A new thru zero triangle core VCO

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gtrmstr53
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Post by gtrmstr53 » Mon Nov 25, 2019 12:45 am

Just wanted to shout out this module. I bought a kit thru Thonk and liked it so much I bought a second once I had enough saved up. They've been great modules, and the build process was a lot of fun!

Edit: Wrong thread. Well that's embarrassing...
Last edited by gtrmstr53 on Mon Nov 25, 2019 11:51 am, edited 1 time in total.

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Post by devinw1 » Mon Nov 25, 2019 11:26 am

Hey gtrmster, I think you are probably talking about the EN129 kit that Thonk Sells?

Anyway, yes that looks nice but there should be a thread for that. This thread is about a new thru-zero I'm developing, which isn't (but will be!) a module.
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devinw1
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Post by devinw1 » Sat Dec 07, 2019 8:59 pm

Got the boards done and a panel too. Will order the prototype boards tomorrow! :tu:

I probably could have squeezed it all onto 2 boards (the middle one is not super dense), but I just kept the layout really clean and open and went with my standard "triple stack"

Image from Solidworks attached.
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Post by Orgia Mode » Mon Dec 09, 2019 4:06 pm

Really great looking 3D model.

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Post by J3RK » Tue Dec 10, 2019 2:51 pm

That does look good!

I'm way behind on my current projects, but this has me eyeing the empty vector rails I have laying around. :hihi:
Are you lemon? Does your head come to a nub?

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Post by devinw1 » Tue Dec 10, 2019 3:55 pm

I feel ya..

I've got a small pile of other people's stuff I still need to build, on top of all my stuff. :D :D



Boards are supposed to be here this Friday and a prototype is on order so I should have the first proto unit built with some testing and such some time next week. :tu: :tu:

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Post by devinw1 » Sat Dec 14, 2019 6:48 pm

So, I've been doing some more tweaking/testing as I wait for the first PCBs to show up, and I've found a strange little issue that I hope ya'll might be able to help with. It goes like this:

For high modulating frequencies, when I am turning the modulation amount pot, the carrier freqency, right before, during, and after the transition into TZ modulation will increase for a "bump" in a 2-3 degree range of the modulation amount pot. So say the carrier is at 500 Hz, as you turn the modulation up you hear it go to say 510Hz, then back immediately to 500Hz like something is adding DC to the core's reference current in a "speedbump" in the road. The effect is noticable the higher the modulating frequency. At modulating frequencies below 2-3KHz you can't even hear it really, but say up at 5+ KHz you start to hear it more.

If I put the scope on the feed to thru the reference current resistor in the expo converter, I don't see the signal jump up or down. I just see the modulating freqency start to "reflect" back over the zero line from the full wave rectifier as it should.

Any ideas?

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Post by devinw1 » Sun Dec 15, 2019 5:44 pm

Well, I found one problem that made the issue a lot better. I had the feedback capacitor in the expo converter op amp wired wrong, and was also too big a value. I put it in correctly (from - to out) and made it a 100p and things are much better. It still does bend up for a small section of knob rotation when modulating at high frequencies though. Interestingly, I recently built up the EN129 TZVCO and it does a similar thing at high frequency modulation, only it bends up and stays up whereas mine has a "bump". I can make the bump go away if I completely remove the 100p cap.

Perhaps to make further improvements I need a different op amp for the expo converter? (Currently using LF413).

In the meantime, I think I'm going to just build up a set of the boards that just came in since everything else is working nicely.

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Post by guest » Sun Dec 15, 2019 6:07 pm

im trying to understand the issue here. for high modulation frequencies, if it passes through zero, the output of the VCO pitches up for a short bit as it crosses? is this a function of the CV into the expo converter? do higher CVs make it worse?

ive found i can go as low as 10pF for the feedback cap in an expo converter.
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Post by devinw1 » Sun Dec 15, 2019 10:08 pm

Yeah, for high modulating frequencies, as soon as the signal to the expo starts crossing over zero (ie the rectifier is kicking in and folding the wave over), the pitch of the carrier bends up a little and then as you make that signal bigger (more modulating), it bends back down. This is for 100p capacitor in the expo.

