Isms gate character variance

Hi. I did some random panning stereo patching with my isms lpg’s and noticed that the characteristics was very different.
The middle one have considerable longer decay time and sound much more lively. It’s also more difference between the lpg and vca modes.
The other two have much shorter decay (any at all?) and the only difference between the log and vca mode is that you can notice some low pass filtering if you have very weak signals to the level input. But it’s hard to hear since it also sounds more dull due to the lover volume.
It this variation normal?
I’ve not owned any vactrol gates before so I don’t know if the tolerances are usually this large.

yep, this is expected with vactrols and lpg.

The middle one clips in vca mode when the level is a teletype trigger out (9.9 V). The other two doesn’t clip. Are you sure this is normal?

not sure about clipping, but decay variance is normal.

indeed, vactrols are know to be each unique in response.

I often feel the need to attenuate a trigger when striking a lpg, to make it sound softer…

@Galapagoose may have something to say about the expected behavior of the gates.

There’s definitely a lot of variance in the vactrol response - this difficulty in consistency is undoubtedly part of the reason they fell out of favour or real EEs. Conversely, for the musically oriented folk, this variance is one part of why vactrols have become so well loved. Each instrument is truly unique, and there’s many techniques that are only capable with a gate of long character, and similarly for short.

In the Gate circuit of isms, the VCA mode is much faster than the LPG / LPD modes. They’re designed to be opened with +5v signals, and a little over that is generally fine too. While it won’t damage the Gate, I would probably suggest attenuating the full 10V triggers from teletype before hitting the gate. The Atten’s are conveniently located immediately next to the Gates for this purpose. Pulsewidth, as well as volume will drastically affect the character you get out of the Gate when sending a trigger.

I understand. However, the variations on mine is (imho) beyond musical variations. I have recorded some noise gating and made some DC measurements.

Test setup

VCA-mode, level not connected, in not connected, out to a DMM.
Level knob on min, 9, 12, 3 and full.


Gate 1

0, 0.07, 0.72, 5.67, 5.71

Gate 2

0, 0.52, 3.39, 8.41, 8.41

Gate 3

0, 0.094, 0.95, 5.56, 5.56

As you can see both saturate at 12 o’clock but the middle one (the ‘nice’ sounding one) has much higher output.

When I tried the same setup and measurements except in lpg mode I found something interesting.
Level at min was 0 but with know at 9 o’clock the voltage was drifting upwards on gate 1 and 3. It looks like it’s a capacitance building up charge. It started slightly below 2 volt and went up to ~2.4V. It’s a lowpass filter so I guess thats expected.
Gate 2 was totally stable at 4.26 V.

I tried to connect a buffered mult with 100kΩ (typical eurorack) input impedance between the output and DMM (the DMM might have to high input impedance and let the charge build up) but the result was similar.

The range of Gate 1 and 3 are 0 to ~4.85V with saturation around 2 o’clock while the second gate have equal range but the saturation point somewhere around 11-12 o’clock.

Sound samples

Same setup for each gate.
Sound samples recorded using isms square LFO to level, white noise to in and out to center.

gate-1.m4a (325.5 KB)

gate-2.m4a (325.9 KB)

gate-3.m4a (407.7 KB)

Regarding the middle gate it does sound like something isn’t quite right with it. Funny that you like the sound of it the best though. It’s possible that we could arrange a fix for the middle Gate, but it will end up much closer to the other two Gates, rather than the other way around. Feel free to email / direct message if you’d like to arrange this further.

I’m assuming the audio recordings are in VCA mode. This mode typically has very little attack/decay in response to the CV changing, so the audio seems to be pretty close. The feeling of the Level knob is definitely quite different across the various modes - this is a feature of the classic buchla design and I felt it was important to follow that.

The voltage drift you’re noticing is most probably the vactrol’s time-constant in play. One of the many interesting elements of vactrols is that they are slower to respond to changes when they are nearest the ‘dark’ point (lowest volume in our case). This is why you get a sort of exponential decay when triggering. There is an interactive element introduced when in LPG / LPD modes that exaggerates this behaviour, hence the ‘ringing’ effect.

No, the audio recordings are made in the lpg mode. That’s why I think the middle one is the good one since it’s the one with some amount of decay.
Anyway, I’ve sent you a PM.

Oh that’s fascinating. Could you post a similar clip but using a slow triangle LFO so as to show the the frequency / volume response?

The reason I assume the middle channel is acting up is that with no audio input, the ‘envelope’ output should only get to ~5V peak. Another great way to ‘hear’ the speed of the decay is to patch the ‘envelope’ (no audio input) out to a VCO’s pitch input and then trigger the Gate. This makes it easier to hear the shape the vactrol’s decay.

Ok. I teared down my current patch and have done some recordings:


First is the LFO -> attenuator -> lpg-in, lpg-out -> Ripples freq (4pole LP, max resonance, no input)
Both the square and triangle LFO is used (not the same time). The attenuator is at approx noon, 2, 3 o’clock and max for two cycles.

sweep-lpg-1.m4a (1021.4 KB)

sweep-lpg-3.m4a (1.2 MB)

sweep-lpg-2.m4a (1.1 MB)

The range is about the same (in contrast to the vca mode) but the attenuator curve and decay time is very different.


The second set is the triangle LFO straight to the gates in vca mode. Same self-oscillating filter.

sweep-vca-1.m4a (228.7 KB)

sweep-vca-2.m4a (258.0 KB)

sweep-vca-3.m4a (226.2 KB)

Here the range is much bigger on the second one.

Thanks for the tip about using frequency instead of loudness for analysing the decay time. The ear is much more sensitive to frequency changes than loudness. Especially in the low range.