"Variable Slope Filter / Phaser "
- Powerfull 36dB/Oct LPF (6-pole Filter)
- Variable Filter Slope (voltage controlled)
- Phaser with equal poles, or spread poles (voltage controlled)
- "Everything between filter and phaser" (combination of APF and LPF stages)
- Unique on-board LFO with volzage controlled waveform-crossfading
- Audio-Rate and Random Sample & Hold Modulation
- Standalone FX (with 16V AC wallwart) or +/-15V Synthesizer Module
- No need to implement everything - omit half the components and still have the 36dB/Oct LPF ...
back to JH.
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
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
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
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.)
Filter / Phaser part
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
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
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
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,
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
Copyright J. Haible (C) 2009