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Discrete OTA VCF

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The Synthesis Technology E440 is a lowpass voltage-controlled filter with 3 simultaneous outputs.
The input is any audio source (can be from line level to synth level) and the output is via the
different slopes of the filter: 2pole (-12dB/oct), 3pole (-18dB/oct) and 4pole (-24dB/oct). There are
3 CV inputs to control the cutoff frequency plus a separate CV input for the resonance. The E440
will self-oscillate at high resonance and can be used for a sine VCO (each output is phase shifted
45 degrees from the prior one) with 1V/Oct tracking (trimmable) over a 4-octave range. This
response is also temperature-compensated. An external audio mixer module can be used to blend
the 3 outputs into “in-between” slopes and by inverting them relative to another, different
frequency responses.


»  RESONANCE: amount of filter resonance. This knob setting is added to whatever CV is plugged into the
   RES CV jack. The filter will self-resonate (with no external applied CVs) at the ~9 tick mark.

»  FM1: filter frequency modulation standard attenuator for whatever is applied to the FM1 jack.

»  FM2: filter frequency modulation reversing attenuator for the CV plugged into the FM2 jack.
   The 12:00 (straight up) position is maximum attenuation: rotating CCW will add the inverted signal
    to the FREQUENCY control while rotating CW will add the applied CV.

»  FREQUENCY: the cutoff (or self-oscillation initial frequency). The VCF will internally add all 4 CV
   inputs together to determine the filter’s cutoff frequency. The range is 20Hz to ~ 18KHz.

»  DRIVE: this is a combination attenuator for the AUDIO IN and a booster amplifier (~2X gain).
   The filter can be over-driven to clipping (with a large voltage output swing, ~14V pk-pk). You can
   use this control to boost line-level signals as well. The unity gain setting is ~ ‘4th' tick.

»  CV inputs. The range of each input is -5V to +5V. FM1 and FM2 are on attenuators, while
     1V/Oct and RES CV are not.

»  Audio jacks. AUDIO IN can be any level up to 14V pk-pk. The 3 outputs are the lowpass
   responses. All 3 outputs share the resonance and cutoff CVs. You will note that the relative levels
   of the 3 responses are slightly different, which is resonance dependent. This is just the nature of
   the electronics. The 4 OTA stages are not gain-matched and the filter caps are +-5%. This will
   cause each stage to vary slightly in gain relative to the others. Consider this part of the E440’s charm.

Technical Details

»  Eurorack Format
»  CV Inputs: -5V to +5V, DC to 10KHz. RES IN is 0V (no resonance) to +5V (full resonance).
»  Audio input: 50K nominal input impedance, 100mv to 14V pk-pk.
»  Outputs: -7V to +7V, 20Hz to 18KHz
»  Power: +12V @23ma, -12V @23ma. nominal
»  Jumper options: JP1 – Bass Boost Enhancement OFF or (ON).


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