thank you Helmut for your patience and answers. Now, it's more clear in my mind.
Besides, some answers didn't satisfy me : R4 and especially D4 and R12. How does my amp can work since it is in this configuration ?
If I figure it out well, electronics can only be understood by practicing but certainly not by reading books like I do. Silly of me.
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--- In LTspice@..., "Helmut" <helmutsennewald@...> wrote:
Hello Jean,
My answers are below.
--- In LTspice@..., "jean_claudeabeille" <jean_claudeabeille@> wrote:
OK. I agree, on your schematic, the amp works fine. Problem is :
why did you changed, added components and changed some wires :
- output on R4 in place of C5
R4 has to be ideally connected to the output as I did. Then the
offset of the amplifier output will be only the offset of U1B.
If we connect R4to the net "C5", the offset of U1B will be
multiplied by the gain 25 of the amplifier.
- D5 instead of Q5
I don't have the model parameters for a Vbe breakdown. You also
don't know what breakdown voltage you get. 7v, 8V, 9V?
- D4 added
1.5V of one LED is too less.
- Q11 PNP instead of NPN
You had a NPN symbol but named it a PNP. That's a "foul play". :-)
The simulation will simply not work as intended.
I replaced it with a PNP to leave the PNP-type. Alternatively
one could had rename the NPN with a NPN-type.
- U1A output connected to I-
- deleted R12.
I have shown the only correct usage of unused opamps. To make
this even more save, connect a 10k resistor between GND and
the +input of U1A. (I had forgot this resistor.)
This is not the real schematic, the one I uploaded.
What does mean : "Watch this voltage. Select R5!" ?
You should look that the opamp output U1B is not at the rails.
If this is the case and your circuit is correctly wired, you
should try a slightly larger or lower value of R5 to bring the
opamp output into its linear region.
I understand nothing about this :" This control loop has to be slower then the lower corner frequency of your amplifier". Can you develop this ?
At very low frequency of a few Hz, this loop will cancel your
AC audio signal. It will behave like having a highpass with
a few Hz in the input.
How do you worked to get this solution ? What is the trick ?
I have 35 years experience with analog circuit design.
At last, what is the job of D3/D107,
It limits the input differential voltage during power-up
and down or if somebody connects an AC-coupled pre-amplifier
or the input voltage is faster than the amplifier can follow..
D102/D103.
They avoid that you expensive amplifier will be destroyed due
to reverse bias during power-up and down.
I apologize wasting your time with so basic questions,
but I try to understand.
I read the first part of B. Cordell's book and learn a lot
but not enough to understand this damn amp. I asked some
help to B. Cordell but he answered to read chapters 1-4.
Am I stupid or something?
You had made a mix of a few schematic drawing mistakes and a
very few design errors. The combination makes it hard to get
the first working simulation. Now it's working.
PS:
I like the idea with Q14. It limits the maximum current of Q8.
Best regards,
Helmut
