# Envelope Filter - Pea Co. Pedals



## Peaco (Jul 25, 2014)

This was my first run of Envelope Filters. Completely hand wired. Based off the MXR schematic I kept it mostly stock except for dealing with volume drop issues. This is one of my favorite circuits that I have done so far. It is super clean sounding. Its like 1970's Dead all night long.

Here is a sound sample.

https://www.youtube.com/watch?v=zhJiKUr3IpE

Here are some shots.


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## Peaco (Jul 25, 2014)

So I was tinkering around during the week and decided to rehouse my personal envelope filter. I took the stock MXR version and added an emphasis control, a switchable reverse sweep and a mid boost switch. I messed around with a bunch of values and was able to make it overall brighter. They are notorious for volume drop so I managed to more output of it as well. I think it looks pretty decent.


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## mhammer (Nov 30, 2007)

While Steve Giles' mod for getting downward sweep by merely inverting the pulse width with the leftover invertor section is pretty brilliant and elegant, if you ask me (and I have it built into a few of these), it suffers from "starting" at the wrong point. While upward sweep in envelope-controlled filters is fine starting out from the lowest possible filter frequency, downward sweep should NOT start out at the highest possible filter frequency. In fact, that's part of what the Mu-Tron/Q-Tron circuit does when you switch drive direction. It doesn't just produce "the opposite" as Steve's mod does, but sets the start point in the sweep a bit below the highest point. So, ideally, one wants to implement the addition of the invertor section, but use a 3PDT toggle to adjust the pulse width when the extra invertor is added.

Note, as well, that the circuit is actually a state-variable filter. You can get lowpass, bandpass, and highpass outputs from it, albeit at rather different volume levels. On mine, I just have the added Q/emphasis pot, sweep direction, and a range-select switch. Good enough, I suppose. 

I always did like the MXR unit. One of the few units out there with a usable Attack-time control. You can insert an Attack control into just about any envelope-controlled filter, but the nature of the majority of envelope follower circuits is that you lose too much envelope strength for the small variation in attack time that you get. The MXR circuit doesn't have that problem, and the resulting wide variation in attack time allows the pedal to match the feel of a song much better. Though nobody really uses it with that circuit, varying the decay time is also quite useful.


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## Peaco (Jul 25, 2014)

Mhammer, thanks for chiming in and shedding some light on this. I always felt that the reverse wasn’t quite a reverse. I understand the inversion, but it just kind of sounds weird. I do like it though, and I think it could be useful in certain situations. 

I have zero experience with the tron type filters. I have been eyeing their BOM’s though. Maybe that will be a fun comparative project for a rainy day. You have intrigued me. 

So far I really like the MXR circuit…. I really like the 70’s Dead Shakedown street type of sound and this does it very well. I also really like experimenting with it. I would like to learn more about modifying decay… It is also something I have been reading about regarding compression. Very interesting!

Thanks again!


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## mhammer (Nov 30, 2007)

The decay time will be set by R17 here: http://generalguitargadgets.com/wp-content/uploads/mxr_envelope_filter_sc_mods.gif

The circuit "works" by turning the CMOS switches in U1 and U2 on and off, for different amounts of time. Staying off longer mimics a higher resistance. The circuit built around A1a/b/c is a high frequency clock with a given pulse width (i.e., on time and off time). The envelope voltage at R23 combines with this clock pulse to arrive at _changes_ in the on and off time, making the filter sweep. The "reverse sweep" mod simply routes the clock through an additional (previously) unused invertor section so that however long the clock pulse was "off" is now the amount/proportion of time it's "on".

The change needed to make the reverse sweep work optimally is somewhere I that HF clock section. I wish I understood more about it, so I could know what to modify.

Finally, note that not all 4069 chips will "work" in that circuit right off the bat. And part of that seems to be the threshold for triggering a pulse in the clock circuit. R14/R16 divides down the supply voltage to insert a bias into pin 1. It can happen that the required bias is a little higher or lower that what is set by those 2 resistors. One can either stick obsessively to a brand of 4069 chips that you know will always work, OR you can tinker with that bias until you get it working with the chips you have. Brand of 4069 is moot as far as the filter sections and envelope follower go. It only matters as far as the specific threshold needed for generating a clock pulse.


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## Peaco (Jul 25, 2014)

Thanks for the info and the much needed insight! Definitely helping me understand it more. I am going to play with R17 and see what the changes are like. I wonder if having variable decay would be a useful function?

I don’t hate the reverse sweep, but I don’t love it. Based on your info maybe I can tinker with that as well. 

