301 - Yamaha CS-15 mods: info and modification description (contribution by Don Solaris)

The original CS-15 is a great monophonic synth from the late 70's. It has a specific dual 12 dB multimode filter structure and a dual signal path. However these two filters are permanently set into parallel connection. This can be sometimes limiting, specially when more filter power is required (i.e. 24 dB response). One of the mods here will provide your CS-15 with serial filter connection - the 24dB mode. Of course, switch is included so that you can always bring back the original unmodified parallel filter routing the 12dB + 12 dB mode. One of the features that make CS-15 specific (next to parallel filters) is the ultra fast LFO that will go all the way up to 100 Hz. There are no many analog synths with LFOs that can go that fast (most end at 10-15 Hz). The potentiometer is used to tune from 0.1 Hz to 100 Hz. If you look at the numbers, this is quite a big range, and we said majority of LFO modulations are performed in up to 10 Hz range. The same is applied on CS-15's LFO potentiometer. That means, you can precisely tune from 0.1 Hz to 10 Hz (over 80% of potentiometer's turn ). However, going from 30 Hz to 100 Hz occupies only 10% of the potentiometer's turn. So if you were to perform some kind FM or Ring Mod effects, this might become a tricky job, as it requires precise tuning, but instead potentiometer jumps from i.e. 20 Hz into 40, then 70 then 100 Hz. Please note, this doesn't have anything to do with potentiometer's condition (cleaning). No matter how good you clean it, the same problem will exist, simply because this is the physical precision limit of the potentiometer. The only solution is to expand the precision in the high frequency range. In other words, we will expand 10% of potentiometer's turn into some 80% or more. Switch is included to provide the original unmodified version.

2010-05-11 14:41:56