The oscillators´ tuning is set with their own octave switches and tune controls, the first sets the master tune and the tune control for 2nd VCO sets the detuning. The octave selectors allow for a LFO mode and a "wide" mode with a full range of 10 octaves on the tune controls.
Each VCO has two CV inputs for changing the frequency, one with 1V/octave and an attenuated one. Another CV socket with 1V/octave affest both oscillator´s tunig. The pulse width control is manual and voltage controllable with an attenuated input per VCO. A rotary switch selects the waveform that is available at the "wave out" sockets (sine, triangle, rising and falling sawtooth, square and pulse), the 2nd VCO has in addition individual outputs for sine, falling sawtooth and pulse plus a white noise output. Oscillator synchronisation between VCO1 and 2 is done with a switch.
The expansion modules 501E and 501M add more features to the 501D
5U module, Moog form factor, two units wide
Has power connectors for COTK and synthesizers.com