North Suburban HAMMOND ORGAN Society

~ X66 Percussion, cont'd ~

The percussion signals from the piano, banjo and sinewave (flute) percussion preamps are routed through the percussion stop tabs and then appear at the input of the circuit below, which Hammond calls the percussion anti-thump amplifier. Right at the input we see somewhat of a high-pass filter which is to suppress any low frequency transient thumps that might have made it past the percussion filter circuits. Once again, I shall introduce my opinion here which is that the lower end of X66 percussion voices that use harmonic synthesis harp, celesta, marimba, chimes, xylophone and glockenspiel is slightly weak in proportion to the upper end. Swapping the 0.27 uF capacitor (1st one in the diagram when going from left to right just before the 4700 ohm resistor) with a slightly larger one will make the entire percussion somewhat louder with emphasis on lower frequencies. It will also tend to allow a little of the initial thump sound through if you make it too large. Experimentation to find a good compromise is the way to procede here.

There is one more important section associated with the percussion and which perhaps I should have shown first, but I wanted to maintain the continuity of this present discussion of the various amplification and filtering elements associated with the percussion.


Figure 30.

The final section of interest in the percussion section is what hammond refers to as the percussion switchboard. The percussion switcboard is what determines which actual percussion keyers will sound depending on what percussion stops you have in use. Because the chimes, marimba and glockenspiel are made by harmonic synthesis, it's necessary with each of these to sound several harmonically-related keyers from a single key. The percussion switches determine which keyers will sound from each key.

Here is the description of the operation of the percussion switchboard from the X66 service manual.

percussion system switchboard section

Figure 31. One of four assemblies which together make up the percussion switchboard. To borrow a pipe organ term, this is an example of unification, where a single rank of pipes is made available via electrical switching at a number of different pitch levels. On the X66, Percussion tones which are synthesized from two or more sine wave frequencies (Chimes, Marimba, Glockenspiel and Xylophone) are really made by unifying the appropriate individual sine wave keyers for each pitch as required.

To produce the percussion voices, 7 harmonics are used. They are keyed in varying combinations depending on which percussion tab is depressed. To accomplish this, percussion keying voltage is routed from each upper manual key to a group of 7 transistor switches. With 7 transistor switches and 61 playing keys, a total of 427 transistor switches are used to activate separately or in combination, 84 percussion keyers. The 427 transistor switches make up what we term the Percussion Switch Boards. There are four of these boards in the organ.

As can be seen on the schematic, these 7 transistor switch groups activate keyers corresponding to the fundamental, 1-1/4, 2nd, 4th, 5th, 10th, and 3rd harmonic of the key being played. The low ends of the base biasing networks of each of the individual switchboard units are grouped together by harmonics, i.e. all fundamentals are tied together, and all 1-1/4 harmonics are tied together, etc. These groups are then routed to the percussion tab switch contacts. With all percussion tabs up, -5 volts is applied to the low end of these base biasing networks. Under this condition none of the electronic switches can be activated. When a key is depressed, +11 volts are picked up from the percussion keying bus bar, and applied to the tops of all of the base biasing networks associated with that key.

Assume for the moment that a key is depressed, and all percussion tabs are up. Under this condition, +11 volts is applied to the top end of each 10K resistor and -5 volts to the bottom of each 4.7K resistor of each transistor switch associated with the key depressed. The voltage at the junction of these resistors, 2-10. which are tied to the base of each transistor switch, cannot rise above 0 volts. When a percussion tab is depressed, the -5 volts is removed from the low end of the 4.7K resistors of all the transistor switches used to control the harmonics of the particular percussion voice.

The low ends of these resistors are now routed to ground. Under this condition, when a key is depressed, +11 volts are again applied to the top of all the 10K resistors of each transistor switch group corresponding to the keys depressed. With no opposing voltage applied to the low ends of the selected 4.7K resistors the base voltage approaches approximately +3 volts. Only those transistors which have been selected by the percussion voice tab or tabs, by having the low ends of their base biasing networks grounded, will be turned on.

Another source of keying voltage for the percussion switch board is through the Arpeggiator switch. This voltage comes from a separate keying bus in the lower manual. Note that the contacts that make against this bus are interconnected in octave intervals. By using this arrangement, any key or keys which are depressed on the lower manual will feed keying voltage to all of the individual switches of the Arpeggiator corresponding to the keys depressed. In other words, any "C" key depressed on the lower manual will apply voltage to every individual "C" switch of the Arpeggiator. The Arpeggiator switches are normally open. When the Arpeggiator rollers are stroked and a lower manual key is depressed, keying voltage will be applied to the appropriate switch board terminals.

The above diagram is that of a typical percussion switch and also shows one section in detail. A number of components are located in what is called a couplate which we might consider in a sense an early integrated circuit where a number of components are included in one assembly. Hammond illustrates a typical couplate in detail at the bottom of the picture.

The next section to look at is the pedal division of the instrument. Pedal tones likewise consist of both approximate sine wave pitches and also complex waves which are formant-filtered. It seems logical to follow the percussion section with the pedal section inasmuch as, in a sense, the pedals are also a specialized form of a percussion as far as how they are handled in this instrument.


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