VC Phaser using the LM13600

[jorgen.bergfors at idg.se]

Hi all.
I designed and built a test circuit inspired by the EH Small Stone. Instead of the CA3094, I wanted to use the LM13600. The test circuit consists of an exponential voltage to current converter and two all-pass stages. In an actual phaser more than two stages of course would be used. The Small Stone has four, some other phasers have six. Phase shift is around zero at 0V CV and increases with increasing control voltage.

The following was measured om my test circuit:

Frequency Phase shift (0 to 10V CV)
18 Hz 45 - 350 degrees
37 Hz 30 - 340 degrees
75 Hz 20 - 330 degrees
150 Hz 15 - 300 degrees
300 Hz 10 - 250 degrees
600 Hz 0 - 250 degrees
1200 Hz 0 - 110 degrees
2400 Hz 0 - 55 degrees

As can be seen, there is some phase shift at lower frequencies even when the control voltage is 0 volts. The frequency response falls to -3 dB at 24 Hz. This is the worst case, at 5V CV. At 10V CV it seems to be flat down to DC. At 0V CV it goes down to 18 Hz. There is no HF damping (below 20 kHz).
Noise and distortion seems to be low. I haven't measured them, but they aren't noticable on the scope or at listening tests.
Note that phase shift is higher att low frequencies. Is this common to all phasers or are there phasers that are the other way round (higher phase shifts at high frequencies)?

/Jorgen