VC Phaser using the LM13600

[jorgen.bergfors at idg.se]

Hi Paul. The frequency shift at lower frequencies with 0V CV is probably due to the fact that I had to add a 2M2 resistor to ground, from the OTA output. Without it, the DC level will sometimes drift with no CV (the  OTA turned off). The load impedance in my circuit is very high because I used JFET opamp buffers instead of the LM13600 Darlington buffers. If I had used the buffers, I suppose the load resistance would have been lower than 2M2 ohms. The Small Stone used the Darlington buffers in the CA3094, so I suppose it is worse than my circuit in this respect. It is possible that I could improve it by using an even larger resistor, but 2M2 was the largest that I got.


/Jorgen





MIME:pfperry at melbpc.org.au on 99-01-05 13:39:41
To:  Jorgen Bergfors at IDGSE
cc:  Subject: Re: VC Phaser using the LM13600 

At 10:59 AM 5/01/99 +0100, you wrote:
>Hi all.

>The following was measured om my test circuit:

>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)?

perhaps this is a function of the filter capacitors?
Try making them all one quarter the size..

paul perry melbourne australia