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Help me debug my Cmoy! (please.)

post #1 of 8
6/24/11 at 1:06pm
Thread Starter 

I everybody, I'm new here.

 

I just finished my first build attempt at a cmoy pocket amp, as described here:

http://tangentsoft.net/audio/cmoy-tutorial/assy.html

 

... a lot of you will know thid build, I guess.

 

After a lot of errors, solder bridges, etc., I got the thing into a state that should be working (my multimeter tells me of no connections where there shouldn't be any, voltage on the +v of the chip is 9.49 and on the -9.50).

 

Nothing gets hot when I turn the device on, now (this has happened, the chip was pretty hot to touch)

 

All ground (in, out, power) go to the same rows on my perfboard)

I'm not using a poti for volume control (I plan to only use the volume control on my mp3 player), the in signyl goes directly to the cap*

 

Now, when I plug this into my computer and headphones into the cmoy, the signal I get from the cmoy is recognizably the music I should be hearing, but:

 

noisy (hissing, a little bit like an amp that should be clipping)

 

less loud than when I plug my headphones in directly!

 

 

*While writing, I notice that I've probably made a mistake by leaving out the poti without any replacement R. Sounds plausible?

Will check with the build tutorial.

 

 

 

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post #2 of 8
6/24/11 at 9:03pm

I'm pretty sure you don't NEED a potentiometer. I've skipped using pots in other builds (just jumpered the holes), and they worked no problem.

 

Do you have R5 (resistor) installed?

 

Are you using metal film resistors? Carbon film/composition will make quite a bit of noise in comparison to 1% metal film.

 

Pictures I guess would be helpful because there could be a few problems. Top and bottom of board might help.

 

 

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post #3 of 8
6/24/11 at 9:10pm

You don't "need" a pot, but you should probably stick a 10K resistor from input to ground in it's place.

 

The high noise is likely because you have a gain of 11x, way too high for any reasonable source/headphone combination, perhaps something like 5x will be better (replace R4 with one of the 4k7 resistors you use for virtual ground).  If you still have noticeable hiss, add 10-100R resistor on the output (R5).

 

If you're actually noticing that the CMOY is quieter than directly to source, you probably have messed up somewhere and you're routing too much signal to ground instead of through the amplification.

 

If the opamp is really getting hot, something is very wrong; if it used to get hot, you might have burned the chip.

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post #4 of 8
6/25/11 at 2:21pm

The op-amp should not get hot. Maybe a tad bit of warmth, but not hot.

 

And yes, you're gain is way too high. I believe my gain is somewhere in the neighborhood of  4.

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post #5 of 8
6/26/11 at 5:39am
Thread Starter 

To answer from top to bottom:

 

I don't have the r5 installed (yet)

 

I'm using 5% tolerance resistors (and took measurements with my multimeter to match the channels closely). I don't know if I baught metal film or others.

 

I'll try the 10k from input to ground and the lower gain

 

About the hot chip: I fixed the mistake that led to the chip getting so hot (mixed up + and - on the socket ... ) and tried with the setup with another one (I had bought 2), both sounded the same.

 

How do I see where I messed up and route too much signal to ground?

 

 

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post #6 of 8
6/30/11 at 11:43am
Quote:
Originally Posted by martin1917 View Post

To answer from top to bottom:

 

I don't have the r5 installed (yet)

 

I'm using 5% tolerance resistors (and took measurements with my multimeter to match the channels closely). I don't know if I baught metal film or others.

 

I'll try the 10k from input to ground and the lower gain

 

About the hot chip: I fixed the mistake that led to the chip getting so hot (mixed up + and - on the socket ... ) and tried with the setup with another one (I had bought 2), both sounded the same.

 

How do I see where I messed up and route too much signal to ground?

 

 

Do you have the jumpers in in place of the R5 resistors? These complete the feedback loop for the opamp, and without it, the amp will not work.

 

One trick that I picked up from Tangent is to remove the opamp, and with the cmoy power off, ohm out the pins of the socket to ground (virtual), noting any differences from what you think it should be on the schematic. If everything here looks OK, then ohm out each pin to the adjacent pin, again referring to the schematic. This should show whether your feedback loop is actually connected, if you have any shorts between adjacent pins, etc. Also double check all solder joints.
 

 


Edited by Pars - 6/30/11 at 11:49am
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post #7 of 8
7/2/11 at 12:34pm
Thread Starter 

I have the the jumpers instead of r5

 

I added the 10k between signal in and ground, plus I ohmed out the chip socket with no power to the board. Everythin cheks out (now, I had to fix a few things) - Thanks for the tip!

 

What's not checking out is the power side of things: I'm getting roughly -10 and +2 V (my to 9V Blocks are somewhat depleted), R between + and gnd is 2.7 kOhm, between - and Gnd 0.7 kOhm. Should this be 4.7?

 

I don't see where I messed up the connections.

 

No when I put power to the board and give it a signal in, I get a very loud signal out in a reasonable quality (hissing noise whith very low sig in, bad quality with a loud sig in (to much gain), fine in between) - only on one channel!

 

I'll give it a thorough check tomorrow, but I'll be grateful for tips where to look!

 

 

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post #8 of 8
7/2/11 at 2:00pm

R from + and - to GND should be the same.

Use fresh batteries so that the rail voltages

are equal.

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