MXR Envelope Filter Build Report

[jmasciswannabe]

Just finished completing my build of the MXR Envelope Filter using Tonepad layout. This is the first project that I have found on there where I should have checked the debug threads first. I love the Tonepad site and all of Francisco layouts, but this one should be updated because it is pretty unusable with the current c4 value. It made debugging extra difficult. Anyhow, all the chips came from futurlec and are NXE. No problems there. I didn't have to adjust r16 or mess with any voltages. Both the backwards and range mods give the pedal a little more versatility. Emphasis mod is essential. I ended up replacing R5 to 120k. This made the threshold knob more usable. Prior to this it was oscillating towards the last quarter of it's rotation.

I am very pleased with this envelope filter. I have build the McMeat and actually prefer the simplicity of this one. Easy to dial in the sound you are looking for and the tracking is pretty good as well as pick dynamics. It was mentioned at one time by Markus that the HP, LP and BP are tappable off IC2, but I decided to leave well enough alone.

Dee-lite ful!!



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[soggybag]

I was happy to see that someone built this recently. I just etched a board last night and started soldering this morning.

I used the GGG layout. I made a couple small changes so that I could chop off two corners and the PCB would fit snuggly in a B sized box.

Looking C4 it seems this is just a low pass filter. Did the original value cut too much of the high?

I notice there are a few odd resistor values, 3K6, 62K, 240K, 22M and 5.1M.  What did you do for these?

[soggybag]

I just soldered up all of the parts. Made a few resistor values by soldering a few resistors in series.

I made the 22M with two 10M and a 2M2. I made something close to 5M1 with some values that I had on hand.

For the 240K I subbed a 220K. I also subbed a 3K3 for the 3K6.

[jmasciswannabe]

I am notorious for the resistor is series trick and having resistor teepees all over the place. For the 22m I had to use three! From what I read C4 is supposed to cut out noise above a certain level. The Hammer said, in more or less words, that it was a bit overkill and recommended a .047. Most people who use a .01 didn't have any complaints about the noise.

[soggybag]

I just got everything soldered up and gave it a preliminary test, which had sound but no envelope filter effect. Have to give it a closer look tonight.

[jmasciswannabe]

Aw man.....major bummer. What chips are you using? If they are TI you are more than likely going to have issues. I remember reading changing r16 from a 100k to a 62k can help this, though. Best of luck debugging bro!!

[soggybag]

I don't think the chips are TI. I remember reading about issues with TI chips. If I understand correctly the issues revolve around getting the oscillator to oscillate. I checked this first with a scope, and I can see a high frequency oscillation.

My next step is to check the envelope. See if I can see the envelope on the scope. Maybe check the oscillator to see that it is changing frequency when the envelope is running through it's cycle. Then a little time with the audio probe...

[soggybag]

After jabbing the thing with various probes for fifteen minutes, I found I forgot to solder a jumper across the pads for the direction switch mod. Now I'm getting the filter effect. Yay.

Not sure that it's working 100% yet. I'm hearing whistling along with the filter. Sounds very synthy and high pitched, like the Q is turned up almost to the point of oscillation.

Not sure what's going on. I did leave out that .1 cap on the output. The high pitched sound could be n artifact of the oscillator that needs to be filtered. I'll have to experiment with cap values here.

[Mark Hammer]

Upward and downward sweeps are not pure inverse versions of each other.  If one looks at schematics of any of the autowah pedals that have direction switches, you'll see that they do not simply invert thigs.  Rather, the starting point of the downward sweep is strategically located a little ways from the treble end of the sweep, simply because the ear wants to hear something when the sweep starts.

The direction-reverse in the modded MXR-EF/A-Gua, as elegant as it is (and please remember to thank Stephen Giles for this innovation) is a pure flip, without any of the compensation that normally accompanies the direction reversal in pedals like the Mu-Tron/Q-Tron.  You'll also find that a Sensitivity/Threshold setting that works for upward sweep, may need readjustment for downward sweep.  It may also be appropriate to adjust the 62k/100k pair for the downward sweep.

[soggybag]

Thanks for the comments Mark. I figure the direction is going to be imperfect. It's hard to beat the simplicity of this mod so I'm to put up with it's warts.

I was looking at the oscillator with the scope. It's pretty interesting how the system produces a PWM that controls the 4066 switches. It got me to thinking. Would it be possible to make a phase shifter with this arrangement? Seems you could rearrange the filter section to create a two stage phaser. Then use the envelope section to create an LFO that controlled the HFO and use the 4066 switches as the adjustable resistance. if that makes any sense.

[Mark Hammer]

Well, if Stephen Giles doesn't post while I'm typing this, I'll simply note that this is EXACTLY what MXR did for the 2000 and Commande series of phasers.  They simply replaced the JFETs (and requisite matching and biasing) with a 4016/4066, a clock circuit, and some fixed resistors, to emulate the changing resistance of an LFO-modulated JFET.  Stephen has posted a number of circuits implementing that.

The nice thing about it is that no matching is required.

[soggybag]

I'll have to try and find a schem for that phaser, it sounds interesting.

On my way to work I had the idea that MXR EF could also be re-purposed as a Filter Sample and Hold. Seems there are five inverters in the envelope section. You could make an oscillator out of two or three. Use two more along with a transistor as a noise source. I'm not sure how this would work, but I bet you could use the extra 4066 switches as the sample and hold mechanism. There would also be an inverter left over. My limited ability tells you'd need to add a FET on the output of the sample and hold to buffer sample voltage. I don't think the inverter would be able to do this.

[Davefx]

Hello everyone..  Been out of the DIY fold for a while and am happy to say I feel better now and am back at it:)
  Getting ready to put the finish on the MXR EF and I'm actually building it for my bass player.  He uses active pickups. Should I do some initial changes to the input or output filters (already changed C4 from tonepads layout to .01 as suggested) or should I leave them be and see how it goes?  I built in all the mods..  Any changes to the range switch mod that would be better for bass?

As usual, appreciate any help from the local gurus:)

Dave