Phaser (2)

idea: Elex-team

Last week, we were introduced the all-pass filter — a curious circuit that does not flash an input signal but rather slows it down. This filter forms the heart of our phaser.

Important for the final phasing effect is the number of these filters that the signal has to pass through: the more filters, the stronger the effect. Of course, we can't put a random number of filters in series, but with six we have achieved good results. Below you can see the complete scheme.

An input stage has been built up around op-amp A1, which provides the necessary impedance matching. A9 and A10 together form a slow triangular oscillator. Depending on the position of P1, the frequency is between almost 0 Hz and about 2 Hz.

The all-pass filters are built around A2, A4, A3, A6, A5 and A8. A2 / A4 are mainly responsible for the low tones, A3 / A6 for the mid-tones, and A5 / A8 for the high frequencies. (The theoretical operating points are at 500 Hz, 3 kHz, and 10 kHz.)

The phasing effect is created by sliding the delayed signal "back and forth" and then mixing it with the original signal. A7 is responsible for that mixing (or summing); P2 allows the gain of this stage to be set between 0 and 6 dB.

The transistor at the output of A10 drives LEDs D1 ... D6, which in turn illuminate the light sensitive resistors (LDRs) R23 ... R28. The effect is clear: as the LEDs light up more or less brightly (at the rate of the triangle at the output of A10), the value of the frequency-determining resistor changes with each all-pass filter (because there is always an LDR parallel to it). The result: the signal that passes through the delay section is shifted back and forth in its entirety in the rhythm of the oscillator frequency.

Finally, the "inverted" sum of the original signal and the processed signal appears on the output of A7. Because that sum changes continuously, the amplitude of the output signal will be low at certain frequencies and high at others. This creates the "comb-shaped" characteristic that we saw in the first episode.

In the next and last episode, we will go into some details and the construction. A larger version of the schedule can be downloaded below.