My Perpetual Technologies dac P3A decided to take a turn for the worst.One channel is distorting.I put ascope on the + input to the I/V stage and I got the following image.Now I am not a digital expert and I was wondering what to do next in terms of diagnosing the problem.Do you think it is possible that the cs4397 or one of the other main chips is responsible and can this be replaced-how can you desolder smd?
DAC fault finding
Hi Protos,
Your photo of the positive input to the I/V converter is pretty meaningless and will not help with fault finding.
This is because this circuit is a) a differential to single-ended filter, NOT a current to voltage conversion stage, and b) differential so measuring half of the signal is useless.
I suggest you (if possible) measure both simultaneously and subtract the negative signal from the positive... This will then show you the actual signal which would then be filtered by the circuit. Compare this to the output. if it is the same the DAC is at fault.
Check that all signals are present and accounted for. Check that they make sense. i.e. that fBCK is 64xfLRCK. Find, or source a single tone to the DAC (test cd etc) and listen to the distortion. Is it ticking (indicates a clocking error) or is it a distortion that looks vaguely sinusoidal (indicates a problem with the analog output stage).
Is anything getting hot? As this indicates several things (HF oscillation, clocking, an internal fault etc).
Good luck with it!!
apollyon25
Hi Protos,
Your photo of the positive input to the I/V converter is pretty meaningless and will not help with fault finding.
This is because this circuit is a) a differential to single-ended filter, NOT a current to voltage conversion stage, and b) differential so measuring half of the signal is useless.
I suggest you (if possible) measure both simultaneously and subtract the negative signal from the positive... This will then show you the actual signal which would then be filtered by the circuit. Compare this to the output. if it is the same the DAC is at fault.
Check that all signals are present and accounted for. Check that they make sense. i.e. that fBCK is 64xfLRCK. Find, or source a single tone to the DAC (test cd etc) and listen to the distortion. Is it ticking (indicates a clocking error) or is it a distortion that looks vaguely sinusoidal (indicates a problem with the analog output stage).
Is anything getting hot? As this indicates several things (HF oscillation, clocking, an internal fault etc).
Good luck with it!!
apollyon25
Hi Protos,
I modified one of these Perpetual Technologies dac P3A couple of years ago. They run HOT, so you may have some cold solder points on the PCB.
Use a test CD (Technics for example) and start tracing the problem from the "back" (from the RCA connectors), OR from the RF digital input. It would probably be easier for you to start from analogue out RCAs.
It’s a bit crowded inside with a lot of SM components, so if you are not sure what to do, take it to service!
Nick
I modified one of these Perpetual Technologies dac P3A couple of years ago. They run HOT, so you may have some cold solder points on the PCB.
Use a test CD (Technics for example) and start tracing the problem from the "back" (from the RCA connectors), OR from the RF digital input. It would probably be easier for you to start from analogue out RCAs.
It’s a bit crowded inside with a lot of SM components, so if you are not sure what to do, take it to service!
Nick
Thanks for the replies.
I further checked and measured and this is what I have:
1.cs4397 outs measure identical and clean on 1khz sine
2.There is no apparent distortion on the scope at -20db with the opamps
3.If I take out the output opamps (unfortunately I had to destroy these opa 627 to take them out but I have some more) and measure the output of the cs4397 AFTER the first series resistor 1K I find that the +R output is double that of any of the other outputs.This is the first point where you can measure the distorted sine wave when the opamps are in.
4.Might this lead to clipping in the summing amp?Would such a result on the scope indicate partial clipping?But why is this distortion/clipping obvious on the + input of the first opamp and not on its output only?
5.I resoldered most of the network but the problem persists.The problem appeared suddenly during normal play so there was no physical stress on the PCB/components and none that I can see now.
I further checked and measured and this is what I have:
1.cs4397 outs measure identical and clean on 1khz sine
2.There is no apparent distortion on the scope at -20db with the opamps
3.If I take out the output opamps (unfortunately I had to destroy these opa 627 to take them out but I have some more) and measure the output of the cs4397 AFTER the first series resistor 1K I find that the +R output is double that of any of the other outputs.This is the first point where you can measure the distorted sine wave when the opamps are in.
