Some days ago I posted a question in the VCV Rack group:
Is there a filter that I haven't modelled and I'm overlooking?
I asked this because in my research regarding analog filters, I have seen that many filters follow the same basic architecture. For example, after the famous Moog ladder filter, there are variations using diodes instead of transistors, using less stages, using OTAs, using VCAs, using Blackmer cells, etc. One of the reasons there are so many variations of the same filter is because Moog patented the circuit. In order to circumvent the patent other manufactures took the same operating principle and made their own versions.
A four pole ladder filter tends to sound the same to other ladder filters. However changing the nonlinear components adds variation to the sound. Many people does not realise that many filters are basically the same. We tend to associate filters to brands: Moog, Roland, Korg to mention a few. Moog and many Roland synthesizers had, in principle, the same kind of filter.
I received a lot of answers to my post. Most of them were suggestions about which brand I should model. Other suggestions were in fact filters that I have overlooked, mainly because by looking at the diagram, the filters look familiar. I don't have all that gear that was mentioned, and I haven't even been close to those synthesizers. All the work I do is based on the circuits that I can build.
Some of the suggestions that I found interesting from the modelling perspective were:
MS-10 filter (I have modelled the MS-20 only)
Oberheim version of the State Variable Filter
Roland IR3109
CEM3394 and CEM 3320
VCS-3
ARP 4072
Mutable Instruments Ripples
Roland Jupiter Filter
Serge VCQF
Elka Syntex
Some of the filters that I have covered (which means making models based on similar circuits) are:
MS-20
Roland/Moog ladder
Polivoks
Steiner-Parker
Wasp
Buchla
State Variable
After checking filters suggested I started seeing the patterns; ladders, state-variable, Sallen-Key made with the different core components that I mentioned above.
Testing them all would take me quite a while. Instead of doing so, I decided a different approach. One of the things that has taught me the most is trying to do projects under constraints. In the case of the electronics, when I was a teen living in Mexico, it was hard to get electronic components in my town. So I had to do circuits based on the components I could get. If you make music, other artistic activity or engineering, you know that constraints are good to boost creativity. My approach was to create a filter from scratch using the components I had in my boxes.
I have a some of the common components used in DIY synthesizers. I also have a library of models that I have been developing for many years. With those two things I started doing simulations at the same time of creating a circuit. I used the simulations to define parameters, then I corroborated the result with the circuit.
This is what I have so far. I have designed from scratch a ladder filter. This ladder filter has features that I like, for example, a bit of warm distortion, gain compensation and lots of reconfiguration.
I think it sounds good. Still sounds like a ladder but it has its own personality compared to the other filters I have modelled.
If everything goes as planned, maybe in a future we will have a new analog (and digital) filter called: VOOG.