The Hammond X66 Organ
X66 tone wheel generator signal synchronizer.
The signals from the tone wheel generator are approximately sine wave signals
and they are of very low amplitude, only about 0.02 volts RMS. The synchronizer
is actually an amplifier, and it also changes these signals to square waves.
In
operation, the signal from the appropriate tonewheel generator coil enters
the synchronizer on terminal D1 through the .047uF capacitor listed as
C3. To make this simpler to reproduce, I am using the original Hammond
company part number designations. Note that the signal appears on the
base of transistor Q1, is amplified there and appears on the base of transistor
Q2 to be amplified further. The signal on Q2 collector (terminal D5) goes
on to the appropriate keyer for that particular note. The signal at terminal
D6 goes to the appropriate percussion keyer. A portion of the signal
also goes through the 10 K resistor (attached to the wire for terminal
D5) and goes out along the wire which terminates with an arrow. This is
the input to the first frequency divider.
In converting
the approximate sinewave input from the tone generator to a square wave, this
amplifying circuit is actually "clipping" the waveform which action
in conventional amp-lification would normally result in severe distortion.
However, it handles only one particular frequency. Therefore, the clipping
is not only not a problem, but is actually a benefit in that the essentially
square wave which this circuit outputs is more useful for the subsequent circuitry
in the organ than it would be if it was a sinewave. There are twelve of these
synchronizing amplifiers, and each handles only one frequency. Therefore,
they can clip the waveform significantly and yet the severe distortion caused
by the clipping is not only not a problem but is actually beneficial.
At this point it is worth stating that some electronic organs such as the traditional Hammonds use keyed audio, that is, the key switches actually apply the audio signals to the appropriate following circuitry. Other instruments, such as the Wurlitzer 4600 series, are keyed generator instruments, where the keys control the operating voltage to the tone generators. In keyed audio instruments the audio signals are being generated all the time. In keyed generator instruments, the audio signals only exist when particular tone generating elements are keyed or turned on by DC operating voltages from the playing keys. A further subcategory of keyed audio is that of electronically keyed audio, where the keys control DC voltages which turn on the appropriate audio keying circuits which then let the audio signals proceed through the subsequent circuitry of the instrument. The big disadvantage of directly keying audio signals is that the instrument tends to take on a "telegraph key" effect with tones beginning and ending instantly, often with an audible key click or pop. Electronically keyed audio and keyed generator instruments are free of this major flaw. The X66 is, with respect to the drawbars, the percussion section and the pedals, an electronically keyed audio instrument and as such is musically superior to the traditional Hammonds. The formanted voices are directly keyed audio, but as we shall later see, serious key clicks are not a problem when keying sawtooth waveforms into formant filters.
At this point it is worth stating that some electronic organs such as the traditional Hammonds use keyed audio, that is, the key switches actually apply the audio signals to the appropriate following circuitry. Other instruments, such as the Wurlitzer 4600 series, are keyed generator instruments, where the keys control the operating voltage to the tone generators. In keyed audio instruments the audio signals are being generated all the time. In keyed generator instruments, the audio signals only exist when particular tone generating elements are keyed or turned on by DC operating voltages from the playing keys. A further subcategory of keyed audio is that of electronically keyed audio, where the keys control DC voltages which turn on the appropriate audio keying circuits which then let the audio signals proceed through the subsequent circuitry of the instrument. The big disadvantage of directly keying audio signals is that the instrument tends to take on a "telegraph key" effect with tones beginning and ending instantly, often with an audible key click or pop. Electronically keyed audio and keyed generator instruments are free of this major flaw. The X66 is, with respect to the drawbars, the percussion section and the pedals, an electronically keyed audio instrument and as such is musically superior to the traditional Hammonds. The formanted voices are directly keyed audio, but as we shall later see, serious key clicks are not a problem when keying sawtooth waveforms into formant filters.
