"Brazen Prophet" super simple 4-stage fake phaser

[Taylor]

Following up on my interest in a simple phaser that doesn't require special parts, which began here, I've been playing with some ideas. I tested out Tim Escobedo's Q+D VCF, and found it to be really nice for how simple it is. Of particular interest is his CV scheme. I haven't ever seen that done elsewhere and I totally don't understand why it works. I might start another thread to discuss that. But, put simply, it does work, and that's all we need to know to put it to use.

The idea here is much like my earlier CMOS idea, and also much like ROG's Phozer. What's unique about this circuit is that it gives you a very phaser-like sound with very few parts, and most notably, no optocouplers, LDRs, FETs, etc. No rare parts, nothing to match, no trims to set up to tune it in. Just a quad opamp and some passives.

The LFO is the one Rick used in his Little Angel. I don't know whether that's an original idea of his or a stock LFO, but I like that it gets pretty sine-ish with just a single opamp. Triangle might be nicer, but I'm aiming for ultimate simplicity here. The input goes to a buffer, then splits to 2 stages based on the Q+D VCF - these create 2 peaking filters with a slightly lowpass response. That's analogous to a 4-stage phaser with negative feedback. The resonance in the lower-tuned filter is cut a little (1k in series with the 4n7 cap pair) as it got a little woofy with full resonance. These stages are tuned to different frequencies and controlled by the LFO.

[Taylor]

I will be doing a bigger version with a switch hitter amp on the end and more controls. This will allow for a resonance control which can change the filters from peaking to notching. I'm also going to try out a CMOS inverter version. If it works, we can get a fake 12-stage phaser with just 2 chips!


Note: you do need a glowing blue resistor for the negative feedback loop around the lower right opamp, otherwise the mojo will die.

[Strategy]

Wonderful stuff Taylor, posting a layout? Very inventive!
Strategy

[Taylor]

Thanks Strategy. Like most things, it's a jumble of other folks ideas.   But I think it's pretty promising.

I will definitely be posting a home-etchable layout once I tweak it some more - I just came up with the idea while trying to sleep last night, then built it and tweaked it this evening, so it could benefit from some tweaking before I call it "done".

PM coming at you.

[markeebee]

Elegant, Taylor, elegant.

I think any hairy-arsed numpty can make a novel circuit by throwing half a dozen handfuls of components at it, but it takes a true ninja to make something cool with hardly nuthin'.

Little Angel, Tiny Tremolooooooo, Noise Ensemble, now this - we've been quite spoiled recently.

[markeebee]

I think any hairy-arsed numpty can make a novel circuit by throwing half a dozen handfuls of components at it.....

I can't, by the way, so props to the hairy-arsed too.

[deadastronaut]

nice taylor... cant wait to hear this neat little circuit...so the quad would be an  tl074 i guess!.


ive got a glowing blue resistor..i shine an led on it...

[frequencycentral]

Neat and simple - I like it a lot. The LFO by the way is very much stolen from the Phase 45 LFO, and as you say, really simple but effective. I've used it in a few different circuits. I'm wondering if the input buffer is needed, or could be omitted and the opamp used for another differently tuned filter stage - the Phoser lacks an input buffer, as does the Nurse Quacky, which also has a similar filter. BTW, I breadboarded a single filter like those a couple of months ago together with the S/H circuit stolen from the Maestro FSH, in an attempt to make a super simple S/H filter, but I was dogged by bleed from the noise generator, maybe I should revist it, as the actual effect sounded really cool (apart from the bleed).

[Renegadrian]

I'll have a vero at it, but first a noob question...where do you get the 4.5V?! the voltage divider I see has a 100k and a 47k, with the online calculators it outputs 2.8V from a 9V source...I guess there should be two 100k there?!

[frequencycentral]

^^^

Yeah the P45 LFO has equal value resistors in the divider, I tweaked them for the Angel as it's running into a PT2399 running at 5v. Maybe Taylor would like to tweak it back to 100k/100k for the Prophet?

[Renegadrian]

Also which is the orientation for the 10µ output caps!? negative towards out!?

[frequencycentral]

negative towards out!?

Yups!

[Taylor]

The schematic is drawn in simplified form - not shown are the details like vref divider, power supply filtering, bypass switching, LED, etc. which is the usual style for showing new concepts.

Neat and simple - I like it a lot. The LFO by the way is very much stolen from the Phase 45 LFO, and as you say, really simple but effective. I've used it in a few different circuits. I'm wondering if the input buffer is needed, or could be omitted and the opamp used for another differently tuned filter stage - the Phoser lacks an input buffer, as does the Nurse Quacky, which also has a similar filter. BTW, I breadboarded a single filter like those a couple of months ago together with the S/H circuit stolen from the Maestro FSH, in an attempt to make a super simple S/H filter, but I was dogged by bleed from the noise generator, maybe I should revist it, as the actual effect sounded really cool (apart from the bleed).

Yeah, in my real life version I actually don't have a buffer, but I have it after another pedal just because it overdrives easily and I needed a volume knob in front of it. As I tweak it I'll see if the buffer really is needed or not - I'd certainly be happy to free up an opamp for another filter!

Renegadrian, thanks for your willingness to do a vero layout, but you may want to hang back just a little bit as some changes will likely be made to this in the coming days.

[R.G.]

There was a published design from a couple of decades ago using a similar setup. It happened to use an unavailable VCF IC, so it never got much exposure. It did a phaser the obvious way, two cascaded notch filters, like this. I can't remember exactly where it was. Possibly a synth circuit.