I tried 10p and it works more like the EN129 where the pitch of the carrier bends up a little and stays that way as you add more modulating, returning to bend down again once you pass back below zero crossing.

I'm thinking (and this might be way wrong :D) that it's because right as you cross zero, the expo suddenly sees a higher frequency component, since the tips of the signal are now folded back down. And you get a different effect of this based on the filtering of the capacitor in the expo (depending on the value). If you go too big with the cap, the high frequeny modulation doesn't even get thru and the output looks like when you have a fixed reference current and just modulate my 2 JFETs that reverse the core (looks like a reversing type sync on a VCO).

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Post by guest » Sun Dec 15, 2019 10:29 pm

do you have a full schematic for the vco? is there a fullwave rectifier for the modulation input to the exponentiator? also, how are you measuring the carrier shift? is it by listening to it, or spectrum analyzer, scope shot, etc?
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Post by devinw1 » Sun Dec 15, 2019 11:52 pm

Here is the attached full schematic. The only difference is on my breadboard, I'm not using the VCA section for envelope. I'm just coming in with my modulating signal right at the high pass filter before U3B.

I'm using my ear to hear the pitch bend, but I also have it on the scope and see the triangle go higher (more cycles on the screen). You can see it pretty easy since we're talking like 500Hz carrier and say 5kHz modulating so the carrier keeps it's shape pretty well.
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Post by guest » Mon Dec 16, 2019 1:00 am

what is the shape of the modulation signal? i would assume as it goes through zero, the frequency would go to zero (and not pitch up). but, if youre using a square wave, it would just flip. have you probed the output of U4B to see if bumps up for that time period as well? i know that fullwave recitifiers can sometimes glitch during the transitions, and this could be getting passed through to the expo. a larger feedback cap on U4B would help here, or perhaps a different fullwave topology.
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Post by devinw1 » Mon Dec 16, 2019 11:03 am

I was using a triangle wave to modulate. I had meant to try a sine again last night but didn't yet. I assuming those transitions would be smoother potentially as handled by the rectifier.

I was probing the output of U5B and not seeing any jumps, but that's a good point, I should just look right at U4B output tonight.

As for the feedback cap, I have tried different values, and it does change the situation. For anything above about 50p, the pitch bend goes up a little at the transition, then back down to normal as you push further into modulation. For small values of about 20p or less, the pitch bends up a little at the transition and stays there as you push higher into modulation. For larger caps like 1n, the modulation breaks down at high frequencies and you basically just see the core reverse at zero crossings.

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Post by guest » Mon Dec 16, 2019 10:56 pm

can you post a scope shot? im a bit confused, as i would expect a triangle wave to ramp the carrier frequency down before it goes to 0Hz at the crossing, so the output frequency wouldnt be constant at any point in the cycle.
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Post by devinw1 » Tue Dec 17, 2019 12:38 am

edit: video attachment didn't work. Try this youtube link:

https://www.youtube.com/watch?v=BGVJou92zyc

Let's see if this works. I've attached a video.

This is the output U4B (sound is output of the core thru speakers in my shop). The output of the full wave rectifier doesn't contain the distortions you see on this signal. I'm not sure what that is all about.

Interestingly, the high freqency transition spike looking things you see on scope go away if I disconnect the JFET that flips the polarity of the comparator. Disconnecting the other one (the one on the OTA) doesn't change anything..... Disconnecting either of course kills the sound but I don't see how there's something getting picked up on the modulating signal from the core itself! Very odd.

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Post by guest » Tue Dec 17, 2019 1:10 pm

take a look at your powersupply rails on the scope. you have some small value resistors in there, and on transitions it might be pulling the powersupply low for a short bit.

also, is the signal AC coupled on that scope shot? it looks like its dipping negative and bouncing around a bit. there are 2 markers on the left side of the screen, and i cant tell which one is supposed to be zero. if its the bottom one, then the signal makes more sense to me.
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Post by devinw1 » Tue Dec 17, 2019 1:20 pm

Good point on the power supply. This is all still on the breadboard, so there might be something funky going on. I do try to have 100n bypass caps on all the ICs though.

