MXR Envelope Filter

[jhaible]

Hi Perry, and diy list,

I just got the schemos of the MXR envelope filter - thanks !

You asked for comments - This is how I understand it:

All 4069 gates, except 3 gates in the PWM section, are used
in linear mode. Think of them as opamps with negative inputs
only, limited DC gain (but remarkable gain at higher frequencies),
and a rather high output impedance.

Three gates from A-2 form a biquad filter in the audio path.
There are two integrators, one of them with an extra resistor
(R-19) to set the Q of the filter. U-1 and U-2 form variable
resistors in combination with the 47k resistors, and pulse
width modulation. (This is not "switched capacitor" stuff.
In SC filters you emulate a resistor thru a C and switches,
and you vary the resistance by changing clock frequency.
In our circuit, the resistance is changed by altering the
pulse width, or "on/off-ratio" of the switches.)
On the right, below the audio path, there's the PWM circuit:
one gate set as integrator, two gates set as schmitt trigger,
a fourth gate used as comparator.
The control path is a fixed gain inverting amplifier (1 gate),
a variable gain inverting amplifier (next gate), a passive,
AC-coupled rectifier and peak detector, an inverting buffer
stage (next gate), and a manually adjustable low pass
filter (next gate). Then the control path meets the 
comparator for PWM.

JH.