"Brazen Prophet" super simple 4-stage fake phaser

[Taylor]

Ok, here's the updated version. I have not yet built this, but I tweaked the LFO a bit in simulation to get it more triangular, and also to make it capable of going both faster and slower than before. I'll never pass up an opportunity to make anything into a pseudo-ring modulator.

Besides the LFO, I've added more diodes to hopefully allow more headroom, and added a switch to switch between traditional sound and bi-phase, where the 2 filters are sweeping in different directions.

Of course, this leaves us with an unused opamp, and I've got all kinds of ideas for utilizing that. Also, a depth pot would be easy to add, and a resonance control for the filters would be very useful but requires a dual gang pot. But before we add those goodies I'd like to test out the basic circuit. If anybody has time to breadboard it I'd be interested to know what you think.

[Renegadrian]

v1 vero is ready. I have to correct it to v2 specs as soon as someone says it's ok.
btw, cv pot is like a starve control!?

[Taylor]

No, cv pot is like a manual sweep control or LFO bias. It could be a trimpot as in the Phase 90, but I find i useful as an external control.

[Paul Marossy]

Ooo... I'm keeping my eye on this one. Sounds like it might be pretty cool.

[Taylor]

Ok, I built the above circuit, and it does work, with a caveat:

Although the additional LEDs increased headroom, they also changed the frequency range too much for my liking. It seems limited rather than expanded. I used blue LEDs rather than the spec'd red, which have a larger forward voltage. I would now recommend 2 red LEDs per LED chain.

I also added the resonance control. Notice the 2k resistor between the lower opamp's output and the 4n7 cap. If you replace that with a pot (10k-20k would probably be ideal) you have a resonance control for that filter, analogous to a feedback control on a standard phaser. The same can be done between the upper opamp's output and the 1n cap currently connected to it. You can use a dual gang pot to control both filters at the same time.

Now, what to do with the unused opamp:

1. make a real triangle integrator LFO (I know a certain someone who likes that idea).
2. add another filter stage to emulate a 6-stage phaser.
3. I think if we put the whole audio path in a negative feedback loop, we get notches instead of peaks. We don't see this opamp-nesting done in stompboxes much, but I don't think it's a crazy thing to do.

Edit:
Yep, number 3 does work.

[Skruffyhound]

Good work Taylor, I'm looking forward to breadboarding this.

[Taylor]

The "biphase" thing is not really working the way I'd like. the 2 sweeps need to be offset somehow for them to be sweeping through useful ranges at the same time. I tried a single LED for the LED chain going to 9v, with a series trimpot. Tweaking this got it more in range, but not to my satisfaction.

The trick might be something like this: connect the standard CV pot directly to the lower filter, but in the upper filter, connect it within the right side LED chain. Then have the "biphase" trimpot connected within the other LED chain. That way the CV pot controls both filters when in traditional mode, but it's switched out of the upper filter in biphase mode, and the trimpot takes over LFO bias duty only for the upper filter. This should help focus the frequency centers for the opposing filters to a range that sounds best.

[frequencycentral]

3. I think if we put the whole audio path in a negative feedback loop, we get notches instead of peaks. We don't see this opamp-nesting done in stompboxes much, but I don't think it's a crazy thing to do.

Edit:
Yep, number 3 does work.

On my modular synth I can put a signal through a lowpass filter, then mix (in the correct ratios) the output of the lowpass filter with the original signal, ensuring first that the output of the filter is inverted with respect to the original signal. My lowpass filter is now effectively acting as a highpass filter, courtesy of phase cancellation.

[Taylor]

Yes, originally I had planned to do it as a switch hitter amp for the clean signal, which would allow to pan from inverting to non-inverting mix, with the idea that it would do as you say due to cancellations. For some reason I started to doubt that this would work and decided to do the thing with nesting the filters in the NFB loop, but actually the switch hitter is probably a solid idea. Thanks for confirming that that would work.

Overall I'm pretty happy with this idea.  It's easily expanded in a few different ways. Right now the LFO works very well in medium to high speeds, but not very good at slow speeds. An integrator triangle generator would be better for those who like slow phasers; the inverting trick would be good to get actual notches, which will sound more authentically phaser; and using the last opamp for an additional phaser would be great for those who like more extreme sounds. I've yet to try the CMOS version I was thinking up, but if it works I'll do a version which is analogous to a 12-stage phaser, with proper triangle LFO, and true notches, all with just 2 chips and passives. 

[Renegadrian]

reading what you wrote, it seems that v1 works better...that is verified, isn't it!? Maybe I shall try it without the buffer at the input...

[Taylor]

Well... the first one, as drawn will not be too usable because it doesn't incorporate the CV pot, the distortion is a little too much because it only has a single LED per filter, and the LFO is a little too limited. I would recommend building according to the rev2 schematic, except for now I'd leave out the biphase switch and only use 2 LEDs per filter. It sounds quite good like this.

However, I expect there to be some tweaks and a few different versions with different goals over the next week or so. This is an idea in development. So it might make more sense to just breadboard or perf for right now if you want to check it out. If you're looking more for a finalized effect to go right on your pedalboard, probably a good idea to hang on a bit until we get somewhere that we can call "done".

[Renegadrian]

right on, I'll vero the "final version" when done.

[SISKO]

Although Leds and transistor are much more consistent than jfets, wouldnt you just need diode matching for this circuit anyway?

I think that led matching would make the notches equally spaced trough the sweep of the LFO and both the same depth.
This would start to sound less like two low pass filter and more like a real phaser.

Im not really sure about this, as i didnt breadboard the circuit yet. Just a tough.

Also, at slow speeds: why doesnt sound good? It doesnt have enough depth? Or is it just the oscillator not having enough "linear ramping"?

[Taylor]

No diode matching is necessary. You're not going to get consistent depth through the sweep with a circuit like this, regardless of matching. The idea here is "quick and dirty" so there will be some tradeoffs - that said, I think it sounds quite nice.

At slow speeds, the sweep both looses depth and, as you say, doesn't have linear ramping - it becomes somewhere between a sine and pulse wave.

[Taylor]

For those interested in potentially building this, what sort of configuration would you be most into? I have thought of a few different versions:

1) single chip, has a better LFO for better slow sounds
2) single chip, allows for peaks or notch filters (the current one only does peaks - notches are the more "classic" phaser sound)
3) single chip, same as the current one but with an extra filter for more "wet" or extreme sound
4) two chips, has all of the above

I can make schematics showing all of them, but I figure it might confuse people to have to decide between 4 different schematics/layouts. Give me some feedback on what most interests you among these. Here is a little poll for you to choose what interests you most.

[frequencycentral]

I voted for option 4 (cool to vote for something you actually care about), but I do think there should also be a 'starter' version - in keeping with the 'super simple' concept, probably option 2.

[Taylor]

I'll probably do the full featured one (option 4) and another one, because as you say I really like the idea of a single chip phaser.

[deadastronaut]

yep. both options sound good to me.....simple is nice!...especially for breadboarding and learning!. nice one taylor, look forward to hearing it/them..

[Thomeeque]

..I really like the idea of a single chip phaser.

Well, you could use quad op-amp chip, but it would be probably dirty cheating, right? T.

[Taylor]

I'm already using a quad. If you're going to troll, please pay closer attention.