This may be a bad way to start asking questions, too complicated a circuit, but it is the one I am working on, and want to learn as I go.
Do I understand this circuit, (and ab amplifiers in general) correctly? I am sure I have many things confused, but I am trying to put down my understanding, writing things out helps me solidify my learning, and would like to learn more and make sure I am not learning things incorrectly. So, in advance, sorry for so many questions, and thanks for the help!
Clearly I am a noob, but I have successfully repaired several amplifiers, have just ordered honey badger boards, and have this Rotel I am messing around with.
Rotel AB amplifier section from Rx-1052. 100 watt per channel spec. Several of the functions on the front panel are non functional, or semi functional. So my idea was strip everything out of the case that is not related to the power amp, and try to repurpose this into a straight 2 channel amplifier. I have already cut out everything except for the PS portions and the speaker output relays from the rest of the main board, and I have correct V at the rails going to the amp section. Amp is currently not functional. But as I piece it back together, I want to be able to work through each stage of the amp to really understand how it works, and how to test each stage.
Circuit in question is attached.
CN104 is signal input, and black pin is +18V.
CN602 came from the main CPU, one pin in to the bias pin on the CPU, and the other comes out from the posistor. Both pins are used to shut the amp down in case of fault.
CP203 is from the Power supply. Rails are +-52V
CP 204 is from the posistor, a thermistor, attached to the heatsink. (just out of sight. Goes out via black pin on CP602 to main CPU.
Looking at the Left Channel of the Power Amp:
Rails:
+-52V. R679 and R681 drop the voltage down from +-52 to an appropriate voltage for the input stage and VAS. Full +-52v is used by the Current amplification stage.
Input Stage:
Q601 and Q603 are the input stage, and they act as a differential amplifier. They are oriented as Common Collectors?, and provide voltage and current gain? Also commonly called a Long Tail Pair. R653, Q607, R617 make up the Constant Current Source (CCS) for the input pair. This ensures there is a current flowing through the differential pair, as well as define the voltage? to amplify the input at Q601.
The base of Q603 is receiving the negative feedback from the output, so there is a difference for the differential pair to amplify. Because the differential pair will only amplify the difference between the two signals. If the signal coming in to the base of Q601 and Q603 was the same, then output would be flat. Correct?
Signal goes from the collectors of Q601 and Q603 to buffers? Q611 and Q613, and then to the Voltage amplification stage.
Questions: What is a good procedure to test the input stage?
What purpose do Q609 and the c615 r651 c617 network serve? They seem to act as a return to the main CPU, to the bias pin, (main CPU is gonzo.) So, if there is a negative or positive voltage outside a certain range, it would put the amp into protect (would need a new protect circuit). Does that sound right?
Input stage gain is set by resistors on the negative feedback resistor r611 and r619 correct?
And that completes the input section, correct?
Voltage Amplification Stage
Q611 and Q613 act as buffers with high impedance receiving the signal output from the differential pair. Q605 appears to amplify the voltage on the positive side. At Q605 voltage gain is set by r639 / r629 yes? So 4700ohms/560ohms = 8.4 times the voltage at Q605 base. That higher voltge signal goes to Q615.
Where does Q617 get increased negative voltage gain from? There is no opposite to Q605 providing a larger signal to Q617. Why is there no corresponding pair transistor for Q605? How does the negative side of the signal voltage get amplified?
Bias - Q615 and Q617 sandwich the bias circuit where bias is set by resistors prior to arriving at the base of Q623. R621 r637 and R649 set bias for the amplifier. Correct?
Is bias voltage feeding through C611 into the Negative Feedback (NFB) and into the negative input to bias the differential pair? This is wrong I think. Differential pair doesn't bias, just q615 and 617 are biased? And that bias carries through the rest of the amp?
What is the correct procedure to test the output of the VAS after verifying that the input stage is working properly?
How would I test the bias when the output stage is disconnected?
How about testing the overall function of the amp through the VAS, while output stage is still disconnected?
Remove R669 and c603 (between Q619 and 621) and replace with two 100R and grab signal from the middle of the two 100r?
Sorry for all the questions. Clearly a noob here.
