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Voltage Controlled 4-Stage All Pass Filter


My own take on the classic audio effect. Again, using the SSM2164 quad VCA to provide four stages of phase shift. This module is very much designed as a building block, with the input buffer, the filter, and the output mixer treated as separate blocks, with only minimal connections between them. Ideal if you want to chain multiple VCAPFs together to build a monster multi-channel many-stage phaser.

It turns out that this circuit toplogy had previously been proposed by Hoshuyama in 2004. In that circuit Hoshuyama proposes a 2-pole circuit comprising a combined feedforward/feedback combination, whereas the idea presented here comprises a series of 1-pole VCAPF sections cascaded.

One thing to note is the relationship between poles (or stages) and notches. For n poles there will be n/2 notches, so a 4-pole APF will produce 2 notches.

It can also be used as a standalone phaser with the addition of a couple of 10k pots. The audio samples below are all produced by a single VCAPF in this way.

This module is ideal for controlling with my VCLFO, providing slowly sweeping phaser effects across two or three audio channels.

  • 100k input impedance buffer amplifier
  • on-board jumper for direct connection

  • four stage phase-shifting all-pass filter
  • from 0° up to around 680° phase shift
  • CV controllable, with adjustable sensitivity and bias
  • two signal inputs - source signal and feedback

  • output mixer, combines output of all-pass filter with dry unprocessed signal
  • wet signal tapped off for feedback ("emphasis") pot

  • short-circuit protected output buffers
  • input levels up to ±5V


Sound Samples (MP3)

Here are some samples made with the VCAPF acting as a mono 4-stage phaser controlled by my VCLFO. No credit is claimed for artistic merit or performance abilities! File sizes are approximate. Note also that these recordings have been through a lot to get here - recorded into a minidisc recorder (ATRAC compression) then turned into an MP3 (yet more compression). All were recorded with maximum effect and about 20% feedback.


Documentation

For interested parties the following documentation is provided.
Please note that all these documents are Copyright © 2009 Neil Johnson.

Notes

  • The signal connections are marked on the PCB as follows:
    • BUF/I and BUF/O are for the input buffer's input and output respectively.
    • IN is the main input, FB/I is the feedback input.
    • DRY/I is the dry input to the output mixer.
    • FB/O is the tapped output of the last filter stage suitable for feeding back to FB/I.
    • MIX/O is the main mixed output.
    • CV is the control voltage input.
  • The white area near the trimmers is for you to mark the board with some identifier (I usually write the date of completion).
  • I have specified polypropylene capacitors for the phase shifters. You can also use polystyrene capacitors if you have them available.
  • Components marked 'DNF' are Do Not Fit - I put them in for experimentation (I find it easier to not fit a component rather than to bodge it on later if I change my mind).
  • Yes, IC6 is missing. I think it fell down the back of the sofa...


Completed Module

There are at least two build variations of this module. Shown here are the two trim pot variations - single turn trimmer on the left, 10-turn trimmer on the right

   


Availability

The only special component is the SSM2164P (DIL package version). This is available both from Magic Smoke (they sell the CoolAudio clone V2164), Farnell and Digikey.


Copyright © 2001-2017 Neil Johnson