Living in sync

A little while ago this LFO mod came to my attention:

That caught my interest, since I’d just ordered a PCB for the Music from Outer Space Voltage Controlled LFO, which has a lot going for it, but not sync. Sync is useful if, for instance, you want to create a vibrato-like variation in pitch for a series of notes. You might put a triangle wave from an LFO into the oscillator CV. But when you play a note, whether it starts from the lowest pitch and goes upward, or from the highest pitch and goes downward, or from some intermediate pitch and goes one way or the other is essentially random, unless the LFO has sync capability. If you put the keyboard gate into an LFO sync input, though, you can guarantee the note will start at the “beginning” of the LFO waveform — or, as we’ll see, at the “halfway” point.

Let’s take a look at this circuit. I don’t claim to understand every bit of what’s going on here but I have the general idea. This was designed with a particular LFO in mind, one in which the shown 1 nF C1 is the integrating capacitor, but the caption says it should work for most triangle-core LFOs.

C31 and R33 differentiate the input sync signal to make short positive and negative pulses, as shown: a positive-going pulse at the leading edge of the input gate and a negative-going pulse at the trailing edge. IC3a acts as a comparator with a threshold near +1 V (I’m assuming +V is +12 V), so its output is always -V except during one of those positive-going pulses when it’s +V. D1 and R34 had me confused for a while — I think they’re needed when resetting to the positive peak, as discussed later. Anyway, the outcome is that the gate of JFET Q1 is held at about -V except when a sync signal comes in, and then the negative voltage is removed briefly. RC for R33 and C31 is 100 µs so the gate pulse is about that long, actually closer to 150 µs.

The JFET is being used as a switch here. It’s off when there’s a negative voltage on the gate, that is, the source (bottom terminal) and drain (top) are disconnected. The switch is on, the source and drain are connected, when that negative voltage on the gate is removed. When the switch is on, what you have with IC1b, R37, and R36 is an inverting amplifier with input + 12 V and gain 3.3k/6.8k, so the output will be about -6 V.

Of course it doesn’t change to -6V immediately, capacitor C1 resists that. The caption says it charges up through R36, but in fact it’s R37 and not R36 that together with the capacitance of C1 determines the charging time. That’s not obvious to me, but a simulation shows it. Anyway, RC here is 3.3 µs, so it takes several times that long for the output voltage to reach about -6 V. But the switch is closed for about 150 µs, so it has plenty of time.

Someone on the LMNC Discourse group was having trouble getting this mod working with a different LFO, one designed by David Haillant. In that LFO the capacitor is selected with a switch to be either 10 nF or 1 µF instead of 1 nF. In the 10 nF case the charging time constant is 33 µs. That’s long enough that the capacitor doesn’t fully charge in the time the gate is open, as seen in this simulation plot:

The orange line is the input sync signal, the brown line is the voltage on the JFET gate, and the blue line is the output of the op amp. You can see the latter is still decreasing a little when the JFET switch is turned off, and then it rises from there. To make the cap charge fully during the gate I cut R36 and R37 to 680 and 330 ohms, giving a charging time constant of 3.3 µs again:

The cap charges to -(330/680)*12 V = -5.8 V. However in this design the triangle wave starts at about -7.6 V, so what you see happening is it goes down from -5.8 V to -7.6 V before starting up again. So I changed R37 to 430 ohms, because -(430/680)*12 V = 7.6 V. Then it works as intended:

Here it is over a longer time period with several sync pulses:

All of which looks fine. (Note also what it says in the caption: If you want to reset the triangle wave to the maximum instead of the minimum, you can do that just by connecting R36 to -V instead of +V. I didn’t simulate that.)

What’s problematic is when you switch the LFO range. Then you have a 1 µF cap and the charging time constant is 100 times longer, 430 µs with the smaller resistors. I don’t think you’d want to drop those resistors to 6.8 ohms and 4.3 ohms, so the alternative would be to lengthen the gate pulse by increasing C31 and/or R33. Say make C31 1 nF. Then there’s a delay of about 15 ms between the leading edge of the sync trigger and the start of the new triangle wave, which might be too long for the higher LFO rates. You’d maybe need to use a DPDT switch to change both C31 and C1 between small and large values.

But I’m more interested in the MFOS LFO. That has no switch, just a single integrating cap of 20 nF. With a resistor around 330Ω the charging time constant is 6.6 µs, still much less than the gate pulse width of about 150 µs, so that should be good. The triangle wave goes between about -5 and+5 V so the R37/R36 ratio should be a little smaller, but it’s close to what we want. I breadboarded the LFO and the mod. In the LFO I substituted TL072 for LF442 and TL082 and 2N5457 for MPF102. For the mod I didn’t have a 2N3819 JFET and didn’t have another 2N5457 available so I used the first other JFET I could find, a J112 of which I seemed to have only one.

After a few minor difficulties, it worked! It reset to a slightly too low negative voltage but that’s just trimming. Aside from that it looked fine. Below you see the gate voltage in blue — mostly -12 V, except in the center when the sync signal comes in — and the triangle wave output in yellow, syncing to the minimum in the center.

But then I tried connecting to -V to reset to the maximum — and it didn’t work. Even with no sync signals coming in, just connecting to -V caused the oscillator to stop working.

Eventually, for reasons which turned out to be irrelevant, I found some J113 JFETs and substituted one of them for the J112. Syncing to the minimum still worked. I tried connecting to -V and that worked too!

Why the difference? J112 and J113 aren’t that much unalike. They share a datasheet, in fact. The datasheet quotes different numbers for them, but their characteristics vary over a fairly large range and the ranges for the two overlap: V_{GS}(off) for the J112 is -1.0 to -5.0 V and for the J113 is -0.5 to -3.0 V. So if I read it right, it could be you could buy a J112 and a J113 and find they behave identically. If so that suggests some J112s would work in this application, or some J113s wouldn’t, or both. That in turn suggests the design here is sitting on some hairy edge where you don’t really want to be. But that’s as far as my understanding goes. What I do know is, you can sync the MFOS LFO both ways.

Edit to add:

I ended up trying five different J113s, all from the same order so likely from the same manufacturing batch, and all worked.

After getting some more J112s I tried four of them, all from the same order, in addition to the one I’d tried from a different order, and none of them worked. I also tried one J111 and it didn’t work.

And I tried five 2N5458s, all from the same order, and they all worked.

It’s good that you can sync the triangle core to the maximum because if you sync to the minimum and look at the sawtooth output, it starts “in the middle” at 0 V and goes down, then jumps up and returns to 0 V. That’s probably not what you want for a synced sawtooth.

Whereas if you sync to the triangle maximum, the sawtooth starts “at the beginning”, going from maximum to minimum.

Likewise, though it’s hard to see it here, the pulse wave starts on the rising edge if you sync to triangle maximum, and half a cycle later (which is not the falling edge except in the square wave case) if you sync to minimum — the latter being, again, probably not what you want.

Here is the sine waveform with both types of syncing — both useful, as with the triangle wave:

So there we go. Yeah, long and somewhat arcane post, sorry. Point is, it can be made to work. I’ll design an auxiliary PCB for the mod — it wouldn’t be hard to do on stripboard but as usual I’d rather have it on PCB, especially if I end up making another of these LFOs. In fact I have a board for a Eurorack version and maybe I could kludge the sync into that too.

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