Theory of operation behind step sequencer pedals

[swinginguitar]

What's the general principle behind those such as the goatkeeper, seek wah, etc?

a controller switching in and out each pot in sequence?

[Digital Larry]

Here's a guess.

Use a CMOS 4051 8-to-1 multiplexer along with a low frequency square wave running a 3 bit counter to develop the address for the mux.

http://www.cmos4000.com/cmos/4051.html

Each pot goes from +V to GND and the wiper of each pot goes to one of the mux inputs.  The mux output goes to the thing that is getting moved around (voltage controlled low pass filter, ring modulator carrier, etc.).

[Mark Hammer]

What's the general principle behind those such as the goatkeeper, seek wah, etc?

a controller switching in and out each pot in sequence?

Typically, such devices will use a "counter" that provides the same output at each of a succession of different pins on whatever chips are used (frequently a 4017 counter, but it's not the only fish in that sea).  The counter is stepped through its count by a clock.  The output will be a logic-high voltage of 5V or whatever is being used to signify logic-high.  That voltage is then divided down by a pot.  The counter steps through each of the individual outputs until it gets to the end then restarts at the beginning.

The voltage produced at the wiper of each little pot is then used to control something else.  Some sequencers will allow you to set how many steps are used.  Some will allow you to adjust how "step-y" the steps are, and perhaps include a "glide" control that lets you smooth the transition between the steps to that they are more like up and down ramps.

These sorts of primitive sequencers should be differentiated from more complex synthesizer sequencers that not only use many many more steps, but may use memory for each "step", storing several parameters, that are then deployed when the governing CPU/MPU arrives at that step.

If you are familiar with S&H units, like the Maestro/Oberheim FSH-1, then you are familiar with the manner in which individual control voltages associated with a "step" can produce interesting sounds.  In the case of S&H units, the "sequence" is essentially an infinite one that never really returns to the start.  And, rather than have a predetermined voltage at each of the known steps, it has a voltage derived from a noise source that serves as the "next voltage level".

Many sequencers will include a "manual step" function that stays at a given step so that one can adjust the pot where you want it before moving on to the next step.  In some respects, the classic flip-flop circuit that we use for both octave duividing and stompbox switching (i.e., in Boss and similar pedals) is a two-step "sequencer" that has the same output for each step, goes from step one to step two, and then back to the beginning for the next step.

Some synth designers, back in the analog mono days, would use a sort of superfast sequencer to produce waveforms.  So, if I had a 16-step unit that could go from step 1 to 16 in a fraction of a second, AND if the speed with which it zipped through those steps could be varied, then I could use it to produce a wavshape of my own design, instead of using an oscillator and being "stuck" with triangle, square, ramp, and PWM.  Press a keyboard key, and my "sequencer" zips through 16 different voltage levels, producinga voltage event that is heard as a note.  If my keyboard output voltage dictates how quickly the sequencer clock ticks, then I step through those 16 levels more quickly for higher notes than for lower ones, still producing the same waveform, but at different pitches..

[swinginguitar]

Mark - in the case of a volume stepper, such as a trance gate, what is the signal flow?

So let's say you have 8 steps - 8 pins on the counter each connected to a pot....does the input signal pass through each in series...or somehow each is connected to a single "master" pot....or...?

[Mark Hammer]

There is usually a single master control element of some kind.  That can vary from circuit/effect to circuit/effect.  So, for example, the gain of an OTA in most compressor circuits will be dictated by how much current is fed to the OTA's control pin (Iabc pin).  If one places an LDR in series with whatever is feeding that pin, and then lights an LED on the basis of whatever voltage is feeding the LED from each step of the sequence, then the gain of that OTA will change as the sequencer steps through the sequence.

Alternatively, consider something like the venerable Dr. Q (or Dr. Quack, or Nurse Quacky, et al.).  It changes the center frequency of a bandpass filter, by feeding the base of a transistor (used like a voltage controlled resistor in this application) with a voltage corresponding the the amplitude envelope of the guitar signal.  If, instead, we fed that transistor base with the divided-down voltage from each step of a sequencer, the center frequency of the filter would be shifted around, in stepwise fashion, according to the pattern we've set.

That doesn't mean there HAS to be only one single control element, but however many control elements there might be (e.g., a couple of FETs), they are all ganged together so that they each change in the direction dictated by the difference between this step and the last one.