Please HELP! Cmoy 2 problem

[BlakeN 0]

I have been beating my head trying to figure out what is wrong with my cmoy 2. After quite a bit of trial and error this is what is happening.

 

When both channels are active I get horrible distortion at higher volume levels. If I remove one of the opamps there is no distortion. It doesn't matter if its the left or right opamp.

 

 

What I have done.

 

Removed the POT and shorted connector.

Resoldered all connections and looked for shorts.

Removed the sockets I was using for the resistors and soldered the resistors in direct. (the board doesn't look pretty but it still works)

Tried 3 different 9 volt batties (all were new out of box)

Replaced LM6171 with a spare.

 

Please help me

[heady 0]

I have been beating my head trying to figure out what is wrong with my cmoy 2. After quite a bit of trial and error this is what is happening.

 

When both channels are active I get horrible distortion at higher volume levels. If I remove one of the opamps there is no distortion. It doesn't matter if its the left or right opamp.

 

 

What I have done.

 

Removed the POT and shorted connector.

Resoldered all connections and looked for shorts.

Removed the sockets I was using for the resistors and soldered the resistors in direct. (the board doesn't look pretty but it still works)

Tried 3 different 9 volt batties (all were new out of box)

Replaced LM6171 with a spare.

 

Please help me

 

Hi, could it be your power capacitor is not working properly or seriously underpowered?

 

[BlakeN 0]

Is there any test I could do with a simple multi-meter to tell? I don't have any spare parts on hand to test with, I am using 2 nichicon 1000 uF 6.3v caps. I was thinking the same thing but I don't know how to test other then ordering 2 more caps.

 

Here is a link to the caps I bought

http://www.mouser.com/search/ProductDetail...647-UPW0J102MPD

 

[heady 0]

BTW - what opamps are you using? The values of the caps look ok. Some DMMs can measure capacitance but I have no idea if that will help.

 

How about some close-up pics of the pcb?

Edited October 11, 2006 by heady

[BlakeN 0]

I used the OPA2134 . I don't have a digital camera so I can't do pcb pics right now but it looks bad anway hehe after removing the sockets and resoldering the resistors and all other components. After doing all of that it didn't make any difference. Also the amp will play at full volume through either left or right with no distortion just not both at the same time so I cant imagine its a cold solder joint. Now I did think about a short between the channels but I used a magnifying glass to check and couldn't find any.

 

I also measured the resistance on all of the resistors using secondary points to make sure there was good contact with the pads. The second time arround I did destroy 1 pad but luckly the trace was on the top side of the board so I soldered it there and it was fine. Once again the distortion was happening before I resoldered all the points and is exactly the same afterwards.

 

My DMM won't measure capacitance

 

I think at this point I might just have to buy new electrolytics so I might as well buy new resistors and sockets for my 2nd board and give it another go.

Edited October 11, 2006 by BlakeN

[heady 0]

I used the OPA2134 . I don't have a digital camera so I can't do pcb pics right now but it looks bad anway hehe after removing the sockets and resoldering the resistors and all other components. After doing all of that it didn't make any difference. Also the amp will play at full volume through either left or right with no distortion just not both at the same time so I cant imagine its a cold solder joint. Now I did think about a short between the channels but I used a magnifying glass to check and couldn't find any.

 

I also measured the resistance on all of the resistors using secondary points to make sure there was good contact with the pads. The second time arround I did destroy 1 pad but luckly the trace was on the top side of the board so I soldered it there and it was fine. Once again the distortion was happening before I resoldered all the points and is exactly the same afterwards.

 

My DMM won't measure capacitance

 

I think at this point I might just have to buy new electrolytics so I might as well buy new resistors and sockets for my 2nd board and give it another go.

 

I don't know what else can be wrong. Unless, as you said, there's a short somewhere. Sorry for not being more helpful. I don't think it is the caps, since the caps you used are the right ones.

 

[jasonhanjk 0]

Hi, sorry for coming in late.

Can you do a few quick measurements on the pin so I can help to diagnose? The cap in the spec seems ok. Also check for correct connections for your input and output stereo jack. Make sure it's not solder wrongly.

 

In DC mode with referrence to V-:

OL

OR

OG

+pin of C7

+pin of C6

 

And finally the voltage across C4.

 

[BlakeN 0]

I will take those measurements when I get home tonight. Here is some more info I posted in another forum (headfi.com)

 

 

Here are the parts I ended up using... BTW the RN60 vishay resistors don't fit on the pcb.

 

opa2134pa

6.3v 1000uF nichicon upw electolytics

63v 1uF 5% WIMA polyesters

LM6171BIN

Vishay 1/4th watt resistors

ic sockets

 

The problem:

Distortion and crackling that increases with volume.

 

I have resoldered all points

Removed the POT and just shorted the contacts

rewired both input and output jacks

used aligator clips instead of jacks

tried 4 different 9v batteries all of which were new.

replaced the LM6171BIN with a spare

Scrubed the board with a toothbrush and 97% isopropyl

 

Here is the kicker, if I remove 1 opa2134 everything is fine. Left or right it doesn't matter.

 

If I hook it up to my 13.8v 20A regulated benchtop psu its fine in both channels.

 

I measured vdc accross both C6 and C7 when the 13.8v psu is plugged in at its 6.8 it is 4.6 with a 9v battery.

 

I ran the amp 24hrs with the 13.8v psu and the caps were room temp... all the parts were room temp.

 

Edited October 17, 2006 by BlakeN

[BlakeN 0]

With no load

 

OL 4.5

OR 3.94

OG 4.5

C7 4.41

C6 8.81

across c4 8.76

 

At loud volume

 

OL 4.5

OR 3.7

OG 4.5

C7 4.35

C6 8.7

across c4 8.68

 

 

I have another bit of odd information

 

I have two sets of headphones

 

One is 55Ohm the second is 115Ohm

 

Both have bass distortion but only the 55Ohm crackle. The 55Ohm set has an inline volume control. If I increase the resistance to 95Ohm the headphones no longer crackle but still distortion. Remember this only happens on battery power the amp works perfectly at 13vdc

 

Thanks for the help

 

[heady 0]

I believe it is because the LM6171 minimum operating voltage is +/- 5V and it seems you are getting below that.

 

To test this theory, do you think you can take out the LM6171 and jumper it with a wire? This bypassing is described in the CMOY2 build thread. The result will be a changed sound but it should not have the problem you described.

 

Just my guess ok? Let jasonhanjk give the authoritative answer.

[jasonhanjk 0]

Hmmm. From Blake's result, the problem lies with the OR pin, the OPA2134B opamp.

Is that opamp socketed? Change a new socket may help, I had previously encounter a bad contact socket. What you can do is using a cutter and cut the socket to 8 pieces. That should make removing it easier.

 

The LM6171 should be ok. The min +-5V is required for the opamp to meet the manufacturer spec.

[BlakeN 0]

It is socketed. I will check the connections on that socket and the solder joints and the connection between r5 and r6. If they all look good I will rip it out.

 

Thanks

Edited October 18, 2006 by BlakeN

[jasonhanjk 0]

Good luck...