Low Pass Gate
This module is a variation on the Classic Buchla Low Pass Gate module
developed by Peter Grenader and released here as his gift to the Serge DIY
From Electronic Music Wiki:
A lowpass gate is essentially a low pass VCF configured to
behave like a VCA. The lowpass gate accepts an audio input and a control
signal, in the manner of a VCA. When there is no control signal present,
the filter's cutoff frequency is in the subsonic range, well below the
audio frequencies; therefore, no audio passes the filter. Applying a
control voltage causes the cutoff frequency to rise significantly, into
the upper end of the audio range, so that most of the audio at the input
now passes. If the output of an envelope generator is presented to the
control input, the lowpass gate will shape the note envelope as a VCA
would, but with some characteristic differences.
The concept of the lowpass gate originated with the Buchla 200 series
modular synthesizer series, which offered a lowpass gate as a module in
the series. The Buchla design used a vactrol to process the control
voltage input; the vactrol had certain non-linearities which made the
lowpass gate a good module for imitating some tuned percussion sounds.
By feeding in shor
t bursts of noise, or just impulse spikes, the lowpass
gate could be made to produce sounds that resembled hand drums, congas,
steel drums, or marimba. The original Buchla module is highly sought
after and has inspired a number of imitators over the years.
From Peter Grenader:
By design, the Low Pass Gate is a Sallen Key FIlter. A
Sallen Key is a two pole filter topology. It is sometimes called VCVS,
or Voltage Controlled Voltage Source. The Sallen Key topology is often
seen in a unity gain filter, although the addition of gain resistors is
an obvious enhancement. The designer is cautioned, however, that
changing the gain also changes other filter characteristics. Like the
other single opamp topologies, changing gain will also affect frequency
and filter type (Butterworth, Chebyshev, and Bessel).
The Sallen Key topology is suited to operation from a single supply.
Current feedback amplifiers can be used, with the restriction that the
connection from the op amp output to inverting input must be the
recommended feedback resistor value. This is true whether or not this
resistor is part of a gain stage. The Sallen Key topology cannot be used
with fully differential amplifiers.
Variable center frequency is obtained by changing the value of two
resistors simultaneously - R1 and R2 iin standard Sallen Key filter
circuit applications. A quick look at the configuration of the VTL5C3/2
vactrol gives a clear analog to this and why it is ideally suited for
Adding the this, the natural tendencies for the VTL5C3/2 Vactrol to
attenuate high frequency spectrum information unequally to low
frequencies manifests the sought after 'ringing' associated this
specific LPG circuit while set in the 'BOTH"mode.
Each LPG here has a 470K Ohm resistor, which was added by me over the
original Buchla circuit as part of a feedback loop added which
compensates the the 3/2 vactrols only going down to 100 ohms and
attenuates the audio bleed associated with that device. You can
experiment with values anywhere from 1 meg to 0 ohm (link). However -
the lesser the value, the greater the bleed and the greater the ringing.
This is the rub - the more you attenuate the bleed - this can only be
heard audibly if you use the LPG in passages where the LPG processed
event is the only sound happening - the more you step on the ringing. A
suggestion: In later model Plan B Model 13 Timbral Gates, I would fit one
gate with a 470K ohm and omit it the other altogether, yielding a dual
personality dual TG.
Please note that these boards can be broken into two piece - two
separate PCBS one gate per - suited for non Serge system application
where the six inch height my be problematic. When doing this, use a pair
of cutters (dikes) to separate the two by cutting the four connecting
bridges in the center of the board. Attempting to break the boards with
force by itself may not give you the result you want and is not
recommended. f you elect to break these in two, the three 0 ohm
resistors shown in the component diagram are omitted and each board must
be fitted with a four position power connector and the two flanking 10uf
The Low Pass Gate has been specifically designed to process AC
signals within the audio bandwidth only. There are two capacitors that
govern this: a .047uf capacitor leading to the signal input of the LPG
and a single 10uf electrolytic leading to the the vactrol's resistive
element (not to be confused with the two 10uf's that flank the power
connector). Replacing those two components with a jumper wire or zero
ohm resistor will allow for sub-audio AC or DC voltage gating but be
aware that varying the amplitude of these types of signals via the
control input will result in slewing - a portemento effect. The ringing
of the the vactrol will also contribute to these types of artifacts. For
this reason, this modification may not be particularity useful for music
applications outside of providing a DC offset/level with sloppy
Finally, the campaign advertisement which appears on this board for
Mssr. Papoon was a standing joke between Gary Chang and Kevin Braheny
Fortune while they wre both employed at the Serge plant on Santa Monica
Blvd in Hollywood, before the company moved to Haight St. in San
Fransisco. Legend has it Kevin put a sticker with the same message on
Gary's Synth panel while it was being built and it remains in place
there to this day, the only graphic on Gary's otherwise infamous
graphic-less eight panel instrument.
Enjoy this board! It was my second built project after the Milton
Sequencer and a big reason why I began producing modules commercially.
For me the sound associated with the LPG is essential in the creation of
electronic music via an analog synthesizer!
I extend this board to the synth community free of profit to me. It
was my intention to do this as a gesture of heartfelt thanks for the
support I've received in recent months from the community since taking
care of the debt associated with Plan B's internal problems in the early
part of 2009. I only hope that this seemingly inconsequential diligence
on my part serves to parallel your generosities. Thank you again and
Papoon for President!
Before you start assembly, check the board for etching faults. Look for
any shorts between tracks, or open circuits due to over etching. Take this
opportunity to sand the edges of the board if needed, removing any
splinters or rough edges.
When you are happy with the printed circuit board, construction can
proceed as normal, starting with low profile components such as resistors
and diodes first, followed by successively taller components.
Take particular care with the orientation of the polarized components,
such as ICs, electrolytics, diodes, and transistors.
When inserting the ICs in their sockets, if used, take care not to
accidentally bend any of the pins under the chip. Also, make sure the
notch on the chip is aligned with the notch marked on the PCB overlay.
The trim pots control how much gain there is in the CV system. Adust
them so you get adequate sweep with your system's CV levels.
Jumper/shunt J1 (present on both gates) affects the ringing and can be installed or
omitted per the builder's personal taste.
The board can be split down the center to make two independent PCBs. If
using them joined, three links are needed to link the two sides. The power
connector can be omitted from one side.
The mode switch selects between VCA mode and Low Pass Gate mode.
|Example wiring for the Low Pass Gate. The wiring for both
parts is identical. |
Each unit has a trimmer that controls how much gain there is in the CV
system. Adust them so you get adequate sweep with your system's CV levels.
- The module will work on +/-12 volts or +/-15 volts.
- PCB info: 6" x 2" with 3mm
mounting holes 0.15" in from the edges.
- Vactrols are currently available through Thonk UK
Article, art & design copyright 2011 by Ken