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Cmoy amp stopped working. Troubleshooting ideas?

post #1 of 24
Thread Starter 

Ok a while ago i built a cmoy amplifier from parts ordered from digikey and it actually worked perfectly with no problems. I was mostly using it in my car to amp my fuze for use with my sound system and i decided to hook the power up to the dc 12v from the cigarette lighter so i wouldn"t have to keep replacing the 9v battery. This worked perfectly for quite a while until i let my friend borrow it and after a month or two it stopped working. Now I'm not skilled with electronics by any means so I have no idea what could be wrong with it but I would like to try to repair it. Any suggestions?

post #2 of 24
I would take a digital multimeter and check all the connections to make sure all components are good and in working order.

If the multimeter shows that everything is good, then I'm not really sure.
post #3 of 24
Thread Starter 

Ok I have a multimeter but how exactly would I go about doing that? Just checking the resistance im assuming? And what exactly should I be looking for?

post #4 of 24
Cornman - just some basic electronic debugging technics:

Standard tests:

- perform a visual check, defective parts, burned areas, broken lines on the pcb
- regarding the higher voltage:
- are there any parts, especially capacitors, labeled with a lower max. Voltage than applied (12 volts).
- Voltage in a car can be higher than 12 volts when engine is running.

(If possible, remove the opamp for next test):
- check that all connections are ok (connectors, cables, ...lines on the pcb)
- set the multimeter to 200 ohm range
- test all lines on the pcb by meassuring endpoint to endpoint should always be nearly 0 ohm

- Solder all points on the pcb again to be sure there are no weak soldering points

Functional tests with opamp:

As you described the failure (and there is only one activ part - the opamp) could be that a high voltage spike has destroyed the opamp

- use a battery in line with the multimeter in dc current 200 mAmpere mode to check the current floating when amp is switched on.

If current is ok somethink like some milliampere
- let the battery connected to the amp and
- test with multimeter set to 20 volt range that the battery voltage can be seen whereever it should (see the schematic).

If this is okay but amp not working i would
- just replace the opamp
- check (again) that the electrolyt capacitors are ok,
- damaged ones by high voltage often can be identified (expanded ends housing deformed).
after each step
- Check with signal and headphone if ok now

- follow the signal path from start to end to see that also here everything is ok

(if the opamp is replaced and should work or you think the opamp is ok)
- if not for example the opamp is defective

in each step the "not finding" the correct result implies were a failure is so broeken line > repair that line etc ..

good luck and success !
Edited by GermanGuy - 6/5/13 at 12:55am
post #5 of 24
Thread Starter 

Ok ive tried everything i can think of, checked connections, resoldered all connections, tested all resistors, still nothing. Do you think its most likely the op amp?

post #6 of 24

if you have checked all the points i mentioned including signal path it should be the opamp !


- did you check the capicitors in the signal path ?

- really nothing to hear (any hiss or at least something like this ?

post #7 of 24
Thread Starter 

ok finally i have gotten somewhere! I was using headphones to test it and i got nothing so i decided to plug some speakers into it and i can actually hear some music playing from only the left side when the pot and source are turned up all the way but it is very quiet and distorted sounding. Is this of any help?

post #8 of 24
Thread Starter 

Im currently tracing the signal as described here http://tangentsoft.net/audio/trouble.html playing a 1000hz sine tone from audacity and im getting .6 volts from ground to input jack as described in step 1 but when i get to step 3 (from ground to + in on the op amp) its giving me 2 volts??? That doesn't seem possible since its supposed to be the same if not slightly less. Can anyone make sense of this?

post #9 of 24

Any chance you can show us a picture? Anyway, I'll check for a blown cap first, Though 12V should be fine for a cmoy, you need to be sure all the parts you used are rated at least for 6V. If your friend use a 15V adapter (or a unregulated 12V adapter), it could fry the 6.3V cap (15/2 = 7.5V). Also, if you are using a socket for the opamp, make sure it is tightly fitted.

post #10 of 24
Thread Starter 

How exactly do you check for blown caps? They all look fine, is there a way to test it with a multimeter? Ok here's a pic if that helps.

post #11 of 24
Just google blown capacitors. You will get lots of articles and if you click on 'pictures' lots of pictures showing how that looks like. On your picture the two big caps look ok.

Have you replaced the opamp?
Edited by GermanGuy - 6/5/13 at 2:13pm
post #12 of 24

Hi cornman


In your opening post you mention plugging your CMoy into your cigarette lighter while using it with your car's sound system. Don't do this!


