Yamaha CS-15
Modifications

written by Don Solaris

About CS-15 mods

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.



Third thing we will do has to do with signal routing. It will let you have a White Noise generator outputted into VCF2. Why is this important? Well, after you put filters in series, White Noise would end up in VCF1. This is not what we want, since in serial connection the VCF1 is located behind the VCF2, therefore White Noise would not be processed by both filters, only one.

All three mods can be done within few hours max. Below you can download a PDF document with detailed description, images and schematics on how to do this mods. You can also download a CS-15 service manual from here.  In short, this document contains:

  • 24dB filter modifictaion /w switch for original 2 x 12 dB routing
  • extended LFO precision for high frequency range
  • noise / external input routed to VCF2



Files

Yamaha CS-15 Modifications

Yamaha CS-15 Service Manual



Sound examples with modificed CS-15 (various synth noises)

Please note, there is no music here. By the time i was recording this i didn't have a MIDI/CV converter yet, so making some useful multitrack was near impossible. Recording below just contains an analog noise / drone bath. About 70% of recording is dry, plus there is no EQ of any kind, no low end removal (below 20 Hz), or DC offset, just a raw analog signal!! 
 CS-15 Don Solaris: Modifications Demo 1 - 06:08 - Most of noises you will hear  in this demo are made using modifications described above. 
CS-15 Don Solaris: Modifications Demo 2 - 03:51 Same as above, part 2. 



08. 28. 2007.
Don Solaris