|Exclusively Analogue Universal Active Filter
The Exclusively Analogue Universal Active Filter is a fully voltage controlled filter with simultaneous
highpass, bandpass, lowpass, and notch outputs and is based on the Eµ 2120 UAF module. It contains an 1120 UAF submodule.
Each of the three signal inputs passes through an attenuator, then the signals are summed and pass through the filter to appear at each of the outputs. The highpass signal is filtered 12 dB/octave below the cutoff frequency, the bandpass 6 dB/octave on both sides of cutoff, and the lowpass 12 dB/octave above cutoff. The notch output is unfiltered except for a 40 dB notch near the cutoff frequency. The Notch Frequency control will vary the ratio of the notch centre frequency to the cutoff frequency several semitones to either side.
The cutoff frequency is exponentially voltage controlled by the cutoff frequency inputs. The keyboard switch connects either of two keyboards at precisely lV/octave. The Q of this filter is exponentially voltage controlled over a range from 1/2 to 512, at one volt per factor of two change. The Q is numerically equal to the gain of the filter at the cutoff frequency. As Q is increased, the filter becomes extremely resonant at Fc. Up to 54dB of gain can be achieved. The notch output is ineffective at high Q. With such high gains, it is very easy to cause the filter to distort. At high Q, the signal input attenuators should be set for low levels.
With sufficiently high Q, fast signal transients such as pulse and sawtooth waveforms will cause the filter to “ring” at the cutoff frequency. The keyboard percussion switch patches a keyboard’s gate and trigger signals into the filter to use this effect. On depression of a key, the trigger signal causes the filter to ring with a decay set by the Q control. When the key is released, the filter Q control instead becomes active, allowing a damping effect if desired. The final Q control has no effect with the keyboard percussion switch off.