I was well advised about the 4069’s via Tonepad build notes to stay away with TI chips. With my first round of builds I lucked out using RCA and if memory serves me Motorola (cant quite remember). This time around I am using ST HCF4069UBE and NXP HEF4066BP. Hopefully I can find some combinations that work well with these chips as they are plentiful. As of right now I have 62k in R16 but haven’t touched R14. Maybe I’ll mess with that a little bit as well. Threshold seem not as effective as it could be. 

You seem very knowledgeable… Do you do effects too? I’d love to see your projects. 

Again thanks a bunch. I am really liking this circuit especially now!


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## mhammer (Nov 30, 2007)

I traced out my 1978 pedal way back when, and it used RCA CD4069UBE chips. Interestingly, much like the changes made to early vs later MXR Phase 90 pedals, the 1st issue Envelope Filter had a slightly different "emphasis" (resonance) value resistor than a later issue in the 1980's. I think the first ones used 200k, and then they switched to 240k. As you will note in the GGG schematic I linked to, the modded version of that circuit uses a 47k fixed resistor and 500k pot to go from much lower emphasis/resonance settings, to much higher than stock.

It would be a simple matter to insert a 560k resistor in that location on the board, and then use a 3-position toggle to place two other resistances (470k and 100k would yield good choices) in parallel to get the stock resonance, and lower than stock. I know folks feel the urge to make things continuously variable, but quite often three choices covers pretty much all the bases, and allows for a simpler and more compact control panel.

Yep, I've been making these things for far too long. Here's a nice little article I wrote about these filters. Not quite as masterful as the stuff that my buddy RG writes (he's an engineer, and I'm most assuredly NOT), but apparently people find it informative: http://www.geofex.com/article_folders/ecftech/ecftech.htm A slightly more recent version can be found here: http://hammer.ampage.org/files/Autowah.PDF
I think the two versions are also a little different in terms of the manner in which the graphics were handled, so best to snag both in order to see everything you were meant to see.

My stuff has been posted around from time to time, and some members here are happy owners of things I've made.

Postscript: Stumbled onto the relevant drawing over lunch. Highpass available from pin *6* of A2 (using the GGG drawing as reference point), bandpass available from pin* 2*, and lowpass from pin *12*. Note that the Emma Discumbobulator is actually a souped-up version of the MXR circuit. The drawing I have shows a single-transistor buffer at both the input and the output. It also shows R14/R16 as _both_ being 100k. Recognizing that you can't always put your faith in reverse-engineering drawings (hell, sometimes even _factory_ schematics are wrong!), nevertheless, that may have some relevance to the 4069 chips used by Emma in their unit. Finally, what Emma has labelled "Width" on their unit is actually the emphasis control.


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## Peaco (Jul 25, 2014)

I would really love to get some original MXR stuff just to hear a baseline of sound… If that makes any sense. After having the EF in my head all day I spent some time with it today…. Depending on guitar I use sometime the threshold is useless from 1 to 5 on the dial….It was worse with humbuckers… It just kind of got muddy or I would lose the sweep. Emphasis, as I mentioned before still has some issues. I am obviously not done tuning it in lol. Looks like I will be taking it apart! You mentioned the phase 90. I have been working on a phase 45 recently. I need to put more time and research into it. 

I really like simplicity of adding parallel resistors. Sometimes it seems like I am playing with dials more than I am playing. 

WOW thanks for linking me to your paper… I just scrolled through quickly. I think that will be a great value to my efforts. Much appreciated! This will make for some great reading while I am “working” lol. I love finding people who are into what I’m into but much more experienced than myself. So thanks for your input. There are only a handful of local musicians that are playing through my pedals. It’s a hell of a feeling though.

Oh man! I’ve never even heard of the Emma! I am in overload now! Doesn’t a buffer at input mess with attack? I have only tried mine with guitar straight in. I got some major research coming my way! I think I will be coming out with a great EF next time I get a few hours in my shop! 

So is everything at http://hammer.ampage.org/ your work?


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## Peaco (Jul 25, 2014)

Half way through your paper... Mind blown....


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## mhammer (Nov 30, 2007)

Well, clasp your hands firmly to the side of your head, and don't let any little bits get flung too far. 

Some of the stuff I posted on my "site" (it's an archive, more than anything) is mine, but a great deal of it is simply articles scanned from sources that people younger than a certain age won't be able to find anymore, whether in a library of any kind, or even sending away for back issues. RG Keen's GEOFEX site predates mine and _does_ contain *his* work. He's an engineer (formerly with IBM, now with Visual Sound), and I'm anything but.

