MXR Envelope Filter

[Rayman]

Just built the MXR envelope filter from the Tonepad layout.  Does anyone know if it is possible to use the unused CMOS (pins 8 and 9 from IC1) to make an inverting gain stage?  I want to place it at the end of the circuit to make up for volume loss and to invert the signal so I can use a passive blend with the guitar signal.

[cobra94]

Hey Rayman,

Can't help you with the unused portion of the IC but here are some mods I got from somewhere to get more volume.  I'm not 100% sure about R16 now that I look at it but all these mods have been in my unit since I built it a couple of years ago.  Let us know how it works out for you.

Part    From      To
R1      3.6K       1K
R2      200K     100K
R16    100K      16K
C2      .05uF     1uF

Cheers,
Peter

[Rayman]

Thanks Peter.  I'm still wondering how to use pins 9 and 8 to invert the signal.  Seems so simple but when I wire it up I get a bad low volume distorted sound.  Does anyone have an idea how to use the unused CMOS to just invert the signal?

[cobra94]

Rayman,

I don't even know if this is possible to use this as an inverter but here is something to try.  I'm not sure where you are connecting the extra inverter but for what I propose I would connect it after R1.  I think you need to provide a bias voltage after R1 before injecting it into the inverter (the data sheet shows the acceptable input voltage is 0 - Vdd which is slightly a little less than 9V).  Don't forget to add a dc blocking cap after the inverter (another C2) before you connect to the output.

I'm sure someone else with more knowledge will chime in to confirm or correct this.

BTW did you try the volume mods (I would do that first).

Cheers,
Peter

[Rayman]

Hey Peter,

I gave up on the spare CMOS pins.  I ended up adding an inverting BJT stage.  I wanted to invert the signal so I could add a blend pot with the dry signal (based on a previous Mark Hammer post for a Dr. Q (or Quack?)).  I had some success and really like sounds I can get. 

I changed C2 to 1u and R1 to 2.2k.  I made R2 220k and added another 220K to V+ to bias the input to a 2n5089.  I made the collector resistor 3.3k and the emitter resistor 4.7k ( I know that's less than unity, but volume is a perfect match to the bypass signal?).  From the collector I have an output cap of 1u going to one lug on a 200K pot.  The other lug of the pot goes to the pedal input via a .1u resistor, and the wiper goes to the output.  I'm certainly no expert so I'm sure this could be improved upon, but I like what I hear.

I also changed the decay 1M resistor to 470k to shorten the decay.  Many thanks to the Mark Hammer posts in this forum.

Take care,

Ray