2810: An ARP Odyssey

Up to this point I've been talking about monophonic keyboard circuits. ARP figured out a clever way to cheat, making the synth duophonic without much extra. First let's figure out what the difference is between this circuit (from the later Odysseys) and the Moog versions.

I should first mention there is still a keyboard voltage buss not shown where the contacts make, uhhh contact just like the Moog examples. In this schematic the + power supply (arrow pointing up) is 15 V. R13 and R17 form a voltage divider between 15 V and ground. This gives 15*R13/(R13+R17) = 3 V at the + input of the op-amp. The voltage of the - input of the op-amp will be the same. So the "keyboard low" voltage is 3 V. How about the current? It is just the current flowing through R11 and R12. The current flowing into J2-6 (up through R11 and R12) is (15-3)/(R11//R12) where R11//R12 is the parallel resistance of R11 and R12. This gives 0.83 mA. Once again this is 1/12 V per note. There is a major difference though. The current is flowing into the "keyboard low" so the keys are decreasing in voltage as you move up the keyboard. The highest key (3 octaves up) is at 0 V.

It is pretty simple to invert this voltage later to get an increasing V/oct scale. Two questions come up first, is this high or low note priority? Since the "keyboard low" is held at a specific voltage, the buss voltage will only depend on the current supplied and the resistance up to the lowest note held. So this is still low note priority like the Moog examples. The second question is why make the keyboard voltage backwards in the first place? If you've been paying attention you've probably guessed it has something to do with being duophonic. That explanation will be the topic of the next post.