The toggle-with-parallel-resistors thing is interesting. Personally, I'm fine with it, but many consumers feel psychologically hampered. Case in point. I've known Tim Larwill, who makes the Retro-Sonic pedals, for a while now. He was making (and still makes) a clone of the Ross compressor (just like Keeley, and so many others do). We were discussing its internal workings, and I mentioned the opportunity to include a gain-recovery time control (which many other companies incorrectly label "Attack" on their compressors). I noted that a 3-position toggle would work well, and recommended the resistor values to use, noting also that the vast majority of players would likely set such a control to the fastest and slowest settings anyway, since the audible outcome was so heavily dependent on picking style, that it would be hard to tell where you had set it unless it was at the fastest or slowest setting. Heck, the middle setting was probably just for show. Well, he implemented it, and then after a year or two, I noticed that it had been replaced by a pot. I asked him about the change and he replied that customers wanted it and felt somehow limited by the 3-way switch, like they were somehow _missing_ something. Of course, that sort of thing didn't stop EHX ( http://www.ehx.com/products/soul-preacher ), but I guess they're big enough that their name and reputation obliges people to be satisfied with the controls they provide.


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## mhammer (Nov 30, 2007)

Just to follow up, one of the challenges with using toggles, whether 2 or 3-position, is the avoidance of switch-popping. Very often, the parameter one wishes to change involves the connecting and disconnecting of elements (mostly caps) that have a DC voltage sitting in them. And suddenly giving them a path to drain off results in an audible pop. If musicians would remember to ONLY set their controls and switches when the pedal is in bypass mode, that would be one thing, but it _would_ be expecting too much. And when those pops are going through a loud amp, they ARE annoying. So, the challenge is to figure out ways to have 2 or 3 settings, controlled by a toggle, without producing popping.

Here's one example, relevant to your current interests.

The frequency range of the EF's sweep is set by a pair of 1000pf caps. If one wishes to change the range so that it sweeps higher, both those caps would need to be a smaller value (and the same value). But using a toggle to select between one set of caps and another would result in audible switch-popping. What to do? Keep in mind that avoiding popping requires that anything that might have DC voltage on it has a path to drain. So our objective is to keep both ends of the caps connecting to something at all times.

Keep in mind that caps in series do not _add_ their capacitance. Rather, the result is given by 1/Ca + 1/Cb + 1/Cn... = 1/C. If I were to place two 1000pf caps in series, their combined total capacitance would be 500pf. If I were to use a toggle to shunt one of the caps in that pair, I would have 1000pf of capacitance, and the 2nd cap would effectively be out of circuit. But better yet, both of those series caps has had a continuous path to drain off, such that shunting either one will be _silent_. So, wanna insert a "Range" switch in the EF, similar to the Mu-Tron? Select a pair of cap values, and do the math. If we used a 1500pf and 1000pf cap in series, their combined series capacitance would be 600pf. We could use a DPST (or DPDT) to shunt the 1500pf caps in each pair, and switch between the 1000pf and 600pf ranges (600pf would be a bit less than an octave higher). OR, we could use a DPDT on-off-on to shunt either the 1500pf OR the 1000pf caps, giving us 1500pf, 1000pf, or 600pf. And so on. The central feature here is that we have not left a lead from any cap "hanging", so as to introduce popping when connected.

On a distortion/fuzz I recently built, I used a slightly different approach. I wanted 3 different treble-cut presets. Normally, that would involve a cap to ground near the output, to form a lowpass filter. But lifting, and connecting that cap would result in switch-popping. So, instead, I connected two different caps through medium-value resistors (220k, I think) to ground. As such, they posed very little treble bleed. I used a 3-way toggle to shunt the one 220k resistor, OR the other, or neither. The caps always had a path to ground to bleed off DC, and shunting the resistor introduced no audible popping noise, enabling tone changes on the fly without annoying switch popping.

The same sort of logic can be applied in many different ways.


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## Peaco (Jul 25, 2014)

I am pretty sure some gooed brain matter leaked out of my ears during last nights slumber..... Still reading....


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## Peaco (Jul 25, 2014)

Well I am done reading….I definitely understand more but at the same time I am more confused! Seriously though your article helped tremendously! Although I will have to read through a dozen more times that is great for reference and additional ideas. Thanks!

I understand more about changing the position of the reverse sweep. I find that I don’t mind how high the start position is… I don’t like how low it goes… If I could I would limit that. 

