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CMoy major issues! At a complete loss at this point! Need Help!

post #1 of 8
Thread Starter 

Let me begin with an apology to all the good people here that put up with me. I know I've posted a lot about this CMoy of mine but at this point I wouldn't be posting unless I had already worked on the thing for the past 6 hours to no avail.

 

It was working fine earlier today


I finally got my custom case (bottom) printed out today and I started temp mounting things in it with duct tape to see how theyd fit.

 

Everything fit ok and I was ready to test it again because some of the connections had been strained at times and I wanted to make sure everything was still ok.

 

Then it did the following.

 

It was showing signs of a bad connection to power or low battery- It would sound OK at first and then quickly (3-8 seconds in) degrade to a grindy distorted sound that was terrible. I started resoldering things (even replaced wire from V- to the DIP-8 V- pin) but to no avail...in fact, its gotten worse. I run my CMoy on 3 9v batteries currently and when measuring voltage off all three in series I got 25 Volts. I then measured their freshness (used a meter that had a built in function for it) and they were all near full. I switched out the oldest battery anyway but it didnt help. I have noticed that when i place my finger over the bottom of the right side of the DIP-8 Socket, specifically when my finger rests over V- and inverted input, the distortion goes away. its quieter than it should be but it goes away...or at least it did go away...now its gotten to the point that it takes /some/ of the distortion away doing that. Also when i pull my finger off it gets way louder for a moment and less distorted and then sinks back into its miserable state.

 

Pulling all the pieces out of the case did not help.

 

ANY IDEAS GREATLY APPRECIATED

 

EDIT: I have gone through tangent's headamp troubleshooting tutorial but my issue wasn't listed there so I just did some generic stuff (tested voltage at different places and connection between wires. I have not been able to Ohm out the socket yet as I really need to get to sleep but that will be the first thing I do tommorow.)


Edited by backspace119 - 2/6/14 at 10:09pm
post #2 of 8
Thread Starter 

Ok so I had some time to ohm out most of the socket this morning (ohmed the wires first again to make sure there wasn't a fault. I got a 26 K connection while I got a 108k (going up) connection between v- and the pin beside it. (input + iirc)......any help?

post #3 of 8
Quote:
Originally Posted by backspace119 View Post
 

Let me begin with an apology to all the good people here that put up with me. I know I've posted a lot about this CMoy of mine but at this point I wouldn't be posting unless I had already worked on the thing for the past 6 hours to no avail.

 

It was working fine earlier today


I finally got my custom case (bottom) printed out today and I started temp mounting things in it with duct tape to see how theyd fit.

 

Everything fit ok and I was ready to test it again because some of the connections had been strained at times and I wanted to make sure everything was still ok.

 

Then it did the following.

 

It was showing signs of a bad connection to power or low battery- It would sound OK at first and then quickly (3-8 seconds in) degrade to a grindy distorted sound that was terrible. I started resoldering things (even replaced wire from V- to the DIP-8 V- pin) but to no avail...in fact, its gotten worse. I run my CMoy on 3 9v batteries currently and when measuring voltage off all three in series I got 25 Volts. I then measured their freshness (used a meter that had a built in function for it) and they were all near full. I switched out the oldest battery anyway but it didnt help. I have noticed that when i place my finger over the bottom of the right side of the DIP-8 Socket, specifically when my finger rests over V- and inverted input, the distortion goes away. its quieter than it should be but it goes away...or at least it did go away...now its gotten to the point that it takes /some/ of the distortion away doing that. Also when i pull my finger off it gets way louder for a moment and less distorted and then sinks back into its miserable state.

 

Pulling all the pieces out of the case did not help.

 

ANY IDEAS GREATLY APPRECIATED

 

EDIT: I have gone through tangent's headamp troubleshooting tutorial but my issue wasn't listed there so I just did some generic stuff (tested voltage at different places and connection between wires. I have not been able to Ohm out the socket yet as I really need to get to sleep but that will be the first thing I do tommorow.)

 

Quote:
Originally Posted by backspace119 View Post
 

Ok so I had some time to ohm out most of the socket this morning (ohmed the wires first again to make sure there wasn't a fault. I got a 26 K connection while I got a 108k (going up) connection between v- and the pin beside it. (input + iirc)......any help?

 

Which previous thread is this in relation to? 

Why not put this in that thread? 

post #4 of 8
Thread Starter 
Quote:
Originally Posted by nikongod View Post
 

 

 

Which previous thread is this in relation to? 

Why not put this in that thread? 

 

It's not the same issue as a previous thread and the discussion there was about something a bit different. I'm sorry if this is improper etiquette I'm still new to forums writing.

post #5 of 8
Quote:
Originally Posted by backspace119 View Post
 

I got a 26 K connection while I got a 108k (going up) connection between v- and the pin beside it. (input + iirc)......any help?

 

What gave you the idea that this was a useful measurement?

 

I think you're just charging up the input capacitance on the op-amp's input here.

 

Ohms mode turns the meter into a current source, so without a path "out" of that node, the current charges up the input cap and the tiny gate capacitance of the TL082's input JFETs, developing a voltage, which the meter interprets as a change in resistance.

 

Quote:
Originally Posted by backspace119 View Post
 

It was showing signs of a bad connection to power or low battery- It would sound OK at first and then quickly (3-8 seconds in) degrade to a grindy distorted sound that was terrible. I started resoldering things

 

When you hear hoofbeats, think horses, not zebras.  Low battery is far more likely than a failed solder joint on a recently built circuit. When solder joints fail, it is usually after months or years, not hours.

 

Quote:

when measuring voltage off all three in series I got 25 Volts. 

 

Is this with the bad sound symptom occurring, while the amp is under load? If not, it's really not a good test, since batteries are weird. When you take the load off a dying battery, the terminal voltage rises.

