Sparkle Boost driving AMZ Presence Control?

[Kultur Vultur]

If I wanted to tack the AMZ Presence Control at the end of Andy's Sparkle Boost, would I need a gain recovery stage or buffer afterwards to compensate for tonestack losses or impedance issues?

I tried to model this in LTSpice IV but for some reason I can't get JFETs to work in it...

[Mark Hammer]

The BMP tone control eats up a lot of signal.  One should keep in mind, however, that the clipping diodes in the BMP limit the amplitude of the signal that reaches the tone control.  The Sparkle Boost output from the drain of the JFET is not constrained in the same manner as the clipping stage in the BMP.  So, there may well be enough signal left over after the tone control, to provide a volume boost, relative to bypass.

The larger question is whether there is "enough" signal left over.  If you want to overdrive something else with the modded Sparkle Boost, the passive signal loss of that tone control network may require a recovery stage to provide the required output level to push subsequent devices hard enough.

If you do mod the SB in the manner described, you can drop the 100k resistor that runs parallel to the volume pot.

[Kultur Vultur]

The larger question is whether there is "enough" signal left over.  If you want to overdrive something else with the modded Sparkle Boost, the passive signal loss of that tone control network may require a recovery stage to provide the required output level to push subsequent devices hard enough.

If you do mod the SB in the manner described, you can drop the 100k resistor that runs parallel to the volume pot.

Yeah, I was looking to overdrive the input of my amp with this project (not a big fan of distortion pedals but the agressive filtering you get in a BMP-style tone control is very nice). As far as making a clean gain recovery stage, does the BMP gain recovery stage run clean?

EDIT: Would I need that 1uF cap after the JFET drain?

[Mark Hammer]

Yes, the BMP recovery stage would do just fine....or you can simply cascade two Sparkleboosts with the tone network between them!

If you were to do that, you drop the 2.2M input resistor n the second one (because you won't need it for de-popping).  You also raise the .022uf input cap in both to .047uf or even .1uf to keep some bass.  The 1uf output cap is not absolutely essential since  DC is blocked at the output, but it probably would help the tone network to behave normally if separated from the drain by the 1uf cap.

[Kultur Vultur]

I actually modelled your idea in LTSpice already, Mark.
It's a miracle how I got LTSpice to work with JFETs...




The frequency response is better with the cascaded Sparkle Boosts than with the BMP gain recovery stage, there's more availble output, and the gain knob is much more functional. If you leave Rgain close to its clipping threshold ( ~1K for my low-output strat, I'll probably put this on an internal trimmer for versatility's sake), then the gain control of the second stage has a much larger usable range. Before you would have had to keep the gain nearly "wide-open" to get any distortion at all but know you can go "from clean to mean" better.

A few component values needed changing (some you suggested), but otherwise this was pretty simple. Perhaps a tonestack-defeat switch would be an interesting mod...

Any input, folks?

[cheezit]

I'm playing with the Sparkle Boost and found this thread.  Kultur Vultur, did this work out as a "keeper" circuit?

The original point of the second stage was as a recovery after the loss from the tone circuit, but as you described how you did it, it sounds like the second stage is the one that was clipping, and the first was set to 'clean' gain.  So the tone stack is pre-clipping.  Seems like the first gain stage should be the one doing the clipping...

[Derringer]

I've been tinkering with a BM for a while now. Just yesterday I setup a fet recovery stage and I like it better than the BJT recovery. The fet makes it seem "amp-ier."

I just used the variable gain FET booster design from Jack's page.

http://www.muzique.com/lab/boost.htm

[aziltz]

this sounds like a great idea!