Your sound system's ground connection is likely tied to vehicle ground (the neg terminal of your car's battery). When you power your CMoy from the cig lighter, this ground becomes the negative rail of the CMoy's power supply, NOT the virtual ground used to reference CMoys incoming sound signal. This virtual ground is 9/2 volts above the negative rail. That means: the moment you connect the CMoy audio jack to the vehicle's audio jack, you are (probably) shorting its virtual ground (4.5 volts) to its negative rail (ZERO volts). Look for "Why can't a cMoy be powered from a vehicle's +12V accessory supply?" in http://www.jdslabs.com/faq.php?page=cmoy#


The bottom line is: it's OK to plug your CMoy into the vehicle's cigarette lighter, or into the audio jack of your vehicle's sound system... but not both at the same time!!!


If this is your problem, you should verify the voltages at both of the power rails (battery's terminals) and at the virtual ground (audio jack ground pin). The rail splitter (TLE2426) is internally protected against short circuits. Maybe the opamp was damaged.


good luck with this

post #13 of 24
Thread Starter 
Ok so the big caps are rated for 35v and the smaller ones are rated to 100v so I doubt thats the problem. Also thanks for the info Gaudeamos I had no idea about that but it makes sense now that I think about it. What exactly is the rail splitter? I looked up the part number and it looks like an op amp to me and I don't have any other parts that look like that. Also even if the op amp was damaged that still doesn't explain why I'm getting double the input signal voltage on the inputs for the op amp when it should be identical to the original voltage if not slightly less. Also should there be any voltage from ground to op amp + inputs when there is no source plugged in? Because with nothing plugged in its still reading 1 volt. Edit: Ok I think I'm getting somewhere. With the op amp out and just taking voltages from ground to the socket Im getting .6volts from ground to + in on both channels (same as ground to input) but nothing from ground to - in on both channels. Should there be voltage from ground to neg in with the op amp out?
Edited by cornman - 6/5/13 at 4:11pm
post #14 of 24
Thread Starter 

Ok I just replaced the op amp and it works!! So that was the problem the whole time, I could have saved myself a lot of trouble by just trying that first lol. Thanks for all the help I really appreciate it!

post #15 of 24

re. 'rail splitter': I didn't realize that your circuit (as you've followed Tangent's instructions) has a passive virtual ground - consisting of R1+/- and C1+/-. A rail splitter is a voltage regulator IC device that generates an exact midpoint voltage between its 2 inputs. It's an improvement over the basic CMoy design that you've implemented. See http://tangentsoft.net/audio/cmoy-tutorial/tweaks.html#vground. Sorry about that red herring!


OK... I'll share my experiences when things were not as they should be during my CMoy construction. Firstly, I found that the amplifier section of the curcuit is pretty robust. Indeed my opamps have taken quite a bit of abuse (only one supply connected, output shorted to supply rails, etc) and are still working fine. Any problems were usually traced to the power supply. So, I think that you should start by validating your circuit's supply.


The following steps may overlap some of GermanGuy's advice, but I am focusing on the power supply.  Before you follow any of these steps, convince yourself that the action(s) make sense to you.


1. Remove the opamp from its socket.


2. Calculate the theoretical current draw on your battery:

a. I'm assuming 9V at the battery; measure your battery's actual voltage when disconnected from the circuit.

b: LED current: (9-2)V / 10,000 Ohms = 0.70 mA (2V is a typical forward voltage drop across a red or green LED; you can measure this in your circuit across the LED terminals when it is illuminated)

c: Virtual ground bias current: 9V / (2 * 4,700 Ohms) = 0.96 mA

d: So, the current should be no more than 0.70 + 0.96 = 1.66 mA (actually less, depending on the battery's internal resistance). Anything in addition to that would likely be leakage through the filter capacitors (C1+/-). There is no other route for current to flow, right?


3. Measure the current draw from the battery when connected to the circuit (let us know if you need help with this). If it's more than your calculation, then you've likely got a problem with the electrolyte filter caps. They don't necessarily show visible damage if they're blown, but they often short-circuit internally, resulting in exaggerated leakage current. That's what happened to me (my caps) when i got the polarity wrong.


4. Measure the total supply voltage (voltage between battery's terminals with battery connected to the circuit) Measure the supply voltages referenced to virtual ground; these partial voltages should measure exactly half of the total supply voltage (+/- 2% assuming you've used 1% resistors as specified in Tangent's parts list). Again, if there is an imbalance, you've likely got a problem with one of your filter caps. The cap with the highest leakage current would be the one with the lowest partial voltage reading.


5. Verify that the voltages at the opamp socket's V+ and V- pin positions are the same as at their respective battery terminals.


5. If the measurements in (3), (4) and (5) were as expected, you have validated your power supply. If the measurements in either (3) or (4) were not as expected, then remove the filter caps from the circuit and repeat the measurements. If the readings are still incorrect, I'm afraid your problem lies elsewhere. In any case replace the removed caps with new ones; don't reuse any that you've desoldered from your pcb.


Let us know how this turns out.

cheers, Ewald

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