Dual-core VCO project

[dslamnig]

This is my first VCO design, I thought I'd share it with the group hoping for comments, suggestions, advice. It's still on the breadboard, there's still time to change it for the better.

I've started out with the Triangular/Square-Wave VCO example in the LM13700 datasheet (https://www.ti.com/lit/ds/symlink/lm13700.pdf), thinking about making a simple LFO. But the project kept creeping all over my breadboard, turning into a full-fledged VCO, dual-core at that. Now I'm running out of connector wires which is a sure sign of maximum circuit complexity - time to wrap it up.

I've tried to keep things as simple as possible, cutting as many corners as I could while maintaining good waveforms. In the end it's not as simple as I'd like it to be, but I did manage to keep it down to 2 OTAs, 2 comparators, 6 opamps, 2 FETs and 4 BJTs. For that you get triangle, variable pulse and rising and falling ramps (because I'll be using it as a LFO, too, and there's no way to get deep space proximity alert alarms right without both). Also there's hard sync on all waveforms. The frequency range is about 0.1Hz to 25kHz, good enough for my old ears.

Triangle core

The triangle core is classic, with the LM311 comparator as the polarity switcher. I didn't want the slew rate of an opamp here, and I especially didn't want to fiddle with a discrete BJT Schmitt-trigger. LM311 is pretty mediocre as comparators go, but it has an open emitter output which proved to be very useful.

Saw core

Tri core up and running, I tried out a couple of tri-to-saw conversion methods and was not satisfied with results. There's always a connection glitch in the middle, and the slope of the jump from one to the other polarity is so-so. So I added a saw core which is slaved to the tri core. It's just a cap charged by an OTA and discharged by a FET on every tri cycle.

Since the discharge has a more-or-less constant duration, there is some drop in saw peak-to-peak voltage at high frequencies, >10kHz. The same thing causes "normal" saw cores to go flat at high freq. But since the master core is triangle, this VCO has very good tracking - with some saw amplitude loss as a tradeoff.

Triangle-to-pulse

The tri-to-pulse circuit also uses the LM311 which despite mediocrity does better switching than an open-loop opamp. The pulses are 0V to +8V, IMO better for LFO triggering and sync than zero-centered pulses. The open emitter output allows it to go down to absolute zero, would not be possible with collector output.

Sync

It took me a long time to get the sync part right. For some convoluted reasons I'll not go into here it's not enough to sync only the tri core - the saw also has to get some. But the result is great fat hard saw sync. The synced pulse also sounds great, but it can be hard on the triangle. Tri would benefit from some type of soft sync, but as I said I ran out of wires.

Expo converter

The expo converter is lifted from Tom Henry's VCO-555 (https://electro-music.com/forum/topic-54623.html), with some minor resistor value mods to account for the +/-12V supply and the fact that it was feeding two OTAs in parallel, so I needed more juice. Also, I'm using a 1.87k +3300ppm tempco resistor instead of 2k +3500 in VCO-555. This is a great circuit that worked perfectly right off the bat, all due credit and respect goes to the author.

Please take a peek at the schematic and give me your thoughts.

Rev 1: Added missing R43 to tri-to-pulse part.
Changed R18 value for greater range.

Rev 2: R21 removed from tri-to-pulse part.
R13, R28 changed values.

Schematic updated.

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DualCoreVCO_schematic_rev2.png
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[dslamnig]

Here are 1kHz waveforms on my ancient Hameg, taken from the breadboard version. Due to the sprawling nature of development there are loads of stray capacitances and contact resistances - I expect things to get sharper when I solder it together. I'll post some audio then.

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DualCoreWaves.jpg
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[dslamnig]

Making the expo module

1. Parts
2. Thermal paste BJT sandwich with tempco on the side
3. The finished product

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[dslamnig]

Rev 1: Added missing R43 to tri-to-pulse part.
Changed R18 value for greater range.

