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Small Stone color switch

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10/4/1999 2:41 AM

Small Stone color switch

What exactly does the "color" switch on the Small Stone do, both sonically and electronically? It's been almost 20 years since I've heard one, so my memory is dim. Looking at the schematic, one half of the DPDT switch routes the signal, and the other half seems to change the LFO in some way. Looking at the output, and considering the technology of the times, I'm guessing that straight and shifted signals are mixed together by the pair of 27k resistors, and exit via the .1uf cap. Based on this, I'm also guessing that what is being routed via the color switch is some regenerated signal via the other 27k resistor and 6800pf cap preceding it. The 100k resistor that gets engaged when the color is "on" serves as the other half of a resistive mixer prior to the input buffer (which isn't needed when the color switch is off, so it gets shunted). If the color switch alters the resonance, does this mean that the 27k resistor on the signal side of the switch can be replaced with some sort of variable resistance to produce variable resonance, and can the 6800pf cap from the output of the last stage be altered to produce different bandwidths of regenerated signal?

I'm also kind of curious as to what the LFO side of the color switch does. Is there something that can be altered to produce variable depth (sweep range)? If so then we may well have a 2 or 3-knob Small Stone here instead of the one-knob job.

Not to gild the lily here, but sometimes you need just a smidgen of what the "magic" button on an EH effect does, rather than a whole lot of it. I'm wondering how to do that.

10/4/1999 1:45 PM

I'll take another look at the schematic. I had this all worked out when I was making the "Little Rock" phaser layout. The color switch is just regeneration, and I believe it's set by the ratio of the two 27K resistors as you mention, but I'm going from a fairly foggy memory here. If so, pot for one of the 27K's should work, as should making them a fixed 47K and a 47K pot.

As I remember, the color switch may not have done anything much to the LFO. I think that was an artifact of how it was drawn.

I'll take a look at it again.

10/4/1999 4:20 PM

Ok, first look.

The mix of dry and phased signal is done at the junction of the 30K from the two-transistor input preamp/buffer and the 27K from the output of the phase line.

The regen is taken from the junction of the 3.3K/4.7K to ground from the output of the phase stage through the 0.0068uF/27K to the color switch. The signal side of the color switch either passes the signal from the 27K/0.0068 directly to the input mixing point at the junction of the 100K/0.0068 at the input or shunts the regen signal to ground. The 270K resistor is also shunted to ground to load the input a bit to prevent a gain/level change.

To vary the amount of regen, make the 4.7K resistor a 5K pot and take the signal off the wiper into the 27K.

The other side of the color switch does do something to the sweep, but I'll have to puzzle over it a bit.

10/4/1999 8:47 PM

While you're puzzling, it is worth noting that the unusual use of an OTA as the LFO results in the CA3094 not only serving an oscillator, but also as the device determining the same LFO characteristics that would normally be determined by an INITIAL and WIDTH control, in addition to RATE.

Although it is wise practice to reduce the sweep width when going for faster sweeps, one normally doesn't really fiddle with resonance in a compensatory manner when changing the sweep speed. So, I'm wondering if the LFO side of the color switch changes the "initial" value to something more appropriate to higher and lower resonance settings.

Just a thought.

10/4/1999 5:50 PM
nic	Mark, I find that the small stone is a pretty good effect in it's off state. Not that it sounds terrible, but the bleedthrough is enough depth! nic

10/4/1999 9:29 PM

I built myself a little box (actually a wah pedal housing) with a SmStone in it. I wired it up exactly as it had been, and from what I learned the half of the "color" switch that isn't involved in the feedback slows the sweep down when the regen. is increased. It may change the waveshape as well, I didn't scope it out. I replaced the 100K resistor on the switch (for regen) with a pot. This allows some color mixing... Also, this is one of those wonderful 70s dinosaurs that eats a bit of your signal when "on" to balance levels between "engaged" and "tone suck" (bypass.) Found that out when I threw a DPDT in there.

Farrow
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