MXR Style Envelope (Vowel) Filter

[Mr. Lime]

It's been some years since I did my last try of my own circuit but I recently felt I should start something again.

A circuit that always caugth my attention was the old MXR envelope filter from the 70s.
It's said that it's PWM envelope detector has a quick and pleasant response and that the pedal generally is one of the best envelope filters for guitar and bass.

Since only two gates of the 4066 bilateral switch are used, I thought about a use of the left over two.
Adding another 4069 CMOS inverter IC offers enough gain stages for another state variable filter plus an input buffer and a summing stage to adress the volume loss some people report with the MXR clones.

My goal is smoething like the schematic shows; two antiparallel filters where the upper one has options for the MXR standard lowpass output and a bandpass output.
The lower filter takes use of the highpass output and a clean bypass for the bass frequency fundament that I feel is missing in my previous autowah pedal builds. The second filter can be removed for more classic envelope filter sounds.

Since I want to adress potential optimizations on the MXR filter circuit, I would add a switchable input jack for the envelope detection part so a clean guitar signal can be applied while the filter part is used in the fx loop of an amp.
Same goes for an expression pedal input that should allow to insert a 5V CV instead of the envelope detector.


What do you guys think?
Has anybody experience with antiparallel filters and might have an advice?

Is there a reasonable benefit to replace the inverters of the filters with opamps?

Thanks in advance.

[R.G.]

See: Geofex.com: Sing-Wah

and

1999's Human Voices and the Wah Pedal

[Mark Hammer]

1) Having adapted the MXR circuit to provide BP or LP, note that the LP volume will be greater than the BP volume.  Not insurmountable, but requiring planning.

2) Ever since I first bought one in 1977 or so, it has been one of my all-time favourite envelope-controlled filters, primarily because of its variable attack time, that allows one to match the "feel" of the sweep to the pace of the tune where it will be used.

3) Personally, I don't see as much value in having a highpass section as in having two different bandpass ranges, that may have different time constants.  Remember that when different formants are brought in is as critical to the "speechiness" of the result as their respective level.

[R.G.]

I agree Mark. Two bandpasses will be more like the vocal tract actually does. One resonant low pass like wah filters actually do, coupled with a second bandpass filter.

A minor complication with vowel makers that occurred to me when thinking about the Sing-Wah was what vowels would actually be sung. The simple cheap and cheerful approach is to just make the resonances move somehow, and uncontrolled, through the vowel space of LP frequency versus higher bandpass frequency. That is, does it sing "ooooo - aaaahhh - eeeee" as you rock the pedal, or some other sequence? Does it avoid "uuuuuhhhhh"? Are the formant frequencies set for male voice or female voice ranges?
I probably over-analyzed it, but it sure seems to me that some kind of control is needed for the, um, more discerning guitarist. I considered putting half a dozen momentary foot switches on a pedal, each one enabling a vowel sound. Want an "aaaaaaaaaahhhh"? Press the "AH" button. The innards would then need to pick the combination that did that. Using a rocker, you could make a control voltage that the circuit picked off for a given sequence. The formant selectors was a move in that direction.

[Matthew Sanford]

I might be off, but on the moving resonances I've been thinking on something similarish. Thinking on the CMOS Handbook "Caver's Lamp", using average of PWM to change the voltage level powering it, would it be possible to do something similar with a cv or envelope to use it's average voltage to power a variable resistor (LDR, FET, others) to create a resistance in relation to it so as the CV or envelope changes so does the resistance, in this case creating the appropriate resistance for the resonance desired? I'm sure it would take a bit to hone it down, and if I'm way off base just ignore my nonsense, you know. But... could it?

[R.G.]

Yes, it's possible. You could also just PWM the LED on time and let th slow response time of the LDR average it out to smooth. For a few other approaches, you could look up switched-capacitor resistances and so on. IIRC the MXR Envelope filter was once a switched-capacitor filter.

There was once a wah pedal (um, FX-17 comes to mind) that was an RF oscillator that was tuned by a sliding metal plate on the rocker changing the capacitance of its tuning circuit. That was then run into an RF frequency to DC converter driving a voltage controlled filter.

[ElectricDruid]

The FilterFX is such a filter. It's the typical state-variable filter like described here and those are frequently used because you can get the three outputs if you like (HP, BP, and LP, all at once!). You also get independent control of resonance, so the SVF is a great design for envelope filters. There's several ways you can control them (PWM of 4066 switches, for example, or using 13700 OTAs) but I chose the "vactrols" approach you mentioned and then controlled the LED brightness using PWM (Ok, PDM, strictly speaking) from a StompLFO. RG is right that the LDR's smoothing effect is very helpful here.

