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Bergfotron VCP

Voltage Controlled Phaser

Courtesy/Thanks to: Jörgen Bergfors

Description: from the homepage

This is a simple voltage controlled phaser, based on the LM13600 OTA. The phaser core is inspired by the Electro Harmonix Small Stone. In order to use the linearizing diodes in the LM13600, I had to make some changes to the circuit though. My module contains eight phaser stages, but you can tap the signal after four. The Small Stone has four stages.

The phaser stages are controlled by an exponential converter of the same type that is used in many VCFs and VCAs. There is a modulation input and modulation amount knob.
The feedback amount is contolled by a knob. The signal is fed back from the fourth or eighth phaser stage, selectable with a toggle switch. Changing the feedback to include more stages, creates more resonance peaks. There is one peak/notch for every two stages.

With feedback after four stages and the phase knob at full, there are resonance maxima at approximately 2,4 kHz and 18 kHz. There are notches at 1,2 kHz and 6 kHz. With the phase knob turned to 7.5, the peaks are at 720 Hz oand 13 kHz, wheras the notches are at 360 Hz and 1,4 kHz.

With feedback from stage 8, I measured the following (frequencies approximate):

10 V CV 7,5 V CV

480 Hz 140 Hz notch

1 kHz 300 Hz peak

1.6 kHz 480 Hz notch

2.5 kHz 720 Hz peak

3.5 kHz 1 kHz notch

5.6 kHz 1.7 kHz peak

3.5 kHz notch

15 kHz peak

Obviously, with full CV, the highest notch and peak are above the audible range.

To avoid overdriving the phaser stages, which can produce nasty clicking or static in the sound, I used green LEDs as limiters in the feedback path. Turning the feedback knob beyond approximately 5, causes the phaser to oscillate, when there is no input signal. With a strong input signal, more feedback can be applied, before oscillation starts.

I'm really satisfied with the sound of this module. I was really surprised to find that it is virtually noise-free. You would think that cascading eight OTAs would create a lot of noise. But for some reason it doesn't.

I also like the action of the feedback control. You can create everything from slight resonances to aggressive overdriven sounds. And by feeding it noise, you get the trademarked J.M. Jarre whooshing sounds.

By using all eight stages, and no feedback, you can add vibrato to any signal. This works best for signals which doesn't have too high overtones though.


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