Can I do this? Query about effect cancelling

[Mark Hammer]

I was tweaking an EA tremolo today, and I started thinking about having a stereo unit, but with two entirely independent LFOs to achieve different sorts of textures, and more interesting auto-pans.  What I'm trying to get my head around is how to bypass a stereoversion with a 3PDT.

I'll be referring to the GGG twist on the EA Trem.

Now, the circuit revolves around an LFO feeding the gate of a JFET, and varying its drain-source resistance.  The JFET is in the path of a cap from source to ground (well, 180R away from the source pin).  That bypass cap will increase the AC gain of the JFET when the path to ground is less than the 1k2 resistance it runs in parallel with, such that the gain goes up and down as the JFET provides a lower and higher resistance path to ground for the cap.

What I was planning on doing was duplicating everything that comes after Q4, and feeding that second circuit via another 10uf cap at the source of Q4; mono in, stereo out.

Since the input impedance of the circuit is reasonably high, leaving the circuit input tied to the input jack, and switching other things, ought not to degrade the tone.  The question is what to switch.

If I use the stompswitch to cut the ground connection of Q2, I cancel the modulation effect.  Do it once for each path, and I still have a 3rd set of switch contacts to disable a status LED.

But here's the challenge.  What happens to the output level when I do that, and how do I arrange for a reasonably balanced output level for effect vs bypass?

[aron]

Wouldn't you preset the output level of the output before the switch? That way you get around unity gain either way?

[Gurner]

If there's too much signal level change when you lift Q2's source ground connection, another option would be to switch in a preset DC level to Q2's gate (so rather than give Q2's gate the lfo feed, you give it a preset DC level)....this way, you could tweak the DC level on the gate so that there's no audible signal level change between 'effect in' & 'effect cancel' (after all, the LFO signal is just a slow moving DC level)

For those like me, who aren't familiar with the circuit, here it is...

http://www.generalguitargadgets.com/pdf/ggg_eat_sc_improved.pdf?phpMyAdmin=4a28f86a515b7883e7bc35a68d4e7b6d&phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

[duck_arse]

I tried a stereo pan trem thing a while ago, but it fell over due to too many oscillator/envelope complications. I started with the slow gear audio section, doubled the shunt fets and output buffer. that circuit does some weird (ie I can't quite fathom it) method of V/2 switching to bypass. I think.

these days I'd mix the slow gear and trem face shunting methods and the ea oscillator, and probably end up with a cardinal .....

[Paul Marossy]

What I'm trying to get my head around is how to bypass a stereoversion with a 3PDT.

How about using a 4PDT? PPP has them...

[Mark Hammer]

I have 3PDTs, and I'd like to keep the wiring down to a blessed minimum....though some may interpret this as liking to make things difficult for myself! 

If it can't or shouldn't be done, I'll accept that.  I'm not committed to making the pedal at this point.

[Paul Marossy]

I have 3PDTs, and I'd like to keep the wiring down to a blessed minimum....though some may interpret this as liking to make things difficult for myself! 

I see. I myself like to take the path of least resistance since I get so much of it everywhere else in life. 

[Fender3D]

Why don't you just short Q2 D and S for bypass?
I think it's more "natural" having tremolo dimming signal when working

[Mark Hammer]

Why don't you just short Q2 D and S for bypass?
I think it's more "natural" having tremolo dimming signal when working

I'll have to see what the impact is on level.  Shorting those two would have the effect of maxing gain.  If I was bypassing the output, that wouldn't be a problem.  On the other hand, if I short out D and S with a small-value resistor, like 1k5, I would likely cancel the modulation, for the most part, without raising the gain very much.

Thankfully, this is experimentation I can do on my existing EAT, without having to commit to build another.

[brett]

Hi
+1 for Aron's suggestion: Take the stereo outputs from the R12s (set to unity/2 V). Bypass the whole thing(s) with the 3PDT - mono input to mono output.
If you want a transition from stereo to mono tremolo, just 3PDT the single in and single out of the 'extra' channel (there will be a volume change if this is coupled to a second 3PDT without a volume change between clean and mono trem).
cheers