In difference to lowpass or highpass filters, the gain of an all-pass filter resides constantly over the range of frequencies at which it is used, but it does offer a frequency-dependent phase shift. Several all-pass filters may be cascaded to produce a phasing unit for use in electronic music.
The phasing effect is produced by phase-shifting a signal and then summing the original and phase-shifted variants of the signal. Figure 1 presents the basic electronic circuit of a first classification all-pass filter.
The phase-shift is conditional on the relative values of R and C and on the input frequency. At low frequencies, C has a very high impedance and the circuit simply functions as an inverting amplifier, so the phase-shift is 180°. At high frequencies the impedance of C is low and the circuit functions as a non-inverting amplifier with zero phase-shift.
The gain of the filter depends on the relative values of Rl and R2. In this case, Rl and R2 are chosen equal so that the gain is unity. The graph shows the phase-shift v. frequency curve for the filler.
The complete circuit of a phasing unit using all pass filters are shown in figure 2. Six all-pass filter stages are cascaded, so the total phase-shift at low frequencies can be up to 1080″! The use of a total of ten op-amps in the circuit may seem rather excessive, but as eight of these are LM324 quad op-amps the total package count is only four ICs.
IC1a functions as a unity gain input buffer and IClb to IC2c are the six filter stages. The direct and phase-shifted signals are summed by lC2d. The proportion of phase-shifted signal and hence the depth of phasing can be adjusted utilizing P4.
The degree of phase-shift at a particular frequency can be varied by FETs Tl to T6, which function as voltage-controlled resistors. By varying the gate voltage the drain-source resistance can be increased or decreased, thus altering the effective value of ‘R’ in each all-pass filter and hence varying the phase-shift.
This may be controlled either manually utilizing P3 or may be swept up and down automatically by the outputt of the triangular wave generator consisting of IC3 and IC4. As the gate voltage of the FETs must always be negative the output of this oscil- lator swings between ‘ 2 and – 6 V.
The oscillator frequency may be varied by means of PI, and the best phasing effect occurs at frequencies between 0.5Hz and 1Hz. At higher frequencies (around 4 Hz) the phasing effect is lost but a vibrato effect is obtained instead.