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CMOY not working!

post #1 of 16
2/1/11 at 9:20am
Thread Starter 

Hey.  I've spent ages on my CMOY amp following TangentSofts tutorial, but it just doesn't want to work.  The power circuit is fine, I've tested the voltage at the right points using an 18v power supply and it splits to 9v each in the right places and 18v across the OPA2135PA op-amp +and-.  When the headphones are plugged in, I can switch the power on and off and I hear a quiet 'pop'. I've assumed this is just the power kicking in. The circuit seems fine! I've disassembled and reassembled, but I'm getting the exact same problem! Every time I check something then try it again, my heart starts pumping, but every time I'm disappointed as it doesn't work!!!!

Any help or pointers would be greatly appreciated!

 

(The op-amp isn't inserted in these pictures but the little notch is on the side furthest away from the capacitors.)IMG_0196[1]

 

IMG_0198[1]IMG_0197[1]

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post #2 of 16
2/1/11 at 9:58am

Your opamp socket is in backwards, though from what I can tell, it is wired according to tangent's guide. With the opamp out, use your DMM set on ohms scale and measure each opamp socket pin to virtual ground. Compare the two sides to the schematic. For example, on pins 3 and 5 should show 100K to ground (R2). If everything looks ok here, ohm out adjacent pins, again referring to the schematic. Make sure you can see the feedback loop (pins 1-2 and 6-7).

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post #3 of 16
2/1/11 at 10:12am
Thread Starter 

Surely it doesn't matter which way the socket goes in as its just extends the actual op-amps pins?  As long as I put the op-amp the right way round it should produce the same effect.

 

Thanks for the tips, I've been doing it bareback up until now, I've only been using an old analogue "Avometer 8". I'll have to get myself a multimeter. I'll post the results.

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post #4 of 16
2/1/11 at 10:17am

Sure it doesn't matter electrically... until the first time you forget and put the opamp in backwards. Then it will matter electrically, to the chip :)

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post #5 of 16
2/1/11 at 10:33am
Thread Starter 

There's hardly any current running through the whole circuit.  Is this normal?

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post #6 of 16
2/1/11 at 10:35am

How and what are you measuring?

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post #7 of 16
2/1/11 at 11:03am
Thread Starter 

I'm not sure what i'm supposed to be measuring for the current lol. What would be the best place to check?

 

I've had a couple of ideas on possible things that are wrong.. My headphones could be too low impedance or something - only Sennheiser HD202ii

or the virtual ground is somehow wrong? I'm a bit of a newb at electronics, I don't really understand virtual grounds

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post #8 of 16
2/1/11 at 11:28am
Thread Starter 

On my first circuit I had the op-amp the wrong way around. Would this have any adverse effects on the op-amp? will I need to replace it?

Thanks again!

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post #9 of 16
2/1/11 at 11:30am

The easiest way to measure current on a battery powered Cmoy is to set the power switch to "off" and put your current meter across the terminals.

 

If you dont have a power switch break the connection at the battery (a 9V can easily be mounted 90* off) and insert the meter there.

 

What current did you get, and what current were you expecting a few posts up? It should be pretty close to the quiescent current of the opamp you used, look it up in the datasheet. Some opamps run on very very low currents.

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post #10 of 16
2/1/11 at 11:48am
Quote:
Originally Posted by nickst3p View Post

On my first circuit I had the op-amp the wrong way around. Would this have any adverse effects on the op-amp? will I need to replace it?

Thanks again!



If you turned it on while the op-amp was the wrong way around, it's time to buy a new one.

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post #11 of 16
2/1/11 at 11:53am

Your opamp is likely toast if inserted backwards. Did it get hot?

 

The "virtual ground" is simply a reference which is halfway between the + and - terminals of the battery. In the Cmoy, it is derived by the two 4.7K (IIRC) resistors running from +/- battery to the center strips on the ratshack board. Since you have already measured 1/2 battery on the virtual ground, it is functioning just fine. It is used as a reference point for measurements. So when someone tells you to measure from virtual ground, they are telling you to use a point on the two center strips as the "ground" reference and measure from there.

 

Battery powered amps such as the Grado RA-1 will do things a bit differently. Using two batteries, the point where the + of one is connected to the - of the other will be used as the "ground". Same concept really.

 

Read this for more information: http://tangentsoft.net/elec/vgrounds.html

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post #12 of 16
2/1/11 at 11:55am
Thread Starter 
Quote:
Originally Posted by nikongod View Post

The easiest way to measure current on a battery powered Cmoy is to set the power switch to "off" and put your current meter across the terminals.

 

If you dont have a power switch break the connection at the battery (a 9V can easily be mounted 90* off) and insert the meter there.

 

What current did you get, and what current were you expecting a few posts up? It should be pretty close to the quiescent current of the opamp you used, look it up in the datasheet. Some opamps run on very very low currents.


The current across the switch terminals was about 15mA.  The dual op-amp im using, the Burr Brown OPA2134PA, has an input of (plus or minus) 2.5V to (plus or minus) 18V and sucks 5mA.

 

So the current and voltage are fine it just doesn't seem to work!  Do you know if accidentally inserting the op-amp the wrong way round and turning it on can stop it from working properly when its the right way round?  

 

The only other logical explanation is that there is some sort of ghost sitting in my op-amp, stopping it from working. :(

 

Cheers

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post #13 of 16
2/1/11 at 12:19pm

Installing an opamp backwards is almost guaranteed to make it blow up.

 

15mA draw on an amp that should be sucking down only slightly more than 5mA is cause for alarm.

 

I agree with the others, get a new opamp.

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post #14 of 16
2/1/11 at 2:08pm
Thread Starter 

@Pars,  I didn't notice the op amp getting hot, but it definitely doesn't seem to be working.  I'll order another one, should come tomorrow at some point.  I'll bang it in and let you know what happens!

I'm extremely grateful for all your help

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post #15 of 16
2/1/11 at 2:14pm

The 5mA would be per channel, so 2 x 5mA = 10mA. An LED will be another 1-2mA. The analog meter is probably hard to read real accurately, so 12mA or so might kinda look like ~15 on it. Check the resistances at the socket (without the opamp in) to see if you can find anything amiss.

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