"Variable Slope Filter / Phaser "


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A mean little module from my JH-3 Modular.

6 filter poles, realzed with OTAs and combined FET / Bipolar buffer stages.

Variable Slope function for 6-pole VC LPF.
Voltage controlled Pole Spread for Phaser mode.
Zeroes switcahbel in pairs, i.e. a lot of combinations of all pass and low pass stages. Positive and negative feedback. Positive and negative wet/dry mix. Rather special overdrive behaviour (Dirty!), and (IMO) pleasant way of breaking into oscillation.

Audio Sample 1 (Arpeggio from OB-8, with delay and reverb. First few seconds dry - then playing with phaser.)

Audio Sample 2 (This is something I recorded a few years ago. I don't remember all the details. But it was a Split on the OB-8, bass sound on left hand and organ sound on right hand, and the whole thing was processed with the Variable Slope Phaser / Filter to get that "pulsating" sound. Lots of delay and reverb as well, obviously.)

Audio Sample 3 (First few seconds: Unprocessed input signal. Then all kinds of modulation, mostly sample and hold and self-modulation, high resonance. Towards the end of the clip, input signal is removed, and phaser runs on the edge of self-oscillation. Don't turn it up too loud - danger for your ears and speakers! Everything is modulated with the on-board LFO only!)

Audio Sample 4 (Living VCOs thru Varislope Filter: Everything from Fat and Smooth to Harsh. Fat because of 36dB/Oct Filter Slope! Only using the on-board Modulation LFO and S&H, plus an external envelope generator.)

Audio Sample 5 (Mostly Phasing. Input signal is a ARP-Omni-like sound from the JH-3200. First Dry. Then starting with rather ordinary phasing, and on to stranger things.)


The Variable Slope Filter / Phaser will get a very special on-board LFO that is derived from my JH-4 synthesizer.
Voltage controlled Rate and Depth, of course.

But there is is more:
You will be able to crossfade between waveforms with a potentiometer, or with another control voltage.
Starting from ccw end, the waveforms will be:
Triangle, fading into Sine, fading into Square, fading into Sample&Hold, fading into audio rate self-modulation.

Let me explain that choice:
Triangle fading into Sine means rounding the edges.
Square fading into S&H means rhythmic modulation with more or less randmness thrown in.
S&H fading into audio self modulation means mixing and fading two different flavors of madness. Especially with high resonance.

Video of LFO Waveform Morphing


 Variable Filter Slope, Low Pass Filter Mode:



Variable Pole Spread in Phaser Mode:



3 Stages in Alll Pass Mode, 3 Stages in LPF Mode:



Building a Prototype

Well, the first prototype was part of my JH-3 Modular, of course.
But for testing the PCB layout and fine-tuning the circuit for which I intent to offer PCBs, I wanted to build a standalone effect. For this, I'm using the same tabletop enclosure that I've used for my Wasp Filter Clone some years ago. (Pultgehäuse from Conrad Electronic, Best. Nr. 520772 - Manufacturer is APW)
The focus is on the on-board LFO modulation, so you won't find any potentiometers for external CVs on this front panel. (The circuit allows a lot of external CVs, of course.)

Here's a screenshot of the panel in Frontplattendesigner (Schaeffer):



Input Level - Audio signal input level control
Feedback - Resonance Loop. Can burst into self-oscillation. Inverted or non-inverted feedback for unusual effects.
Mix - Wet / Dry mix of filtered and unfiltered signal. Inverted or non-inverted mix. Mandatory for phasing; interesting for LPF mode because of phase cancellation effects.
Ouput Level - Output attenuator for audio signal
APF / LPF - Switches to change the function of the 6 filter stages from All Pass Filter (Pole/Zero pairs) to Low Pass Filter (Poles only). 3 Switches for 3 pairs. (6 individual switches would be possible as well.)
Mix / Filter - In "Filter" position the "Mix" potentiometer is disconnected for 100% filter signal without any crosstalk from the dry signal (important for powefull 36dB/Oct LPF effect).
Clean / Dirty - Dirty applies some dynamic load to the single-ended buffer of the last filter stage.
Active / Bypass - True hard bypass
Slope - Spreads the pole frequencies of the individual stages. This means less filter steepness in LPF mode (ccw end is 36dB/Oct), and wider notch spacing in Phaser (APF) mode.
LFO Amount - Modulation amount of on-board LFO
LFO Wave - Crossfade between LFO waveshapes: Tringle -> Sine -> Square -> Sample&Hold -> Audio-Rate Self-Modulation (see video).

Also possible, but not on Front panel:
Slope Control Voltage Input
LFO Amount Control Voltage Input
LFO Waveshape Control Voltage Input
LFO Rate Control Voltage Input
External Filter or Phase Control Voltage Input (for ADSR etc.)
External 1V/Oct Control Voltage Input (no tempco, though.)



Schematics

Filter / Phaser part
LFO part

Component Overlay

Component Values  (as silkscreened onto the PCB)
Unmarked Transistors = BC560C or BC560B
Transistors marked "245" = BF245A
Small Diodes = 1N4148
Big Diodes = 1N4002
Bridge Rectifier is B80C1500 or similar

Using the On-Board Power Supply

MOTM and Synthesizers.com and general +/-15V supply version

Reference Designators
Component overlay for 36dB/Oct LPF (variable Slope) only
(LFO section omitted, no electronic switches, no on-board PSU)


Bill of Materials

Full Filter/Phaser Version with on-board modulation BOM

 NEW: Variable Slope Filter version (no modulation, no phaser) BOM


NEW: Adjustment

1. Measure the voltage at the lead of R82, on the side where a tiny arrow and the number "3" is printed on the PCB, against GND. Turn R84 (note the number "3" near that trimpot!), until you read approx. -7.5V. The "Cutoff" potentiometer should be at ccw position for this test, or not yet connected.

2. Measure the voltage at R71 (where the arrow and the number "1" is printed), and adjust it to -7.5V, with R71. (note the number "1" near that trimpot!)

3. Same procedure, number "2": Measure at R76, adjust for -7.5V with R78.

4. Same procedure, number "4" - R88, R90.

5. Same for "5", R94, R96.

6. Same for "6", R100, R102.

7. Play with Cuotoff potentiometer and see if the manual control range is the way you like it. If not, play with R84 (number "3") until you're satisfied. Do *not* readjust the other trimmers from previous steps, though!

8. Adjust R6 (Feedb_Adj) for the self-oscillation to set in at the point of the Resonance potentiometr's travel that is convenient for you. *Attention*: The point of self-oscillation will change dramatically when you change the filter configuration (either with Slope potentiometer, or with LPF / Phaser switching). Protect your ears until you know what can happen - the effect can be quite dramatic.

9. Adjust R137 (Noise) to get a similar level in Sample & Hold mode, as with the other modulation waveforms.

 Final Remark: None of these adjustments is overly critical. You may even get good results without the trimmers "1", "2", "4", "5" and "6".



- will be continued -
For more information, please contact
Juergen Haible


Copyright J. Haible (C) 2009