See if you can take DC voltage readings on all the pins of U1 (the NJM2068) to ground (metal shell of the gain switch). Pins 3 and 5 especially should read 0Vdc. I suspect your source device may have DC on the input signal feeding the O2. Since the O2 doesn't have any input coupling caps any source DC will whiz right through and stop after the pot with C13 and C14.
Amarphael:
After re-reading what you wrote the problem may not be source DC. Sounds like you measured some low voltages on the input resistors, and your other O2 is working presumably with the same sound source. BTW I should have said "near 0Vdc" on pins 3 and 5 since there will be some tiny voltage equal to the njm2068 input bias current times 10K or so.
But... a couple other ways to get the -5.8Vdc and -6.6Vdc on the pot are the NJM2068 being bad, or something gone wrong in the feedback loops on both sections of the chip to cause very high stage gain, so the NJM2068 is acting more like a comparator than an amplifier. That -5.8Vdc and -6.6Vdc are getting kind of close to the +/- 7Vdc clipping level that happens if that stage is all the way "on" in either power supply rail direction, which is what a comparator would do.
So then another thing to check here is that you didn't accidentally get the U1 (NJM2068) and U2 (NJM2903D) chips swapped. They are pin compatible and would actually run swapped, but the results would be something like you are getting. The power management circuit would probably work pretty normally with the NJM2068 in for U2, but not so for the gain stage with the NJM2903D in for U1. The other thing to try is after taking those U1 voltage readings above, power the O2 down and remove U1, then power it back up and make the same voltage readings on all the U1 IC socket pins with that chip out. Probably a good idea to leave your headphone unplugged to avoid any damage until the problem with the pot voltages is resolved.
After re-reading what you wrote the problem may not be source DC. Sounds like you measured some low voltages on the input resistors, and your other O2 is working presumably with the same sound source. BTW I should have said "near 0Vdc" on pins 3 and 5 since there will be some tiny voltage equal to the njm2068 input bias current times 10K or so.
But... a couple other ways to get the -5.8Vdc and -6.6Vdc on the pot are the NJM2068 being bad, or something gone wrong in the feedback loops on both sections of the chip to cause very high stage gain, so the NJM2068 is acting more like a comparator than an amplifier. That -5.8Vdc and -6.6Vdc are getting kind of close to the +/- 7Vdc clipping level that happens if that stage is all the way "on" in either power supply rail direction, which is what a comparator would do.
So then another thing to check here is that you didn't accidentally get the U1 (NJM2068) and U2 (NJM2903D) chips swapped. They are pin compatible and would actually run swapped, but the results would be something like you are getting. The power management circuit would probably work pretty normally with the NJM2068 in for U2, but not so for the gain stage with the NJM2903D in for U1. The other thing to try is after taking those U1 voltage readings above, power the O2 down and remove U1, then power it back up and make the same voltage readings on all the U1 IC socket pins with that chip out. Probably a good idea to leave your headphone unplugged to avoid any damage until the problem with the pot voltages is resolved.
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Yeah, I did measured all of them but by the time i submitted the post i got logged off and it MIA and i've remebred only those two... Anyway, reading are as follows w/chip and w/o:
pin1 w/ -5.81V, w/o 0V;
pin2 w/ -2.3V, w/o 0V;
pin3 w/ 0V, w/o 0V;
pin4 w/ -11.7V, w/o -11.7V;
pin5 w/ 0V, w/o 0V;
pin6 w/ -2.3V, w/o 0V;
pin7 w/ -5.81V, w/o 0V;
pin8 w/-6.6V, w/o -0.004V.
the NJ2068 it self is fine because i swapped the other from the working unit but the readings are the same. Also, no i didn't reverse u1-u2, it's 2068 and 2903 respectively.
So how do i check those feedback loops?
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The problem is pin 8 of U1, the positive voltage rail. That should be somewhere around the opposite of the negative power supply rail on pin 4, +11.7Vdc or so. With that positive rail missing the NJM2068 is just doing the best it can with all the other voltages.
Most likely your Q1 mosfet isn't turning on. See what voltages you are getting on pins 4 and 8 of U2, the NJM2903, during a time when you are getting that -6.6Vdc for pin 8 of U1, with U1 installed in the socket. Also see what you are reading on pins 1 and 7 of U2.
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23vAC is ~32V peak when rectified. The recommended limit for the 7812 voltage regulators from most manufacturers is 35V. Fluctuations in the mains supply may cause this voltage to be approached or even exceeded. In the UK mains voltage is specified to be +/-10%.
Of course you could have been measuring on a high day.
Although the voltage will probably be slightly less when the transformer is loaded, the caps in the power rectifier section are only rated 35V. Again, this is slightly more than they will be exposed to, but for longevity they are usually only exposed to 2/3 of their rated voltage.
You will probably get away with using this transformer, particularly in the short term, but there will be 20V dropped across the regulators, causing them to run hot. The increased temperature within the case will shorten the life of the caps.
