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"In 1974 Yamaha released their first ever synth, the SY-1 which was also available in the rare form of the CSY-1 which we have here, a funky 2 manual organ with the SY-1 built-in and playable from either the upper keyboard or the 13 note bass pedals.
The sound is amazing, organic, unique and very expressive too thanks to Yamaha's excellent pressure sensitive keyboard (aftertouch) which can be assigned to various parameters. The presets are pants as you'd expect but get tweaking and it really comes to life. It's something a bit different. The sound is in my opinion very similar to the mighty Yamaha GX-1's monosynth section and I have been told that they even share the same filter circuitry, if not more. (The GX-1 was made in 1975). If you can't find a GX-1 for sale or just don't have £100,000 or so to pay for one but simply must have that bubbly warm early Yamaha sound then here's your solution.

From the CSY-1 operation manual, "Do not be alarmed if sound rattling (sympathetic vibration) occurs. All materials have critical resonance frequencies at which they vibrate. Since the CSY-1 produces continuous tones, it is only normal that some will cause windows, shelves etc. to rattle."
The CSY-1 has it's own amplification/speakers built in and even has a real built in Leslie type 2 speed rotary speaker, the synth sounds great through this and the spring reverb is very sweet too.
Have a listen to Royksopp, see if you can spot it, it's one of their favourite synths. You can get an idea of the sounds on Youtube. Search for Yamaha SY1 / SY2, there are a few demos, also search for a Youtube video called Synth Barocco, the soundtrack is exclusively CSY-1. You can also download the whole CSY-1 manual if you go to the Yamaha online manual library."