Scope Design: JFET Input EXICON Output 10 Watt Class A

[prasi]

Hi again prasi, is it worth changing the silk screen on your pcb for the value of C3 back to 220pF (was 47pF) as lineup has shown in post #49?

I will wait on your reply before I order my pcb's, thanks.

Hi Gary,
there is no need to change gerbers and silk again.
one can use 180p/200p/250p /220p from one's part bin.

 

[prasi]

here is an ultra compact "AKSA" style (outputs on sides) diy only version just for fun. Can have doubled up power traces for fab houses.

 

[Vunce]

Excellent compact layout Prasi!
Glad I waited, this version allows more chassis options for me.

 

[lineup]

The target max output is 9.5-10 Watt Class A into 8 Ohm.
It depends a bit on the actually supply voltage from the trafo.
A guess is +/-16 to +/-17 VDC rectified from transformer.
The trafo is of course 2x12VAC. Can be 100VA-200VA.
If both channels are driven by one and same trafo I should say we should use 200VA.

 

[lineup]

This is the final schematic.
The only change form previous is C1=220pF.

 

[rsavas]

How about

1) some RFI suppression at the input? R=220, C= 220pF
2) Some bulk decoupling for VCC,VEE? Prasi shows them in his schematic and layout.
3) emitter followers for U3,U6,U8

 

[prasi]

Excellent compact layout Prasi!
Glad I waited, this version allows more chassis options for me.

Thanks Vunce,
here are gerbers. It might be slightly challenging stuff for some, but for you , its simple.
and also diy pdf attached for thimios.

 

[yoaudio]

Ordered!

 

[gary s]

Well done prasi, I am with Vunce on your compact version. Thanks again.

 

[CentralAsian]

How well it behaves if loaded by a capacitor (say 10n), straight to the amp output, with everything else disconnected (Zobels, load, inductors etc.) ? If it is not able to handle at least 10n capacitive load it will IMHO be prone to oscillation when fully built, even with no capacitive loads present.

 

[lineup]

Capacitive load.
That is what the wirewound inductor is for. In the Zobel.
Don't think this will be a problem.

 

[avtech]

@gary s, if you end up ordering 10 of those compact boards, perhaps I could trouble you for a pair?

 

[lineup]

I have tested with a speaker model RLC load.
I had to add C5=10pF across the feedback resistor. And change C1=100pF.
This was done simuating with the oscilloscope.
Resulting squarewave at 1kHz can be seen in 2nd image.

 

[CentralAsian]

Capacitive load.
That is what the wirewound inductor is for. In the Zobel.
Don't think this will be a problem.

This was precisely my point. Stability of amplifier with all that coils at the output and tested in simulator is just a half of the story. If amplifier is not able to handle 0.01 uF at the output without the coils (when run in simulator), then it is a good probability it won't be stable when actually is assembled, even though the coils and such all connected properly in real life.

 

[gary s]

@avtech - have sent you a PM.

 

[thimios]

What is the led type for LED1?

 

[lineup]

Red LED. The most usual.

I have tested more.
It looks like C5 should be 22pF. For perfect 20kHz squarewave.

 

[lineup]

This was precisely my point. Stability of amplifier with all that coils at the output and tested in simulator is just a half of the story. If amplifier is not able to handle 0.01 uF at the output without the coils (when run in simulator), then it is a good probability it won't be stable when actually is assembled, even though the coils and such all connected properly in real life.

I will test with 100n across 8 Ohm resistor as load. Without the Zobel inductor.

It is not stable.
Not in 100nF and not even in 10nF.
What can be done about this?

With Zobel it is stable even in 1uF

 

[CentralAsian]

Well, you stil can try to assemble it and check if it is reasonably stable in practice. But generally you would need to add some compensation here and there, Miller or shunt, depending on you preferences. Yes, the THD, bandwidth and speed will have to be sacrificed. In my opinion (a lot of people will agree, and a lot -disagree) though humble 20-30 v/uS should be well enough for 50 or less Watts amp. And THD anything below .005%@20KHz is (in my opinion) not worth fighting for either, sacrificing stability.

 

[mtarr]

Nice amplifier design. Amplifiers with voltage gain in the output stage have more variability in their frequency response as the load (speaker) affects the gain of the output stage. It may be necessary to allow for multiple compensation options to accommodate various loads.