If the amplifier fails after a comparatively short life, you will have little reason to complain. The voltage from the transformer is certainly in the range that would be considered risky. The documentation recommends transformers in the range 13.5-20v no load.
Of course you could have been measuring on a high day.
Although the voltage will probably be slightly less when the transformer is loaded, the caps in the power rectifier section are only rated 35V. Again, this is slightly more than they will be exposed to, but for longevity they are usually only exposed to 2/3 of their rated voltage.
You will probably get away with using this transformer, particularly in the short term, but there will be 20V dropped across the regulators, causing them to run hot. The increased temperature within the case will shorten the life of the caps.
If the amplifier fails after a comparatively short life, you will have little reason to complain. The voltage from the transformer is certainly in the range that would be considered risky. The documentation recommends transformers in the range 13.5-20v no load.
Yeah that's what i was thinking but the 2903 readings were exactly righ to begin with:
pin1 -11.7V
pin4 -11.7V
pin7 9.5V
pin8 11.7V
Other four are also ok.
Q1 measures (from the back of the board):
left: -11.7V
middle: -11.7V
right: -11.2V
Q2 reads the same first two, 9.3V on the right.
But i do have the 'imbalance' where D4 (unbended) is 0.4V lower than D3 (banded) when powered on, but are fine (less tan 0.1V) when powered off, ACV connected. Which should be fine according to the TS as it refers to the latter situation (powered off).
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Interesting! You are narrowing it down. That leftmost pin of Q1 with the -11.7Vdc on it (looking at things from the back of the PCB with the jacks at the top) is the +11.7Vdc input from the power switch and is hooked directly to pin 8 of U2, where you measured the +11.7Vdc, by a tiny trace on the top of the PCB that runs under a bunch of components.
On the back you probably had your meter lead on the Q1 lead sticking out. If you can get your meter probe on the actual copper pad around that leftmost Q1 pin you may get the +11.7Vdc. May be a bad solder joint there or a bridge. Try reheating all 3 Q1 pins with the soldering iron for about 2 full seconds each. Then also take a look between them with a magnifying glass for bridges and good solder coverage all around the pins. Can be really hard to see bridges between those mosfet pins.
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Problem with Objective 2 output
Hello,
I'm new to diyAudio and new to diy amplifiers too. I was wondering if anyone could help me with a problem I have with my newly built O2 amp.
The sound of my amp is great, but from time to time, if either the input or output cable moves, the sound in of my left earphone disappears. I can get it back when I jiggle the cables, but from time to time (whenever the one of the cables move to much) the sound disappears again. I have no multimeter at hand, but my best guess is that the 3.5 input and output are broken, but it seems as if the problem doesn't occur as often when my batteries are almost empty. Could someone confirm this, or give suggestions on what the problem might be?
Any help will be greatly appreciated!!
Hello,
I'm new to diyAudio and new to diy amplifiers too. I was wondering if anyone could help me with a problem I have with my newly built O2 amp.
The sound of my amp is great, but from time to time, if either the input or output cable moves, the sound in of my left earphone disappears. I can get it back when I jiggle the cables, but from time to time (whenever the one of the cables move to much) the sound disappears again. I have no multimeter at hand, but my best guess is that the 3.5 input and output are broken, but it seems as if the problem doesn't occur as often when my batteries are almost empty. Could someone confirm this, or give suggestions on what the problem might be?
Any help will be greatly appreciated!!
Hi all, I need help. Not unexpectedly, but still frustratingly, I am having problems putting together my O2. To summarize the situation:
I populated the board, saving mosfets for last. Had an ESD mat that connected to a ground at outlet and my body via wrist strap. My soldering iron (Metcal SP200) is supposed to be ESD safe and is plugged into an isobar in a lab.
My R5 resistor (270kohm) read funny ~150k or ~80k depending on how I used the leads on my DMM. I left it and went ahead with power testing. Everything was within normal ranges until I got to U2 Pin 2, which read high near 9v. The other pins were ok except Pin 7, which was a negative 8v. According to the troubleshooting guide, either my U2 or my Q2 or my R5 are wrong or broken.
After a week of waiting I got replacements for all 3. First I swapped the U2 since that was in a socket. Same measurements as before. I then swapped the R5 resistor. Got better readings ~120k for resistance, but U2 pins all had same problem. I then replaced the Q2 mosfet, being as careful as possible with handling. Still roughly the same measurements.
I am stumped. I checked for solder bridges, reflowed nearly every connection, re-tested several times. Anyone know what I can check next? Maybe I am still doing something wrong with mosfet handling despite all my precautions. I have one more spare mosfet in case that is still the problem.
Thank in advance.
I populated the board, saving mosfets for last. Had an ESD mat that connected to a ground at outlet and my body via wrist strap. My soldering iron (Metcal SP200) is supposed to be ESD safe and is plugged into an isobar in a lab.