 

More than once, I've had a battery that measured in-spec under no load, but collapsed to near zero volts when a small load is placed on it.

 

Quote:

 I then measured their freshness (used a meter that had a built in function for it)  

 

Most battery testers are just dumbed-down voltmeters.

 

The better sort of battery tester drops a load across the battery, then checks for voltage sag. In effect, it measures ESR, not voltage. This is a far better indicator of battery health than terminal voltage.

 

This is why I suggest doing the measurement with a DC voltmeter under load. It approximates the action of the better sort of battery tester.

 

Quote:

when my finger rests over V- and inverted input, the distortion goes away. 

 

This suggests the amp may be oscillating. One of the tests in my troubleshooting guide was an amp quiescent current (Iq) measurement. The results can confirm or refute that guess.

 

An output DC offset voltage test could also be illuminating.

 

While you have the amp set up for a quiescent current measurement, try it with the headphones plugged in, and with them out. If the current measurement drops from ~40 mA to ~10 mA, that's pretty much a dead cinch for oscillation, and it would mean the op-amp is having trouble driving your heapdhones' cable capacitance. Another problem with the TL082.

 

Quote:

EDIT: I have gone through tangent's headamp troubleshooting tutorial but my issue wasn't listed there 

 

Although the guide is structured as "if this, try that", a lot of the tests and measurements it asks you to do tell multiple stories. Just because none of the tests or results clued you into the problem, doesn't mean one of us couldn't look at the same results and see the problem. Do all the tests that make sense for your amp, and post the results here.

 

Also, traditionally this is where you post high-res pics of the board, top and bottom, and we criticize your soldering job. :)


Edited by tangent - 2/7/14 at 4:42pm
post #6 of 8
Thread Starter 
Quote:
Originally Posted by tangent View Post

 

What gave you the idea that this was a useful measurement?

 

I'm sorry, It said on your page to ohm out the socket.....(the TL082 is out)

 

Quote:

 

More than once, I've had a battery that measured in-spec under no load, but collapsed to near zero volts when a small load is placed on it.

 

 

I measured while it was playing music at roughly 80% volume

 

Quote:
 

Most battery testers are just dumbed-down voltmeters.

 

 

this one showed mA off the battery (said norm for a 9v was 25mA for its measurement purposes and all were within 22-25mA)

 

Quote:

This suggests the amp may be oscillating. One of the tests in my troubleshooting guide was an amp quiescent current (Iq) measurement. The results can confirm or refute that guess.

 

Did this before I posted. Not out of the ordinary (7mA idle and 47mA iirc. I do know I looked up ratings and it was ok).

 

Quote:

An output DC offset voltage test could also be illuminating.

 

It was a bit high when i measured before I posted. But nothing out of the ordinary and people told me it was ok to use my headphones unless it was more than 100mv. (the measurement for left was around 43mV and right was ok (16mV))

 

Quote:

While you have the amp set up for a quiescent current measurement, try it with the headphones plugged in, and with them out. If the current measurement drops from ~40 mA to ~10 mA, that's pretty much a dead cinch for oscillation, and it would mean the op-amp is having trouble driving your heapdhones' cable capacitance. Another problem with the TL082.

Did this before I posted too, it was normal as well.

 

As for hi res pics I didn't have a camera at the time (still don't) but next time I have an issue I'll try to get some pics.

 

Because you had mentioned in an email that it seemed I was being a little un-self sufficient I tried my best to cover all my bases before posting this time. I have resolved the issue at this point (it was a bad opamp. grabbed another TL082 from radioshack and it works fine now) While I have you here, is it possible that my circuit killed the amp or was it more likely my testing/playing with it or a sudden short that killed it? (next time im at the shack ill pick up another and run them stacked until my shipment with the OPA2132 finally gets here.)

 

EDIT: about the self sufficiency. The reason I thought it was a bad power connection was because i searched the issue and found this thread. But the guy simply said "solved" with no explanation on how he solved it


Edited by backspace119 - 2/8/14 at 5:09pm
post #7 of 8
Quote:
Originally Posted by backspace119 View Post
 

It said on your page to ohm out the socket....

 

Where?  I can't find it.

 

 

Not out of the ordinary (7mA idle and 47mA iirc. I do know I looked up ratings and it was ok).

 

47 mA is not normal. Your amp is almost certainly oscillating.

 

This also explains the relatively high DC offset. 100 mV isn't a magic threshold value. It's an arbitrary value having to do with headphone driver damage, not IC performance characteristics. For a JFET-input op-amp like the TL082, 43 mV is quite high.

 

You can try adding bypass caps and adding R5. Both can help.

 

Or, put a more suitable chip into the amp. A TL082 isn't really a suitable chip for the CMoy pocket amp.

post #8 of 8
Thread Starter 
Quote:
Originally Posted by tangent View Post
 

 

Where?  I can't find it.

 

 
 

 

47 mA is not normal. Your amp is almost certainly oscillating.

 

This also explains the relatively high DC offset. 100 mV isn't a magic threshold value. It's an arbitrary value having to do with headphone driver damage, not IC performance characteristics. For a JFET-input op-amp like the TL082, 43 mV is quite high.

 

You can try adding bypass caps and adding R5. Both can help.

 

Or, put a more suitable chip into the amp. A TL082 isn't really a suitable chip for the CMoy pocket amp.

I have a burr brown on the way. The power measurement for idle was most certainly correct but under load I would really have to check again. As for the socket measurements I thought i had seen it on your page but it must have been someone on the forum in a different post who mentioned something about this.

 

I understand where R5 goes but I'm not entirely certain where these bypass caps would go

 

I don't think any of this will really matter because I will be buffering the output soon. (if its oscillating that would matter but as I said I have another chip on the way)

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