Checked and correct.
Checked and correct.
Checked, and it's around minus 12.
Checked, and it's around plus 8-9.
Checked R24, connected with no bridges, etc. C16 looked OK, but I removed and replaced in case it was faulty. No bridges or shorts.
Checked and I do have continuity in all places.
Unfortunately, I have the same problem I did before I started this new round of tests. Pin 8 of the empty sockets still reads about -1 volt. Pin 4 values are correct at about -11.84v. I rechecked all the soldier joints and don't see any bridges. I used a grounding strap when I soldiered in the replacement Q1 (as with the original) so hopefully didn't damage with ESD.
I'm discouraged with this but hopefully I'm close to fixing it.
Thanks for your help so far. Any other ideas would be very helpful. Thanks in advance.
- Andy (Steve Mannix)
Hmm... well based on those measurements it all still points to Q1 being duff 
When you are reading continuity from pin 8 of the opamps to the drain of Q1 are you actually measuring to the actual lead of Q1 or its solder pad on the PCB. If so could there be a break there around the pad.
If not then there is a problem with these Q1's you are using. To prove the amp you can just link out Q1 from source to drain and the amp should all function apart from the low battery cut off.
Q1 is a "P channel" FET and so the source connects to the point of highest potential which is the incoming +12 volts. The output is taken from the drain. Now the FET turns on when the gate is at a lower voltage (more negative) than the source. With pin 1 of the opamp at minus 12 volts you should have a "source to gate" voltage of around minus 24 volts. That is why FETs of at least a 25 volt Vgs rating are specified. That will turn the FET fully on and it should act as if it were just a switch linking source and drain.
You can measure the source to gate voltage as a confirmation if you want. The source is at 12 volts as already measured. The gate should be at the same potential as pin 1 of U2 but will read slightly different due to the high value of R24, although with a DVM that difference will be very small.
If the voltage is OK, and all the continuity to and from the FET is OK then something is wrong with those parts.
When you are reading continuity from pin 8 of the opamps to the drain of Q1 are you actually measuring to the actual lead of Q1 or its solder pad on the PCB. If so could there be a break there around the pad.
If not
then there is a problem with these Q1's you are using. To prove the amp you can just link out Q1 from source to drain and the amp should all function apart from the low battery cut off.Q1 is a "P channel" FET and so the source connects to the point of highest potential which is the incoming +12 volts. The output is taken from the drain. Now the FET turns on when the gate is at a lower voltage (more negative) than the source. With pin 1 of the opamp at minus 12 volts you should have a "source to gate" voltage of around minus 24 volts. That is why FETs of at least a 25 volt Vgs rating are specified. That will turn the FET fully on and it should act as if it were just a switch linking source and drain.
You can measure the source to gate voltage as a confirmation if you want. The source is at 12 volts as already measured. The gate should be at the same potential as pin 1 of U2 but will read slightly different due to the high value of R24, although with a DVM that difference will be very small.
If the voltage is OK, and all the continuity to and from the FET is OK then something is wrong with those parts.
Hmm... well based on those measurements it all still points to Q1 being duff
Update - I replaced Q1 for a second time, no change. Decided to recheck everything else you talked about before and decided to replace R24. That was it!
Thanks for all your help with this. I haven't done the comparator check or the audio checks but all looks good so far.
Regards
Andy (Steve Mannix)
I wouldn't have thought the resistor was it, either. I think it was the soldering. When I rechecked it, the joint looked suspect, so I thought it was worth replace with another one (the lead to cut too close to the board to reflow it very well).
I'm not sure I still have it.
So I got my first O2 built now and all the initial DIY testing went fine and measured correctly + the amp works great. I'm a little surprised by the strong turn off "bump" sound though. The turn on sound is really quiet. I also tried pulling a battery with the headphones connected and the sound it madewas really loud but I guess that's normal as the DC on the output is quite a lot in that situation.
Regarding measuring the DC Offset thump on power off, is this acceptable way to measure it?