My R5 resistor (270kohm) read funny ~150k or ~80k depending on how I used the leads on my DMM. I left it and went ahead with power testing. Everything was within normal ranges until I got to U2 Pin 2, which read high near 9v. The other pins were ok except Pin 7, which was a negative 8v. According to the troubleshooting guide, either my U2 or my Q2 or my R5 are wrong or broken.
After a week of waiting I got replacements for all 3. First I swapped the U2 since that was in a socket. Same measurements as before. I then swapped the R5 resistor. Got better readings ~120k for resistance, but U2 pins all had same problem. I then replaced the Q2 mosfet, being as careful as possible with handling. Still roughly the same measurements.
I am stumped. I checked for solder bridges, reflowed nearly every connection, re-tested several times. Anyone know what I can check next? Maybe I am still doing something wrong with mosfet handling despite all my precautions. I have one more spare mosfet in case that is still the problem.
Thank in advance.
Hello.
I have a slight problem with my O2.
After few hours of running on batteries, the amplifier does not come back to life after switching it off. Plugging in AC power for a brief moment while the switch is on lets it come back to life and work much longer.
This issue does not seem to occure when batteries are fully charged.
Could it be that MOSFETS suffered from ESD and "get stuck" ?
I tried testing them with almost depleted batteries and the result was rapid on/off cycling.
Maybe using the old values for R9 and R25 would resolve my issue?
Best regards.
Edit: I suspect that my batteries are 6 cell.
I have a slight problem with my O2.
After few hours of running on batteries, the amplifier does not come back to life after switching it off. Plugging in AC power for a brief moment while the switch is on lets it come back to life and work much longer.
This issue does not seem to occure when batteries are fully charged.
Could it be that MOSFETS suffered from ESD and "get stuck" ?
I tried testing them with almost depleted batteries and the result was rapid on/off cycling.
Maybe using the old values for R9 and R25 would resolve my issue?
Best regards.
Edit: I suspect that my batteries are 6 cell.
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HV, you could try new resistor values. It may help. Personally it's difficult for me to see your problem.
fraggler, your O2 malfunction is tougher. Your going to have to pause, take a deep breath, and attack it again. Inspect and inspect again. Think about it and think about it some more. Measure and measure again. In other words, the opposite of your R5 troubleshooting.
TWIMC, I have no ESD mat on my bench, and haven't used a wrist strap or my ESD-safe soldering iron in quite a while. I was, though, able to solder my O2 FETs with no problem. By just halfway understanding the ESD failure mechanism and avoiding it. That's more important than anything you can find in a catalog, IMO.
PS Yeah, this was sort of a "tuff luv" post. Don't be offended please.
fraggler, your O2 malfunction is tougher. Your going to have to pause, take a deep breath, and attack it again. Inspect and inspect again. Think about it and think about it some more. Measure and measure again. In other words, the opposite of your R5 troubleshooting.
TWIMC, I have no ESD mat on my bench, and haven't used a wrist strap or my ESD-safe soldering iron in quite a while. I was, though, able to solder my O2 FETs with no problem. By just halfway understanding the ESD failure mechanism and avoiding it. That's more important than anything you can find in a catalog, IMO.
PS Yeah, this was sort of a "tuff luv" post. Don't be offended please.
Sofaspud, tough love is fine
I am learning about the circuits concurrently with building. Unfortunately, it is self taught from scattered resources that I have found online (even the wikis here are incomplete). I tried working down the power chain testing each piece to see if they were measuring properly in the circuit (everything measured fine out of the circuit), trying to find where the break point was. I guess what I was looking for was help with the next level of knowledge, as I know what everything does, but not how or well enough to know what might actually be wrong. I also learn better through discussion.
If you can't help with the actual troubleshooting, do you know of any guides that might help me learn enough to troubleshoot it myself?
I am learning about the circuits concurrently with building. Unfortunately, it is self taught from scattered resources that I have found online (even the wikis here are incomplete). I tried working down the power chain testing each piece to see if they were measuring properly in the circuit (everything measured fine out of the circuit), trying to find where the break point was. I guess what I was looking for was help with the next level of knowledge, as I know what everything does, but not how or well enough to know what might actually be wrong. I also learn better through discussion.
If you can't help with the actual troubleshooting, do you know of any guides that might help me learn enough to troubleshoot it myself?
The problem appears to be confined to U2. Not the simplest of comparator circuits. You measured 9V on pin 2, when it should be approx -9V. Pin 7 measured -8V, when it should be closer to +8V. Basically "upside down." Measure all the U2 pins and post the results so we can see just what all the inputs and outputs are.
Or check the leads on your DMM. I know that sounds funny, but you never know.
Any progress? Just trying to keep the discussion moving here until you're enjoying listening to your O2.
I don't want to hijack this thread with general troubleshooting philosophy. That could be given it's own worthwhile thread if you'd like. Sort of a supplement to the Recommended books thread.
I don't want to hijack this thread with general troubleshooting philosophy. That could be given it's own worthwhile thread if you'd like. Sort of a supplement to the Recommended books thread.