Understanding the MXR Envelope Filter

[rogeryu_ph]

Slacker,
Copy that. I thought was the resistor  I did audio probe the sound like tremolo. I'll try your instruction and many thanks. Also I see that why the sound is there even without power. I'm using 105 tantalim for 1uf is that okey? the neg side is going to output.
Mdh,
I'm using HEF4069 maybe it's from Philip. Once I tried slacker's instruction and it's okey maybe I only have to shop for several brand. Many thanks also.

Roger

[George Giblet]

> oscillator using 1 gate of a 40106, 1 cap and 1 resistor that works just as well as the more complicated oscillator used in the original. This could be used to debug the circuit to try and identify whether the issue was with the "clock"

Yes that's an easy one to whip up.  It could be used for debug but I wouldn't go modding the original to incorporate it.

I have some notes on the oscillator section.  They say 35kHz, 2.3Vp-p with 2.3V DC offset; presumably at the triangle waye output, the inveter that goes to the 47k.  A square-wave output would be present at the inverter output that connects to the  small cap (10p).

I suppose all you need to do is check the oscillator is working.  A multimeter set to Frequency connected to the *square-wave* output would do.  You could also measure the DC voltage at the square wave output.  If it's 0V or 4 to 5V then there the oscillator might not the oscillating if it's around 2.3V +/- 0.5V then it's probably working.

A CRO would make life easier.

> Like Paul pointed out this circuit seems to be a bit touchy as to whether it works or not but no one ever seems to have got to the bottom of it. The only solution that seems to work is switching to different manufacturer's 4069s, but that's not solving the underlying problem.

I would not expect that circuit to be too badly behaved.

There are many inverters operating in linear (amplifier) mode.  The original circuit used 4069UB,  the unbuffered version.  If some people are unknowingly sub'ing the buffered version that might be a problem.

Another thing to try would be to inject into the junction point of the 1M and 22M  resistors, near the envelope detector output.  Perhaps by connecting a 470k resistor (experiment) from there to ground.  That should force the filters to change frequency and let more highs through.  Play a signal through the unit and switch slowly between resistor and no resistor.  You should be able to hear the filters working (or not working if something is wrong).   If it works then everything is working (to some extent) - the only thing unchecked is the inverters (amplifiers) around the threshold control.

[rogeryu_ph]

When we say oscillating it whine high pitch is'nt it? That what I hear on the audio probe on IC1 and IC2 and also IC3 but when attack is full sound like a pump boat and there is a beat and when the attack is reduce it becomes faster. And the sound fairly low you need to crunk the amp just to hear but the guitar sound is like little bit tremolo..

Thanks
Roger

[George Giblet]

There diferent types of oscillation!

There is intentional and unintenional oscillation.  The oscillation you mentioned is unintentional high frequency oscillation.  You get often get this in high gain circuits the frequency may be so high that you cannot even hear a whine.  The motor boating you are experienced is also unintenional this is low frequency oscillation.

The oscillation I was talking about is intentional oscillation.   The circuit needs an oscillator to function, the PWM generator that feeds filters.  This oscillation frequency shouldn't get into the audio, and it is not intended to do so, it's kind a behind the scenes block of the circuit.  For the MXR circuit you probably won't hear much on the audio probe, the 35kHz is beyond your hearing.

Early in the thread slacker gave a good suggestion.  If you increase the capacitor on the oscillator by factor of 10 to 100 it should put the oscillation into the audilbe range.  The intention here is not to make the circuit work, it's not a mod.  It's just a way you could confirm the oscillator is working using the tools you have.  If you had an oscilloscope you would just probe the circuit with out having the change the part values because a cross can display the high frequencies.

A couple of other things:

- Do you have a multimeter?

- It is normal for this circuit to produce and output when the power is off, don't worry about that behaviour.

