SDE mods vs 4PM filter


how do the SDE mods for the SMR4 MKII compare to the 4PM filter?
If I have a 4PM, which mods do you think are still worth to implement and which don’t?
Are some even applicable to both filters to extend their sound ranges?

What is your experience?


I gave it a short try to use different diodes on the 4pm like on the SDE… It didn’t really work out but that’s mostly because i didn’t try hard enough.
I think most of the SDE mod are much more towards dirt and distortion than the 4PM different filter flavours.
I think the most interesting thing to try to implement to the 4pm might be the filter feedback. Also the output drive should be a simple mod.
All other mods are carefully designed and tuned for the SMR4 and will need a lot of experimentation to get them right on the 4pm.

Yes, that is what I have heard.

How do the SMR4 mods compare to the standard 4PM? Is is worth investing the additional resources (parts and deskspace) for the effects? If so, for which?
For example, I really like the sound of the germanium diode in the sample files :slight_smile:

If you like the smr4 sde sounds you best build an smr4 sde!
A 4pm will always sound different. An extra shruthi is always worth It.

^ agree

I don’t have a modded SMR4 but the 4PM filter and resonance modes are superb.


Hi Karg, and thanks for showing an interest in my SDE mods

1. I reverse-engineered the SDE filter shapes from the 4PM, so they will sound almost identical.

2. The Filter FM mod is also inspired by that of the 4PM, but goes a lot further in depth - not necessarily ‘dirty’ but certainly highly squelchy

3. The filter Flavours (probably the most popular of my mods) would work on the 4PM, but they are highly signal level-dependent (which is probably why shift didn’t get very far - I spent many hours adjsting component values to optimise the various flavours for the Shruthi-1). However, if you particularly like the GERM option then this should work well, since the germanium diodes start clipping at a comparatively low signal level - just try soldering one in across the first opamp in the filter chain.

4. As shiftr mentions, Feedback is easy to implement, and should also work well with the 4PM. It can also create lots of different effects, acting like a tone control at lower settings to out of control mayhem at higher ones.

5. Output Drive is another easy one that should work on almost any filter with an output stage whose gain can be varied.

Hope this helps!


filter feedback is what my 4PM needs ,whats the easiest way in a… i mean very easy way ?

inserting a jack from the output back in to the input?
Otherwise :
reading schematics from the SDE mods

and comparing that to the 4PM schematics

Really it’s easier than you think.

Thanks for the replies everyone, I think I know now what to do…

Actually, another, this time very practical, question :slight_smile:

Does anyone know good 2pole-6position switches that have the standard D-shafts so that I can use the very same knobs as for the potentiometers?

Unfortunately no, but you could use one like this with a 6mm plastic shaft and cut it to the right lenght and make the D-shape with a file :slight_smile:

Ok, that was my alternative plan… :frowning:
Thanks for letting me know.

Hi Karg,

Just spotted this post. My solution was to use these knobs, which are available in both 6mm and 6.35mm shaft diameters, so you can use identical looking knobs for both pots and switches.

Hope this helps!


Thanks :slight_smile: