Doepfer A-127 VC Triple Resonance Filter

Module A-127 is a triple resonance filter unit. It contains three separate voltage controlled band pass filters. Each filter has it's own LFO (triangle waveform) with LED display and adjustable frequency and amplitude. Instead of the internal LFO an external control voltage may be used to control the filter frequency. In this case the external voltage is fed into the external CV jack (with integrated switch to turn off the LFO signal) and the amplitude control of the LFO affects the level of the external CV signal. Each filter is equipped with the following controls: LFO frequency, LFO/external CV amplitude, filter frequency, filter resonance and filter amplitude. In addition to the mix output for all three filters each filter has a separate audio output. The original audio signal can be added to the triple filter mix signal with a separate control. All of the 3 filters share a common audio input with attenuator. The filter audio inputs are very sensitive so that distortion may intentionally be used to create new sounds - if desired.
The frequency control range of the bandpass filters is about 40Hz...6kHz, the frequency range of the LFO's is about 0.02Hz...20Hz (= 1 min ... 1/20 sec per cycle).
Each of the three filters can used also as a 12dB low pass (with resonance control) instead of band pass. For this a jumper has to be changed on the filter board in question. In this case the module can be called no longer "resonance filter" but "triple low pass filter". It is also possible to modify only one or two of the filters to low pass.

The A-127 is a versatile module for sound modification. Here are some application samples:

• reproduction of resonances (fixed filter frequencies, resonances and amplitudes)
• triple internal or external CV controlled sweeps for very complex filterings
• filter effects similiar to vocoder and/or synthetic speech using specific external CV's to control the three filter frequencies (e.g. coming from the Sequenzer A-155 or MIDI-CV Interface A-191)
• MIDI controlled filter settings or sweeps (e.g. in combination with A-191)
• random filter sweeps using random voltages (e.g. from A-118)
• triple S&H filter effects (in combination with S&H A-148)

For more detailed information please look at the English user's manual A127_man.pdf

In addition a breakout module A-127BOM is available. This expansion module offers the following additional functions:

• notch filter for each sub-unit
• separate outputs for lowpass, highpass, bandpass and notch for each sub-unit
• separate audio input with attenuator for each sub-unit

The breakout module allows the separate use of each of the three A-127 filters. In addition the filters can be patched parallel or serial. Especially the serial connection of three notches generates very complex phasing-like structures. For parallel filter connection the existing mixer of the main module may be used or an external mixer. For serial connection the filters are patched in the desired sequence at the breakout module. A jumper is used to define the filter type for each sub-unit for the mix unit of the main module. 

Remark: The breakout module can be used without modification only in combination with the new version of the A-127 (pcb printing "Version 4"). Only the new version has the expansion connector available, that is required to connect the breakout module without soldering. It's also possible to combine the breakout module with older versions of the A-127 (versions 1, 2 and 3) but in this case soldering is required to connect the breakout module to the main module (soldering 4 wires to each filter unit). The instructions how to connect A-127BOM and A-127 main module are available here:A127BOM.pdf. This document also describes how to wire older versions of A-127 to the A-127BOM.

Note: The front panel printing of the first production series is faulty. The highpass and bandpass are mixed up. If you cannot accept this drawback we ask you to wait for the second production series (about middle of 2015). The correct printing is BP right from LP and HP below LP.