[rogeryu_ph]

Guys,
Finally RCA CD4069UB made it! I shop for National, ST micro, Harris but to no avail. Strange why only RCA. I did change only the IC2. The IC1 is Philips and IC3 is Harris. Everytime I change ic the VR varies RCA vr become only 3.14V I don't know if this is ideal but the sound is huge. I did the emphasis mod and the filter range mod it's ok next week i'll try the reverse. Thank you so much Slacker, mdh and George for this. BTW I changed back r16 into 100k

Roger

[George Giblet]

The good thing is everything is working.   The bad this is we still don't know what the problem is!  I looks like it is related to IC2, but is it?  Did the change in Vr fix something else?

The chip voltage on the original I measured was 4V.

The only "bad" thing about the original circuit I could see is that IC1 pin 8 is left floating.  This is a no no for CMOS so it probably should be grounded.  The original circuit actually left this input floating but that doesn't make it right.

[rogeryu_ph]

No I think. co'z when i debug this before, I have for 4069 chips which when i measure the VR i got also 3.15v but not working. This make me suspect that the vr is lower so I stick to 4.5 pair of chips but still to no avail. So I run out to buy 4 more brand.  For awhile I suspect maybe that I have problem on my built circuit and need to build another one. The vr with different combination has different voltage. I got one measure 5.6V some 4.9v other 4.1v but as i said stick to combination with 4.5v which has sound like motor boating sound and some whining or whistle when full the emphasis knob and attack. So I change  back it into 100k and tried the new 4 chips again which RCA comes alive.

The big question now is what can I do with the rest of 4069

Thanks again,
Roger   

[rogeryu_ph]

Guys, out of curiousity I swap IC1 to IC2 it does'n't work!!! I measured the vr it's 4.3V!!!! Before is 3.14v. I immediately return after this co'z i'm alarm it may damage the ICs well it works again, strange


Roger

[slacker]

Glad you got it working. I haven't got much time at the moment but when I do I'll try and get some 4069s that don't work and see if I can figure it out what the problem is.

[George Giblet]

> Guys, out of curiousity I swap IC1 to IC2 it does'n't work!!!

Thanks for doing that test, very interesting.  Seems like a biasing or gain issue - are you sure these are unbuffered (4069UB)?

One thing you can try is to tie a resistor from the output of the inverter back to the input, use say 1MEG.   Power the inverter from 4V and/or 9V and measure the voltage at the output of the inverter.  Tie unused inputs to ground.  (If you like, repeat on a few gate or ICs).

What that tells you is the self biasing point for the inverter. It's usually just under half the supply voltage, say 45%.  If a CMOS gate biases to around that figure it should work with reasonably high gain.  If it's way off there's something weird going on with the devices.

Going beyond that, use the above circuit but this time add a 10MEG to the input of the inverter.  Connect the free end of the 10MEG to Vr and measure the output voltage, now connect the free end to gnd and measure again.  From those two voltages you can see the gain of the CMOS gate and the symmetry of the swing.

[rogeryu_ph]

I'm not sure if these are unbuffered George. Definitely these chips are not defective maybe it's the manufacturer output variation to their standard co'z I put the same PhilipsHEF4069 on IC1 but the difference was one with no sounds at all the other has fairly low sound both don't wah.
BTW I don't expect this kind of good sound i'm getting for this project but really i'm impressed with the outcome of the wah sound!!! Great project.

Roger

[thedefog]

One strange is even the circuit has no power the I still get fair amount or little bypass sound!!  Bypassed is ok. I already change all the out board wiring and DPDT and switch craft still strange!!

That's normal with no power to the circuit the signal can still pass through C3, R7, R12, C2, and R1 so you will get some sound.

Just for starters, I'm curious if the filter works at all.  Replace the switches (IC3) with 22k resistors (try a few different values in the 1k-50k range) and see if it sounds like a stuck wah.

R18 and R25 resistor is the one I should replace with 22k? (i'm using Tonepad Schem)

No what you need to do is remove IC3 and put the 22k resistors between pins 10-11 and 8-9 of where IC3 was. If you've used a socket just plug the resistors into the socket. IC3 basically acts a a variable resistor so replacing it with normal resistors will make the filter work. If this solves you volume problem then you've got the same problem as mdh had and the only fix might be to try some different 4069s.
[/quote]

When I put 22k Resistors into 8-9 & 10-11, I got the stuck-wah sound. So the whole issue with these is definitely just the TI chips. My 4066 is a Fairchild chip, but my 4069s are TI. I'm ordering replacements for them now. I'll post when they come in to confirm. Good thing I socketed these.

