Voltage controlled phaser

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.

Let's hear it

Noise through phaser set to 4 stage feedback and LFO-swept.

Noise through phaser set to 8 stage feedback and LFO-swept.

LFO triggering envelope. Phaser set to high resonance and LFO-swept.

Sawtooth with phaser used as a formant filter (slightly swept by LFO).

Even overtones waveform, phaser and Minimoog clone VCF.

Construction

This is a fairly simple module to build, as long as you are able to make the circuit board for it. The board layout is a little changed, from the one you see on the pictures. It is no longer two sandwiched boards, but one larger board.
I have used a SPDT toggle switch to select feedback from stage 4 or 8. If you want, you can use a rotary switch instead. Then you can select the feedback from any of stage 2, 3, 4, 5, 6, 7 or 8. There are solder points for wires on the circuit board, marked with the corresponding number.

Schematics

Circuit board layout

Component placement plan

Alignment

Preferrably, use a sine wave as input signal, and turn the Phase knob all the way clockwise. Adjust the trimmer at stage 1, while monitoring the signal at output 4 (the output of stage 4). Adjust it so that there is no DC offset on the signal.  When this is OK, do the same with the other trimmer, while monitoring the signal at output 8.

The completed module seen from the side.

The completed module seen from behind.

The completed module seen from the bottom.