Notes by Peter Grenader:

The Expo converter circuit used in the M15 is about 80-90% responsible for it's sound - it's reaction to audio bandwidth FM. It's a discrete driver - no op amps in the expo circuit whatsoever. Sounds great, but tracking is tougher than using other methods. 6.5 to 7 octaves shouldn't be a problem. 10? It'll never happen. This is due to two reasons:

1) While most VCO's drive their expo with either an op amp or an transconductance amp such as the 13700, the Model 15 uses an all-discrete push-push driver into the expo's matched NPN pair. If the trans/op amp circuitry was incorporated into the M15 it would undoubtedly increase the tracking range. However with that the instrument would lose a large part of it's sound and we are not willing to make that trade off. Even with the tightly-match NPN pair in the expo converter (Linear Systems LS318), the nature of it's performance is such that the response is bowl shaped and attempting to increase those limits past what's called the 'linear zone' will result in the far edges going sharp (which manifests itself like the center actually going flat). Again, somewhere around six octaves will put you well within the linear zone limits.

2) The Model 15 uses a single intergrator/pot configuration is responsible for tracking across it's entire range. To compensate for the linear zone shortcomings of expo converters mentioned above, some manufacturers (Moog for example) incorporated an octave switch. What this does is limit the range of the expo converter to it's linear zone and extending that range with an octave switch which changes the capacitor in the core's integrator, thus shifting the range of the VCO up or down the audio band. This configuration will give you better tracking because it's handled in steps and requires less performance from the expo converter. In short, the single frequency pot is a great feature but comes wth some penalty.

Back in the halcyon days of modular analog synthesizers, the range of the keyboards were scaled to support the VCO's tracking range. Sometimes only two octaves, at best 4 or 5. By the time digital and polyphonc instruments were introduced tracking was handled by an entirely different method and keyboards grew as a result. With the introduction of MIDI to CV converters, these keyboard controllers where capable of genrating analog outputs, the assumption being they would allow an analog VCO to be controlled across it's entire range. In reality however the VCO's would not perform up to the limits of most of these larger keyboards for the reasons listed above. Usually the VCO is (unfairly) acused while in truth the range of the keyboard elipses it's capabilities.

If your type music requires a large tracking range - you may want to look into one of the other Eurorack VCOs on the market which incorporate an octave switch configuration. There are a few to choose from. Actually, the Aries VCO is about the best in the world for tracking if you can find one. If you want the Buchla 200 sound, then either pay $2k for a 258, or get a Model 15 which tracks a lot better than the 258 ever did. That's what the M15 was designed to do.

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