herbert janssen

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Kurzweil K150 Fourier Synthesizer

Here is some information about the K150FS, a rather rare additive rackmount synth. I'm interested in anything about this beast (it could really use more support, IMO), so if you have additional information, documentation, sowftware and especially more of those precious instrument models, please contact me.

Hardware and user interface

The K150 is a rather huge machine: 4 height units, very deep and a considerable weight. On the front panel you will find a huge red alphanumeric LED display, 24 buttons including a numeric pad, a volume knob and the headphone output. The back panel has the MIDI trio, a jack to connect two foot switches, a cassette interface and the mono out jack (yes, there is only 1 mono out).
The K150 is 16 voice polyhonic, but instead of voice stealing, it implements partial stealing which is of course more economic. The downside is that with "only" 240 oscs, heavy layering etc may result in what is essentially 3 voice poly. Nevertheless the partial stealing may have a particularly nice effect on shifting chord pads and the like.
The sound hardware uses a 16 bit DA converter at about 20 kHz. The rolloff of the anti aliasing filter is rather smooth but the instrument models may compensate for this. Main CPU is a 10MHz 68000.
The user interface is - well - usable. The one line display and the OS have their limits but after a few days you get everywhere with a few button pushes. What leaves most other synths way behind is that all parameters are in real units: semitones, cents, dB and Hertz. This adds up to the general impression that someone was really trying to do it right.


The MIDI capabilities are quite amazing: omni, poly or multi mode, 16 part multi timbrality, 35 (!) control destinations may be freely defined to any MIDI CC, pitchbend, aftertouch or poly pressure. The patch memory can hold up to 256 sounds but only one multi mode assignment. When patches are switched, pressed notes keep sounding, very nice!

The synthesis engine

The K150 uses 240 hardware oscillators that can each be switched to produce either a sine wave or one of two noise samples. Each oscillator may use its own 256 stage (loopable!) amplitude envelope. The definition of an additive spectrum including its dynamics can only be done on the FS version of the K150 and it can only be done via the external SOUNDLAB software that runs on the Apple ][ (be it real or emulated on a Mac). Such an additive sound model is called an "instrument" on the K150 and additionally to the preset sound models you may define up to 64 in the user RAM. Each instrument may contain up to 64 partials which in contrast to e.g. the Kawai K5 can have any frequency, i.e. the spectrum does not have to be harmonic.
On the K150 itself, the user can access only the higher level parameters - which are however quite powerful. You can have up to 3 keyboard regions with up to 7 layers for each region for all sorts of splits/stacks/velo switches and so on. For every layer you can select one of the low-level instrument models and then globally assign controllers, set up the very flexible pitch LFO, add chorus/delay effects (not via DSP but by stacking up voices) and apply timbre shifts.
I find the parameters offered quite nice and very "musical": e.g. the response to attack velocity and the timbre is nicely adjustable - even via MIDI CCs if you like. Together with a fader box and maybe an arsenal of pedals, switches, joysticks and wheels you can do what the terminology implies: adjust an instrument to the musical context or playing style. It is really a fun to play expressively.
What is essentially not possible is to completely change the character of a sound in realtime by other means than crossfading between layers.


The overall sound character is a little dull and thin like on most early digital synths, but compared to e.g. the Kawai K5 it seems to be much less limited and more versatile. The bass is definitely "there", the noise level is basically OK.
Concerning the presets that my Version 1.6 ROM came with: The piano, clarinet, acoustic bass and acoustic guitar patches are very realistic given that this is an additive synth. Things like the electric pianos are nice but in this arena there's no comparison to FM of course.
The only reference recording I know for the K150FS is Wendy Carlos "Switched on Bach 2000". In the liner notes she says that she used three of them - together with other early digitial pearls like Synergies and a SY-77.


If you are interested in the spectra of acoustic instruments have a look at the sharc timbre database.
There is a file with discussions about additive synths.
And another file with Kurzweil K150 discussions.

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