Instruō dåpf Dual All-Pass Filter

Description

Introducing dåpf, the first dedicated effects processor in the Instruō product line. The Instruō dåpf is a set of two analogue all-pass filters. All-pass filters don't attenuate frequency amplitudes directly, but rather alter the phase relationship of frequencies across the spectrum. They appear in many voltage-controlled filter topologies and serve as a core building block within DSP. The normalled I/O of dåpf allows for cascaded or independent use of the two all-pass filter circuits, offering everything from classic phase-shifting to psychoacoustic stereo enhancement and synthetic Doppler effect.

Features

• Two parallel all-pass filters
• Independent attenuverters for voltage control
• 1-Pole/2-Pole configuration for channel 1
• Cascaded normalisation
• Global 1 volt per octave tracking

Installation and Specifications

Installation

1. Confirm that the Eurorack synthesizer system is powered off.
2. Locate 4 HP of space in your Eurorack synthesizer case.
3. Connect the 10-pin side of the IDC power cable to the 2x5 pin header on the back of the module, confirming that the red stripe on the power cable is connected to -12V.
4. Connect the 16-pin side of the IDC power cable to the 2x8 pin header on your Eurorack power supply, confirming that the red stripe on the power cable is connected to -12V.
5. Mount the Instruō dåpf in your Eurorack synthesizer case.
6. Power your Eurorack synthesizer system on.

Note: This module has reverse polarity protection. Inverted installation of the power cable will not damage the module.

Specifications

• Width: 4 HP
• Depth: 30mm
• +12V: 60mA
• -12V: 60mA

Overview

The Instruō dåpf module features a vertical layout with several controls. On the left side, from top to bottom: a 1V/Oct Input (12), followed by Channel 1 Input (1) and Channel 1 Output (2). Below these are the Pole Toggle (3), Channel 1 Phase Slider (4), Channel 1 Phase CV Input (5), and Channel 1 Phase CV Attenuverter (6). On the right side, from top to bottom: Channel 2 Input (7) and Channel 2 Output (8). Below these are the Channel 2 Phase Slider (9), Channel 2 Phase CV Input (10), and Channel 2 Phase CV Attenuverter (11). A small indicator labeled "dåpf" is at the bottom, with "12dB" and "6dB" markings near the Pole Toggle, and "1V/OCT" near the corresponding input.

Key

• 1. Channel 1 Input: Direct audio input for the first all-pass filter.
• 2. Channel 1 Output: Filtered audio output for the first all-pass filter. If unconnected, signal passes to Channel 2 Output.
• 3. Pole Toggle: Selects the filter slope: -6dB/Octave (1-Pole) or -12dB/Octave (2-Pole).
• 4. Channel 1 Phase Slider: Adjusts the center frequency, affecting phase shift (180° for 1-Pole, 360° for 2-Pole).
• 5. Channel 1 Phase CV Input: Bipolar control voltage input for the Phase Slider.
• 6. Channel 1 Phase CV Attenuverter: Controls the depth and polarity of external modulation for the Phase Slider.
• 7. Channel 2 Input: Direct audio input for the second all-pass filter. Normalizes to Channel 1 Input if unconnected.
• 8. Channel 2 Output: Filtered audio output for the second all-pass filter.
• 9. Channel 2 Phase Slider: Adjusts the center frequency, affecting phase shift (360°).
• 10. Channel 2 Phase CV Input: Bipolar control voltage input for the Channel 2 Phase Slider.
• 11. Channel 2 Phase CV Attenuverter: Controls the depth and polarity of external modulation for the Channel 2 Phase Slider.
• 12. 1V/Oct Input: Bipolar control voltage input for 1V/Oct tracking, summing with Phase Sliders and CV Inputs.

Channel 1

Input (IN): The channel 1 Input (left) provides direct access to the channel 1 all-pass filter.

Output (OUT): The channel 1 Output (left) provides the filtered signal present at the channel 1 Input. If no cable is connected to the channel 1 Output, then the signal present at the channel 1 Input is processed and passed to the channel 2 Output. This configuration can be used for mono-to-mono processing.

Pole Toggle: The Pole Toggle sets the slope of the all-pass filter. If the toggle is in its left position, the profile is set to -6dB/Octave (1-Pole). If the toggle is in its right position, the profile is set to -12dB/Octave (2-Pole).

Phase Slider: The Phase Slider shifts the centre frequency of the all-pass filter processing the input signal. If the Slope Toggle is set to -6dB/Octave (1-Pole), moving the Phase Slider from its minimum to maximum position results in a global phase shift of 180°. If the Slope Toggle is set to -12dB/Octave (2-Pole), moving the Phase Slider from its minimum to maximum position results in a global phase shift of 360°.

Phase CV Input: The Phase CV Input is a bipolar control voltage input for the Phase Slider. Control voltage is summed with the Phase Slider and scaled and/or inverted by the Phase CV Attenuverter.

