Resonance audible even when no notes are played
  • Hey, finally got around to finishing setting up my Shruthi with SMR4 board... loving the sounds so far! Just working on tuning the filter (I agree with a previous comment here, it would be great if someone could put up a newbie-friendly video tutorial for this showing what we are meant to be listening for) and I've noticed that while Resonance is set to max I can hear this sound bleeding through even with no notes triggered. I am guessing this means a problem with the amp section.

    Saying that, I can also hear noise coming through when playing notes with a cutoff of 0 - am I right in assuming that 0 is meant to not let any sound through? If so it seems like the filter is not working 100% too.

    Any suggestions on how I can solve these (or pointing out how wrong I am with these various assumptions...!) would be greatly appreciated.
  • First of all, a cutoff of zero will let some sounds go through for a couple of reasons: 1/ there is filter tracking so the cutoff value dialed on the filter page is shifted up/down depending on which key you play on the keyboard (and depending on the patch you play on, random values, velocity, etc… check in the modulation matrix that nothing affects cutoff) ; 2/ each stage of the filter generates some noise at the output, so even with a cutoff of 10 Hz you have some background noise adding up ; 3/ a 4 poles filter attenuates with a 24dB/octave slope, not a 200dB/octave slope :)

    On the SMR4 board, the VCA bleeds a bit – I have been pussyfooting over this when doing the v0.7 revision but there was no way (layout-wise) I could add the extra trimmer to keep it quiet – there is such a trimmer on the SSM2044 filter board. Note that, when the resonance is not set to 63 (ie, when the dominating sound source is the oscillators), the bleed is harder to observe since the oscillators are silenced “in software” when the slowest envelope reaches zero.

  • Thanks for the instant reply! Just good to know that it isn't something I've done incorrectly in the build/setup.

    Understood about the filter, makes sense when I think about it...

    Is a VCA trimmer something we could add as a mod to the 0.5 board if we could get hold of the correct value trimmer? Would be very grateful if you could perhaps hint at the best places to take some small wires from on the PCB for this please... :) I agree that this isn't a major problem in the majority of cases (not likely to be creating too many patches with full self oscillation) but I was looking forward to at some point putting the shruthi backwards through a passive DI into some guitar fuzz boxes and other effects (my favourite way to get even more enjoyment out of my synths and keyboards), some of which have huge amounts of gain or are otherwise sensitive to unwanted sounds so I am interested in removing as much extraneous noise from the output as possible. I understand this is asking a lot from a synth kit which is half digital, and all budget, but I'm still hopeful :)
  • Pin 2 of IC4 is the summing node where the VCA CV is scaled / filtered / rectified. The bleed comes from the fact that the “0V” coming from the digital board is not exactly 0V, but a few mV. So you need to subtract a few mV here. Try connecting pin 2 to a 1M or 330k resistor connected to the wiper of a trimmer whose extremities are connected to 0 and -5V. The trimmer acts as a voltage divider, and through the large resistor it generates a small negative current that will suck the VCA CV down.

  • Meant to say - thanks v much for this, will bookmark it - definitely be useful at some point.