Do I understand this circuit, (and ab amplifiers in general) correctly? I am sure I have many things confused, but I am trying to put down my understanding, writing things out helps me solidify my learning, and would like to learn more and make sure I am not learning things incorrectly. So, in advance, sorry for so many questions, and thanks for the help!
Clearly I am a noob, but I have successfully repaired several amplifiers, have just ordered honey badger boards, and have this Rotel I am messing around with.
Rotel AB amplifier section from Rx-1052. 100 watt per channel spec. Several of the functions on the front panel are non functional, or semi functional. So my idea was strip everything out of the case that is not related to the power amp, and try to repurpose this into a straight 2 channel amplifier. I have already cut out everything except for the PS portions and the speaker output relays from the rest of the main board, and I have correct V at the rails going to the amp section. Amp is currently not functional. But as I piece it back together, I want to be able to work through each stage of the amp to really understand how it works, and how to test each stage.
Circuit in question is attached.
CN104 is signal input, and black pin is +18V.
CN602 came from the main CPU, one pin in to the bias pin on the CPU, and the other comes out from the posistor. Both pins are used to shut the amp down in case of fault.
CP203 is from the Power supply. Rails are +-52V
CP 204 is from the posistor, a thermistor, attached to the heatsink. (just out of sight. Goes out via black pin on CP602 to main CPU.
Looking at the Left Channel of the Power Amp:
Rails:
+-52V. R679 and R681 drop the voltage down from +-52 to an appropriate voltage for the input stage and VAS. Full +-52v is used by the Current amplification stage.
Input Stage:
Q601 and Q603 are the input stage, and they act as a differential amplifier. They are oriented as Common Collectors?, and provide voltage and current gain? Also commonly called a Long Tail Pair. R653, Q607, R617 make up the Constant Current Source (CCS) for the input pair. This ensures there is a current flowing through the differential pair, as well as define the voltage? to amplify the input at Q601.
The base of Q603 is receiving the negative feedback from the output, so there is a difference for the differential pair to amplify. Because the differential pair will only amplify the difference between the two signals. If the signal coming in to the base of Q601 and Q603 was the same, then output would be flat. Correct?
Signal goes from the collectors of Q601 and Q603 to buffers? Q611 and Q613, and then to the Voltage amplification stage.
Questions: What is a good procedure to test the input stage?
What purpose do Q609 and the c615 r651 c617 network serve? They seem to act as a return to the main CPU, to the bias pin, (main CPU is gonzo.) So, if there is a negative or positive voltage outside a certain range, it would put the amp into protect (would need a new protect circuit). Does that sound right?
Input stage gain is set by resistors on the negative feedback resistor r611 and r619 correct?
And that completes the input section, correct?
Voltage Amplification Stage
Q611 and Q613 act as buffers with high impedance receiving the signal output from the differential pair. Q605 appears to amplify the voltage on the positive side. At Q605 voltage gain is set by r639 / r629 yes? So 4700ohms/560ohms = 8.4 times the voltage at Q605 base. That higher voltge signal goes to Q615.
Where does Q617 get increased negative voltage gain from? There is no opposite to Q605 providing a larger signal to Q617. Why is there no corresponding pair transistor for Q605? How does the negative side of the signal voltage get amplified?
Bias - Q615 and Q617 sandwich the bias circuit where bias is set by resistors prior to arriving at the base of Q623. R621 r637 and R649 set bias for the amplifier. Correct?
Is bias voltage feeding through C611 into the Negative Feedback (NFB) and into the negative input to bias the differential pair? This is wrong I think. Differential pair doesn't bias, just q615 and 617 are biased? And that bias carries through the rest of the amp?
What is the correct procedure to test the output of the VAS after verifying that the input stage is working properly?
How would I test the bias when the output stage is disconnected?
How about testing the overall function of the amp through the VAS, while output stage is still disconnected?
Remove R669 and c603 (between Q619 and 621) and replace with two 100R and grab signal from the middle of the two 100r?
Sorry for all the questions. Clearly a noob here.