I would really like to try out the HP,LP,BP switching… From what I understand about your post is that the MXR is definitely capable of doing all three. That tells me what type of filter it is (the type names escape me at the moment…. Umm its not notch type? NM), but if I do implement that change I will get variable volumes between the modes. I would be very interested in having a look at that emma drawing you speak of…. I went and checked it out on you tube… It is very cool!

So based on your parallel, three resistor emphasis switch…. I could do basically the same type of setup for switchable decay time? Three position switch with three parallel resistors, three options OR 2 resistors switchable in series for two choices…. Is that right? Lol

One of my favorite parts of the article is the ripple section. I didn’t know exactly what that was that I was hearing but now I do! LOL although some of those sounds get pretty cool I would like to improve on that a little bit. 

So again, seriously thanks for the input. I’m still mind blown! I have parts for one more that I intend to sell. I’m going to experiment more on mine and hopefully do up something very cool! I think I will keep the emphasis pot for now (I bought the pot already) and if it will work the way I think it will give it the decay time switch and if I can visualize how to rig it… the three pass modes. 

I am so stoked to experiment and I don’t even know what section/issue to start with!


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## mhammer (Nov 30, 2007)

Peaco said:


> Well I am done reading….I definitely understand more but at the same time I am more confused! Seriously though your article helped tremendously! Although I will have to read through a dozen more times that is great for reference and additional ideas. Thanks!
> 
> I understand more about changing the position of the reverse sweep. I find that I don’t mind how high the start position is… I don’t like how low it goes… If I could I would limit that.
> 
> ...


*Ideally, if we were pros (which neither of us are!), this would all be done on a breadboard. But since we're discussing experimentation with an existing pedal, the temporary solder joint is the way to go.

PM me with an e-mail address, and I'll send you the Emma drawing I have.
*


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## Peaco (Jul 25, 2014)

I can’t wait to get some time with this circuit again! I am going to get all the values and mods sorted out on mine and I have enough parts to build another which will be for sale. Very excited!
I am going to have a listen to that Funny Cat… I find the ripple interesting… 
I am definitely not pro lol. Just a wannabe that REALLY likes to build. I do have a bread board that I use from time to time but it is actually really crappy. Sometimes I don’t know if it’s the circuit or the board that is giving me issues lol. I have been using PCB to test the ECF (notice I use the proper acronym ) This circuit is just crazy for me on the BB, it makes more sense for me to see the traces.  

Thanks again for everything!


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## Peaco (Jul 25, 2014)

Again, Mark thanks for all your help. This is the newest Envelop Filter. I am not sure I like it better with, or without the decal.


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## mhammer (Nov 30, 2007)

That's a nice neat build. Blemish-free!


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## Peaco (Jul 25, 2014)

Thank you sir! Im starting to get into your OS stuff now  Im hooked!


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## sulphur (Jun 2, 2011)

This baby is comin' to papa! 8)


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## Peaco (Jul 25, 2014)

Sulphur, she will be home soon! Thanks again!


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## Peaco (Jul 25, 2014)

Did up another one of these! I friggin love this circuit! I think I am going to keep making them in yellow.


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## zurn (Oct 21, 2009)

Clips pls!


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## sulphur (Jun 2, 2011)

This was my third, or forth EF and by far the most useable.

The Subdecay Prometheus almost has too much to tweak and not a ton of range.
My TWA Little Dipper is a fun unit, very vowel like, but again little range.

Both of those pedals seem to get most use in the middle third of the controls,
where the PeaCo unit has a broader useable range,
meaning that you can get more use out of the whole range of the knobs.

This is a keeper!


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## mhammer (Nov 30, 2007)

I don't know if you have or play a bass, but if you set it for slowest attack, and very low Q/emphasis, and then feed that into a chorus, it does a really nice Jaco Pastorius sound, even when you aren't playing fretless. Clearly it isn't the _same_ as fretless, but the slow-attack and less emphatic sweep provides a drone-like tone, in a way that fretted basses can't normally nail.

Nice build, BTW. But stick the locking nut for the stompswitch on the_ inside_. Prettier that way.


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## Peaco (Jul 25, 2014)

Zurn. Clips are on the way... Ill post here when I get them!

Sulphur Awesome man! Just Awesome! I am very glad that you are digging it! Thanks again!

Mhammer, I actually just borrowed my buddies bass to do some experimenting Ill give that a try and let him know as well. He is a huge fan of Jaco stuff!

I really dig this forum, great people! Thanks again for everything fellas!


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## Peaco (Jul 25, 2014)

Hey fellas, 

As promised some new sound slips of the ECF.

https://www.youtube.com/watch?v=qKUBsZor05s

I hope everyones day is going well!


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