Early VCS3 and Synthi A featured a 18db/oct filter. Then it became 24db in 1974.
Technically, early filter is a 4-poles 24db behaving more like a 18db.
This mod ads a 5th pole (as on later Synthis) to the early Synthis, thus makes the filter a 30db behaving like a 24db. More info about this weird feature in this excellent article of Tim Stinchcombe.
Personnaly, I don't think this mod is very useful. It's most noticeable effect is a displacement of the cutoff point. In some complex patches it can be more drastic, but I almost never use it.
Pot to replace with a push-pull one :
- Filter "frequency"
This mod consists in simply adding a 100nF capacitor and a 100 ohms resistor in series to the filter circuit between diodes D16 and D23's cathodes.
