Rational VCA

Anyone have some insight into this unit? My search-fu skills revealed nothing.

It has controls for “num” and “div”. Those aren’t controls I was expecting to find in a VCA, and I can’t quite work out what they do.

It allows you to multiply the input by a specific fraction e.g. send an offset =1 into the rational VCA and set the num = 1 and the div = 2 and the output will be 0.5.


Brilliant. Thanks!

You know, this had nothing to do with me asking this question. But thinking about this topic:

If I’m thinking about it right, this unit could actually provide about 30dB of gain? Plus of course you could stage a couple of them in series. I wonder if that would work for bringing an instrument level signal into the 301 without any additional hardware…

I’m not sure… it’s a great question and it would be good to have a definitive answer :slight_smile:

I may try it this evening if I can track down the right adapter amongst my mess o’ stuff…

Hehe yeah!! I have a box - it’s the box of cable doom but there’s always something that’ll do the job in there!

Well worth investing in some 1/8 to 1/4 leads - I have about 20 of them and use them all the time - they’re very useful!


Ha. I have a box like that too. Except the box is my entire studio room. :slight_smile:

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I tried this and it didn’t work. Guessing that the input impedance on the 301 ins are not set up for “Hi Z” inputs.

The second rational VCA did amplify the noise floor quite well. :slight_smile:

Rational numbers pop up in music a lot. Just intonation and rhythm for example. If you have a linear FM CV and you want to adjust by a rational factor then use the Rational VCA. If you have a time CV and you want to adjust it by a given time signature then use the Rational VCA.


Scratch this. It DOES work. Turns out the guitar I used (one I haven’t played in a while) needs a little service on its output jack. A single rational VCA with a ratio of 18:1 amplified the signal nicely up to modular levels.

So basically, no additional hardware needed to bring an instrument or line level signal into the ER-301.


Amazing - thanks @Joe

Great! Thanks for this description, I just realised I never actually used the rational vca

I still don’t understand why you would use this instead of a regualr vca, can you explain this more dumbed-down? so for linear fm, is a rational vca better used for preserving fm harmonic ratios? i think i get that. but what do you mean by a time cv adjusted by a time signature? and what other ways would you use this kind of vca?

The Rational VCA now makes a lot of sense to me and I think it’s one of the more useful units! Yes, setting up frequencies that are related is a good use case.

Here’s another use case I could think of off the top of my head.

You have a clock signal, and you’re feeding it into a Period-o-Meter unit. You’re using its voltage to set the delay time on a variable delay. You could put a rational VCA in the chain that’s controlling the delay time, and then you’d have voltage control over a tempo-synced delay time. So you could set it to 1/4 note, 1/8 note, and you control the division with another modulator.


my bad!

Just to be clear, I don’t think it will work as a clock divider or multiplier - those are on the feature request list. The rational VCA only changes the amplitude of a signal. In the example I was giving, it is multiplying/dividing a DC voltage coming out of the period-o-meter. In this case it would accomplish something similar.


Here’s a custom unit that does this, for illustration.

  1. On an empty chain, insert a custom unit
  2. Load this preset into the custom unit.
  3. You should hear a triangle with random pitch driven by a steady gate with a delay
  4. Drill down into the custom unit and find the rational VCA which is connected to the variable delay’s delay time
  5. Turn the divisor up to some different number, like 3,4, 8, etc. (num=2 and div=3 kind of spooks me, for some reason. :smiley_cat:)

Tempo Sync Delay.zip (2.1 KB)