But anything that generates a PWM signal could control those LEDs - envelope follower, whatever. If you set up two FilterFX filters in series or in parallel, you could tweak the caps in one of them to give them a bit of frequency separation and then use the same control signal to move them about.

[Mr. Lime]

Thanks a lot for the response!

I'll consider switching the highpass to a bandpass, since even the lowpass sounds quite bright in the demos.
The volume change is a valid point I might adress with a trim pot in place of the mixing resistor (R9).

I already built a FilterFX but hardly use it to be honest. I guess I'm just having more practical use with an envelope controlled filter than a lfo and the MXR circuit has been on my to-do list for over a decade.

Also I think I'm happy when it turns out to be just a different kind of MXR wah at the end 

[ElectricDruid]

I already built a FilterFX but hardly use it to be honest. I guess I'm just having more practical use with an envelope controlled filter than a lfo and the MXR circuit has been on my to-do list for over a decade.

Sure, I was replying mostly to Matthew's message. Sorry for the derail.

That MXR schematic is certainly interesting. The 4066 switches and PWM to control the integrators is reasonably conventional (although not common) but *why* did they choose to use 4069 inverters in a linear mode instead of proper op-amps?!

[PRR]

*why* did they choose to use 4069 inverters in a linear

At one time, it was fashionable. "MOS is tube-like!??" Also I suspect after the peak of CMOS Mil/Ind popularity there was an over-supply and a price crash. Price/profit trumps perfection.

[Mark Hammer]

Bear in mind that, for a period, MXR went essentially "CMOS crazy" in many ways.  There was the phase shifter that used PWM control of phase shift, and the analog delay that used CMOS to provide a "tracking filter" that offered maximum usable bandwidth for any given delay time.  IMHO, the envelope filter was simply one more instance of this exploration of PWM and CMOS capability.  Not suggesting it had nothing to do with what others have suggested, but those weren't the only factors.  I'm sure we've all stumbled onto some little trick that had never occurred to us before, and we proceed to beat it to death with a stick.

[duck_arse]

Adding another 4069 CMOS inverter IC offers enough gain stages

connect pin 13 IC1F to pin 12 IC2F, then single pole switch between pin 12 and other pin 12. you don't want a floating cmos input next to a "High Frequency Oscillator".

[ElectricDruid]

...and we proceed to beat it to death with a stick.

No, no, no! We merely "develop it to its fullest extent", or "explore the entire realm of its possibilities"! You make our careful engineering process sound so BRUTAL! 

Point taken though. PWM does have plenty of possibilities, especially where you need to control lots of something at the same time, like multi-stage phasers or multi-pole filters. I can see why MXR might get into it. I suppose it's also a way to differentiate your stuff, if that's something you're doing and no-one else is.

[R.G.]

[...] we proceed to beat it to death with a stick.

Stick, nothing!! I prefer a sledge hammer, and then burying it under old textbooks.   

[Eb7+9]

but *why* did they choose to use 4069 inverters in a linear mode instead of proper op-amps?!

... proper op-amps??

that's Keith Barr pushing his design muscle (cuz he had one) ... still, there is another pretty obvious reason for doing it - as demonstrated here :

https://m.youtube.com/watch?v=KLswgNkhzbY&pp=ygULQXJkdWlubyB3YWg%3D

[ElectricDruid]

that's Keith Barr pushing his design muscle (cuz he had one) ... still, there is another pretty obvious reason for doing it - as demonstrated here :

https://m.youtube.com/watch?v=KLswgNkhzbY&pp=ygULQXJkdWlubyB3YWg%3D

Ok, so please explain what the *other* "pretty obvious reason" is for us simple people who aren't getting it from your video link. Thanks!

Are you thinking "the sound"? Honestly, I'm not clear where you're at.</curious>

[Aleph Null]

Ain't got nothing insightful to say, just want to watch this thread.

[R.G.]

I think that the link to his own "Viva Analog" youtube video is an attempt to say
"Look - I cleverly did a wah using an arduino" or to drive more views to his youtube videos. Just guessing.

I could be wrong, though. I'd like to hear a clear statement of the pretty obvious reason. It seems to have gone over my head, at least.

J.C. - tell us what's obvious, please.

[PRR]

just want to watch this thread.

If you are 'saying nothing' just to get notified, there's a quieter way. Bottom of thread, click "No Alerts Or Emails", change the setting to Alerts, or Emails, as you wish.

[Aleph Null]

just want to watch this thread.

If you are 'saying nothing' just to get notified, there's a quieter way. Bottom of thread, click "No Alerts Or Emails", change the setting to Alerts, or Emails, as you wish.

Good to know! I took "no alerts or emails" to mean the opposite.