Low-Pass Gate

1. CV Bias Knob Voltage added to the CV inputs. Input jack and black knob are for modulation, white knob is for manual bias adjustment.
2. Ping Input Connect a gate, trig or clock signal to trigger the low-pass gate. When no other CV signal is applied, this will produce a short envelope perfectly suited for sequences or percussions.
3. Ping Level Knob Adjusts the level of the pulse generated by the Ping feature. Input jack and black knob are for modulation, white knob is for manual level adjustment.
4. CV 1/2 Inputs Control the amplitude and frequency of the first and second channels of the low-pass gate. The gate needs 1.4V to open. The LEDs show the current CV level. CV 1 is normalled to CV 2 when the latter is unconnected.
5. CV 1/2 Knobs Attenuators for CV 1 and CV 2 inputs.
6. In 1/2 Inputs Audio signals to be processed by the first and second channels of the low-pass gate. Out 1 is normalled to In 2 when the latter is unconnected.
7. Out 1/2 Outputs Output signal for the first and second channels of the low-pass gate.

Overview ⚓︎

A low-pass gate (LPG) is similar to a VCA, but it varies the brightness of the sound together with the volume.

The Multiphonics LPG emulates a circuit using photoresistive opto-isolators, which reacts more slowly than a typical VCA. When triggered by a gate signal, it has a quick attack and a decay similar to a plucked string. As such, it can be used without an envelope generator.

This LPG has two channels that can be used independently as two separate LPGs, or together in series. Since no two photoresistive opto-isolators are identical, we designed each channel with a different frequency response curve and decay time.

When used separately, the filter in each channel has a 6 dB/oct frequency response and a long decay. When used in series, the combined two-stage filter has a 12 dB/oct frequency response and a short decay.

Usage ⚓︎

Each low-pass gate channel works like a VCA: when a voltage is applied on the CV input, the signal from the In jack will pass through the corresponding Out jack. Varying the CV level will affect both the amplitude and high frequency content of the signal.

For a 6 dB/oct low-pass slope, you can patch either channel like in the following graphic:

Unlike a VCA, a LPG has a highly non-linear behaviour:

• Below 1.4V, the gate is fully closed; no signal passes through.
• Starting at 1.4V, the gate opens very quickly with a logarithmic curve.
• At 6V, the gate is nearly fully open.
• At 10V, the low-pass cutoff frequency reaches a maximum of around 15–16 kHz.

This graphic compares the CV response of a LPG and of a State Variable Filter.

In Depth ⚓︎

Patching a 12 dB/oct LPG ⚓︎

In addition to using this module as a 6 dB/oct LPG like we saw above, it is possible to use both channels in series for a steeper 12 dB/oct filter with a shorter decay time:

The arrows on the module’s faceplate show the normalled connections that make this possible:

• When CV 2 is unconnected, it gets its signal from the cable connected into CV 1, so that both LPG channels are controlled by the same CV signal.

• When In 2 is unconnected, it gets its signal from Out 1. By chaining the two 6 dB/oct channels together, we get a 12 dB/oct LPG.

Pinging the LPG ⚓︎

A typical way to use a LPG is to open it with a short pulse to take advantage of its acoustic-sounding decay envelope. Since Multiphonics trigger signals are too short to fully open the LPG and patching a Pulse module into a LPG just to ping it is a bit of a hassle, we decided to add a dedicated Ping trigger input to the LPG.

When the Ping input is triggered or manually activated by pressing the orange button, a short pulse at the chosen level is sent to CV 1 and CV 2 to open both LPG channels. This can turn the incoming audio signal into a plucked or percussive sound, whether a 6 or 12 dB/oct topology is used.

The voltage level of the pulse generated by the ping feature is set with the white Level knob. That level can be modulated with the input jack and dark grey attenuverter to the left of the white Level knob.

Modulating the ping level is a great way to add dynamics to a sequence.

CV Bias ⚓︎

The white CV Bias knob at the top-right of the module can be used to inject an additional voltage to both LPG channels.

If you were to use the LPG as a plain low-pass filter, the CV Bias knob would be the filter’s cutoff knob.

The CV Bias knob has a modulation input jack with an attenuverter to its left. Modulation can be used to do add some movement to both LPG channels using an ADSR or a LFO. This works better when the CV 1 or CV 2 levels are set to around 50%.

This image shows how to patch a tremolo or filter sweep effect using a LFO or envelope modulation on the CV Bias: