"Brazen Prophet" super simple 4-stage fake phaser

[Thomeeque]

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

O-oh, sorry, that was really stupid

OK, when I'm at it may I yet troll about sound samples, are there any already, please?

Thanks, T.

[Gurner]

My suspiscion is that this one isn't going to give you that 'well fed' feeling that a multi stage phaser can nourish - from what I can gather, this implementation is playing the differing phase shift of two filters off against one another ....innovative yes, pleasing...hmmm. I look forward to the sound clips!

On a more positive note, I can save you one cap in your build - the output DC level wrt both those final opamps is sitting at the same level.......and anyways, there's 2k sitting in between them which caters for any slight DC imbalances - the two caps on their output pins can be binned ....one cap is suffice on the final output.

[Renegadrian]

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 want to vote, but first I have to understand better! I'd say one quad, but with the most options it can give - so it is #3!?
Does it require a lot more components compared to #2 or #1?!
I'd like to see a circuit simple to build but with a good range of sounds, from subtle to mayhem! 

To say, Little Angel meets the above requirements for me. Simple but good sounding.

[Taylor]

from what I can gather, this implementation is playing the differing phase shift of two filters off against one another ....innovative yes, pleasing...hmmm. I look forward to the sound clips!

Well, no, there's really nothing to do with phasing happening here. It's simply 2 filters, tuned to different frequency ranges, swept by the same LFO. I don't claim it to be particularly innovative, but I do think it's pretty sweet for such a small, no-fuss circuit.

Thanks for the suggestion on the output caps - you're quite right. 

[frequencycentral]

I do think this is innovative - don't do yourself down. I've played around with single stages of that kinda filter and they sound cool. The peaks/notches on a 3 stage version will certainly be sonically very interesting. I'll certainly be building the complex version of this  - really looking forward to it reaching fruition.

[Gurner]

from what I can gather, this implementation is playing the differing phase shift of two filters off against one another ....innovative yes, pleasing...hmmm. I look forward to the sound clips!

Well, no, there's really nothing to do with phasing happening here.

There must be else you wouldn't get the phasing effect! (I'm meaning phasing here to be audio signals being summed with differing phases)

There'll be a different phase response appertaining to each filter, isn't it a case of by applying the CV, the phase slope will alter with the LFO - & since the signal output of these different 'phase slopes' are being summed you're gonna have phase notches?

[Taylor]

No, notice I called this a "fake" phaser. If you read the first post I think it will be clear. These are not allpass filters, they are bandpass filters. By putting them in an opamp's NFB loop, we get notches, but it has nthing to do with the 2 filters' phases being different or anything to do with summing - the reason there are two is not to get any cancellation between them, but to get 2 peaks or notches instead of one, as you'd have in a 4-stage phaser.

[slacker]

Yeah Gurner's right there is phasing going on. Here's a simulator plot of one stage



The trace with the peak is amplitude and the other trace is phase with the scale on the right. As the peak sweeps up and down the frequency range the phase plot roughly follows it with the phase inverting at around the frequency of the resonant peak. The shape of the phase plot changes somewhat as the resonant peak moves through out the frequency range but in any position different frequencies have different phases.
Mix the outputs of two of them together and a given frequency from each will have a different phase, this will change as the filters sweep, so you've got phasing. Especially in the range between the two peaks you get cancellation giving a notch.

EDIT: this wasn't in response to you Taylor, you posted while I was typing.

[Gurner]

No, notice I called this a "fake" phaser. If you read the first post I think it will be clear. These are not allpass filters, they are bandpass filters. By putting them in an opamp's NFB loop, we get notches, but it has nthing to do with the 2 filters' phases being different or anything to do with summing - the reason there are two is not to get any cancellation between them, but to get 2 peaks or notches instead of one, as you'd have in a 4-stage phaser.

I'd already read your first post (before making my prior posts)....I didn't even mention an allpass filter as clearly that's not what's going on here - I think you might be misinterpreting my reference to phase shift as thinking I mean 'traditional' (in the stompbox sense of the word)  phasers....I don't, you may call it a fake phaser, but you're essentially playing two different phased signals off against each other (ie summing signals with phases that aren't 100% in sync), which will yield an ahem 'phased' sound.

Those 'peaks' have come about by using two separate (animated) filters  - & when filtering you get a phase shift. So, you may be using peak filters ...but IMHO the outcome is you'll get differing phase shifts at different frquencies when putting a signal through each filters, thereby creating moving notches when they're summed.

[Taylor]

Hmm, well in all respects I'm always willing to concede that I could be wrong.   I know you know more about EE than I, I just often assume that people are misunderstanding me as that is often the case. (Poor communication skills on my part.)

I plan to put this through real life frequency analysis once I build a final version, so we can see exactly what's going on here. If you're right, then my design is much more clever than I'd thought!    

[earthtonesaudio]

In general, all filters shift phase.  Most filters give a frequency-dependent amplitude.  The exception is the all pass filter, which is probably why it's the one people often call a "phase shifter."

In fact, the only reason you hear notches in a "real phaser" is because of the additional process of mixing in the dry signal.  Without the dry signal, you get vibrato with no tonal change.

The only thing I would suspect this parallel-bandpass topology incapable of... is vibrato.

[R.G.]

Taylor, one really interesting thing to do with this kind of filter is to sweep the two filters with LFOs that are not the same.

You might want to read the stuff at GEOFEX about vocal tract filtering.

[Taylor]

Taylor, one really interesting thing to do with this kind of filter is to sweep the two filters with LFOs that are not the same.

You might want to read the stuff at GEOFEX about vocal tract filtering.

Right, I was hoping to do that by inverting the diode orientation on one of the filters, to get one going down while the other goes up. This is cheap because it can use the same LFO with no extra opamp needed for inversion, but I haven't gotten it to sound very good yet. On the verge of some "yoyoyoyoyoyeeee" type sounds ala the Ludwig Phase II, but still kind of lacking due to the compromise of the design.

What might be nice would be to try a quadrature oscillator, and use the sin and cos outputs for the different filters. That gets the job done with only 2 opamps while having them staggered instead of just in anti-phase. Totally independent LFOs would be fun too, as you say.

The best part about this idea, to me, is that it's really "immediate" so it's possible to implement all kind of interesting ideas like the formant filtering you're talking about, changing the peak spacing is easy, etc. I would expect there to be tons of versions of this as time goes on. I kind of wish I had picked a less pompous name for this one.  It was a take-off of "brazen head", ancient experiments into speech synthesis thought to be prophets.

[culturejam]

Taylor, do you have a current/working version of this to post? I wouldn't mind having a go at this on the breadboard.

[deadastronaut]

pompous or not...lets hear it...especially the yooooooowwwweeeeee...great description btw....

[R.G.]

Right, I was hoping to do that by inverting the diode orientation on one of the filters, to get one going down while the other goes up. This is cheap because it can use the same LFO with no extra opamp needed for inversion, but I haven't gotten it to sound very good yet.

The problem with that is that the diode-resistance setup needs to have the diode sitting at nearly its active forward drop at DC to be off, and only moves about 0.2V to fully on. You can't just invert the diode on one side without also trying to invert the DC bias on the diode. If you do, you get a big dead spot where neither one is doing much. Also, you have to have a CV which can both source and sink current, probably at the same time.

I think you might be able to get something like this to work if you used each diode in series in the middle of two resistors, one to V+ and one to ground, bypassed the center to ground with a BFC and fed the control voltage to the middle point. That forces you to put the resistor ends of the diodes to the T filter points, perhaps with a couple more BFCs, and worry about the interactions of the resistances. It would be a tight fit for whether this would be more or less parts and PCB space than adding another opamp. An opamp, on average, is four pins, and needs another four pins of resistors to do the inversion. It's one of those "what are the costs" questions I referred to in "PCB Layout for Musical Effects".

As a meta-discussion on LFOs, I've found that LFOs are quite picky. You have to know and correspond to the necessary input range of the thing being modulated, in both AC and DC terms. The variety of things being modulated make this a design task each time, not usually something intuitive.

What might be nice would be to try a quadrature oscillator, and use the sin and cos outputs for the different filters. That gets the job done with only 2 opamps while having them staggered instead of just in anti-phase. Totally independent LFOs would be fun too, as you say.

I built something similar with a dual OTA for each filter and a PWM'd control voltage from a PIC back when I was messing with the Ludwig. It was fun. An 8-pin PIC can supply two PWM'd outputs and have up to four pot or switch inputs on the other pins. Easy enough to get quadrature without the analog messing.

A three-phase integrator LFO locked at 60 degrees is interesting too, and can be done with CMOS inverters.

[thedefog]

http://www.diystompboxes.com/smfforum/index.php?topic=85658.0

I made a layout a while back for the Q&D VCF. I built mine into a busted WAH pedal and also added in a simple LFO with Triangle/Square modes. It sounds really nice and I actually have used it a number of times in recordings. I'm thinking an envelope trigger on this would work really well, tapping into that CV source. Maybe an all in one pedal with mode selector (LFO, Phaser, Envelope trigger) is in order here 

[Taylor]

Taylor, do you have a current/working version of this to post? I wouldn't mind having a go at this on the breadboard.

I was hoping to get some time to draw up some more final versions, but I might not be able to this week. If you look at my last schematic, rev2, that will work pretty well with these changes:

-make each LED chain 2 LEDs instead of 3. Try red or blue (red lower forward voltage=more distortion, maybe better sweep range)
-forget the trad/biphase switch for now. Connect upper LED chain to ground just like the lower one

Some things to play around with:

-the 1n/4n7 caps that make the "T" in each filter tune the base frequency.
-add the resonance control I mentioned in reply #44. Makes it much more versatile.

[nepalnt21]

i know this is really old, but i love this idea! does anyone have sound clips? schematics? updates?

i especially like the idea of each notch being swept by a separate lfo.

[Taylor]

I don't think I developed the idea much beyond whatever I posted in this thread, but it looks like the image links are dead, so I'll try to track down the schematics on one of my old computers.