4.Might this lead to clipping in the summing amp?Would such a result on the scope indicate partial clipping?But why is this distortion/clipping obvious on the + input of the first opamp and not on its output only?
5.I resoldered most of the network but the problem persists.The problem appeared suddenly during normal play so there was no physical stress on the PCB/components and none that I can see now.
If the output is anything like this http://www.diyaudio.com/forums/attachment.php?s=&postid=286357&stamp=1071911103
where is the distorted voltage?
where is the distorted voltage?
The first point at which it can be seen is right after the 1k resistor at the output of the cs 4397.As I said before the output of the cs4397 before the resistor is clean and absolutely symmetrical on all four diff outs..If I remove the opamps the sine wave after the 1k resistor has no distortion but it is double in size compared to the other diff signal or even to the symmetrical signal in the other correctly functioning channel.
I am completely stumped because I measured and compared readings with the multimeter from one channel to the other between most of the nodes connections etc and they match so I cant see a bad connection or a failed resistor or cap (which would be surprising in an opamp stage with low VA).
I have substituted the opamp but the problem is the same .It has something to do with why the +R output just after the dac is swinging double what it should be.However I still don't understand why the distortion is appearing all the way back up to just before the output of the DAC since I dont see feedback there according to your circuit.
Since the the swing is double the others I would assume (resistive divider theory) that the impedance to ground following the 1k resistor would be off spec and lower than it should.However that is not what my multimeter is telling me because it is reading similar impedances to ground.
Slightly off topic but why does the above schematic sum up the two diff outputs in one opamp then take its output plus one of the two diff outs and sum them again in the second opamp?
I am completely stumped because I measured and compared readings with the multimeter from one channel to the other between most of the nodes connections etc and they match so I cant see a bad connection or a failed resistor or cap (which would be surprising in an opamp stage with low VA).
I have substituted the opamp but the problem is the same .It has something to do with why the +R output just after the dac is swinging double what it should be.However I still don't understand why the distortion is appearing all the way back up to just before the output of the DAC since I dont see feedback there according to your circuit.
Since the the swing is double the others I would assume (resistive divider theory) that the impedance to ground following the 1k resistor would be off spec and lower than it should.However that is not what my multimeter is telling me because it is reading similar impedances to ground.
Slightly off topic but why does the above schematic sum up the two diff outputs in one opamp then take its output plus one of the two diff outs and sum them again in the second opamp?
protos said:
No, it's only drawed a little bit complicated. The Signal runns thru the 10 Ohm Resistor to pin 2 and to the output.
It's also an active feedback for the negative input of the first op.
Don't know why they put the output from the (2nd) op to half negative supply via 5k (10k || 9k9).
Regards
Jobstens
Errr,protos, its not my circuit. It from a thread started by someone called protos.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=9350&highlight=P3A
When you measure the + and - outputs are they completely isolated bar the 1k resistors? It could well be a duff chip.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=9350&highlight=P3A
When you measure the + and - outputs are they completely isolated bar the 1k resistors? It could well be a duff chip.
Thanks guys.
In the end the original suspect was guilty.
When I replaced the last output chip everything was ok.Because of the strange architecture when I took out of circuit the last(2nd) chip and put in a new chip in the 1st stage I could see distortion so I naturally thought that the problem was not in the second stage but before that.However the fact that the 2nd chip was either dead or out of circuit probably affected the feedback to the first chip and produced this distortion.I still can't really understand the 2nd stage in this circuit and I haven;t seen it before . Can anyone explain?
In the end the original suspect was guilty.
When I replaced the last output chip everything was ok.Because of the strange architecture when I took out of circuit the last(2nd) chip and put in a new chip in the 1st stage I could see distortion so I naturally thought that the problem was not in the second stage but before that.However the fact that the 2nd chip was either dead or out of circuit probably affected the feedback to the first chip and produced this distortion.I still can't really understand the 2nd stage in this circuit and I haven;t seen it before . Can anyone explain?
