The Tone Generators
THIS PAGE STILL UNDER CONSTRUCTION
WARNING: Some of this is rather geeked out.
The basic oscillator used in this vintage Artisan organ is evidently a modified Hartley oscillator. The standard Hartley oscillator has a tapped inductor to provide positive feedback to maintain oscillation, and a variable capacitor coupled thereto forming an adjustable resonant LC circuit. Using a variable capacitor at audio frequencies is impractical, since the largest standard variable capacitors are only a few hundred picofarads (in other words a few hundred trillionths of a Farad), and each one weighs a few pounds and occupies about as much space as an entire octave generator board. Capacitors useful at audio frequencies can be up to ten million times larger than this. Obviously this is impractical. This problem was solved by making the tapped inductor variable and using a fixed capacitor. (Fixed capacitors useful at audio frequencies are usually no bigger than a person's thumb, and often much smaller.) Inductors useful at audio frequencies, even variable ones, can be reasonably small, as can be seen by looking at any of the pictures of tone generators. (The inductors are the largest single component mounted on the tone generator boards.)
Explanation of how the variable inductors work.
Open oscillator schematic in separate window.
The power (75 volts) to each individual oscillator is routed through the console. Only by pressing the correct key with the proper stop turned on is power applied to a specific oscillator.
R2 and C3 form a simple RC circuit which determines how much time is required for the supply voltage to build up to its maximum. This varies with the pitch of the oscillator, being somewhat longer for for the lower pitches to simulate the slower attack of larger pipes. (Bigger (thus lower pitched) pipes require longer to "speak".) The time constants range from about 70 milliseconds at the high end to about twice that for the 32' low C at 16 Hertz. (The actual amplitude build up takes somewhat longer than the time constant of the RC circuit. If you don't understand this, eventually I'll have an explanation here. In the meantime, ask a EE friend to explain.) This delayed attack also eliminates the annoying "pop" which is present with a sudden onset of oscillation, or when the oscillators are free-running and are gated on and off. For those with the proper math background, do a Fourier analysis of one single cycle of a sinusoid and you'll see what happens - you get a big splat of frequencies instead of the single frequency present in a sinusoidal waveform of infinite duration. That's where the POP comes from. By forcing the oscillator to start up from zero each time a note is played, the waveform does not immediately go to full amplitude, and essentially all of the additional harmonic content added by the gradual increase in amplitude is subaudio, so is not heard.
In addition, R2 sets the amplitude of the oscillation, smaller values for the lower pitches give higher amplitudes. This is necessary to compensate for the inefficiencies in converting lower frequency electrical waveforms to acoustic signals, as well as the way humans perceive loudness relative to pitch.
To be continued...
This page last updated at
CHORD site maintained by Dr. William Park. Please address comments or suggestions to parkw@ces.clemson.edu