Attachments
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Ha, well I suppose when it left the factory it did, but it didn't when I got it, one side did, and the other side did not, and messing with it, I managed to zap the other side with a resistor that was the wrong impedance (put two in series to get close and solder joint came loose, smoked a few resistors and the driver), and now neither side works. I have successfully repaired like six different ab class amplifiers, until I zapped this one. But I want to fix it. I can't find the specified driver transistor, and I would like to be able to trouble shoot the circuit with the output stage removed with a different driver pair I have on hand in place. But really, I am hoping to see if someone can help me confirm my understanding of the circuit. The more I wrote the wall of gibberish out, it seemed to become a little more clear, but new questions arose. I seem to know enough to get myself in trouble, and I would really like to know the proper way to analyze this type of circuit and test the stages properly.
I don't even understand the question. "Increased negative voltage gain"?
But I think the answer is: not everything is push-pull or works AB. Think of a simple common-emitter amplifier with a current 'source' load.
Ha, I won't use that phrase again.
Q605 is coming emitter, so it should be a voltage amplifier to the tune of 8.4x the input voltage. correct?
q613 is a common emitter amplifiers, but gain is 1, or unity is that correct? would the voltage of q605 and q613 outputs not need to be similarly amplified? Or am I missing something with respect to q613
Q611, is common collectors, does it serve simply as a buffer to drive the vas at q605 and 615? Why is there not a similar common collector transistor for q613?
Q615 and q617 are common collectors. These are biased by the bias circuit, and are the pre drivers for q619 and q621?
Thanks for looking. Appreciate the help.
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Signals at Q601 and Q603 collectors are opposite phase. Q611 inverts the phase from Q603 to bring the signals back in phase. Q605 and Q613 are emitter follower drivers for VAS transistors Q615 and Q617. Q619 and Q621 are emitter follower drivers for output transistors Q625/627 and Q629/631. Q623 is Vbe multiplier to set output stage bias.
Tikiroo, thank you!
I had been struggling with the purpose of q611, I knew it inverted the input, but I couldn't understand why, I had been missing that the output of q603 and q601 were out of phase. Being a common emitter, q611 inverts the signal, and should be providing no current gain. Voltage gain is set by r631/r633, 560/560 = 1, so unity voltage gain. This ensures that 605 is receiving the same voltage and current signal as 613. Sound right?
Another question around q605, if this is providing current gain, and q613 is performing a similar function, as both are emitter followers, why does q605 collector go to ground and q613 collector goes to positive rail? In both cases, the current for for the emitter is coming from the 560 ohm resistors is it not? So the current and voltage output at the emitters should be the same. With some gain in current. Voltage should be unity gain.
And that higher current drives q615 and q617 to perform the voltage amplification of the signal towards the +- rail voltages.
Am I wrapping my head around this correctly?
I had been struggling with the purpose of q611, I knew it inverted the input, but I couldn't understand why, I had been missing that the output of q603 and q601 were out of phase. Being a common emitter, q611 inverts the signal, and should be providing no current gain. Voltage gain is set by r631/r633, 560/560 = 1, so unity voltage gain. This ensures that 605 is receiving the same voltage and current signal as 613. Sound right?
Another question around q605, if this is providing current gain, and q613 is performing a similar function, as both are emitter followers, why does q605 collector go to ground and q613 collector goes to positive rail? In both cases, the current for for the emitter is coming from the 560 ohm resistors is it not? So the current and voltage output at the emitters should be the same. With some gain in current. Voltage should be unity gain.
And that higher current drives q615 and q617 to perform the voltage amplification of the signal towards the +- rail voltages.
Am I wrapping my head around this correctly?
At risk of confusing things, I was looking at a schematic of an amp I did successfully repair, to compare to the rotel to test my understanding.
This is a diamond d5 600.2 car audio amp. q4 and 5 are the long tail pair, and q19 is unity gain for I and V, and passes an inverted signal to q9 that matches the signal passed from the output of q4 to q20.
schematic attached.
This is a diamond d5 600.2 car audio amp. q4 and 5 are the long tail pair, and q19 is unity gain for I and V, and passes an inverted signal to q9 that matches the signal passed from the output of q4 to q20.
schematic attached.