Not AC coupled on the scope video. The lower mark is indeed zero. I'm just zoomed in and offset to make it fit. The TZ offset is at like maybe 10% so the average signal is sitting like 600mV or so.

I move the trigger level up toward the end to make it lock, and that's why it wigs out a little when the signal goes back down below the trigger.

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Post by devinw1 » Wed Dec 18, 2019 10:58 am

Well, I didn't have much time to look at it yesterday, but I did check the power rails right at the expo op amp and they look quite clean.

I disconnected the rectifier from the expo and just put a 1M resistor to -12V to set a fixed reference current, and I can still see spikes from the modulating frequency on the output of that IC (U4B). Hmmmm. I even disconnected the current from the expo transisitor to the OTA and I still see the same spikes. I'm thinking there must be some kind of ground loop now or something?

I also will try disconnecting the zener bridge and see if anything changes. I know the resistor value is quite low, but the numbers tell me it should only be seeing about 3.8mA.

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Post by guest » Wed Dec 18, 2019 1:54 pm

does your layout use short runs, with components flat to the breadboard? its possible its a radiative thing. when i get a chance ill look at it some more. its odd that the carrier changes frequency based on this, though. does the amplitude of the carrier change (assuming the current is staying constant into the integrator, that would be the only other way for it to pitch shift).
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Post by devinw1 » Wed Dec 18, 2019 3:09 pm

It's definitely not the prettiest. I have a limited supply of the little breadboard jumpers and ran out at some point and have 10-15 wires that go up and over to make connections and are not super short :D. I guess I wouldn't be surprised if this is part of the problem.

The amplitude of the carrier does not seem to be affected when the freqency bump occurs, but as you can see in that video, the modulating signal does get distorted, and I think the overall dc shift that occurs due to this distortion is the culprit. If you look at the video at about 5 seconds the Vaverage jumps about 60mV when the "spikes" emerge where the rectification tips begin to pop out. If I disconnect the FET that flips the polarity of the comparator, the distortion pretty much goes away, but of course then I don't have a modulated VCO anymore so I can't hear if the problem would be present, but my guess is not.

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Post by devinw1 » Wed Dec 18, 2019 7:40 pm

OK, spikes on the expo output was a breadboard issue. Tidied everything up and moved the stuff that drives the FETs away so no wires were crossing over, and the signal looks super clean, no more weird distortion

But, the "bend bump" at high mod frequencies remains! :hmm:

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Post by devinw1 » Wed Dec 18, 2019 11:40 pm

Aha! I've got it!

Despite my efforts of using super low offset op amps on all the mod signal stuff (LF412), I started digging in and realized that there is an offset between when the FWR starts folding the wave (what it thinks a zero crossing is) and when the comparator pops on and starts slamming the JFETs back and forth. If the modulation signal to the core starts crossing over and changing the shape of the wave at not the precise time that the core starts reversing, you hear an artifact.

I rigged up a 100k trimmer to +/- 12 and the wiper to a 470k/10k divider into the zero crossing detector and adjusted the 2 to be even and voila the bend is gone!

I think you notice the effect more at high mod frequencies because the fundamental is more audible then, and so you hear it.

Taking a measurement, I had to trim the zero crossing reference all the way up to 124mV, so the question then is why was there so much offset? Not sure. But, this is not such a hard calibration to do, so I don't feel too bad putting it on the as an adjustment. But, I'll also think about it some more and welcome suggestions.

Anyway, all in all a good night. I can sleep tonight. HA. :deadbanana:

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Post by devinw1 » Thu Dec 19, 2019 12:45 pm

Now that I think of it, I think I should probably just hook a zero crossing detector right off the + leg of the second rectifier op amp. This should show 0 volts until the diode is conducting and the rectifier is clipping/flipping the signal. I think this might work without trimming better. I'll give it shot tonight.

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Post by guest » Thu Dec 19, 2019 2:00 pm

great news, glad you found it. why not take the signal from pin1 of U5A? that should give a strong signal, one way or the other, depending upon the polarity of the input. heck, you might even be able to connect a transistor up to it to switch the JFETs directly. you might be able to cut out a bunch of opamps. U3B, U6A,B would all seem a bit redundant at that point.
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