Thomas Henry VCO-1 for Eurorack

dogfaceaudio · Post by **dogfaceaudio** » Thu Dec 10, 2015 3:54 pm

I'm currently working on adapting the fantastic VCO-1 design by Thomas Henry to an 8HP Eurorack module. I implemented the resistor changes suggested by Oscar over at the electro-music forum, and changed a few small things. Are there any errors in my schematic or things that should be changed? Thanks!

*edit* Direct link to schematic image:
http://s12.postimg.org/51th1c517/VCO1_rev01.png

executiveBlaster · Post by **executiveBlaster** » Thu Dec 10, 2015 4:54 pm

the image isn't working! I'd love to see.

dogfaceaudio · Post by **dogfaceaudio** » Thu Dec 10, 2015 5:40 pm

Added the URL for the schematic image to the original post.

J3RK · Post by **J3RK** » Thu Dec 10, 2015 6:09 pm

How exact are you keeping with the original? I notice that you included reference voltages for the pots for example. If you're open to a little interpretation, I would HIGHLY recommend using the LM13700 instead of the CA3080, as it's still manufactured. Then you have the other half of the LM13700 to either use as a sine shaper, a modulation index VCA, or maybe even a slew for example.

Another thing might be to AC couple the exponential input like the linear input. I would also increase the cap size to allow for lower rate modulations to work better. Something between 1uF and 22uF works well IMO.

You might want to make the integrator op amp a single. I've done it both ways, but settled on using a single op amp after noticing a little bit of odd behavior using dual/quad types.

I absolutely love the VCO-1. In fact, the basic core is where many of my own VCO designs started out. A little while down the road, a little advice from Thomas Henry and Dave Brown (among others) and I'm still using quite a bit of it.

If you're interested a couple of other things I changed are:

Using an LM311 for the core comparator instead of the discrete transistors. The transistors work fine too though.

I would also recommend on the board layout, providing a footprint for a monolithic matched PNP pair for the exponential converter. SSM2220 is nice, and not terribly expensive. (I would also keep the discrete footprints as well. It adds options for the builder.) You could use a THAT 340 and it would take care of the exponential transistors, and also the pair of NPNs in the core comparator at the same time, or better yet the ones in the sine shaper if you keep the NPN based one.

One last item for now. I have a sawtooth shaper that will work with this VCO. If you're interested, I could export a schematic for it tonight.

Anyway, just a few ideas based on my own experience. Your schematic looks good from the short time I looked at it. (at work right now...) Clean and easy to follow.

Happy to chime in again after I look at it more if you like.

nigel · Post by **nigel** » Thu Dec 10, 2015 6:21 pm

It depends on how much you want to change the original circuit, but I would be tempted to replace the 3080 with one half of a 13700. (You could then use the other half to replace the sine shaper.)

[Edit - I overlapped with the last post!]

forbin · Post by **forbin** » Thu Dec 10, 2015 7:20 pm

+1 for replacing the 3080 with 1/2 a 13700 -- personally I wouldn't touch the transistors for the schmitt trigger/comparitor or the sine shaper -- i used 2n2222 for the sine shaper in mine and i recon (it might have been luck) but i have got the cleanest sine that i have out of my analog VCO's

The core of this thing is very similar to the OTA incantations of the 258 VCO clones.

Have you asked Thomas about this? just out of politeness...

dogfaceaudio · Post by **dogfaceaudio** » Thu Dec 10, 2015 7:46 pm

I'm trying to make a slightly more stable version of the original Thomas Henry design, which is why I added in the reference voltage for the pots. Ideally I want to stay as close to the original with this design, later on I'll experiment with changing things.

The CA3080 is still very easy to find so I just left that as is. I did look for a cheap matched PNP pair to replace the two 2N3906's and found the DMMT3906, but the tiny SOT-363 package would've been too annoying to solder by hand. The SSM2220SZ comes in a nice SOIC-8 package so that looks like a good replacement for the two 2N3906's. Do the two 2N3904's in the sine shaper also need to be matched?

Lastly, is it possible to use a 1.87kOhm thermistor instead of the 2kOhm thermistor? I have a few 1.87kOhm thermistors on hand but no 2kOhm ones.

Right now I'm just doing this design for myself, but if enough people are interested I can ask Thomas Henry if he is okay with me doing a run of PCB's to sell.

forbin · Post by **forbin** » Thu Dec 10, 2015 8:07 pm

Ideally the sine shapers should be matched but probably not worth a proper matched pair.

You can get away with 1.87K instead of 2K by adjusting the input resistors to the opamp by 1.87/2 = .935 -- so instead of 100K on the 1V/oct use a 91K or work out a parallel or series pair to get you closer... your just trying to keep the gain the same (Rf/Rin). The other input resistors probably don't matter as much as their effect is adjustable anyway... You may well find that the tracking adjustment will get you in anyway...

Oh and since your putting in a voltage reference to make a quiet 10V you probably should use it for the schmitt trigger and R9...

J3RK · Post by **J3RK** » Fri Dec 11, 2015 12:44 pm

Here is the sawtooth shaper that I mentioned. You can get some other interesting shapes out of it too by feeding it a square with a different phase, or possibly even other waveforms. I haven't tried others, but noticed some cool jagged wave shapes while I was experimenting to get it working by taking the square from different places in the circuit. In the UltraFade VCO, I take the square from the non-inverting input of the LM311. You can probably get it from the transistors in the Schmitt trigger of the VCO-1. If I remember correctly, Scott Stites mentioned that in his VCO-1 article.