Updated schematic in original post.

[dslamnig]

All soldered and still works! As I hoped, the waveforms are much better defined, especially high frequency. I'm getting some CV noise, seems to be coming from the tri cap, the expo module and the input jacks. I expect this to subside somewhat after I mount it on the panel (to be cut and drilled soon), but I'll probably have to put some grounded shielding around sensitive parts.

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soldered.jpg
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[dslamnig]

I'm starting to feel a bit solipsistic here, so let me introduce myself:

I'm Davor Slamnig, but everybody except my wife and mother-in-law calls me Slama. I live in Zagreb, Croatia, and got my first soldering iron when I was seven. I have been wielding it amateurishly ever since, without much regard for theory, breaking Kirchoff's, Ohm's and other laws, always jumping over currents instead of using Wien bridges.

I got the modular bug a couple of years ago. Although I'm building all the modules myself, I decided to adhere to the Doepfer Eurorack standard. I've already built a VCO around the 3340 chip, and I set out to design one on my own. A few months ago I bought my first LM13700 and started playing with it. The result exceeded my wildest expectations - a perfectly usable dual-core VCO with a mean saw.

The components are pretty much run-of-the-mill, I've been able to get everything in local shops except the tempco resistor which I ordered from Germany. But an ordinary 2k resistor worked fine while I was developing the circuit.

Now I just have to make the panel and add some shielding, and it's ready to rock. I'll post some audio samples then.

[blue hell]

solipsistic ... oh dear ... we can't have that ... hi Slama

Looks like you are building a nice VCO there .. and re. things getting sharper .. the scope pics don't look bad from here?

[dslamnig]

[gabbagabi]

always good to see people designing!
big respect for to board-work!
iam a bit confused about the powerlanes, is it point or stripboard?
any chance to see the solderside?

one question: why it is good to have the open emitter on the LM311? I generally dont like them, i always wish those comparators could be more like "normal" opamps - just faster

cheers,gg

[dslamnig]

[electrotech]

Hi Slama,
This is very impressive, especially the fact that it's your first VCO design !
The waveforms from the 'scope screen-shots look excellent too.
Well done.

Andy

[dslamnig]

[dslamnig]

4kHz and 10kHz waveforms. Saw is starting to show the cap discharge time, pulse down-slope starts to bend, but triangle stays true.

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[gabbagabi]

It's neither, it's a "joint hole" board.

Yeah i ordered them myself but they where never delivered.
Afterwards i felt kind of lucky about, they are looking as if they would be full of phenol and other fancy chemicals
But you are still alive and kicking, it seems it is not to bad?

Those little imperfections in the waveform makes your vco sounding different from the others. this is something good! And 6 octaves = wow!

[AlanP]

I'm really impressed by this -- both the circuit itself, and what I'm seeing of it's performance, and also the physical construction, which seems very well thought-out.

[dslamnig]

[dslamnig]

[dslamnig]

The Red Menace!

Ok, it's done.

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DualCoreModule.jpg
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[dslamnig]

Here's some audio, finally. These are raw waveforms through the octaves, tri-saw-pulse.

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RAW WAVES TRI SAW PULSE.wav
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Filename:	RAW WAVES TRI SAW PULSE.wav
Filesize:	11.36 MB
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[dslamnig]

And here's a saw bass line, PWM melody and a Jan Hammer wannabe triangle solo. My 3320 Pro-One clone filter and 3360 VCA are in the patch, with some reverb and delay added.

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Triangle lead.wav
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Filename:	Triangle lead.wav
Filesize:	7.52 MB
Downloaded:	295 Time(s)

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Saw bass.wav
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Filename:	Saw bass.wav
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PWM lead.wav
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Filename:	PWM lead.wav
Filesize:	6.58 MB
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[dslamnig]

Rev 2 - as built:

R21 removed from tri-to-pulse part.
R13, R28 changed values.

Original schematic updated.