Verify FMV/James switching w/ Opamp buffer schematic

[kalegood]

Hello,
Could someone please verify my schematic here; I'm trying to create a pedal that will switch between the FMV and James tonestacks. I've included an opamp buffer from AMZ. This is my first schematic; some things look a bit sloppy to my eye, but I wasn't sure how to clean 'em up.  Vdd (below R6) is supposed to mean power source. If I'm not mistaken, since I have a voltage divider, I would just put 9 volts here.

[PRR]

Many typos.

A passive tone-stack has three terminals: In, Ground, and Out.

The tone in each band is varied from IN (full) to Ground (none).

You have connected In and Ground. Tone will vary from full to full-- no variation.

You are missing the wiper connections on the Fender B-M pots.



Optimizations:

Passive tonestacks have wild input impedance, especially wide-range ones like Fender and James. You usually need a low-ish impedance driver. (Note the cathode-follower in 5F6A.) Unless your complete plan (not shown) has a stage which can do this, better allow for one.

Sticking with your dedicated buffer, if you run two buffers it makes sense to derive a filtered "Vref" bias and use it for both stages. This also significantly reduces power-supply crap (your scheme injects nearly half of any PS crap directly to the output buffer).

With a lowish-impedance driver, if pretty low-Z, it can probably drive the inputs to both stacks at the same time. This reduces your switching by one pole, and may significantly reduce the wiring effort.

Several of your resistors are not needed. You have to think about loading, but these networks work best with minimum loading, don't add load.

[PRR]

Another thing. Both Fender and James, being passive tone networks, have a large overall loss. A proper James is 10:1, Fender depends what tone you want but may be similar. Unless you have accounted for this elsewhere in the system, the output buffer probably wants to be gain of 5 to 20 (nominal 10).

For most useful settings, the loss of the Fender and James will be "different". Especially once you start turning the knobs and looking for exciting new tone shapes. You *may* end up following each tonestack with a volume pot where you can equalize the losses.

[kalegood]

Ok, I've redone the circuit, basically redrawing what you did there.  I was just copying from the Tone Stack Calculator; not quite sure why those excessive resistors are there. The buffer and values are from AMZ.

I've put the opamp buffer on the output again, but from what you're saying, it sounds like I'll need much more than the unity gain it provides. Unless, of course, using the amp qualifies as "accounting for".

This is designed to be tone controls for my Epiphone Valve Junior and other cheap (tone-stack free) amps I hope to build. Another option I considered was to build a preamp out loop into the VJr and throw the pedal in there, as my understanding is this is a tonestack's normal placement. My understanding is also that, if I were to do that, I could do away with both buffers and wouldn't have to worry about the gain issue.

[duck_arse]

you have the R8 and R12 wipers hard to ground. no signal will pass there, lift the short to ground.

[samhay]

you also have the inputs on your op-amps swapped.

[kalegood]

Ok, so I think I'm going to go to an opamp in front and drop the one on the back in favor of an external boost (probably a micro-amp). Which means I may as well go back to the voltage divider in the first schematic, if I'm not mistaken.

I'm a bit confused on my opamp input (probably because I don't really know how opamps work). I've got it lined up just like the AMZ page, third from the bottom: http://www.muzique.com/lab/buffers.htm. But you're saying I need to switch my positive and negative input?

The pots I can figure out, I would think.

[samhay]

You can use the second op-amp to boost the signal. They are pretty good at doing things like that.
Look again at the AMZ page - it's a good resource. You have the + and - inputs switched.

[kalegood]

Ah, yes. Short-sighted on my part.

My understanding (from post #3) is the opamp will convert to low z at unity gain (give or take). However, due to the loss in the circuit, the output will be at fairly low volume levels. So I'll need to add some gain somewhere. Of course, the preamp could do it, but that might be a be noisy? I'm not really sure, as I'm new to all this. This would be my first pedal.

[B Tremblay]

Have you seen the Tonemender? If you're interested, I can share the unreleased updated version.

[kalegood]

Ok, how 'bout now. I've taken the Tonemender opamp sections for my input buffer and output booster.