guidance on footswitchable “second rate control” mod for EHX small clone?

[ctelecaster]

Hi, I've been thinking about taking my first foray into pedal modding by trying to figure out how to do a mod on my EHX Small Clone.

I primarily use it with the Rate set somewhere below halfway for a subtle slow chorus. Sometimes though I like to take the rate up to near max for a sort of faux rotary type modulation. I have been thinking that it sure would be nice to install a second foot switch on the enclosure that could switch between the rate set on the existing rate knob and a second level, whether it be via a second pot mounted topside or an internal trimmer.

I was planning on simply using a 3PDT with each pole switching all three lugs of the two pots. I'm curious if anybody could describe if a more "sophisticated" way to do this exists, such that I might save the third pole for a future bicolor led mod to indicate which "rate" is active. I'd be happy just to get the footswitching rate done now and deal with the LED in the future.

Like I said I'm a bit new to this kind of work. I'm just handy enough with a soldering iron to be dangerous, so any guidance would be greatly appreciated.  Thanks!

[idy]

You will want to confirm, but it looks like the existing rate pot either has pins 1 and 2 together, or pin 1 isn't used at all. ( I have seen both on schematics, and they act the same.) So you really only need one pole to change pots: one pot can always be "hanging" without affecting anything.

[ctelecaster]

You will want to confirm, but it looks like the existing rate pot either has pins 1 and 2 together, or pin 1 isn't used at all. ( I have seen both on schematics, and they act the same.) So you really only need one pole to change pots: one pot can always be "hanging" without affecting anything.

I don't have my unit in front of me at the moment, but looking at Small Clone boards on google this does appear to be true: lugs 1 and 2 linked.

Does this mean that I can let lug 3 hang on both pots and just switch between the combined 1 & 2 on a single pole? Or is it the opposite way around?

[PRR]

SPST

[idy]

It doesn't matter, let 1 and 2 hang and switch 3, let 3 hang and switch 1 and 2. Same difference.
You can check your board to see how it is, but don't think it matters if 1 and 2 are linked. The important pins are 2 and 3.

[RickL]

I agree that PRR's solution is the simplest and that it will work, especially if the intended use of the second speed control is for the fastest or close to fastest speed. But this solution puts the second speed pot in parallel with the existing pot, which means they are not completely independant.

When the second pot is engaged, changing the position of the first pot will (slightly) change the speed, either faster or slower depending on the direction it is turned. You'll notice it more if the second pot is set for a speed only a little bit faster than the first. Also, the first pot will always have to be the "slow speed" control. Adding the second will always make the speed faster.

You might not even notice the difference, and it is what I would do, but if it is important, use a spdt switch to choose one or the other of the pots.

[Mark Hammer]

Hi Rick!   

There are two issues to grapple with.  One of them has been outlined by Rick already (more in a moment).  The other is that faster speeds are generally less tolerant of greater depth.  This is why so many chorus pedals have a speed/rate AND depth/width control, so that sweep width can be dialed back when speed is increased. 

The schematic shows that the depth switch simply connects a 2k7 resistor to ground to bleed off some of the LFO amplitude or conserves it by lifting that ground connection.  Here's my suggestion for a foot-switchable fast/slow. (Many thanks to EHX for the basic drawing that I modded.


The LFO speed is partly dependent on the resistance provided by the 1M pot and series 47k resistor.  The lower/smaller the resistance, the faster the speed.  If we split that 1M into a pair of 500k/470k pots  one can set a maximum/slow resistance and the other set a minimum/fast resistance.  Bridging the "slow" speed pot reverts to whatever the fast one is set to.  Since they are in series with each other, the slowest possible speed occurs when they are both enabled and set to their highest resistance value.

I've replaced the 2k7 bleed resistor for depth-setting with a 5K pot.  This allows the person to dial in how much LFO depth they wish to preserve for faster speeds.

The whole thing provides for fast and slow presets, actuated with a DPDT switch.  Closing both sets of contacts at the same time gets you the faster speed with the desired amount of LFO depth reduction.  If you want it foot-switchable, then I suggest a standard 3PDT stompswitch, with the 3rd set of contacts used to light up two different-coloured LEDs, to let you know what speed range is active.

[ctelecaster]

Thank you everyone, a wealth of great information here. I'll mull this all over and see what route I want to go.

Much appreciated!

[anotherjim]

There is a way to automatically reduce depth with increasing speed and to some extent, it looks like it already has it with the 10uF cap after the depth control. With the 4.7k resistor from the opamp pin1, the cap forms a low-pass filter (3Hz) making the triangle wave from pin1 smoothed to be more sine-like. It also means that as the LFO speed increases, the wave will reduce in amplitude so automatically reducing the depth. You could probably increase that 10uF value if you find it's too much of a warble on fast.

[Mark Hammer]

There is a way to automatically reduce depth with increasing speed and to some extent, it looks like it already has it with the 10uF cap after the depth control. With the 4.7k resistor from the opamp pin1, the cap forms a low-pass filter (3Hz) making the triangle wave from pin1 smoothed to be more sine-like. It also means that as the LFO speed increases, the wave will reduce in amplitude so automatically reducing the depth. You could probably increase that 10uF value if you find it's too much of a warble on fast.

That's very similar to the tactic used in the first-issue (6-chip) Small Stone pedals, except their choice of components starting the "softening" of the triangle around 1.5hz.

[PRR]

...this solution puts the second speed pot in parallel with the existing pot, which means they are not completely independant....

Agree. It is what I thought he was asking for, but it seems I mis-read.