Completely control of Eurorack SVF with external CV

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waldenpotato
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Completely control of Eurorack SVF with external CV

Post by waldenpotato » Thu Oct 01, 2020 9:03 pm

I'd like to be able to control the frequency of four SVFs with one knob. I'm curious if this would be possible.

I'm imagining making a module that has one knob - a control voltage that would represent the frequency knob - and four outputs - all outputs being the same control voltage. I would then plug this control module into the CV1 of each of the SVFs. I think the SVFs would then need to have their CV1 knob to their maximums and their frequency knobs to their minimums.

I have two questions:
1) Is it possible to have complete control of the SVF frequency from the CV1 input as described above?
2) What would the output of my control voltage module need to be? 0V to 5V?


Thanks!

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oozitron
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Re: Completely control of Eurorack SVF with external CV

Post by oozitron » Fri Oct 02, 2020 6:38 pm

A voltage source of 5v (or so) going into an attenuator, then into a 1>4 mult (buffered?) should do what you want.

Why is the question that pops into my head :-) Are you going to put them in different modes (LP, HP, BP, etc)? That combined with offsets would certainly make some interesting sounds. You could also feed their CV inputs with four outputs from a quadrature LFO (I have a Mankato filter that will do this when res is max and it self oscillates). Doing this with BP filters is really cool!

Would love to hear what you create...

Andrew
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waldenpotato
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Re: Completely control of Eurorack SVF with external CV

Post by waldenpotato » Fri Oct 02, 2020 10:01 pm

oozitron wrote:
Fri Oct 02, 2020 6:38 pm
Why is the question that pops into my head :-)
I've wanted to build a polyphonic modular synth for over 12 years. This is me returning to diy synth building. 12 years ago I had no resources, so couldn't really do anything. I'm getting back into it as a pandemic hobby. I should mention that I already have the filter built.

I'd like to build the module as I described as a learning exercise - I want to control the frequency of all four filters from one knob so that they all have the same sound, rather than having to move the four frequency knobs to the same setting. It's a pretty basic setup, but that's what I want. I don't want to "modulate" the frequency, I want to mimic all four frequency knobs with one knob.

I just want to clarify if it's possible using the CV inputs and how they should be configured. Which CV inputs would work for this and what voltages do I need to feed them?

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Synthbuilder
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Re: Completely control of Eurorack SVF with external CV

Post by Synthbuilder » Sat Oct 03, 2020 5:42 am

The CV1 or the CV2 input could be used. Which one you chose will be the one you can live without once you have used it for the master frequency control. This then presents itself as a bit of conundrum since ideally any filter module should have at least two CV inputs available for controlling cut off frequency in a standard synth voice - one for key tracking, and one for envelope modulation. Ideally you want a third input, preferably one with a 1V/octave sensitivity, that you could use for the master controller. Sadly, you don't have that on this module, so you'll have to use one of the inputs. To give you more CV inputs an external mixer module could be used to sum those additional CV sources but you'd need one per filter.

Any master CV control should output -5V to +5V with 0V in the middle. Alternatively, a range of 0V to +10V could be used. I prefer the former since it can both add and subtract from module's current setting. Since CV2 on the SVF can be set to 200% (ie. 0.5V/octave) you could get away with a -2.5V to +2.5V (or 0 to +5V) range if using CV2 as your input. The idea is that you want around 10 octaves of controllable frequency.

Make sure that any output from the master controller can tolerate being shorted to other outputs or 0V. Having a 1K resistor in series with each of the four output sockets will suffice in this application.

waldenpotato
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Re: Completely control of Eurorack SVF with external CV

Post by waldenpotato » Sat Oct 03, 2020 1:12 pm

Synthbuilder wrote:
Sat Oct 03, 2020 5:42 am
... ideally any filter module should have at least two CV inputs available for controlling cut off frequency in a standard synth voice - one for key tracking, and one for envelope modulation.
This might be a little off the original topic, but I think maybe I've never quite understood the "key tracking" or "1v/oct tuning" with filter frequency cv inputs. I don't think I've ever plugged my keyboard's 1v/oct output into a filter's cv input. Is this typical? In my mind, without having thought about it greatly, I've always thought of a filter's cv input for 1v/oct tracking was so that you could play your filter like an oscillator. But I'm not sure if that actually makes sense. Is the 1v/oct tracking on a filter's cv input really just to change the filter's frequency (and tone) based on what key you are playing on your keyboard? Or is it just nice to have a filter's frequency track at 1v/oct like an oscillator because 1v/oct is a nice standard? Maybe I just need to remember that a filter's frequency is really just its "cutoff frequency" and not a frequency like an oscillator might think of it.
Synthbuilder wrote:
Sat Oct 03, 2020 5:42 am
To give you more CV inputs an external mixer module could be used to sum those additional CV sources but you'd need one per filter.
Would a simple three-input and one-output summing mixer work for this? Let's say you insert an envelop and a keyboard's 1v/oct into a summing mixer, would the output of that summing mixer really control a filter's cv input the same as if the filter just had two cv inputs in the first place?
Synthbuilder wrote:
Sat Oct 03, 2020 5:42 am
Any master CV control should output -5V to +5V with 0V in the middle. Alternatively, a range of 0V to +10V could be used.
It sounds like my choice between -5v to +5v or 0v to +10v will be determined by which cv input on the filter I want to use.


Thanks for all of the good information.

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Synthbuilder
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Re: Completely control of Eurorack SVF with external CV

Post by Synthbuilder » Sun Oct 04, 2020 2:57 am

groffcole wrote:
Sat Oct 03, 2020 1:12 pm
Is the 1v/oct tracking on a filter's cv input really just to change the filter's frequency (and tone) based on what key you are playing on your keyboard?
It's really about mantaining harmonic structure as you play up and down the keyboard. If filter cut-off does not track the oscillator frequency then the sound would become duller as you play up the keyboard. In other words, the filter is not moving in tandem with the oscillator so higher harmonics get increasingly attenuated as the oscillator frequency rises.

It's why we have filter tracking controls on filters. And quite often it is nice to have less than 100% filter tracking. Indeed, this must have been so apparent that the designers of the Minimoog back in the late 60s they added the 'off, 1/3, 2/3, full' switch combo for the filter tracking. The 2/3 is rather good actually. It keeps the filter bright enough as you play up the keyboard, but it doesn't get too shrill at the top because the filter doesn't open as much for those higher notes.
Would a simple three-input and one-output summing mixer work for this? Let's say you insert an envelop and a keyboard's 1v/oct into a summing mixer, would the output of that summing mixer really control a filter's cv input the same as if the filter just had two cv inputs in the first place?
Yes. Some filter modules even rely on users having to use external mixers for multiple CV inputs. My own Journeyman and DLF have only one variable CV input, and one fixed CV input. The latter can be connected to an external mixer for more CV control.
It sounds like my choice between -5v to +5v or 0v to +10v will be determined by which cv input on the filter I want to use.
The bipolar -5V to +5V is the one to go for. Although you can get negative action by using the CV1 input of the SVF, you'd have to turn each module to the '-' end every time you wanted to do this.

Tony

waldenpotato
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Re: Completely control of Eurorack SVF with external CV

Post by waldenpotato » Sun Oct 04, 2020 11:03 am

Awesome. Thank you. I’ll have to experiment with this and let you know how it goes.

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