1. Plug in headphones, plug in power, no source.
2. Turn on power of O2.
3. Place ground (black) of DMM at ground [square hole] of P2. Place Voltage meter (red) of DMM at the round hole furthest away from the ground hole. (Left channel I believe.)
4. Turn DMM to measure DC mV and then turn on MAX function. I have a Fluke 115.
5. Turn off O2 and note MAX value.
6. Repeat for other round hole second furthest away from the ground hole. (Right channel I believe.)
My headphones have an impedance of 120 Ohm and I measured:
Off thump Left 17.7 mV
Off thump Right 15.3 mV
1. Plug in headphones, plug in power, no source.
2. Turn on power of O2.
3. Place ground (black) of DMM at ground [square hole] of P2. Place Voltage meter (red) of DMM at the round hole furthest away from the ground hole. (Left channel I believe.)
4. Turn DMM to measure DC mV and then turn on MAX function. I have a Fluke 115.
5. Turn off O2 and note MAX value.
6. Repeat for other round hole second furthest away from the ground hole. (Right channel I believe.)
My headphones have an impedance of 120 Ohm and I measured:
Off thump Left 17.7 mV
Off thump Right 15.3 mV
I've never seen an O2 PCB so I'll have to take your word for the test points you are using. You need to be measuring between ground and the junction of the 1 ohm resistors on the opamp outputs. You don't need headphones plugged in for this.
Any steady state DC present is down to the natural offset of the opamps and it will vary. The design makes no attempt to minimise this offset. To reduce it to zero would need the input bias currents of the four output opamps equalising and that would be done by breaking the direct connection between output and inverting input for each, and fitting a resistor there instead such that the theoretically identical steady state currents flowing out of the two inputs for each opamp were each working into equal impedances.
That would reduce the ever present DC offset but that isn't the same as switch on/off thump.
The thump is measured the same way but unfortunately a meter isn't fast enough to record the values. That needs an oscilloscope. You can prove that to yourself by measuring the voltage across a battery. If the battery reads 1.5 or 9 volts or whatever steady state, now try "flicking" or touching one lead on the battery very quickly. The voltage applied to the meter is still the same but it won't register it correctly. The meter needs a certain time to acquire and display the reading. The thump noises may vary depending on the position of the volume control. That is because of the added "thumps" from the earlier stages.
Nothing you can do to the O2 circuit will stop this thump. To make it silent would need a dedicated mute arrangement on the output.
The thump does not concern me, it is not loud at all, I was more curious about how to measure it.
I do not believe that unplugging or plugging headphones in to a powered on amp is a good idea. Can create distortion as the plug moves past the connection points in the jack. Also, the jacks will eventually wear out if the manufacturers spec sheet is to be believed.
The thump sounds the same without regard to volume position.
LIstening to music through the O2 now, no problems. It has been an educational first build.
Could someone tell me where to find the BOM for the O2? I have a couple of boards that I got from Qusp, I haven't seen or heard from him for a long time now. I am looking for the BOM and circuit layout and I have seen that there is a trouble shooting guide in one of the threads. Thanks in advance to whoever can point me in the correct direction. These boards are for my two kids to build and they are bugging me now to get this done! I wouldn't imagine there are any more group buys of all the components, any info is appreciated.
Steven
Steven
"Toggle" switch are better in this regards. I leave near coastal region & all "push" type switches needs to be cleaned/lubricated quarterly by something like WD40.
The switches are simple bifurcated slide switches. The contacts have a thin coating of what appears to be dielectric silicon grease.
I took my gain switch apart when I had a problem with the low gain position. The left channel dropped considerably below the right channel, and I could find no errors with the resistors. After taking it apart, I could find no problems, so I re-assembled it, tested it, and put it back on the board.
My O2 Headphone amp now workes fine. Go figure.
I took my gain switch apart when I had a problem with the low gain position. The left channel dropped considerably below the right channel, and I could find no errors with the resistors. After taking it apart, I could find no problems, so I re-assembled it, tested it, and put it back on the board.
My O2 Headphone amp now workes fine. Go figure.