Re: Crest CPX1500
Good to see you made some progress with the missing connection to the rectifier. That would explain why Channel B was behaving so badly compared to A!
I am trying to understand the opamp\feedback circuits here, (If someone else could lend a hand, that would be great!!) I may well be getting things wrong at this point!
As far as I understand it, though:
U100-A is set as a buffer for the input signal to the rest of the amp. That's easy enough.
U100-B seems to be set as an inverting amplifier (?) for the input signal, and also taking feedback from the Speaker+ output line (before L100). It seems to drive Q104 and Q105, which in turn drive the output drivers and output stage.
U102-B is used as a comparator, seems like it has some job to do with the GCL function. If its output goes high, then it should make U102-A also produce a high output and turn the DDT LED on, to indicate that the GCL limiter is active.
One half of U101 (Pins 1, 2 and 3) takes input from the buffered output of U100-A and feeds it back to U100-A's input. I am not sure what the purpose of this is, perhaps to lower distortion?
The other half of U101 makes even less sense to me:
The output from U102-B goes through to Pin 5 of U101. This seems to be comparing the output from U102-B's comparison, with the original input signal at Pin 6, out of R102.
If U101's Pin 6 (inverting input), ie: the audio input signal were ever to become higher in voltage than the input to Pin 5, then U101's output on Pin 7 would go LOW, and short the 15V+IA rail to 15V-IA. I am really not sure what purpose this serves, but I am probably misunderstanding the circuit significantly??
I don't know why the amp seems to oscillate (perhaps) and go into short condition now, when you have it powered without U101 and U201, or worse, with them connected.
I am confused though, earlier you said you had signal at the speaker from Channel A, with U101 removed. What has changed between then and now going into a restart loop, except for fixing the -Ve rectifier connection to channel B?
What if you run with Channel B disconnected again? Does Channel A work again without tripping the lightbulb?
Good to see you made some progress with the missing connection to the rectifier. That would explain why Channel B was behaving so badly compared to A!
I am trying to understand the opamp\feedback circuits here, (If someone else could lend a hand, that would be great!!) I may well be getting things wrong at this point!
As far as I understand it, though:
U100-A is set as a buffer for the input signal to the rest of the amp. That's easy enough.
U100-B seems to be set as an inverting amplifier (?) for the input signal, and also taking feedback from the Speaker+ output line (before L100). It seems to drive Q104 and Q105, which in turn drive the output drivers and output stage.
U102-B is used as a comparator, seems like it has some job to do with the GCL function. If its output goes high, then it should make U102-A also produce a high output and turn the DDT LED on, to indicate that the GCL limiter is active.
One half of U101 (Pins 1, 2 and 3) takes input from the buffered output of U100-A and feeds it back to U100-A's input. I am not sure what the purpose of this is, perhaps to lower distortion?
The other half of U101 makes even less sense to me:
The output from U102-B goes through to Pin 5 of U101. This seems to be comparing the output from U102-B's comparison, with the original input signal at Pin 6, out of R102.
If U101's Pin 6 (inverting input), ie: the audio input signal were ever to become higher in voltage than the input to Pin 5, then U101's output on Pin 7 would go LOW, and short the 15V+IA rail to 15V-IA. I am really not sure what purpose this serves, but I am probably misunderstanding the circuit significantly??
I don't know why the amp seems to oscillate (perhaps) and go into short condition now, when you have it powered without U101 and U201, or worse, with them connected.
I am confused though, earlier you said you had signal at the speaker from Channel A, with U101 removed. What has changed between then and now going into a restart loop, except for fixing the -Ve rectifier connection to channel B?
What if you run with Channel B disconnected again? Does Channel A work again without tripping the lightbulb?
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