a dual preamp to check out - build pics added

[onboard]

Here's one of my pedals I want to throw in the ring for everyone to gander/critique/use etc.

My idea was to have a dual-preamp that switched between clean and colored. Some switchable tone shaping with a single gain stage would get the job done, but I fell completely in love with the Tillman preamp and the R.O.G. Fetzer Valve, and wanted an active tone control somewhere. So with some J201's *and* dual opamps in the drawer I thought I'ld go nuts.


The single opamp up top is a plain vanilla gain stage. Set it's volume control for unity gain strictly for buffering, or turn it up. Or down for that matter and have a padded output.

The jfet is somewhere between a Fetzer and Tillman gain stage with R8 and the switchable C4 at the output setting a treble rolloff.  Am I correct in guessing the "bright" switches on amps of yore were actually "dull" switches? In other words, switching the brightness on is actually switching the rolloff out of the circuit.

After that is an active single band graphic EQ from RG's "Simple, Easy Parametric and Graphic EQ's, Plus Peaks and Notches " with part values I had handy.  Since my cap values for C5 and C6 are between the listed values for 1kHz and 2.5kHz, I'm guessing the band is right around 1.5kHz? At least in that neigborhood I hope, since R11, R12, R13, and the pot value are all slighty different too.

Add another switch for true bypass and Bob's yer uncle. I'll get a pic up once the pedal's actually done - the board's finished and the box is drilled and waiting to be, (ahem) painted and finished with graphics.

edit No idea why the TinyPic link to the schem is down...

[toneman]

U invert once on top.
U invert twice on bottom.
T

[onboard]

Is that an issue if they're not mixed? Using a non-inverting opamp for straight  gain would be the fix then.

[onboard]

Something to try on the jfet side is moving the .01uf cap at C4.  Switched in an out after R8 makes for a straight-up treble roloff to get a "bright" switch option. Put the .01uf immediately before the jfet to get a response that almost sounds filtered. This came up a while ago when some asked about the SD Pickup Booster and it's resonance switch changing single coil to humbucker type tone.

Has anyone else used R.G.'s simulated gyrator graphic eq circuit? (the Mids portion after the jfet, from R10 on out) I know there's been some very usable Twin-T type tone control ideas floating around...

edit Make that simulated inductor, the circuit hanging off the pot is a gyrator.

[onboard]

Here's the finished pedal. Something I'ld like to fix are the level controls - they're not at unity gain until around 3 o'clock instead of 12 o'clock which I think is more functional. As is, about the first 40% or so of the pot rotation is'nt that useful. Maybe increasing the pot values to 500k would work?

Something else, the first time I tried the pedal there was no popping from the  bypass switch at all (knock on wood). I've since had the board in and out to switch the jfet's drain resistor, and now the bypass switch pops like mad - but only when the level is dimed. Roll it back a touch and there's no popping. Could it just be from the gain increase at the jfet?

Sorry, our server must be jammed up right now and it's not letting me in. I'll try to upload the pics later.

Hmmm...I can't get a TinyPic url to show either.

OK, Image Shack came through.



Now I'm embarrased about the fake company name I put on the decal...
Scratch that. Actual Audio Inc. sounds cool. I may just have to start up.