MXR envelope filter, circuit analysis

[Mbas974]

ok sorry for poor english...

what I mean... is that this pedal is not noise free like Mr.Q
there are to much high frequency hiss/noise or whathever you want to call it...

I've changed C8 from 100p to 500p
and R14 68K

[Mark Hammer]

I think you should change C8 back to 100pf.   That whole circuit is a high-frequency clock that turns IC3 on and off.

Changing R14 from 62k to 68k might be helpful, but it will depend on the brand of 4069 chip you used.  It might help in some cases, but not in others.

[Mbas974]

Thanks,
chips are T.I. I put 68K after reading that only fairchild work with 62K  

I'll try the POT mod with 2 Res on both sides.

ok...  "that block" is high frequency clock for triggering the switch;
but there is no way to put a kind of limit to the high frequencies (what is the working range of high freq clock) ?

the tonepad circuit use buffers in/out.. will it be helpfull ?

[Mark Hammer]

If a HF clocks produces an unpleasant noise, we usually fix that by moving the clock freq higher and out of hearing range, not lower.

The 3k6/100nf network is meant to keep any clock noise silent.

[StephenGiles]

https://www.dropbox.com/s/s83ut5r3uwfrydd/MXR%20start%20%26%20stop.jpg

Outline for possible way of introducing Start & Stop frequency to the MXR Envelope Filter - I cannot locate my breadboard at the moment so perhaps somebody else would try this out. It is simply to level shift the regular and inverted clock and present a mix of the two at the transmission gates.

No values yet - any thoughts?

Or would it be better between R23 and R26?

[StephenGiles]

https://www.dropbox.com/s/b378wjrywouryoc/MXR%20EF%20with%20Start%20%26%20Stop%20Freq.jpg

Found my old drawing of this beast which now shows idea for Start & Stop Frequency controls - any mileage in this?

[Mbas974]

What is the best way for moving clock freq higher and out of hearing range ?

[StephenGiles]

What is the best way for moving clock freq higher and out of hearing range ?

The clock is at 37khz - if you can hear that, I'll be your manager!

[Mark Hammer]

The clock is at 37khz - if you can hear that, I'll be your manager!

And I'll be pleased to fetch you tea! 

I think, my friend, you need to explore swapping chips to see if you can find a quieter one.  This does NOT mean changing brands (although I suppose you can do that), just plugging in different 4069s to see if one is quieter than another.  It does happen.

Apart from that, I'm not sure what else to suggest.

[Mbas974]

it's hard to me hear 10Khz.... 

what I mean is that changing some values I could have change as well the freq.
so I'd like to better understand what are the RC filter/ leverage I have to correct my mod.

I've only 4 IC... but seems I have to make a stock of it 

Thanks guys 

[Zwachi]

2) There are two cascaded filter sections, formed by IC1e/f. I'm still perplexed by just what sort of filter these make, since they look like they ought to be lowpass but sound like they are bandpass in many respects.

This is just a state-variable biquadrature filter topology realized with CMOS inverters instead of opamps.  I spent a good deal of time trying to track down the schematic for reference and ran into a bunch of dead links and couldn't view it, however, if my memory serves me right, it looked like they took the low-pass output.  The bandpass output follows the first integrator stage, lowpass follows the second integrator stage, high pass precedes both integrator stages.

The high-pass output on the state-variable filter sounds very cool at a high resonance...it's a kind of a synth-wah sound.  I don't know if high Q can be accomplished with CMOS inverters or not--I'm not sure what the gain is, but I know the bandwidth and slew rate are generally fast, however, upping the Q with uncompensated amplifiers may cause the thing to become an oscillator.

If I remember right, the high pass output on the MXR EF is the output of the first CMOS inverter.

One of the things that has never been discussed, and I'm wondering if it is possible, is some sort of mod to the clock/switching circuit that would permit either:
a) footpedal control
b) LFO modulation as is found in the MXR/Dunlop Auto-Q
c) alternate envelope shapes like exponential rise

Certainly!
a) Just connect a DC voltage accross a pot and the wiper goes to where the EF output would normally connect to the circuit.
b) No different than a.
c) I have thought about this kind of thing before, except for a string swell.  Just set an input "Envelope Detection Threshold" comparator to trigger whatever envelope you want.  Use some synth module topology...maybe an envelope generator chip, and you have your exponential rise or whatever, triggered by a certain dynamic threshold.

I'm currently planning to build a MXR kind of envelope filter and would like to add an expression pedal input.
Maybe someone could clarify how to do this? On which point should I connect the new voltage source best?
Has the expression pot to be a interstage divider or does it work as a serial resistence like in my drawing as well?

Thank you so much!