North Suburban HAMMOND ORGAN Society

~ X66 Percussion ~

We now return to look once again at the X66 organ's percussion voices. We already looked at the percussion keyers, but they are only a part of the total picture. The percussion keyers take sawtooth waves from the frequency dividers according to what keys you push. There are both formanted voices and synthesized voices in the percussion section of the instrument, but there is only one set of percussion keyers to handle everything. This means then that the outputs of the percussion keyers are also sawtooth waves, directly from the appropriate outputs of the frequency dividers.

In the percussion section, there are two voices, called Piano and Banjo, which are somewhat suggestive of the actual instruments, and which are derived from the sawtooth percussion keyer outputs by the formant system. However, the remaining percussion voices are all produced by additive harmonic synthesis and therefore must be made up from sine waves. A further aspect of the percussion system is that because they operate by keying a DC voltage to the appropriate input on each percussion keyer, there would also be a thump each time you press a key as that is what happens when an electronic keyer controls an audio signal, unless some measures are taken to reduce or eliminate it.

Fortunately, such thumps are rather low in frequency. This implies that a high-pass filter in the signal path somewhere before the final amplification stages is needed to take the thumps out. However, a high-pass filter can also take out lower frequency musical signals as well, because such a filter only knows frequency and does not discriminate between a low frequency musical tone and a thump.

However, in order to have sine wave signals for the additive harmonic synthesis, you would also need a low-pass filter to eliminate high frequency harmonics from the sawtooth outputs of the percussion keyers. Once again, however, such a filter will not discriminate between high harmonics and higher pitched musical tones. Therefore, the X66 percussion keyers are followed by a number of different filters, each handling typically about a one half octave band of frequencies. Such filters cut off sharply below the particular half octave range of acceptable frequencies and also cut off sharply above the acceptable or "passband."

By doing it this way, the Hammond engineers got the best of both regarding the synthesized percussion. They got sine wave outputs to use for creating voices such as harp, celesta, marimba, chimes, glockenspiel and xylophone, and when you play, there are no initial transient thumps either. And they also could use the unfiltered outputs for formanting to make the piano and banjo voices.

In order to filter the outputs of the percussion voices, these tones are first amplified in an array of small amplifiers referred to as percussion filter drivers. the outputs of the percussion filter drivers then enter the percussion filters. After leaving the filters, they are amplified by the percussion preamps, and then finally the percussion anti-thump amplifier, which also has some degree of low frequency reduction to eliminate any possibility of thumping. Below are reproductions of Hammond schematics of these sections. Once again, you can roll your mouse over the diagrams to get explanatory labels.


Figure 26. Typical percussion filter driver card. This one handles the first octave of percussion pitches. After the first octave, the remaining ones handle half note groupings of pitches. All are essentially alike but the values of the capacitors are scale according to the actual frequency range. The numbers 13-24 refer to actual generated pitches on the X66. The first octave, #s 1-12 is the 16' octave and there are no percussions at that pitch level, so the keyboard percussion actually starts with the note CC, or # 13, the 8' pitch. As before, roll mouse over picture to see labels, or if using a smartphone, tap the picture.


Figure 27. Piano filter for lower pitches. Note that this card also outputs the banjo signal as well. Also note that the input signal enters the filtering section through the capacitor listed as C4. At the same time, the signal continues out unchanged at the right from the upper terminal where it enters the percussion "flute" tone or sinewave filters. A typical example of one of these filters is in the next picture, figure 28, below. Here, the sawtooth signal is filtered to become a sine wave which is used for the percussion voices that are derived by means of harmonic synthesis. These are, the chimes, harp, celesta, marimba, xylophone and glockenspiel.


Figure 28. Typical percussion filter which allows only freqeuncies in its designed range to pass through, rejecting all ithers both below and above its passband.


Figure 29. The three percussion preamps. Their outputs are then routed to the percussion stop switches, after which the entire percussion goes to the percussion anti-thump amplifier, detailed on the next page.


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