J3RK · Post by **J3RK** » Fri Dec 11, 2015 12:51 pm

Oh, and also you can add reset sync to this VCO very easily. Put a FET across the integrator capacitor, and run a comparator into the gate. You can use a transistor based comparator, op amp, or an actual comparator IC. As long as it switches the FET to short the cap you're set. This will allow you to completely stop the core as long as the comparator is high. If you want a more typical sync, stick a small cap in line, (or maybe a more fully featured gate-to-trigger circuit,) to make the pulses short.

You can also add an LFO mode, by putting a switch and a larger cap (100nF works well) across the integrator cap. When the switch is on, the larger cap will take effect since it's in parallel to the normal one.

I believe I mentioned this above, but a THAT 340 IC is around $8, and gives you a matched PNP pair and a matched NPN pair in a single DIP-14 package. This would take care of your exponential converter, and your sine shaper. They also have an SO14 package, but that one always seems to be out of stock at Mouser.

J3RK · Post by **J3RK** » Fri Dec 11, 2015 1:45 pm

forbin wrote:Oh and since your putting in a voltage reference to make a quiet 10V you probably should use it for the schmitt trigger and R9...

I second this.

dogfaceaudio · Post by **dogfaceaudio** » Fri Dec 11, 2015 2:04 pm

I'm assuming I'd have to change R9 to 1.0M if I change the input to the +10V reference?

The resistors around Q3 and Q4 would probably also have to be changed, though I'm not entirely sure how to go about recalculating those values.

J3RK · Post by **J3RK** » Fri Dec 11, 2015 2:10 pm

dogfaceaudio wrote:I'm assuming I'd have to change R9 to 1.0M if I change the input to the +10V reference?

That's what I've done on my newer cores. As far as the comparator goes, I use the LM311, so it's a bit different. You may need to make a few tweaks in the discrete one.

J3RK · Post by **J3RK** » Sat Dec 12, 2015 1:53 pm

Here's my current revision triangle core VCO. I just built it up from scratch because my other schematics were a bit on the illegible side. I added voltage references (my older VCOs just used the rails, which also made the comparator section a bit more complicated than it is with the references.) Attached a PDF as well, as it's easier to read.

J3RK · Post by **J3RK** » Sat Dec 12, 2015 3:13 pm

Also, an OPA134 might be preferrable in the integrator.

dogfaceaudio · Post by **dogfaceaudio** » Sat Dec 12, 2015 9:51 pm

Here's the schematic with some of the suggestions added in. I ended up not using the 10V reference for the Schmitt trigger as I couldn't find a good resistor combination in LTspice to get 10Vpp output, I could only get 8Vpp. I kept the transistors for the exponential converter and Schmitt trigger to stay as close to the original Thomas Henry design as possible for this board. I'll probably end up experimenting with a matched pair on another board design. Are there any issues with this schematic, or does it look good enough to start doing the PCB layout?

http://s22.postimg.org/8fkooskv3/VCO1_rev02.png

nigel · Post by **nigel** » Mon Dec 14, 2015 7:45 am

I'm fairly certain that Q2 is back to front (C and E should be swapped).

J3RK · Post by **J3RK** » Mon Dec 14, 2015 12:20 pm

nigel wrote:I'm fairly certain that Q2 is back to front (C and E should be swapped).

Yes, the emitters should be tied together there. Nice catch!

dogfaceaudio · Post by **dogfaceaudio** » Mon Dec 14, 2015 2:03 pm

Thanks for pointing out the issue with Q2, nigel! That was a huge mistake on my part. Here's revision 0.3 of the schematic. Hopefully I haven't screwed anything else up:

http://s15.postimg.org/nh6jp8atl/VCO1_rev03.png

dogfaceaudio · Post by **dogfaceaudio** » Wed Dec 16, 2015 11:34 pm

Did a quick and messy PCB layout to test this schematic. I sent the design to the awesome people at OSHpark and should have boards in about 2-3 weeks.

Front Panel Board:
http://s16.postimg.org/y3on4qj37/thvco1_panel.png

Core Board:
http://s16.postimg.org/nscac2rdv/thvco1_core.png

calaveras · Post by **calaveras** » Wed Dec 16, 2015 11:56 pm

I was kind of interested in this project if it became a PCB/panel type of party.
But honestly I'm not big on SMT.

just my 2 cents

Ps I know folks say SMT isn't that bad, but I have really shitty eyesight and already have a hard time soldering!

dogfaceaudio · Post by **dogfaceaudio** » Thu Dec 17, 2015 11:38 am

I went with SMD on this one as I wanted to fit the whole thing in 8HP. It wouldn't be that difficult to do a through hole version, but it'd most likely be a larger module.

calaveras · Post by **calaveras** » Thu Dec 17, 2015 12:02 pm

no worries, I just love to whine about my poor eyesight and SMT.

J3RK · Post by **J3RK** » Thu Dec 17, 2015 12:18 pm

calaveras wrote:no worries, I just love to whine about my poor eyesight and SMT.

I hadn't planned on doing a run, and I'm not quite sure when I'll have time, but I have a similar, small VCO PCB (basically a PicoVCO with added features and improvements in a smaller space) all laid out. I definitely don't have time to do a fully fleshed out project, but I could probably squeeze in a run with BOM and schematic in the somewhat near future if there was interest. It's through-hole. Actually the SSM2220 is SMT, but the rest is through-hole.

I've also started lining up all board IO so it will work with a SIL header. (which facilitates control PCBs, something I didn't used to do.)

Anyway, let's not pollute this thread with it though. I'll see what my schedule looks like for the next couple of weeks, and then make a separate thread.

dogfaceaudio · Post by **dogfaceaudio** » Thu Dec 17, 2015 2:00 pm

I'm certainly interested in this small VCO design, would it work in a Eurorack system? How many HP does it use?

MUFF WIGGLER

MUFF WIGGLER

Thomas Henry VCO-1 for Eurorack

Thomas Henry VCO-1 for Eurorack