Need help simulating NAD 2200 amplifier in LTSpice

[pbilous]

Hello all!

I have recently purchased NAD 2200 amplifier but was wondering if I could improve on the schematics. It is probably as good as it is already, but I still wanted to try.

So I have put together simulation in LTSpice from the service manual.

Unfortunately it doesn't word.

Will appreciate if someone could help and point at why it might not work!

Attached are sim files and schematics from the service manual.

Thank you!

 

[WhiteDragon]

Woah Woah woah. Looks like crazy fun.
Before spending much time with it and glazing my eyes over.
Maybe diode string not connected?

 

[pbilous]

Unfortunately that is not the reason why it is not working. I tried with and without diode string.

 

[jan.didden]

What do you mean 'it is not working'?
What happens if you start the sim run?
Do you get error messages?

Jan

 

[pbilous]

When I feed it 1Vpp sine wave, I get half of it at the output load resistor of 4R

 

[jan.didden]

In LTspice? Just follow the signal and find out where it goes wrong.

Jan

 

[Indiglo]

There appears to be an incorrect wire link, D23 is going to the junction of R16 and Q311 instead going to the junction of R48 and R15.

 

[jan.didden]

Nice catch!

Jan

 

[pbilous]

That fixed it!
Also was missing gnd connection at feedback!

Now, why would someone stuck up two differential pairs? It is not easy to stabilize those!

 

[pbilous]

Ok, I think I was able to get max phase margin vs open loop gain out of it.
Phase margin was only 16 deg.
Increased to 27 deg.
While extending OLG from 30dB to 50dB at 20kHz.

 

[pbilous]

As Jan pointed out, C5 and R11 got deleted.
Other resistors R20, R21, R23, R26 are for offset adjustment - I put two separate resistors as I dont know how to do variable resistors in LTSpice

I have also decided to use MUR820 diodes instead of S6K20