A VCA-Based Phaser

[mediatechnology]

yes gain cell is non inverting, looks like voltage input, not current input.

I think it's current in. http://www.analog.com/static/imported-f ... SM2164.pdf

The 30K is the input V-I converter. The 30K wrapped around from the output back to the 2164 input alters the gain control slope (I think) since it's providing NFB current into the input summing node. The sign of the feedback is negative due to the output I-V. I just figured out that the same trick can be performed with the THAT2180 but in a slightly different way because it's non-inverting overall.

To invert the gain control response with the 2180 assume that there's the typical 20K Rin and a Cin to the VCA. The output I-V also has the typical 20K Rfb and a small Cc. Since the input stage of the THAT VCA is inverting (internally by the log converter) summing the overall input to the I-V's inverting node (using a third 20K brought from the left-hand side of Rin/Cin to the I-V inverting in) allows the VCA to subtract the input from the output. In other words, when the VCA Ec is at 0V (and the VCA gain unity) the overall output is 0 because the VCA's output current is being subtracted from the input current. (Assuming resistor match, precise unity gain etc.) As the VCA goes into attenuation, the overall output increases. Instead of a normal Ec vs "G" response it appears that you get 1/G.

Why this is useful in the context of the phaser I'm not sure yet, but a 1/G Ec response may linearize the effect.

EDIT: Wouldn't it be 1-G?

For G=0 the output is 1. For G=1 the output is 0.

As to the series RC network at the input, it does add a pole but I think it's there for 2164 stability (per the datasheet). I think the C that's doing all the work is the large-ish Cfb around the output. So I bet the dominate (single) pole is Cfb.

I think the SSM2164 topology could be adapted to the THAT2180/2162. The cell I show (and have demoed) is more of the classic approach but I'm not sure that when transformed into the other topology would necessarily sound any different. It would appear to save an op amp buffer though and reduce the requirement for BIFETS.

[JR.]

OK, if it is current input, perhaps that is what the R in series with the other filter cap is doing. Still looks like a classic 2 pole LPF and that gives the 180' of phase shift per stage as connected, when the 2x (10k) inverted output goes away the 1x (20k) takes over.

JR

[mediatechnology]

I was thinking about keyed effects based on dynamics and some crude form of frequency tracking in addition to an LFO. I'd like it to mutate a high hat by being keyed from it or follow a guitar part kinda-sorta. (Which I did by knob turning.) Pedal operation for sure. Stuff that made the DynaFlanger interesting.

Also thinking about inverted Cv of parallel dual 4 poles (or whatever #) and sum them so they can do crossover effects with full cancellation in the output as they sweep past one another.

The barber pole stuff looks really involved doing the crossfades. I think people could take phaser modules and then glue them together into a barber pole using a lot of off-board stuff.

One limitation of Vactrols that occured to me was their response time prevents them from being quickly keyed over a wide light/dark range. VCA control is instantaneous.

[emrr]

both of those examples sound cool, and useful for different purposes.

[mediatechnology]

Probably want to settle on 0-5V and +/-5V sweep inputs then. Makes envelope control easy.
Could have a single input with bypassable level shifter to switch between unipolar and bipolar sweep.

Exactly my thought Roger.

Thanks emrr for the feedback on the samples. I was concerned that phasing may have been a part of the 70's fog we wanted to forget.

[emrr]

I think we want to use them from time to time, and nothing available gets the 70's sound we have in our head, so we walk away from the idea most times. I've never played with one in person that really sounded right, excluding real tape machine flanging, and this seems to be on the path.

[JR.]

I made studio flangers in the 70-80s and to mimmic true tape reel flanging, we would have needed two delays so the flanging could sweep all the way through zero.

It seems a modern digital delay based flanger might be too clean.. IMO we need to model the phase and amplitude response of the tape machines of the era, and other path errors that prevent complete cancellation of signals.

I had a pretty well respected studio box (Loft 440 and later 450 delay line flanger), but the BBD based analog delay got squashed like a bug by even the crude digital delays back then.

JR

PS: best sounding clock control for flanger was linear voltage to period converter, which gave a linear delay change from triangular sweep waveform, which made for non-linear comb spacing in flanger result. Regenerative feedback also made for deeper combs and a hairier sound.

[mediatechnology]

I took a little break from working at the O.K. Ranch to test out a few phaser ideas. With brush set-out about another week away the chainsaw and I are enjoying a little separation. I do miss the smell of 2 cycle fuel though.

Bifet op amps for the VCA input buffer and the all-pass sections is a definite must. (I still used 5532s for the VCA I-V.) With R3 "growing" at AC frequencies to several times the apparent 10K due to bootstrapping, the low current noise of the BIFET parts keeps the Inoise x R3' noise levels to a minimum. For convenience I used TL072s. (R3' is the Jolly Green Giant value of R3.)

Due to hum contamination on the protoboard, broadband measurements are a little problematic. The noise floor rises about 10 dB from full bootstrap to none. Doing an FFT and picking a spot frequency, say 1 kHz, results in a noise floor change from ~ -110 dBu to -100. Needless to say this is quieter than any phaser I've ever heard (haven't heard a Vactrol one) and it seems to have excellent high level performance.

I cobbled up a 5532-based Ec driver. Most of the "action" is in the first 10-20 dB of VCA attenuation.

In addition to all of that I tried 4.7 nF all-pass caps versus the 1 nF I was using. With high rate-of-change LFO and 4.7 nF one can develop some cool Vibrato effects. The comb frequency/phase rotation is also low enough for recursion to appear in the sum/difference outputs.

[mediatechnology]

Point taken on the Vos error.

There will have to be a trim anyway to set the maximum gain limit to slightly less than unity. Right now the VCAs have a common Ec bus. If any of them go into gain, positive feedback results. I'm using a pre-bias of 3 mV, about -0.5dB, to keep that from happening. (The room temp VCA gain tolerance at Ec=0 for a 2180B is about +/- 0.15 dB.) I was also thinking about skewing the VCA input and output resistor values to prevent gain.

In any case the pre-bias doesn't seem to be too critical because a fairly large delta-R occurs.

I' only have a few OPA134s so I'll order some.

[mediatechnology]

Yep, the Vos of an untrimmed 5534, though not a lot, at 6 mV/dB can provide significant tracking error in Ec.