JCM800 emu in SMT - who's interested?

[Ajay]

Hey guys,

After making an SMT version of Gaussmarkov's Dr Boogey layout, I thought it would be kinda fun to have the JCM800 emu available in SMT as well. So, as I couldn't find any Eagle designs, I redrew the schematic and made some mods in the tone stack so the board's now capable of either a standard TMB circuit, a simple BMP tonestack or the enhanced version of the BMP tone stack.

To save board space (and hopefully being able to fit the board in an 1590A size enclosure), I used fixed resistors for the drain supplies instead of trimmers.

I'm still tidying things up, but I was wondering if anybody else would be interested and maybe share some thoughts on the layout?

[Kipper4]

I'd like to har a sample if possible please.
Rich

[Ajay]

Okay, here's a first version of the board:
http://www.aronnelson.com/gallery/main.php/v/diyuser/ajay/JCM800-emu-SMT-v01.zip.html
I'd appreciate any suggestions/comments/remarks/etc about this board.

Thanks, Ajay

[Ajay]

Hey people,

There's an updated version available now. In this version, I've added thee bypass capacitors for three drain resistors, so there's more room to experiment. Furthermore, I've cleaned up the layout a bit. You'll find it here:
http://www.aronnelson.com/gallery/main.php/v/diyuser/ajay/JCM800-emu-SMT-v02.zip.html

I'm planning to finalize this design by next week and then order boards. If anybody has suggestions for the layout or the circuit itself, please share them.

[J0K3RX]

Any news on this project? Does it work?

Here's some ideas I had inspired by Mike Fortin

Better resolution...
https://dl.dropboxusercontent.com/u/11178619/Fortin/Image1.jpg


Maybe?

[Ajay]

Oops, completely missed this one 

Although I got a little side tracked by a noise gate redesign (completely from ground up, still working on that) and my x-88/ac-30 clone, I ordered the boards two days ago.

The mods look interesting though and could be implemented rather easily on the board and/or directly on the pots. Some would have to go to SMD pads directly, but still doable IMO. I expect the boards to arrive in two weeks (they're being made in China) and I'll start working on them as soon as I receive them. As I've ordered 10 of them, building up a second one with these mods shouldn't be a problem 

The first mod looks like a frequency dependent gain control. I guess a colleague of mine would be very interested in trying this mod, as he needs a fat lead sound and this mod could just do that. Also the various gain pots and bright switches would help there.
One thing I'm not sure of is the 47u/1k/47u network. Normally with tube amps and fixed resistors this would lower the bias point/output swing, thus increasing distortion. This could also work here, but then we'd probably need a different set point for the drain voltage (not the usual 4.5V, but first measure the resulting voltage after the second 47u cap and then using the half of that as bias point). Also, I think the resistor value needs to be more, as the drain currents are relatively small (I had to use a 33k resistor to get about 4.5V voltage drop, so for 1V or so I guess I'd start with 6k8-8k2). Hey there's room for experimenting . BTW, in the updated schematic I've provided for decoupling of the second and third stage drain resistors which could result in the same effect, but your mod could also be implemented, albeit with some 3d electronics

As for the output stage: I would include a 100k resistor after the tone stack for setting the output impedance.

I'll keep you posted.

[Gus]

Something I posted in the past.  I have not built it.  A sim
http://www.diystompboxes.com/smfforum/index.php?topic=102027.0