Phase CV Attenuverter: The Phase CV Attenuverter determines the depth and polarity of external modulation applied to the phase parameter.

Channel 2

Input (IN): The channel 2 Input (right) provides direct access to the second all-pass filter. If no cable is connected to the channel 1 Input, the signal present at the channel 2 Input normalizes to both all-pass filter channels. In this scenario, only the corresponding phase parameter will affect the signal present at the respective channel Output. This can be used for mono-to-stereo processing. If no cable is connected to the channel 1 Input or channel 1 Output, then both channel's phase parameters will affect the signal present at the channel 2 Input, creating a mono sum of both Outputs. This can be used for mono-to-mono processing with two parallel all-pass filter stages. Attenuating the input signal before connecting to the channel 2 Input is advisable to avoid clipping.

Output (OUT): The channel 2 Output (right) provides the filtered signal present at the channel 2 Input.

Phase Slider: The Phase Slider shifts the centre frequency of the all-pass filter processing the input signal. Moving the Phase Slider from its minimum position to its maximum position results in a global phase shift of the input signal by 360°.

Phase CV Input: The Phase CV Input is a bipolar control voltage input for the Phase Slider. Control voltage is summed with the Phase Slider and scaled and/or inverted by the Phase CV Attenuverter.

Phase CV Attenuverter: The Phase CV Attenuverter determines the depth and polarity of external modulation applied to the phase parameter.

Output Indicator: The Output Indicator provides LED illumination of the signal amplitudes present channel 2 Output.

Global

1V/Oct Input: The 1V/Oct Input is a bipolar control voltage input that is calibrated for 1 volt per octave tracking. Control voltage is summed with the values set by the Phase Sliders and Phase CV Inputs.

Normalling

The normalling of input signals moves clockwise around the panel from the channel 2 Input to the channel 2 Output. This normalling scheme can be used for mono-to-mono, mono-to-stereo, and stereo-to-stereo processing.

Patch Examples

Mono Phase-Shifting

Summary: An oscillator is connected to channel 1's Input and monitored through channel 1's Output. A control voltage signal modulates the phase.

Audio Path:

• Connect the output of a harmonically rich oscillator waveform to the channel 1 Input.
• Monitor from the channel 1 Output.
• Set the Pole Toggle to a desired setting.

Control Path:

• Connect a control voltage signal to the channel 1 Phase CV Input and set the corresponding Phase CV Attenuverter to a desired position. Note: If using bipolar control voltage signals, bias the corresponding Phase Slider so that negative voltage doesn't clip the Phase CV Input.

Serial Mono Phase-Shifting

Summary: An oscillator is attenuated and then connected to channel 2's Input and monitored through channel 2's Output, creating a mono sum of both channels. Two different control voltage signals modulate the phase of each channel.

Audio Path:

• Connect the output of a harmonically rich oscillator waveform to an attenuator to attenuate the amplitude of the waveform. Set the attenuator to a desired level.
• Connect the output of the attenuator to the channel 2 Input.
• Monitor from the channel 2 Output.
• Set the Pole Toggle to its right position to set a -12dB/Oct pole for channel 1.

Control Path:

• Connect two different control voltage signals to both Phase CV Inputs and set the corresponding Phase CV Attenuverters to a desired position. Note: If using bipolar control voltage signals, bias the corresponding Phase Sliders so that negative voltage doesn't clip the Phase CV Inputs.

Mono-to-Stereo Phase-Shifting

Summary: An oscillator is connected to channel 2's Input and monitored through both Outputs. Two different control voltage signals modulate the phase of each channel.

Audio Path:

• Connect the output of a harmonically rich oscillator waveform to the channel 2 Input.
• Monitor from both Outputs.
• Both outputs will need to be externally panned hard left and hard right.
• Set the Pole Toggle to its right position to set a -12dB/Oct pole for channel 1.

Control Path:

• Connect two different control voltage signals to both Phase CV Inputs and set the corresponding Phase CV Attenuverters to a desired position. Note: If using bipolar control voltage signals, bias the corresponding Phase Sliders so that negative voltage doesn't clip the Phase CV Inputs.

Stereo Phase-Shifting

Summary: Two oscillators are connected to both Inputs and monitored through both Outputs. Two different control voltage signals modulate the phase of each channel.

Audio Path:

• Connect the outputs of two harmonically rich oscillator waveforms to both Inputs.
• Monitor from both Outputs. Note: Both outputs will need to be externally panned hard left and hard right.
• Set the Pole Toggle to its right position to set a -12dB/Oct pole for channel 1.

Control Path:

• Connect two different control voltage signals to both Phase CV Inputs and set the corresponding Phase CV Attenuverters to a desired position. Note: If using bipolar control voltage signals, bias the corresponding Phase Sliders so that negative voltage doesn't clip the Phase CV Inputs.