Intellijel Passive LPG 1U User manual

Passive LPG 1U
Passive Low Pass Gate
Manual (English)
Revision: 2021.07.27

TABLE OF CONTENTS 
COMPLIANCE 2
INSTALLATION 3
OVERVIEW 4
FRONT PANEL 5
TECHNICAL SPECIFICATIONS 6
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Passive LPG 1U Manual 1

COMPLIANCE 
Passive LPG 1U Manual 2
This device complies with Part 15 of the FCC Rules. Operation is subject
to the following two conditions: (1) this device may not cause harmful
interference, and (2) this device must accept any interference received,
including interference that may cause undesired operation.
Changes or modifications not expressly approved by Intellijel Designs,
Inc. could void the user’s authority to operate the equipment.
Any digital equipment has been tested and found to comply with the
limits for a Class A digital device, pursuant to part 15 of the FCC Rules.
These limits are designed to provide reasonable protection against
harmful interference when the equipment is operated in a commercial
environment. This equipment generates, uses, and can radiate radio
frequency energy and, if not installed and used in accordance with the
instruction manual, may cause harmful interference to radio
communications.
This device meets the requirements of the following standards and
directives:
EMC: 2014/30/EU
EN55032:2015 ; EN55103-2:2009 (EN55024) ; EN61000-3-2 ;
EN61000-3-3
Low Voltage: 2014/35/EU
EN 60065:2002+A1:2006+A11:2008+A2:2010+A12:2011
RoHS2: 2011/65/EU
WEEE: 2012/19/EU

INSTALLATION 
This module is designed for use within
an Intellijel-standard 1U row, such as
contained within the Intellijel 4U and 7U
Eurorack cases. Intellijel’s 1U
specification is derived from the
Eurorack mechanical specification set
by Doepfer that is designed to support
the use of lipped rails within industry
standard rack heights.
Because this is a passive module, it
requires no power to operate.
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Passive LPG 1U Manual 3

OVERVIEW 
In general, low pass gates (LPGs) control the amplitude and timbre of a signal by opening and
closing a low pass filter. When the gate is closed, no frequencies are allowed to pass through (and
no output is heard). As the gate begins to open, the lowest frequencies pass through to the output.
As the gate opens further, the output grows louder and its high frequency content increases.
Traditional Low Pass Gates use vactrols to control the opening and closing of the filter. Encased
within a vactrol is both a light source and a light detector. Applying a control voltage to the vactrol
lights its internal LED — the brightness of which is measured by its detector. What makes vactrols
particularly unique (and well-suited to musical applications) is their non-linear response to voltage
— in particular, the way in which the internal LED slowly dims (rather than shutting off instantly)
when the control voltage is removed. This sonic coupling of timbre and volume, along with the
circuit’s natural decay, makes the low pass gate particularly adept at producing tones associated
with struck objects, like hand drums or marimbas. Indeed, low pass gates are the main ingredient of
the so-called “Buchla Bongo” sound.
The Intellijel Passive Low Pass Gate 1U is a simple, DC coupled, vactrol based, 1-pole passive low
pass gate, which is controlled by an external CV input. Without positive control voltage, the gate is
completely closed and none of the LPG’s input signal is passed through to the output. Increasing
amounts of positive CV open the gate — making the output signal both louder and brighter. At +5V,
the gate is fully open and the input signal is passed through to the output.
This makes LPGs sound very different (and more “acoustic”) than the traditional complement of
envelopes, VCFs and VCAs that are employed in most subtractive synthesizers. Exciting the vactrol
(via the CV input) with a trigger, noise blast, or short envelope will result in a very natural sounding
filtered envelope that’s reminiscent of the sonic characterics of struck objects, which are allowed to
decay naturally.
In addition, because this is a passive circuit (no power is applied to it), its sonic characteristics are
somewhat dependent upon the audio signal you feed into it, making it a very natural and organic
way to shape a signal’s volume and tone.
Passive LPG 1U Manual 4

FRONT PANEL 
[1] LPG IN
Input into the Low Pass Gate circuitry.
Any signal patched into here will be gated by the
voltage arriving at the CV IN [3] jack, and sent to the
LPG OUT [2] jack.
[2] LPG OUT
The output of the Low Pass Gate.
It contains an amplitude and timbre modified version
of the signal patched into the LPG IN [1] jack. The
amount of amplitude attenuation and timbre
modification is determined by the voltage present at
the CV IN [3] jack.
[3] CV IN & LEVEL LED
A voltage patched into this input will determine the
extent to which the Low Pass Gate affects the signal
arriving at the LPG IN [1] jack before passing it to the LPG OUT [2] jack
At +5V, the Level LED lights bright green , and the gate “opens,” allowing the signal to pass from
the LPG IN [1] to the LPG OUT [2] unaffected.
Voltages less than +5V, but higher the 0V partially “close” the gate, causing a dulling of the
sound (much like a low pass filter) as well as a decrease in amplitude. The Level LED dims as
the CV IN [3] voltage diminishes.
Unlike traditional VCFs and VCAs, low pass gates have a more ‘organic’ response to gating and
filtering a signal, and work particularly well when a short trigger or noise burst is sent into the
CV IN [3] jack. When excited this way, the circuit imparts a more natural sounding
filter/envelope to the input that’s reminiscent of the sonic characteristics of a struck object, such
as a hand drum or marimba.
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Passive LPG 1U Manual 5

TECHNICAL SPECIFICATIONS 
Passive LPG 1U Manual 6
Width
6 hp
Maximum Depth
19 mm
Current Draw
None (passive module)