MXR Envelope Filter - Build Report Part Deux

[Bill_F]

Thought I'd just report in again after playing this bad boy for a bit. All I can say is Peter I can see why this was your favorite out of your purchased pedals. This thing is unbelievable, so versatile and fun. It's not a plug-n-play sort of pedal. All the knobs seem to interact to some degree, but once you start to see how they are working along with the mods (reverse, emphasis, filter) the variations are immense.

Today I was playing a DIY SD-1 into the envelope filter then into a slightly overdriven Laney amp. I was getting tones that reminded me a bit of Michael Shenker. I set the envelope filter to where it was getting hardly any sweep, it sounded more like a wah in a fixed position, very nasally. I couldn't believe how good it sounded, very inspiring.

I built this more as a challenge, thinking I might not use it a lot. I was a bit tired of building distortions (sorry Aron) and wanted to try some things that were different. But now I can see that this thing just has to go onto the pedal board.

Sorry for rambling, but I wanted to let you all know that this pedal is well worth the time and effort it took to build it.

Thanks Francisco, another great project.

Bill

[Fp-www.Tonepad.com]

you're welcome!

[Mark Hammer]

Envelope-controlled filters are probably one of the most fun and least understood types of pedals for guitarists.  Most of the time, I think that they are too smart for the average player.  On the other hand, like so many things in life, having a decent explanation and and an opportunity to take ownership of something (which is where the project part comes in) bridges that knowledge gap easily.

Try this and tell me what you think.  In 1979 I used to do a backwards tape emulation with a cheap Univox compressor, my MXR 6-band EQ and my (unmodified) Envelope Filter.  The pedals were in series: compressor -> EQ -> EF.  The EF was set for slowest attack.  My first one had a lower Q than a later one I bought in 1987 or so.  I think the feedback resistor was 200k on one of them and 270k or 240k on the other one.  The earlier one had less resonance, and probably even a little less would have been better for the effect I'm describing.

The EQ had the lower 3 bands rolled off and the upper 3 bands dimed (the legending on the 6-band said that it had 18db boost and cut though I doubt it actually did). The compressor was set for a pretty hard squeeze and the output cranked.  Compressors have a way of restricting the amount of sweep in the EF, and in this case I set the threshold just at the edge of sweeping.  The Univox had a very long decay (you can probably duplicate it by upping the decay cap in an Orange Squeezer) so the notes would blossom over an extended period.  What would happen with this combination is that the filter would sweep just a bit so that it essentially swelled rather than swept.  Because it took a while to get where it was going, and because the low end was reduced via the EQ, you didn't really hear anything for a bit.  Then as the filter swept up, the roar of the boosted mids and treble would enter into the picture, so that just like tape reverse, the tone became crisper as you moved towards the "beginning" of the note.  Extremely convincing.

Yep, there is a lot to love in that pedal.

[Peter Snowberg]

I'm glad you like it Bill.

I'm really curious how different yours came out from a factory unit. I haven't seen mine for several years (I know it's in a box somewhere :?) and I'm getting inspired to build a replacement using film caps and more pots.

Now that you have it built, it's time to think about mods. Lol! One to think about is a sidechain for the envelope follower. With that addition, you can process a signal before the EF with other pedals that destroy the dynamics, but use a dry (or compressed, or expanded) signal to control the envelope in the EF.

Envelope-controlled filters are probably one of the most fun and least understood types of pedals for guitarists. Most of the time, I think that they are too smart for the average player.

I couldn't agree more. Appreciation requires somebody with a sense of adventure to figure out how to make use of them. At that point the fun begins.

Take care,
-Peter

[mattv]

I'm glad you like it Bill.
Now that you have it built, it's time to think about mods. Lol! One to think about is a sidechain for the envelope follower. With that addition, you can process a signal before the EF with other pedals that destroy the dynamics, but use a dry (or compressed, or expanded) signal to control the envelope in the EF.

By this, do you mean a separate input for the follower? Then I guess you'd split the guitar signal, with one side going straight to the envelope follower and the other through a Big Muff (or whatever) and then to the regular input. Cool.  

[Mark Hammer]

There are two categories of loop-sends that can be applied to envelope-controlled filters.

One is the pre-filter loop that the Q-Tron and Meatball use, where the envelope follower is directly tied to the input stage and uses the "purest" instrument signal it can to derive the envelope signal that will be applied to the filter.  Once that signal is tapped, the pedal can send the audio off to parts unknown where it can be mangled before being processed by the filter.  Great idea, since some of those other effects canseriously screw with the amplitude envelope in ways you don't want (e.g., fuzzes reduce the quiet-loud contrast too much sometimes, and modulated effects like phasers just screw it up big time).

The other is the pre-rectifier loop.  Here, something can be inserted between the pedal input stage and the envelope follower to deliberately (or even randomly)  monkey with the signal envelope or even replace it with something else.  For instance, you can take a whole-band-feed or a drums-only feed from the mixer and feed THAT to the envelope follower so that it sweeps with the band not your own playing alone.

In the case of de-essers, they shape the signal being detected by the envelope follower so that when it is applied to a gain cell (i.e., in a compressor/limiter) it cuts the gain most for sharp sibilants, and very little for bass or mids.

One thing that has got me thinking lately is the idea of delayed envelopes.  In other words, you feed the audio signal to a delay line on the way to the envelope follower, such that you get the same envelope that you plucked, just not *when* you plucked it.  If I can depict it verbally, imagine a pedal with a bandpass filter that when you pick it goes "boooooooooo-WA-cka" (i.e., 100msec delay for onset of sweeping).

In principle, this should be bloody easy to implement.  Since you don't really need much bandwidth (500hz-1khz will be plenty), you can squeeze oodles of delay time out of a single MN3207 or a BL3208, by clocking it ultra-slow.  Since none of that clock signal is entering the audio path little or no filtering or other noise reduction will be needed.  In other words, you could implement envelope delay using nothing more than a BBD and appropriate clock-chip (MN3101/3102) and the usual passive components around those chips.  Further envelope shaping can be done at the rectification stage itself, via the usual attack/decay stuff.

[mattv]

I think Tim Escobedo's Phuncgnosis already has some of these features. The delayed envelope definitely sounds different and interesting. I guess you could even just use a delay pedal with separate dry & wet outputs into the regular & envelope inputs of the envelope filter.

Hmm.. what else? Oh yeah. Find time to implement these ideas!  

[Mark Hammer]

Actually, now that I think of it, if you made the delay-time resistor tied to the MN3101/3102 a foot-operated control.  You could bend and reshape the envelope by samplng out at different rates than you sample in.

So, set the envelope-delay for shortest delay time, play then  immediately slow the envelope delay down and sample out at a different rate.

This is getting twisted!  :twisted:

[mattv]

Not only that, but it seems like using a couple of repeats would allow you to get two or more wahs with only one pluck of the string. I've been pondering something similar for a few weeks now and had yet to think of any good way to do it. This is great!