A phase is, after all, a moveable set of filter nulls and perhaps peaks. How you get the notches is immaterial to the sound. The bridged-T is an OK way to do this.

PAIA stuff from 1968 - 197? used the trick of using the forward resistance of a diode as a tuning method in a Twin T. I think the "Synthespin" did this, and some of the PAIA synth modules did as well. It works well for signals across the diodes down in the 10mV-100mV range, but gets rapidly more distorted as the signal voltage across the diode approaches any significant fraction of the diode forward drop. You can stack several diodes for bigger resistance range and bigger signal tolerance.

The same circuits should work with a bipolar used in its variable resistance region (similar to the use in Dr.Q, Pulsar, and Seamoon Funk Machine) or LDR (bigger signals, low distortion), JFETs as variable resistances, and probably switched-capacitor "resistances".

It isn't a fake phaser to the extent that it makes notch filters. It's a real phaser, as real as the phase shift method of getting moveable notches. It's another point in the continuum of tradeoffs of result/$, precision, and complexity.

[Taylor]

There was a published design from a couple of decades ago using a similar setup. It happened to use an unavailable VCF IC, so it never got much exposure. It did a phaser the obvious way, two cascaded notch filters, like this. I can't remember exactly where it was. Possibly a synth circuit.

A phase is, after all, a moveable set of filter nulls and perhaps peaks. How you get the notches is immaterial to the sound. The bridged-T is an OK way to do this.

PAIA stuff from 1968 - 197? used the trick of using the forward resistance of a diode as a tuning method in a Twin T. I think the "Synthespin" did this, and some of the PAIA synth modules did as well. It works well for signals across the diodes down in the 10mV-100mV range, but gets rapidly more distorted as the signal voltage across the diode approaches any significant fraction of the diode forward drop. You can stack several diodes for bigger resistance range and bigger signal tolerance.

It isn't a fake phaser to the extent that it makes notch filters. It's a real phaser, as real as the phase shift method of getting moveable notches. It's another point in the continuum of tradeoffs of result/$, precision, and complexity.

Ah, very good info, especially regarding the diode trick. This circuit is certainly prone to distortion (but of course so are some other popular phasers). I will definitely play around with more diodes to see if I can get some more headroom. Also, a bi-phase kind of thing should be easy by inverting the diodes in one filter, as per Tim's original Q+D schematic. 

And thanks for the encouragement regarding the "realness" - it was my guess that a peak is a peak, a notch a notch, regardless of how it was obtained. I just put "fake" in there to forestall comments that I'm a dummy for not knowing what a phaser is. 

I've done some more testing of this and it's pretty cool. One great thing is that it will be really easy to add various mods to it because it's such an "immediate" circuit concept. I want to try to add a control for resonance - I know how to do it using a dual pot, but hope to find a trick to do it with a single pot.

[Thomeeque]

..but I'm aiming for ultimate simplicity here.

Why?

Btw. if you really do, then IMO you can dump whole input buffer, seems redundant..

T.

[Taylor]

Simple circuits are cool because:

-more people will build them

-they provide a good stepping stone for beginners

-designing simple circuits that work well is difficult, so offers material for learning more about electronics, flexing our thinking ability, expanding lateral creativity/problem solving

-I'm often too strapped for time to build and debug my more complicated ideas - consequently I've never built the CMOS 12-stager linked in the first post. Whereas I could whip this one up very quickly

-they can be made in small boxes, not that I care about that personally. 

It's mainly the first thing. I'm contributing an idea to the community, so the idea is to make it something that more people will be into. I've already contributed some complex and obscure ideas that nobody has ever built, and while there's plenty of enjoyment in that, it would be cool to see somebody actually build these things. 

[CynicalMan]

I just breadboarded this (it's currently making whooshing sounds on the amp behind me ), and I have a few tweaks to suggest. Firstly, the input buffer is redundant, I assume you put it there anticipating future need, but for now it's not really necessary. Also, I'd increase the mixing resistors to 10k each, just so your op amps don't have to drive 2k loads. Thirdly, I'd mess with the LFO bias a bit. For me, the stock setting had the bias way too low. The phaser sounded like a really bassy pulse, instead of the nice sine-y sound we want. I found that a 47k resistor in parallel with the 10u cap on the LFO's output helped a lot with that. That's even a place for a possible mod: a 1M-ish log pot in parallel with that cap that would shift the sweep range up or down.

The major issue to me seems to be the distortion. I don't have time now, but tonight hopefully I can take a look at the filters in LTSpice to see if you can reduce the gain without affecting the filtering.

Anyway, a very cool circuit. I hope you don't mind if a tag along in the design.  

[Taylor]

I've actually got most of those changes planned/implemented. On my build, I have a CV pot that adds to the LFO so you can shift the sweep through different ranges - without that (as I've drawn it) it's definitely not too usable.

The schematic I posted was kind of just to get the idea across and I wouldn't recommend anybody build it just yet. I will have a tweaked version very soon. I don't mind at all you or anybody else suggesting mods or changes, in fact that's what I hope for! But it might a good idea to wait for my finalized schematic just so that the development is linear instead of parallel paths, which would confuse matters considerably.

Regarding distortion, Tim had a trim pot v. divider on the front end of his Q+D VCF - I'm just turning the guitar volume down. As RG noted above, adding more LEDs should give more headroom, and a fixed v. divider on the front end would do the trick, at the cost of noise.

[Hides-His-Eyes]

..but I'm aiming for ultimate simplicity here.

Why?

simple circuits to do things that previously required complicated circuits are Art.