STORY TIME // Dwyfor Tech origins, “forgettable” Buchla circuits & Korg MS-50 love

For the launch of the Eurorack module Pas-Isel I had a conversation with Dan Phillips the man behind Dwyfor Tech about getting into circuit design, merging Buchla & Korg circuits, making his first module and why he even bothers! Grab a drink and prepare for some story time.

Ben // So tell us how did Pas-Isel get started?

Dan // Pas-Isel first came about when I read an article on the Buchla 191 by the fantastic synth designer Osamu Hoshuyama: [LINK]

He ends it by saying it’s a fairly ignorable module as it requires a load of fiddly trimpots to work right and doesn’t have any resonance. But… In the corner of the illustration, there’s an equivalent schematic centered around an op-amp. I wondered if anyone had ever attempted that? The short answer was… No.

I designed the core of the filter around that equivalent schematic, adding a completely boring feedback loop for proof of concept, and using standard op-amps and small signal diodes for the voltage control element (the original uses transistors configured as diodes – Probably for matching purposes? But not necessary in this day and age!). However, it does have one major drawback – It really really likes to tend towards increased resonance amplitude at high cutoff. So the res gets more and more as cutoff goes up.

That’s when I decided to draw a little inspiration from another filter I truly love, the Korg MS-50: [SCHEMATIC LINK]

Korg MS-50 Schematic from HERE.

Ben // Why the Korg MS-50, over the better known MS-20 or MS-10?

Dan // The MS-50 is a very different beast to the MS-10 or MS-20; it’s actually based around a diode ring. Just like the Buchla, it uses a bipolar CV to get a controllable resistance out of diodes, so I wholesale took the CV generation circuitry from it… But the resonance feedback circuit was also just what I needed. The diodes in there act as a soft-clipping circuit, like in a guitar overdrive pedal, and keep the resonance at a totally constant level regardless of cutoff!

So after a little more tweaking, making a few physical prototypes and asking friends to try them out, making a few modifications based on their feedback… Pas-Isel was born.

A close up of the breadboard for the Pas-Isel filter core.

Ben // In a previous conversation you told me this low pass was actually made up from high pass filters, can you share that with us here?

Dan // Each filter pole is actually a diode-controlled highpass filter, very closely resembling the Moog Horizontal Highpass that’s in the same patent as the iconic Transistor Ladder Lowpass. Each highpass, though, is in the feedback loop of an op-amp – This means that it’s cancelling out the high frequencies of the original signal, and giving an overall lowpass response.

It’s not technically a lowpass response either, in all honesty – It’s a high shelf filter. But after four filter stages, the shelf is about -120dB, so completely undetectable. So for all intents and purposes, it’s a lowpass.

Another fun fact about it, the output of Pas-Isel is inverted compared to the input. This is a deliberate design choice – If you run the output through a VCA and blend it with the input, you can use that VCA for a voltage-controlled resonance patch (assuming nothing else in that signal chain inverts the signal anywhere!).

Pas-Isel main board schematic, find these HERE.

Dan // I suppose the question has to be asked, why I went to the trouble?

Ben // Indeed! So tell us why you’ve got into making Pas-Isel and starting Dwyfor Tech.

Dan // VCFs are my absolute favourite part of any given synth – For me, that’s where the ‘soul’ of the sound sits. That’s what makes one vintage synth sound totally different to another.

But… I got a bit frustrated that wherever you look, there’s a handful of tried and tested ‘classic’ designs that get endlessly recycled. Don’t get me wrong, they’re great… but there’s nothing that’s doing the classic VCF in a new way from the ground up. It’s always a Moog ladder/diode ladder/CEM/Roland OTA in a new mask.

So yeah, that was my ‘mission statement’ going into this, as it were. Do something different to everyone else.

Pas-Isel in Dwyfor Techs Eurorack set up.

Ben // Do you know if other low pass filters, made up out of high pass filters exist? Seems like a rare thing for Pas-Isel.

Dan // Lowpass filters based on inverted highpass filters like this are vanishingly rare in vintage synths. Other than the Buchla, the only one I could find is the Hammond 102200, a Moog Satellite-style preset synth for sticking on top of your organ. They took the approach of sticking an entire four-pole highpass in a single op-amp feedback loop (so you only need a dual op-amp for the entire filter!!), and as a result… it’s another pretty weak offering. Fixed resonance level, high frequency leakage, not worth cloning. In Eurorack, from what I understand there’s the Verbos Dual Four Pole which is more or less a 191 clone with some kind of rudimentary non-self-oscillating res circuit, and LA67’s faithful 191 clone, and… That’s probably it.

Ben // To close our interview I’ll go with a typical final queston, what can we expect from Dwyfor Tech in the future?

Dan // I do want to have a trio – VCO, VCF, VCA. My next module is already in development, Cryfder, a dual VCA based on new-old-stock, out-of-production chips with VFD tube overdrive/mixer. As for the VCO, I do have ideas I’m exploring.

Ben // Thanks Dan for chatting to us, it’s a pleasure to share your story alongside making my demo video.

Find Pas-Isel as a DIY kit (you can request assembled too) on Reverb here // https://reverb.com/shop/dwyfortech

If you haven’t yet, give my video on Pas-Isel a watch HERE. It’s got bags of characters and is well laid out for performative play.

If you’ve enjoyed this little ‘story time’ interview let me know, should I be doing more of these in the future?