I have a thread going over in the car audio section and it was suggested to seek some advice from some of the guru's over here.
Here is a link to get up to speed if any of you are inclined to help out.
http://www.diyaudio.com/forums/car-audio/188798-logic-soundlab-gx802.html
I have drawn out the basic circuit using the program LTSpice, although I don't know how to use LTSpice even 10%. The .asc file is ready for inspection. Maybe someone over here even recognizes the circuit structure just by the component selection and the pictures and can point me in the right direction?
Here is a link to get up to speed if any of you are inclined to help out.
http://www.diyaudio.com/forums/car-audio/188798-logic-soundlab-gx802.html
I have drawn out the basic circuit using the program LTSpice, although I don't know how to use LTSpice even 10%. The .asc file is ready for inspection. Maybe someone over here even recognizes the circuit structure just by the component selection and the pictures and can point me in the right direction?
You can use winzip to zip the file, then upload. Or save it as a jpg and attach.
Reffering to post 23, it looks like the feedback loop is trying to drive the PNP output to correct the clipped waveform. It looks like the PNP output is open between base and emitter. But, the second photo in post 1 shows that it is working with large signals. It's almost like the output stage is underbiased and the feedback loop is trying to compensate. There appears to be a crossover notch in the waveform, obviously not at the center because you are using a speaker load which has a reactance that shifts the phase of the current WRT voltage. Try a resistive dummy load instead. What is the DC bias current flowing through the output transistors with no signal applied? Measure voltage across the emitter resistors and divide by the Ohms value. You should measure about 1.3VDC between the two output transistor bases.
If there is a large crossover gap then the output with highest gain will dominate within the loop for small signal levels. Hfe, not always, but is typically higher for NPNs than PNPs. I've seen this behavior before related to heavy class B (off) output stage bias w/global FB. Hope this helps to point the right direction, it is hard to troubleshoot a circuit by only looking at a few low resolution photos.
Reffering to post 23, it looks like the feedback loop is trying to drive the PNP output to correct the clipped waveform. It looks like the PNP output is open between base and emitter. But, the second photo in post 1 shows that it is working with large signals. It's almost like the output stage is underbiased and the feedback loop is trying to compensate. There appears to be a crossover notch in the waveform, obviously not at the center because you are using a speaker load which has a reactance that shifts the phase of the current WRT voltage. Try a resistive dummy load instead. What is the DC bias current flowing through the output transistors with no signal applied? Measure voltage across the emitter resistors and divide by the Ohms value. You should measure about 1.3VDC between the two output transistor bases.
If there is a large crossover gap then the output with highest gain will dominate within the loop for small signal levels. Hfe, not always, but is typically higher for NPNs than PNPs. I've seen this behavior before related to heavy class B (off) output stage bias w/global FB. Hope this helps to point the right direction, it is hard to troubleshoot a circuit by only looking at a few low resolution photos.
Presently as the amp sits right now, I have a 10K Ohm trimmer installed at R104 and one at R204. One channel has had all of the transistors replaced with the ones Perry suggested in my original thread.
I have been thinking all along something is underdriving the PNP output because the clipped wave form always occurs with low input signal and would "grow" at a slower rate than the NPN, but the negative portion of the signal isn't correct at the driver stage either.
Now, when I adjust the 10K trimmer, I can adjust it so that the signal is nice and clean (no crossover notch) and both positive and negative side are equal and amp sounds good. The trade off here is that it puts about .750VDC accross the speaker terminals and causes the amp to draw too much idle current as well as the turn on/off thumps are terrible.
With the 10K trimmers set back to 10K and an 8 Ohm resistor accross the ouput terminals, I measure 21.5mV accross each emitter resistors which are .15 Ohms = 143mA. I placed my meter in series with the 8 Ohm resistor it shows .65mA. I also measure .795VDC accross the bases of the outputs. The crossover notch is still present, but they sit straight accross from each other (no phase shift).
I have been thinking all along something is underdriving the PNP output because the clipped wave form always occurs with low input signal and would "grow" at a slower rate than the NPN, but the negative portion of the signal isn't correct at the driver stage either.
Now, when I adjust the 10K trimmer, I can adjust it so that the signal is nice and clean (no crossover notch) and both positive and negative side are equal and amp sounds good. The trade off here is that it puts about .750VDC accross the speaker terminals and causes the amp to draw too much idle current as well as the turn on/off thumps are terrible.
With the 10K trimmers set back to 10K and an 8 Ohm resistor accross the ouput terminals, I measure 21.5mV accross each emitter resistors which are .15 Ohms = 143mA. I placed my meter in series with the 8 Ohm resistor it shows .65mA. I also measure .795VDC accross the bases of the outputs. The crossover notch is still present, but they sit straight accross from each other (no phase shift).
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