Figuring out how circuits work.
Hi Protos.
Yes the fault is usually found to be what the original guess was.
Fault finding is a nasty business especially at the junction of digital and analog.
National Semiconductor's staff scientist (Bob Pease) has a VERY good book on analog stuff and I'd thoroughly recommend it! A very bright (if not slightly eccentric - sorry Bob!) guy!
I find that if I cannot figure out what a circuit does I break it up into blocks, and simplify the hell out of them.
Consider what happens at dc, then what happens at HF. What are the voltages doing? What do you think the currents are doing? Consider the loading of following and preceding sections...
For particularly complicated circuits I redraw them on paper (you will find that most people cannot draw circuit diagrams that make sense!). If this fails download a simulation program (even eval versions) and simulate it.
Just remember that just because the simulation says it does something doesnt mean that that is what happens in reality!
Remember that even if you spend an hour or 10 on it you will at the end; understand the circuit better; know why the topology was used; know its problems (or at least suspect them) and add to your knowledge/experience base!
Resist the temptation to ask for the answer! Find it yourself!
apollyon25
Hi Protos.
Yes the fault is usually found to be what the original guess was.
Fault finding is a nasty business especially at the junction of digital and analog.
National Semiconductor's staff scientist (Bob Pease) has a VERY good book on analog stuff and I'd thoroughly recommend it! A very bright (if not slightly eccentric - sorry Bob!) guy!
I find that if I cannot figure out what a circuit does I break it up into blocks, and simplify the hell out of them.
Consider what happens at dc, then what happens at HF. What are the voltages doing? What do you think the currents are doing? Consider the loading of following and preceding sections...
For particularly complicated circuits I redraw them on paper (you will find that most people cannot draw circuit diagrams that make sense!). If this fails download a simulation program (even eval versions) and simulate it.
Just remember that just because the simulation says it does something doesnt mean that that is what happens in reality!
Remember that even if you spend an hour or 10 on it you will at the end; understand the circuit better; know why the topology was used; know its problems (or at least suspect them) and add to your knowledge/experience base!
Resist the temptation to ask for the answer! Find it yourself!
apollyon25
Hi Protos,
just stumbled on this thread.
Good to hear it´s working again!
I simulated the schematic I drew and it seems to work.......
All it does to the input signal is to make it single ended and form some kind of low pass filter with a quite high -3dB point.
Meanwhile I´m not using it anymore. Instead I sent the output of the DAC through a passive filter into a X-BOSOZ for balanced operation. Does make a nicer sound than the OPA627
William
just stumbled on this thread.
Good to hear it´s working again!
I simulated the schematic I drew and it seems to work.......
All it does to the input signal is to make it single ended and form some kind of low pass filter with a quite high -3dB point.
Meanwhile I´m not using it anymore. Instead I sent the output of the DAC through a passive filter into a X-BOSOZ for balanced operation. Does make a nicer sound than the OPA627
William
Unfortunately or fortunately as the case may be during my troubles with the PT dac I decided to order the dddac as a standby and because it was such a good deal.I did not really expect it to be as good as it turned out .In fact it beats my PT combo easily which is astounding when you think of the price difference and the fact that the the PT is a bit of a giant killer itself.
The differences IMHO always were quite obvious and not really a matter of taste or a 'better compromise'.The most obvious were the increased dynamics and better imaging and soundstaging plus more 'live' sound and a feel that you were allowed to see more of the info on the recording.There was no hard digital sound at all and the extension both ways seemed better and more natural.Precise adjustment of the voltages is absolutely necessary though for best sound.
I would not worry too much about balanced output but I can see why you want it.I can't get my head around it right now but Doede is very helpful and could give you the lowdown on it.
I would not worry too much about balanced output but I can see why you want it.I can't get my head around it right now but Doede is very helpful and could give you the lowdown on it.
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