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My Cmoy is Finished!

post #1 of 11
Thread Starter 
Well, I finally finished rebuilding my Cmoy. I built it in stages, checking everything before moving on. It's housed in a nice case that's a bit big, but the switch and jacks are mounted on the end, so I can stick it in my pocket. The case has a clear front, so you can see the red LED when it's on. Pic is attached.

However, the left channel is much louder than the right one! I've even shorted the input reduction resistor (R1 if my memory is correct) on the right channel, but it still isn't equal So what could be the matter? The volume difference is big enough that it's hard to listen to.

Also, the amp distorts the sound. I checked the PS and it outputs +4.62 and -4.65 (or the other way around), so I don't think it's that.

Any tips?
post #2 of 11
Thread Starter 

Another pic

Here's a pic of the back of the PCB, when the case is open:
post #3 of 11
Eric,

Just a guess as I am a real newbie. From my former reading at Headwize the problem COULD be the volume pot not tracking equally side to side. The fix, however, I'm not sure of. Hopefully someone else will elucidate.

BTW, where did you find the OPA's? Still waiting for my order of 50 2134's from Insight - August!!
post #4 of 11
Ok, so what you're saying is the title is wrong... your cmoy is NOT finished.

This is where not having access to the HeadWize historical threads is bad.

Ok, let's go... Also, can you shoot the top of it without the iced plastic in the way? can't see anything!

One channel is louder than the other and there's distortion. Is there distortion in both channels or just one? Give us as much info as you've got.

In the meantime, start at one end and work towards the other... verify each component is going where it should, and each joint is good. Be methodical. Very methodical. This is my mantra.
post #5 of 11
check the feedback loop.... if one of the feedback resistors is incorrect, or there is a cold solder joint, it could become very loud and distort...
post #6 of 11
Thread Starter 
no_iq, I don't have a volume pot. Also, I'm using the 228s, although my 134s just came from Digi-Key (I think they have them in stock now).

Apheared, I've attached a 'collage' of two pics. Sorry about the bad quality, but the board is limiting my max file size to 55kb (!).

Thomas, I've checked the resistors and all appears to be O.K. However, if you look at the pic I posted earlier, you might be able to see that there's a questionable joint connecting the feedback pin to the resistor-I'll resolder it and see if that helps.
post #7 of 11
sorry, its really hard to "see" electrical connections

An easy way to check connections is using a multimeter with a continuity checker. Simply touch two points, and it will beep if they're connected....

Check all the connections in the feedback look, including the connections to ground.... Another possible problem is the inputs, make sure the pot is wired correctly...

BTW, i like to use those same Radioshack PSBs, but you can make wiring a lot simpler by using the two bus strips. Place the IC sockets so the pins "stradle" the two rails... Make them the two power rails (+/-4.5v) and simply make solder bridges (i place a small wire there) between the power inputs and the power rails to connect them.

I prefer using "star grounds" for the ground connection, simply solder a wire to each point labled ground, and solder all the wires together...
post #8 of 11
Thread Starter 
Argh, why does everyone think I have a volume pot?! BTW, I chose to use the bus strips as grounds because there are a lot more ground connections than power connections.
post #9 of 11
If you don't use a volume pot, the distortion might be happening if your gain is set too high. A power supply of +/- 4.6x (from the 9V battery) might not be able to handle high gains. I'm not sure if this is true, since I've never tried it (I use a 15-17V voltage wall wart with a high gain amp setting), but it is a possibility for the distortion part of it.
post #10 of 11
Thread Starter 
Ok, I tried thomas' suggestion and IT WORKS!!! WAHOO!! IT'S AMAZING HOW GOOD IT MAKES MY KOSSES SOUND!! THANK YOU CMOY, THOMAS, APHEARED AND EVERYONE ELSE FOR YOUR HELP!!

See my review in the Headphone Reviews forum.
post #11 of 11
Hello,

Gain should be smaller than Mr. Moy has suggested. I suspect he put a little higher gain to be on the safe side. However, you can drive any phones with gains less than 5.

My Moy amp has Gain of 2. (It drives my K240M, 600 ohm monster.)

Tomo
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