Yet another perf for today: AC30 Top Boost emulator

[Xavier]

JFET emulator of the Vox AC30 Top Boost I have found here

http://www.freeinfosociety.com/electronics/schematics/audio/voxac151996preamp.pdf

Most of the cap values look strange to me, but they should be correct according to the schematic, as I have never seen a non-electrolytic 500uF cap. Let me know what you think !!!

Small board for an emulator. I will try this one.....

[Rafa]

Seems to me not closer than the english channel, if you really want to simulate the AC30 you should give the Moeller simulator a shot, he said its extremely close to the original,
Cheers
Rafa

[KerryF]

That looks pretty cool.  Looks like a good use of my many J201s that I have.  I really wanted to get rid of them on an easy emulator, so I guess this is the one.  Let us know how it sounds.

[Rafa]

Upps the scheamtic and the layout arent cleraly the same circuit
Rafa

[Xavier]

Upps the scheamtic and the layout arent cleraly the same circuit
Rafa

They aren't clearly the same circuit? of course they are, it's just that as I said the perf layout is a JFET EMULATION of the schematic.

I have drawn this one because the parts count is significantly lower than the english channel, and as I've said, I haven't built it yet to judge.

[JHS]

The AC-sim from S. Moeller was designed for direct recording, w or w/o speaker sim, in front of most amps it sounds kinda strange (like an AC30 connected to an amp's input via loadbox) 'cos the tone controls on those amps can' be set to a linear frequency response.

JHS

[Rafa]

JHS:
Have you built it??
I wanted to build it but its expensive and complicated, but now that I know is not good for using as a stompbox, mhhhh, perhaps I wont build it
Have you tried it as a preamp, I mean conected direclty to a power amp ( no eq, or boosting)??

Thanksssss a lot
Rafa

[Meanderthal]

 Rafa- JHS was talking about the English Channel, not the Top Boost emulator.

I believe ROG have a top boost emulator project also...

[bancika]

Good luck finding 500uF non-electro cap (and saving money to get them )
Even 1uF aren't that cheap

[DuncanM]

I would guess that V2 is output - but where does T2 go to?
AND R8 seems to be connected at only one end...



<Aha, maybe T2 is output and V2 is the other end of C5 which is unmarked...>

<Nope, the whole tone circuit looks screwed compared to the AC30 Schem...>

[markm]

Maybe I'm missing something but, I don't see any 500uF caps in the schematic at all 

[Meanderthal]

 Weelll, there's some kinda math formula for calculating the cap values in a tube to fet conversion, based on the fact that tubes have a way of multiplying capacitance... 500 uf does sound crazy. But, math is math, so I guess that's where that came from.
What's up with r8?

[lovekraft0]

Most of the cap values look strange to me, but they should be correct according to the schematic, as I have never seen a non-electrolytic 500uF cap.

Just a heads-up - if those two 500uF caps (C4 and C5) are the ones marked C17 and C27 on the Vox drawing, they're 500 volt caps - C17 is actually 470pF and C27 is 47pF. You might want to redraw your own schematic with FETs and trimmers, just to make sure you have everything correct before you start on the layout.

[Xavier]

Doh..........

SORRY GUYS...........

This is the correct link to the schematic I've used. Now I understand everything..........

BTW

V2 is F7
Output is T2
T1 to ground
R8 must be connected to the cap just above it, I forgot to put a trace.....

Please check this schematic and you will understand what I'm talking about

http://www.freeinfosociety.com/electronics/schematics/audio/voxac30topboostpreamp.pdf

[d95err]

The 500, 100 and 50 in the schematic are obviously pF, not uF.

EDIT: Yes, it is inconsistent since the 25 on the cathode bypass caps should be uF. Goes to show that you can't trust schematics you find on the Internet...

[Xavier]

The 500, 100 and 50 in the schematic are obviously pF, not uF.

EDIT: Yes, it is inconsistent since the 25 on the cathode bypass caps should be uF. Goes to show that you can't trust schematics you find on the Internet...

OK, thanks for clearing this for me, I will correct the layout then....

It's a simple layout and it's very similar to any of the other "JFET adaptations" I've seen, that's why I have drawn it.....

[KerryF]

Anyone build this yet?

[Bucksears]

Bumping this one back up - does the volume control serve as a gain ctrl? Can you add a master vol at the end?

- Buck

[petemoore]

Bumping this one back up - does the volume control serve as a gain ctrl?
  If it's in the signal path working as a *variable divider [see BMP 'Sustain'] then yes...it will control the gain feeding anything past that point.
  *Gain control IS [often] the exact same thing [wired the same] as *volume control, but since what comes after is often multiplied in gain by the circuit, it's 'called' a gain control.

[Xavier]

I guess it's more of a gain control really. I would add a 100KA at the endo of the circuit, but again, I've drawn it as is (with Jfets)