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)