Doug Self Blamless Class B Amp

[Lawrence.M]

I am trying to simulate the Doug Self Class B Blameless Power Amp Design.However with an input signal of 1V at 1KZ I am getting the same at the output. Can anyone tell me what i might be doing wrong.Thanks.

 

[Mooly]

R17? and C12

 

[sgrossklass]

Yeah, the feedback network values are b0rked.

Your gain is 1 + (R13 || R18 + Z(C12))/(R17 + Z(C2)). The time constant of 330R and 1µ is well within the audible range.

Try C12 = 100p...1n, R17 = 470R, C2 = 220µ.

 

[Mooly]

As I recall C12 was marked as 100pF in the documentation supplied with the PCB's and certainly my examples I built (on the official boards) were unstable and oscillated continually with that value fitted. I think 15pF or 18pF was the value used in the original magazine articles.

 

[Lawrence.M]

Thanks very much Mooly and sgrossklass.
I changed the values of the components as suggested by Mooly and I now have around 22v pK.

 

[Kay Pirinha]

I think Q10 has been drawn upside down.
Best regards!

 

[Mooly]

Well spotted I wish real components were as forgiving as models. Just tried reversing it in LT and the amp 'works' although with no bias current.

 

[Lawrence.M]

Thanks Kay Pirinha.Don't know how it missed that one.
Yes Mooly.It didn't make any difference to me when simulating but presumably in reality things would not look so good!

 

[Mooly]

The magic smoke would be set free with a real amp.

 

[IanHegglun]

There was an LTspice simulation of Doug Self's 50W amp recently in post 5 here Simulation: Cyril Bateman's Amplifier-Speaker cable interactions

In that thread PB2 (Pete Basel) was trying to replicate Cyril Bateman's bench test with a speaker cable using Doug Self's amplifier that self destructed due to a strange RF oscillation.from the speaker cable and amp interaction.

PB2's simulation did not oscillate with a simple (lumped) cable model. So it needs more work to replicate the cause of oscillation so the problem can be understood. Cyril did not find the reason before passing (RIP).

 

[sandrohv]

@Kay: Dude you have some serious eyes. No way I would have spotted that one.

 

[Kay Pirinha]

Ok, that really was some strain, due to the not too clear drawing . And I had to veriify that the MJ4502 indeed is a PNP device.

Best regards!

 

[erikovsky]

Thanks Kay Pirinha.Don't know how it missed that one.
Yes Mooly.It didn't make any difference to me when simulating but presumably in reality things would not look so good!

Hi Lawrence,

Do you mind sharing the Multisim file? I am interested too, maybe we can share things with each other.

 

[Lawrence.M]

Hi erikovsky,
No problem.Here is the Multism file.I actually built an amp from the Doug Self Blamless Class B amp design over 10 years ago which I am still listening to.