[thedefog]

I received an off-line query from someone about the MXR EF clone at Tonepad recently.  As usual, it was an "I built it but it doesn't work, did I get the wrong chips?" sort of query.  That's something that comes up often enough that I started to cogitate about it, and it dawned on me that maybe the culprit is not the brand of invertor chips, but rather the specific CMOS switch chip in tandem with the components that result in the amplitude of the clock pulse.  Is it possible that the "on" pulse going from the clock to the switch is simply not enough to turn the switch on for some brands?  If so, then that might mean that one could use virtually ANY brand of CMOS chips if we knew how to change certain component values and adjust the properties of that clock pulse to do its job.

Does that make sense, and does anyone know what to zero in on in the circuit to accomplish that?

That was me... but then I decided to try to "learn to fish," so to speak, rather than to post a debugging thread with voltages.  I freed up a largish breadboard by finally committing my GEO test oscillator and GEO FET matcher to perf, so I should be good to go.

FWIW, with respect to the interaction of the 4066 with the 4069s, I originally had TI 4069s and a TI 4066 in there, but I've also tried a Motorola 4066, which resulted in the same behavior that I described in the PM I sent you.  I'm guessing that the problem is in that clock pulse generator.  This would be consistent with the Tonepad build reports, in that it seems like swapping IC2 out for a non-TI brand improves the behavior of the circuit.  So I think that what I've done here is volunteered to try to become an expert on this portion of the circuit, and work out whether we can make it work consistently with chips from different manufacturers.  I have a long way to go, but I'm willing to try    Maybe I'm naively optimistic, but I think if I can break it down into simple enough blocks, I have a chance of figuring it out.

I've built this project from tonepad with sockets, and I can say that the TI chips do not work at all, the Motorola ones introduce some distortion, and the Fairchild ones work PERFECTLY. So do yourself a favor and order fairchild ones from your parts supplier.

[bobp1339]

I've built this project from tonepad with sockets, and I can say that the TI chips do not work at all, the Motorola ones introduce some distortion, and the Fairchild ones work PERFECTLY. So do yourself a favor and order fairchild ones from your parts supplier.

Question... What Fairchild chips are you using?  I am having the same problem with the Fairchilds...

Thanks
Bob

[Mark Hammer]

If I remember correctly, there is a 62k/100k voltage divider pair, that sets a bias voltage in the free-running clock circuit.  The collective suspicions of several of us here are that whatever bias voltage is required by that sub-circuit for THAT chip may occasionally (and more frequently for some brands) fall outside of what is produced by that divider pair, given their typical 5% tolerance.  The punch line is that some brands will work for some people but not for others.

Just for the heck of it, replace the 100k fixed resistor in that pair with a 68-82k resistor and a 50k trimpot, and fiddle with it to see if you can nail the required bias voltage.  My hunch is that this mod will make it amenable to essentially any brand of 4069.

[bobp1339]

YES!!! Put a 100k trimpot in place of the 100k resistor and adjusted down until the sound stopped. Turned it back up a hair, sound kicked back in with a wicked WAH!!!
Voltage at divider is 1.62v...

Works on both TI and Fairchild.  I have a clone MXR Envelope filter with 2 TI 4069 and 1 TI 4066 and it sings!!!!

I am going to do a layout to include the trimpt tonite. Once I verify it by building it, I will post.

Thanks for the help!!!!!!

peace

BobP

[Mark Hammer]

Excellent!!  Thanks for taking the chance and reporting back.  I'm sure there are others who will appreciate this info.

[bluesdevil]

I was gearing up to build this soon. Thanks a lot for posting this info!

[StephenGiles]

Don't forget the double or even triple filter mod - well worth it!

[bluesdevil]

Don't forget the double or even triple filter mod - well worth it!

Oh hell, I need to look that one up!! Thanks.
Also need to find out how to make a switchable LFO circuit for it like Charlie Moosapotamus mentioned somewhere... I still have some studying to do.