Ever since I built an 8 pole "DynaPhaser" prototype back in the early 80's using a pair of SSM2040s I've wanted to rebuild that prototype using modern VCAs. I recalled that listening to it was such a mindf**k (pardon my English) it was worth revisiting.
One of the things the SSM2040 had going for it was expo control - something that the DynaFlanger never had. I modded a DynaFlanger for exponential control long after it was out of production and it "tracked" a lot better under both envelope and frequency control.
There are a lot of good phaser designs coming out of the DIY modular synth world over at electromusic. Thomas Henry, Jurgen Haible and a lot of other folks have done some great designs using FETs, discrete transistors, Vactrols, OTAs etc. Maxim even has an app note using digi-pots. Most of these designs (excluding the Vactrol) don't do line-level well.
I've only seen one VCA-based design using the SSM2164 from neil's webbly world:
http://www.milton.arachsys.com/nj71/ind ... bsubmenu=3
Schematic:
http://www.milton.arachsys.com/nj71/pdf ... pf-sch.pdf
Neil's design is very clever but I wanted to use a THAT2180 as the basic element for higher performance. The only other cite I found using a true VCA was in a later Aphex 1537 datasheet:
Something about seeing a backwards VCA feeding it's own input bothered me. I didn't immediatley see what was going on. Since the THAT and SSM VCAs are current in/current out I took a closer look at the 1537.
OK, so the Aphex 1537 is current in voltage out. But the simplfied drawing still doesn't make sense. What we want is a voltage-variable resistor.
OK, still odd-looking but closer examination of the resistance versus gain equation shows that the resistor is being
bootstrapped by the VCA.
OK, I'm starting to get it. We have a voltage in/voltage out variable gain block which is overall non inverting. We take a lower-valued resistor and by driving the opposing end with the same polarity we synthesize a larger one and put the whole gizmo under exponential voltage control. Cool!
1) When the VCA is in full attenuation the value of the resistor is unchanged and the "ground" end held at AC ground.
2) When the VCA is precisely at unity gain the value of the resistor approaches infinity. (Well, almost.)
3) When the VCA gain is greater than one all hell breaks loose. (I discovered this in test.
)
So we've got some pretty good dynamic range (of resistance change) available under voltage control.
Note: The following schematic is an earler topology which has since been abandoned in favor of this one: viewtopic.php?f=6&t=368&start=31
The above is a simplified single all-pass network. I cascaded three of these for the demo. The outputs are sum/difference and one is polarity inverted. (I did that for maximum mindf**k.) Control of the demo was done manually and the unit has a good CV vs effect "knob feel" with a linear CV.
R3 is the voltage variable resistor with the "ground" end driven by the bootstrap. An input buffer is required to isolate R4 from R3 permitting a large change in delta-R3. Though not as elegant as Neil's modified all-pass topology (this one uses one more op amp per stage) the THAT2180 should outperform the SSM2164.
When I revisit this (hopefully it won't be another 28 years) I think I'll build a pair of four pole units which can be cascaded or separate with inverted CV responses.