Doepfer A-108 6/12/24/48dB Low Pass Filter

Doepfer A-108 6/12/24/48dB Low Pass Filter

Module A-108 is a completely new voltage-controlled low pass pass filter based on the well-known transistor ladder (Moog ladder). The module has internally an 8 stage low pass filter with different slopes available: 6, 12, 18, 24, 30, 36, 42 and 48 dB per octave. In addition it features an band pass output (i.e. band pass with transistor ladder). In the factory the 4 low pass outputs of the A-108 are internally connected to the filter stages 6, 12, 24 and 48dB. From our first A-108 audio tests this seems to be the best output combination as these outputs generate audible different sounds. E.g. the audible difference between the 48dB and 42dB or between 42dB and 36dB is very little. But if desired any of the filter stages can be connected to one of the outputs.

Remark: Only 4 output stages are available as this seems to be sufficient from our experiences. In the factory the out stages are connected to the filter stages with 6, 12, 24 and 48 dB slope. By changing the internal connections (jumpers or wires) each of the filter stages can be used but only four at a time. Even a multi-way switch could be used but from our results the factory setting (6/12/24/48dB) is the best combination for musical applications. More details below (after the frequency response pictures).

Resonance (Emphasis or Q ) can be adjusted manually right up to self-oscillation, in which case the filter will behave like a sine wave oscillator. The A-108 features an external feedback input that enables the insertion of additinal modules into the feedback path (e.g. VCA for voltage controlled resonance or phaser/frequency shifter for phase/frequency shifting effects). The socket is normalized and internally connected to the 48dB low pass output if no cable is inserted into the feedback socket.

The frequency can be adjusted manually, or by voltage control. Three CV inputs (CV1, CV2, CV3) are available. CV2 and CV3 are eqipped with attenuators.

The filter audio input is very sensitive so that distortion - if desired - is possible even with normal A-100 levels (e.g. VCO output).  Self oscillation will break off at high distortion levels as the internal feedback signal is drown out by the distorted audio signal. This feature may intentionally be used to create new sounds.

In combination with the Voltage Controlled Mixer A-135 and the Morphing Controller A-144 a filter with voltage controlled slope can be realized (i.e. controlling the slope from 6dB to 48dB via CV).

The sketches below show the frequency response of the five filter outputs at three different resonance settings.