Question about shruthi external input


I am building a guitar rack system and I have a Shruthi with the SP filter in it.

I’m planning to run Guitar - Distortion - Shruthi Ext Input - Guitar amp.

My question is:

When the filter us fully open, is that as if it were a “True Bypass” pedal, tonewise?

I know I would be still going thru the VCA. But my concern is how much color will be added.

And is there a way to keep the VCA open without any oscillators on, and without hitting a midi key? (Just plain external input)

Thank you

afaik there is no true bypass, though you can DIY that pretty easily, a bypass switch is not really much of a electronic wizardry, you could even put the Shruthi into a pedal case… though I don’t know how much sense that would have.
An easier solution is to just control the filter with a MIDI pedal so it goes from fully open to whatever you want it to be. I guess it might colour the audio a bit, but that’s mostly a thing of taste and subjective perception, so probably the best thing is to just try it out. If you have a loop pedal (to route the audio in your pedalboard) just patch up two signal chains so that you can switch between a dry audio and the Shruthi and hear for yourself how it sounds (possibly without looking at the pedals and not knowing which one you are listening to… or your judgement will be biased).

Yes: set both oscillators to none, and in the modulation matrix, route an offset to VCA to keep the VCA open. You can also start with one of the few examples of external processing patches (which have the VCA open all the time and no oscillator running).

Ah thanks guys, here’s what I discovered jamming on guitar through my Shruthi for the first time.

  1. The volume of the distortion pedal going into the Shruthi has to be cranked to be at normal pedal level by the time it exits the Shruthi. Is there a volume control of any kind for external signals? Or volume per patch?

  2. On the SP filter, with the filter all the way open, it does kill some high frequencies. Quite a lot actually. On the low pass mode, it’s a lot dark. High pass, it loses low end, etc… with the other filter types.

  3. I am trying to make a 100% silent/muted patch, however I noticed some sound. When I play the keyboard on this muted patch, I am noticing clicking when I hit the keys. All envelopes, modulations, resonance, oscillators, are all set at zero. Is this the DC power supply? I’m using an adapter for the Shruthi, and an adapter for the small midi keyboard, and an adapter for the guitar pedal. All on the same power strip. Is the clicking a grounding problem?

It’s similar sounding to the clicking problem the Korg Minilogue is known to have, though I don’t know if it is the same issue.

I need this silent patch for a specific setting in my rig.

Thank you

  1. There is no software controllable input or output volume, but on most filter boards, there are pads on which you can solder input/output gain potentiometers.

  2. What is the SP filter? It could be a miscalibration of the filter (if it has trimmers), or an effect of keytracking.

  3. No it’s not a power supply issue. There is a lot of corner-cutting on the Shruthi (remember it’s a low part-count DIY kit!), for example:

  • Envelopes and LFOs are generated by PWM, without a very steep LP filter.
  • The VCA circuit doesn’t have any adjustment for offset/bleed, because the oscillators silence themselves when the envelope reaches 0.

And by the way: an ADSR envelope with A, D, S, R set to 0 will not result in a flat signal, but in a click (A = 0: the envelope very rapidly reach maximum level, then very rapidly goes back to zero since D = 0). So you’re probably hearing the VCA, modulated by this super clicky envelope, enveloping residual noise or a DC signal.

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Hmm that’s interesting. I’m using the Steiner-Parker Shruthacon filter from tubeohm.

Might I add that the clicking is not consistent. When playing single keys, it occurs every 3 or 4 notes. But when laying my whole hand on the keyboard, it recognizes the 5 notes I press and it pops 5 times. I’ll give this more time to test, and try different keyboards and power, and envelope settings.

And for guitar, this SP filter may be actually to aggressive, it has 1 sweet spot that goes into self oscillation 1 midi value up (cutoff 44 going to 45). I found it to be very recognizably steppy and piercing. I may have benefited from a 4p or an smr4 in this one.

OK spend some more time with this setup. The All pass filter mode fully open is definitely the most lively/neutral I think for external inputs.

I noticed the popping again. Say if I hit C4, it pops once, if I hit that same key again, no popping. If I hit a different key, it pops once, but not a second or a third time. It may be the keyboard, but I’m not sure. I think I can make this setup work.

I have gotten the popping to completely stop by maxing out both envelope attacks!

It’s a matter of retriggering. The envelope doesn’t retrigger from zero but from whatever level it was at.

And sorry about the SP… I have not designed it so I don’t know about any of its quirks…