Interesting switching arrangement in EHX Small Clone

[Mark Hammer]

I responded to a local ad and picked up a supposedly dysfunctional Small Clone (older folded-metal chassis type) for $25 last night.  Turns out it works just fine; I have no idea what the technical issue was the guy ran into.

When I opened it up to do whatever surgery might present itself as necessary, I was struck by the fact that it used only two lugs of a DPDT switch, yet it still managed to turn a status LED on.  Weird!   So many pedals need no less than 8 lugs to do that task.  What gives?  I initially thought that someone might have installed a new switch for the guy and not known what they were doing.  But the damn thing worked right off the bat when I plugged in.  So what gives?

As luck would have it, I had a schematic someone had posted at some point (don't ask me where, I don't know) of the re-issue SC, showing the switching arrangement.  In the absence of the capacity to post pix from work, I'll rely on words.

A great many commercial time-based and modulation pedals that combine a clean and altered signal use FET-based electronic switching.  This is normally done via a momentary switch and flip-flop circuit driving the gate of a JFET.  The JFET simply allows or blocks the altered signal from being mixed in with the clean signal to produce the effect.  It doesn't "bypass", as such, but simply cancels the effect.  the input and output jacks are wired up to the circuit board, and the signal passes through whatever input and output buffers/circuitry might exist at all times.

As we all know, EHX has a history of lusciously tactile stompswitches.  With the exception of a couple of Russian made products, the overwhelming majority have nice shiny chrome-plated stompers that go "CLICK!".  So, how is this legacy approach to switching integrated/co-opted by EHX into a switching approach that shares more in common with Boss and DOD?

Here's how it works.

As per a zillion other chorus pedals, there is a JFET (2SK30A, a long-time favourite) between the delayed signal and the op-amp where wet and dry are mixed together.  To make sure the JFET gets the right kind of voltage on its gate, there is a diode tied to the gate, just like Boss, et al..  There is also a .047uf cap to ground from the diode, again, just like Boss.  Leading up to the diode/cap junction is a 4M7 resistor.  Hold that thought.

The status LED is hardwired directly to V+.  The stompswitch simply connects the free end of the LED to that 4M7 resistor.  This applies voltage from V+ through the LED and resistor to the JFET gate.  Boom, it turns on the JFET and wet signal comes through.  There is also a 10k resistor to ground from the switch side of the 4M7 resistor.  So when the switch is closed, not only does voltage pass through the LED, 4M7 unit, and 1N914 diode to the JFET, but it passes through the LED and 10k resistor to ground.  Boom, the LED now turns on.

Brilliantly simple, and delightfully elegant.  Doesn't get more parsimonious than that: an LED, two resistors, a diode, a cap, a JFET, and an SPST switch.  Of course, you have to have an appetite for non-TB pedals and treat their buffers at face value, but I find that relatively easy to do.  What I also liked about it was that I had a bunch of X-wing SPDT switches hanging around, so I swapped on of them for the DPDT the pedal came with, and I got to save it for a worthier application.

I didn't really need another chorus, but I'm glad I bought the pedal just to learn that.  I installed a mode switch to get 3 different delay ranges, and will probably sell the pedal off for a couple bucks profit.  The trick I acquired will be the real profit margin, though.

[petemoore]

  That is slick !

[Mark Hammer]

For those who need an actual picture, here"s a picture.

[SonicVI]

yep, a few of the old EH boxes used FET switching. The Small Clone, Graphic Fuzz and Stereo Memory Man are a few that spring to mind.

[petemoore]

The JFET simply allows or blocks the altered signal from being mixed in with the clean signal to produce the effect.  It doesn't "bypass", as such, but simply cancels the effect.
   The status LED is hardwired directly to V+.  The stompswitch simply connects the free end of the LED to that 4M7 resistor.  This applies voltage from V+ through the LED and resistor to the JFET gate.  Boom, it turns on the JFET and wet signal comes through. 
  Does this wet signal increase the output volume when it comes through ?

[Mark Hammer]

No, actually.  You will note that while it seems reasonable that adding a second signal at a mixer should increase overall output, part of the chorus effect is the production of notches.  More than a flanger, as a matter of fact, though not as noticeable because they never really go away.  The notches produced through combination of wet and dry actually reduces the overall output slightly.  Which is why you will notice that the mixing resistor for the wet side is a little smaller than that for the dry side.  That is partly to offset the linear remaining drain-source resistance of the JFET when on, but also to provide roughly equal wet/dry signals.

I was using the pedal this evening, and I couldn't detect any discernible volume balance issues.

[earthtonesaudio]

Everything... in its right place.