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Weston Precision Audio - SF1 Dual/Stereo Filter

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The Weston Precision Audio SF1 Dual/Stereo Filter is a dual mono or stereo linked VCF with additional features that Eurorack sound designers and going to enjoy.  I am a frequent user and abuser of WPA modules including the B2 Kick Drum/Bass, AD110 Analog Drums and PA0 Phase Animated VCO.   

Having owned the B2 for a while, and delighting in the VCF it hosts, I asked Devin if he had considered making a dedicated VCF module.  It turned out that he had, and a longer conversation began.  I was fortunate to have input into the front plate layout as well as link operations during the design stage.   Beyond that, I know nearly nothing useful about electronics and could only nod approvingly as it all came together.  I'm an ideas guy, not a designer...  Fortunately Devin is an excellent designer.  

Full disclaimer: Devin sent me a prototype of the SF1 in June 2022 and I am writing about a module which I was sent for free for the purpose of performing testing and providing feedback.  Of course, Devin has no say in what I write and I have no obligation to WPA.  Having said that, I have the module and he is NEVER getting it back.  

THE BASICS

  • 18hp width.  Pre built only (no DIY)
  • Cutoff and Resonance controls (dual)
  • Cutoff and Resonance modulation inputs (dual)
  • Cutoff attenuation (dual)
  • 1V/Oct control (dual)
  • Link mode (button)
  • Level control with unity at 12oc and saturation further to the right (dual)
  • Stereo Panning (three modes)
  • Pan CV input
  • Dual audio outputs for each VCF
    • Output 1 (for both VCFs) allows a 12 or 24db slopes with low pass, high pass and band pass filtering
    • Output 2 (for both VCFs) allows notch, all pass or phase filtering
    • Output 2 is always 12db and not switchable to 24db

Filter Modes 

As described above, each side of the filter provides dual outputs (L1, L2 and R1, R2). 

  • L1 and R1 outputs: 12db (2 pole) or 24db (4 pole) slope, lowpass, high pass and bandpass multimode filtering. 
  • L2 and R2 outputs: 12db slope, notch, all pass and phased outputs. 

STEREO CONTROL

The SF1 provides three modes of stereo panning: 

  • L/R (all the way counter clockwise) - left input to left output, right input to right output 
  • R/L (all the way clockwise) - inverted: left input to right output, right input to left output 
  • Everything In Between - weights the output in the stereo field based upon the placement of the knob and panning CV

Link Mode

When in Link mode the following controls (on the left side of the module) will control both VCFs:

  • Cutoff and Resonance knobs
  • Cutoff and Resonance modulation inputs
  • Cutoff attenuation
  • 1V/Oct input

Unlinked Controls in Stereo

When in linked mode, the following elements are controlled uniquely on both VCFs.

  • Input Levels
  • Slope
  • Filter Mode

Decoupling the features listed directly above means you have control over each side of the filter in meaningful ways.  

    Panning

    The Pan CV input allows modulation of stereo field placement and can be attenuated to the left or right using the Pan CV knob.  In addition to subtle stereo panning, my testing showed that the SF1 can modulate stereo field placement very quickly using a square wave, which is not something all stereo panning devices can do.  You could use the SF1 as a panning utility very easily and not even engage the VCF features.  

    IN USE

    The layout of the SF1 is very intuitive with the Cutoff and Release orders swapped from left to right so that cutoff is the outer knob for both sides.  When Devin and I were talking about the faceplate design I was concerned that the PAN CV jack input might be an issue, but in practice it works really well.  Everything is spaced out and easy to access.

    Cutoff has an attenuation input and knob for easy control of the most commonly modulated feature in a filter.  Resonance has a CV input but no attenuation knob, so if you want attenuate resonance CV you will need to do that prior to hitting the SF1.  

    Here are some ways that I have used the SF1 in my rack to date.  There are plenty of other things you can do with a Stereo VCF, but here is what I have done so far.  

    • Single VCO input (SF1 in stereo link mode) to stereo output with or without the panning function enabled
    • Dual VCO (Instruo CS-L) in dual mono mode as two discrete voices
    • Dual VCO (Instruo CS-L) in stereo mode as a single stereo voice routed to my stereo effects modules
    • Rossum Assimil8or in stereo mode to add some really fun filtering to drum parts
    • Send and Return target from my 4MS Dual Looping Delay providing surgical filtering of delay feedback loops

    THE SOUND

    So how does it sound?  This is the part where I try to use subjectively coloured words to describe my feelings about the sound the SF1 delivers in my room, through my monitors (Focal Shape 65s), routing my modules (so, so many, now) from my Eurorack into my ears.   

    To get started, lets go with "the SF1 is a high fidelity module that offers incredibly clean filtering in any of its modes."  If I were creating a crap marketing piece for idiots I would write "This isn't your Granpa's filter..." or something terrible like that.  Maybe I would reference the difference between the James Webb Space telescope and the Hubble as a metaphor for the clarity and accuracy of the SF1 vs some other filters. 

    But as superior human beings, we know that nothing in sound production (or art) of any kind is that simple.  Sometimes you want the Hubble because it stretches out the star light in a less accurate but pleasing way. 
    Other times you want the enhanced clarity of the James Webb so you are seeing as closely as possible what is (or was, billions of years ago) actually there.  I used a metaphor which makes no sense whatsoever but also makes complete sense.  I think.  

    The SF1 is definitely closer to the James Webb.  I would recommend the SF1 to anyone who wants a dual mono or stereo Eurorack filter that provides a lot of fun sound design options, and a high level of clarity.  In low pass mode, the cutoff knob is super smooth from full signal moving back to no signal (or whichever mode you choose).  The 1V/Oct inputs for resonance control are super accurate and you can get incredibly fun and musically useful sounds with the input level pots zeroed.  

    Unexpected Saturation  

    A standout feature is the gentle increase of saturation that occurs in the SF1 when you dial the input levels past 12oc.  When Devin mentioned that he was going to include saturation I was not very excited because (to my ear) a lot of saturation sounds in Eurorack are pretty terrible.  I find they often get too angry too fast, and are too clipped to be useful and end up souding nasal. 

    The saturation on the SF1, however, is slow to rise and is very controllable.  It thickens up nicely and makes me smile.  Dial the levels past 12oc and you get more excitement, but not a degradation of your original input.  Perfect.  I would definitely look forward to more sound manipulation modules by WPA.  

    Conclusion

    Pros

    • A great sound design module with a lot to offer.  Multiple features in one module
    • Dual mono, linked stereo, stereo panning, multimode filters, 2 pole and 4 pole, saturation etc.
    • Super clean and precise sound.  Resonance is lovely and very controllable
    • Saturation was a very nice surprise that I actually like a lot - I usually don't
    • Excellent build quality all around.  The SF-1 feels solid and high end to touch as do all WPA modules

    Cons

    The only thing I would say is that if you want an old style filter sound then you should look elsewhere.  The SF1 is very modern and clean, and that is what helps it run so well in stereo where you really need both sides to match in performance more closely than you would expect 'vintage' designs to operate.  Luckily, you are a Eurorack person and you can indeed choose both.

    More information - Weston Precision Audio

     

    09/12/2022