Combining 2+ VCOs into a single module

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Biff
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Combining 2+ VCOs into a single module

Post by Biff » Fri Apr 02, 2021 10:48 am

I’m about ankle deep (read: new and now overshooting) into sticking my foot into projects and I thought it would be fun/useful to create a VCO module that combines 2 VCOs into 1. The main goal is I want it to function similar to a 2 VCO per voice polyphonic synth voice, so having the tuning be controlled by a single knob (but keeping fine tuning independent). The second part was to create an summed output mixer of all of the VCO’s the wave shapes rather than individual outs.

If I wanted to control tuning on one knob for 2+ VCOs, would it be as simple as wiring the tune pot output from the first VCO into the second one (and removing the tune pot on the second and connecting the other pot’s output)? If so, should I adjust the pot’s resistor rate or is it probably too subtle (currently it’s 10k) to need a change?

Second, for the mixer, if I want the overall output to max out at the normal modular level, I’m assuming I’d need to put a resistor on the main output. If so, is the resistor amount going to change depending on how many things are being mixed?

Thank you very very much.

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MikeDB
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Re: Combining 2+ VCOs into a single module

Post by MikeDB » Fri Apr 02, 2021 11:19 am

Probably best to use a simple op-amp mixer to combine the outputs, so as to keep the low impedance output. Does the second VCO have two tuning pots ? In which case yes you should be able to do what you are suggesting, providing the fine tune on the second VCO doesn't affect the tuning voltage on the first VCO. Actually a very good idea, surprised it's not more commonly available.

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Re: Combining 2+ VCOs into a single module

Post by Biff » Fri Apr 02, 2021 1:06 pm

MikeDB wrote:
Fri Apr 02, 2021 11:19 am
Probably best to use a simple op-amp mixer to combine the outputs, so as to keep the low impedance output. Does the second VCO have two tuning pots ? In which case yes you should be able to do what you are suggesting, providing the fine tune on the second VCO doesn't affect the tuning voltage on the first VCO. Actually a very good idea, surprised it's not more commonly available.
Awesome, thank you! I’m using 3340s so I should be able to adjust the fine tune (which is a second pot) in an independent control. I’ll do a little more research on the designing the mixer. I might want to do multiple voices (that’s going to be a lot of 3340), so it’s be nice to have everything manageable under single controls for certain things.

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Re: Combining 2+ VCOs into a single module

Post by J3RK » Fri Apr 02, 2021 2:23 pm

If you're putting them both on the same PCB, it helps to keep the VCOs as separate both physically, and connectivity-wise as you can. If they're interconnected at many points, aren't decoupled well enough, or are too close together, you can get unwanted syncing between the oscillators. Maybe you know this already, but having designed a few dual VCOs, I thought I'd chime in a bit.

I recommend having a single reference voltage circuit for the board close to the power inlet, then make sure the main rails and the reference rails are decoupled fairly heavily across the board. Run as few of these references as possible, and keep them as separate as possible to each VCO.

If you're going to have an output mixer as mentioned above, keep it central to the board, so it's equally distant from both VCOs.

Keep the IO for each VCO on opposing edges of the board.

This should keep them as discrete from one another as possible to avoid syncing, crosstalk, etc.

I haven't worked with 3340s before, so I'm not sure if they're better or worse in these regards to your average non-IC VCO circuit. Maybe it's less of a big deal with these ICs. You may want to look at a PCB for a poly synth that used/uses 3340s, just to see how they're laid out.

Just a couple of thoughts from someone that loves dual VCOs :lol:
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Re: Combining 2+ VCOs into a single module

Post by devinw1 » Fri Apr 02, 2021 3:03 pm

Definitely listen to J3RK. I did a simple dual VCO as one of my first modules and it worked great, but I had learned some lessons after having 3340s sync up on my first pass my Pro-One clone PCBs. The 3340s are definitely sensitive to all the layout issues described above (as are most VCO circuits).

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Re: Combining 2+ VCOs into a single module

Post by Biff » Fri Apr 02, 2021 3:43 pm

Thanks for the pointers :tu:

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Re: Combining 2+ VCOs into a single module

Post by J3RK » Sat Apr 03, 2021 3:47 pm

Happy to help!

I can post some gerber files if you want to look at some of the layouts that worked for me. They’re big boards, so you’ll have to adapt them on a smaller scale, but maybe just the visual might be helpful.

I
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Re: Combining 2+ VCOs into a single module

Post by Biff » Thu Apr 08, 2021 9:04 am

J3RK wrote:
Sat Apr 03, 2021 3:47 pm
Happy to help!

I can post some gerber files if you want to look at some of the layouts that worked for me. They’re big boards, so you’ll have to adapt them on a smaller scale, but maybe just the visual might be helpful.

I


That would be awesome, thanks!

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Re: Combining 2+ VCOs into a single module

Post by J3RK » Thu Apr 08, 2021 2:50 pm

Here's the Gerbers for the second to last version of the FlexWave. This is the "retail" version. I don't have the latest one with me where I am at the moment. This one doesn't have any issues at all, I just made minor improvements on the newest one.
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FlexWaveVCO-Rev1.2 - CADCAM.ZIP
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Re: Combining 2+ VCOs into a single module

Post by Biff » Thu Apr 08, 2021 5:11 pm

J3RK wrote:
Thu Apr 08, 2021 2:50 pm
Here's the Gerbers for the second to last version of the FlexWave. This is the "retail" version. I don't have the latest one with me where I am at the moment. This one doesn't have any issues at all, I just made minor improvements on the newest one.
Thank you very much! This should be very helpful.

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