THAT2180 VCA-Based Phaser All-Pass Section

[mediatechnology]

I've been looking for phaser designs using THAT VCAs and didn't find any. I wanted to use VCAs for fast response and headroom. Google led me to Sam Hoshuyama's very clever design using the SSM2164. I began my experiments using a bootstrapped shunt R approach but the Hoshuyama design was superior. So with the help and support of both Sam and Gary Hebert of THAT I came up with this all-pass section. The THAT VCA, unlike the SSM2164, is non-inverting overall. (The SSM2164 is non inverting but the I-V converter flips polarity.) The following transforms the original design to use a non-inverting VCA teamed up with a THAT1240 to provide a precision all-pass section.



There are sound file demos posted here and throughout the thread:

http://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=6&t=368&start=36

The original discussion starts here:

http://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=6&t=368

I don't post here very often but thought you guys might find it interesting.

Best;

Wayne

[emdot_ambient]

So...the schematic above is 1 pole and then you'd just string together N number of them?

[mediatechnology]

Yes, string them in series with the Ec bused.

A few notes:

Ec should be driven by a bipolar op amp to maintain low Ec bus impedance over frequency.

Bifets are needed in the integrator(s) due to the need for low bias current.

[emdot_ambient]

The slow noise sweep sounded quite good. In stereo with some delay added would be even better

[mediatechnology]

Thanks! There are some A+B A-B stereo demos of the earlier topology earlier in the proaudiodesignforum thread. The new topo essentially sounds the same but has wider range. I hope to get some time to build up four more poles and do a stereo demo of this one.

[frijitz]

[mediatechnology]

Thanks Ian. I haven't tried higher-order structures other than 8X cascaded single pole sections.