- My resistors are gold-banded (10%?) and the op-amp is a
Burr Brown OPA2132PA.

- And yes, I cleaned the flux off.

[motherone]

Greetings from Seatown.. Can you post a picture of the Cmoy? That will be helpful in troubleshooting.

[carlosfm]

Originally posted by burntpotato
I didn't stray from the schematic and parts recommendations on the tutorial website.

- Schematic - http://tangentsoft.net/audio/cmoy-tu...angent-sch.pdf

0.1uF input cap + 100k resistor to ground forms a high-pass filter at... 46.8Hz.

Nice schematic.

Change that cap for 1uF.

[motherone]

Originally posted by carlosfm


0.1uF input cap + 100k resistor to ground forms a high-pass filter at... 46.8Hz.

Nice schematic.

Change that cap for 1uF.

Actually, I believe it sets the high-pass to 15.9hz:

http://tangentsoft.net/audio/input-cap.html

Not sure where you got 46.8hz from. I checked this against other RC filter calculators and it is correct from what I understand.

Tangent even states that the 0.1uF cap is a little on the low side, and he recommends 0.22uF to 0.47uF.

I guess I'll start with the easy questions until a picture is posted:

1) Did you use a 0.1uF cap or something lower in value?
2) Have you double-checked the values for all of your resistors
3) Have you double-checked that you laid everything out correctly? You can still get sound even if you don't wire things up correctly since it will pass through the feedback resistors to the output.

A picture is really optimal so we can see what's going on, though.

[carlosfm]

Originally posted by motherone
Actually, I believe it sets the high-pass to 15.9hz

Sorry, my mistake.
It is 15.9 Hz, Bessel filter.

[burntpotato]

Originally posted by motherone



Actually, I believe it sets the high-pass to 15.9hz:

http://tangentsoft.net/audio/input-cap.html

Not sure where you got 46.8hz from. I checked this against other RC filter calculators and it is correct from what I understand.

Tangent even states that the 0.1uF cap is a little on the low side, and he recommends 0.22uF to 0.47uF.

I guess I'll start with the easy questions until a picture is posted:

1) Did you use a 0.1uF cap or something lower in value?
2) Have you double-checked the values for all of your resistors
3) Have you double-checked that you laid everything out correctly? You can still get sound even if you don't wire things up correctly since it will pass through the feedback resistors to the output.

A picture is really optimal so we can see what's going on, though.

Thanks so much for the checklist! I will check/change those values first before trying to go rob somebody who has a digital camera

[PRR]

Most common mistake on Cmoy is not connecting the various grounds: input jack, output jack, and rail-splitter center-point must all be connected together.

If you don't, the gain "should go to zero", but stray capacitive leakage in the tightly-built amp will spew treble.

For more Cmoy mistake-sharing, his forum is a good place. Someone there may recognize the problem from the symptoms.