Scope Design: JFET Input EXICON Output 10 Watt Class A

[gary s]

@lineup, are you still evaluating and testing this design - or is what you have shown in post #65 and the pcb done by prasi in post #67 the end product. I think a few members here are interested in buying the pcb's and building the amp. Hopefully you can set us straight, thanks.

 

[Kris0603]

Could this design use J49/K134 as outputs, perhaps with modifications? I recently came into a small stash of these.

 

[lineup]

I have tested some more.
It looks as C5 should be 22p.
C1 is still 100p.

Here is the 20kHz squarewave:

 

[gary s]

I am sure they will work, I have J50/K135 TO3 types and intend to build a channel with both types to check the results. Keep the leads as short as possible between the pcb and the TO3 pins.

 

[lineup]

Here is the new final with C5 and C1 correct values.
It is also in the first post in page #1.

 

[gary s]

@prasi - can we trouble you to add the 22pF cap across the 8K2 feedback resistor for the compact pcb design, thanks so much.

 

[thimios]

Solder this at the bottom side parallel to 8k2😉

 

[Vunce]

What about the stability issues?

 

[Ponti17]

Seems quite interesting and worth a small build if no there's no issues with stability.

 

[prasi]

yes soldering c5 from bottom would be ideal.
but for a cleaner build, here you go, c5 (c7 in my sch) =22p and c1 (c3 in my sch)=100p.
except for the resistors, which are now RN55D/0.25W package, the same parts guide as posted earlier can be used. zobel is still 1W min, 2W if you can fit it.

 

[Brijac]

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.

Transitional miller perhaps would be fine for this use case?

 

[lineup]

The bandwidth according to SPICE simulation is now only 350kHz.
THD is like 0.00006%

 

[gary s]

Mant thanks prasi for the updated pcb gerbers - you are a champion around here.

 

[gary s]

Pcb's ordered - looking forward to any member comments here who might also be building this design, although I am in no hurry.

 

[NicMac]

"Be wary of projects which show signs of being based on theory or calculations or simulations only. Look for prototypes and some indication that the author has actually built what he describes".
Link

 

[CentralAsian]

Pcb's ordered - looking forward to any member comments here who might also be building this design, although I am in no hurry.

I think it is a bit premature to order a PCB of undebugged design. It could be easily built on a protoboard.

 

[avtech]

As long as the builder accepts the risk that the end 'thing' may not work as intended or at all, it's not a problem.

And seriously, PCBs are $5 for 10 plus whatever level of shipping fee you want to pay. I can't get a pint of beer for less than $10, so it's hardly going to break the bank to test out a new design.

 

[Vunce]

Well said avtech

I already know that a project such as this is unvetted and most likely will have some bugs or possibly be a complete failure, but its your own decision to give it a go or move on.
Add the Gerbers to an existing pcb order, super cheapo 🤑

 

[gary s]

I have built 2 of lineup's previous designs without a problem, so no guts no glory - we will work it out if there is an issue.

I am in agreement with avtech and Vunce - no harm in trying.

 

[lineup]

I can't see why this circuit would not work.
And using spice is better than build from paper only.
Good Luck to you that chose to build it.
I am waiting reports 🙂