Small Stone Mods

[shawsofhell]

Hi guys, I was just wanting to get a some advice on some useful mods to apply to the small stone design found on the tonepad site.

The mods I was looking at performing were:
1. Bright switch (changing 10uf polarized cap with a .1uf)
2. Univibe like mod (changing the .0068uf caps to other values)
3. Low Pass filter mod (i think this is when you ground one end of the .0068uf cap)

4. I want to have a depth/blend control where i can get a full on phaser sound or a really suttle one. I havent found any designs for this one so some input would be great. I was thinking of adding a pot  around where the 27k and 30k resistors are and changing their values slightly but realized with in one position the colour switch might create some problems with this idea???

[Mark Hammer]

The Tonepad layout is dense.  Dense enough, in my view, that it is inhospitable to *switchable* mods.  Doesn't mean that you can't mod it (or that it isn't a good layout), but having a mod that you can reverse with a switch will be tricky.

The best bet, then, is to use some sort of daughter board for those things you want to switch, or else before you etch the board, expand the layout with pads for additional caps outside the "city limits" of Francisco's existing layout.  You could easily add another half-inch of traces and pads along two edges and it would still fit in a 1590BB.  Run some jumpers from the spot where the caps would normally go to the added pads and stick on some additional pads to go to the toggle switches.

There are a variety of kinds of "bright" that could be implemented: treble boost for the entire pedal output, bright dry but normal wet, normal dry but bright wet, normal wet and dry but bright recirculation.  You need to pick one probably.

A bright dry-only signal could likely be achieved by sticking a fixed resistor and cap (in series) in parallel with the existing 30k resistor to provide a lower resistance path for higher frequencies.  I'm gonna guess that a 10k resistor and a 4n7 to 10n cap would provide the requested extra bite.  Note that there would be not change to the phase-shifted signal, although changing the amplitude of dry signal above a certain range would mean that you lose the ideal 50/50 balance so notch depth would be affected at the treble end.

A treble-emphasized recirculation signal could be achieved by replacing the 0.1uf cap in the recirculation path with something much smaller such that low end is drastically cut.  On the other hand, if all you want is a little more bite but want to keep all the bottom, then you stick another resistor and cap in series and place them in parallel with the 27k resistor and 0.1uf cap.  Again, try a 10k resistor and 4n7 to 10nf cap for the same reasons as above.

If you want to tinker with restricting the notching effect in the bass region so that you don't hear it quite as much as it sweeps down, stick a 10nf cap in series *before* the 27k resistor coming off the last phase-shift stage.  Or, you can swap the 10uf cap for a smaller one, as described.  In the case of the mod you describe, restoring normal functioning involves adding back a large value cap which will likely involve some switch-popping.  In the method I described, the reversal simply involves shunting the added cap which will produce no audible popping.

You can Univibe the SS by replacing the existing 6n8 caps with the values used in the Univibe: 15n, 220n, 470p and 4n7.  Other values will work but using these saves you the bother of doing the math.  Making it reversible to SS specs, though, means either having switch-popping as you move back to the original values, or else working out some math so that you can start out with 2 caps in series for each stage, and shunting one or the other gets you certain desired cap values.  Feasible, but you still have to do the math.

Personally, I wouldn't be a bit surprised if you couldn't do a bunch of what you want using only the last two stages.  So the first two stages stay as they are, cap-wise.  The last two stages have added pads/jumpers so that you can reroute one end of the caps between the junction of the two 27k resistors it normally connects with, and ground (the end tied to pin 6 stays where it is).  Nested within that switching arrangement is the choice of cap values.

For phaser effect depth (not the same as sweep width), you just increase the value of the 27k resistance coming off the last phase-shift stage to the mixing point.  The current resistor values aim for a 50/50 balance so that maximum notch depth is achieved.  Altering the resistance for the wet path reduces its contribution, the extent of cancellation, and the notch depth.

[shawsofhell]

What do you think about changing the 27k resistor with a 50k pot to control the depth?

How loud is the pop you get when switching the caps?

[shawsofhell]

Damn I just realised that a 50k pot when turned to zero would give you zero resistance hence a hell of alot of the wet signal. So how about maybe putting in a 25k pot in series with a resistor? possibly a 20k resistor so i can get a more intense sound as well?

Also with the tonpad layout what are the 2k and 47k resistors for and what is Ca and Cb for?
Also how can I get id of the volume drop everyone talks about when using true bypass?