# CDXL Reissue Mod Ideas



## daeg (Oct 8, 2020)

Eyeballing this build. I've never owned an Envelope filter before and do not play funk, so I'd be looking to mod for greater subtlety. I'll use this thread to collect my ideas and get feedback from others.


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## daeg (Oct 8, 2020)

First idea.

*Add an Attack pot*
The Attack / Decay of the Envelope should be adjustable by changing the RC Time constant of R22 & C10. By adding an A10k pot in series with the 470 ohm resistor, we change the time constant from 29.4ms to 666ms. If we went all the way up to A50k, we could get a whole 3s rise time and an effect that might sound more like an LFO controlled filter.​​The problem that would arise in doing so would be that the R27 would cause loading, and at max 'Attack' (ie the slowest speed) you'd lose half the amplitude of the Envelope signal. We could scale R27 & R21 up to avoid this, but since the following Opamp has a BJT input, that might screw with the bias current. Maybe someone smarter than me could help figure that out.​​


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## daeg (Oct 8, 2020)

Second idea.

*Add a Bias pot*
There is another thread that discusses making the R20 trimpot into an external pot. That's not very exciting because all it could do would be to add distortion to the envelope detectors input by moving the bias point closer to either rail. Now if we could adjust the bias point of the envelope signal itself, we could set how open or closed the filter would be _at rest._​​I don't see any reason we couldn't adjust R21 & R27 to accomplish this, other than things might get wonky in the 'Down' switch position, where the IC2.1 Non-Inverting Pin is biased by R23. We'd probably have to experiment to find the useful ranges.​​​


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## daeg (Oct 8, 2020)

On a side note, can anyone determine if this is a Lowpass filter or Bandpass filter?

To me it sounds like a LFP but on another forum someone described the topology as a Wein bridge oscillator, which would make it a BPF.


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## daeg (Oct 10, 2020)

It looks like I'm going down a path others have already paved.

DIYSB - DOD FX-25 filter-type mod confirmed
DIYSB - DOD FX25 - How does it work?


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## daeg (Oct 10, 2020)

daeg said:


> First idea.
> 
> *Add an Attack pot*
> The Attack / Decay of the Envelope should be adjustable by changing the RC Time constant of R22 & C10. By adding an A10k pot in series with the 470 ohm resistor, we change the time constant from 29.4ms to 666ms. If we went all the way up to A50k, we could get a whole 3s rise time and an effect that might sound more like an LFO controlled filter.​​The problem that would arise in doing so would be that the R27 would cause loading, and at max 'Attack' (ie the slowest speed) you'd lose half the amplitude of the Envelope signal. We could scale R27 & R21 up to avoid this, but since the following Opamp has a BJT input, that might screw with the bias current. Maybe someone smarter than me could help figure that out.​​



Looks like Mark Hammer already tried this years ago and discovered the same problem I was anticipating.


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## daeg (Oct 10, 2020)

I'm also reading many reports that the Decay mod doesn't do much, and that the decay pot on the Mad Professor Snow White also doesn't do much.


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## HamishR (Nov 11, 2020)

Do you ever feel like you're talking to yourself?   ?


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## daeg (Nov 11, 2020)

HamishR said:


> Do you ever feel like you're talking to yourself?   ?


Hah. Yes.

Someday, someone here will have that thought "Hey... maybe I _could_ use an Envelope Filter after all..." and stumble across this thread and find the info useful.


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