Harmonic Foldback

Harmonic Foldback

Hammond ConsoleOrgans have 61 keys on each manual. The number of ToneWheels necessary to produce all 9 harmonics for all 61 keys is 109. Since all console models have either 82 or 91 frequency generators, certain outputs are reused for the upper harmonics of the keys at the upper end of the manual. This characteristic of repeating, for the harmonics of an upper octave, the harmonics used for a lower octave is called foldback. Most of the ConsoleOrgans also have foldback of drawbar 1 on the lowest octave of the manuals; it repeats the harmonic used for drawbar 1 of the next octave above.

Foldback in the Lower Octave

The earliest ConsoleOrgans (with 91 frequency generators) have no foldback of the lower octave; the bottom twelve ToneGenerator outputs are available on the first drawbar of the manuals' bottom octave. On later ConsoleOrgans, the lowest twelve outputs of the ToneGenerator are available only on the pedals.

Since no legitimate (Hammond authorized) explanation has yet been found as to why foldback of the lower octave was introduced, the likely explanation seems to be given in the ServiceManual's description of the various models of tone generators.

The first ToneGenerators were 91 frequency generators. In these generators, the lowest twelve frequencies were produced using twelve wheels with two teeth each. When, subsequently, the 82 frequency generators were introduced, the lower octave foldback was implemented. The ServiceManual states that the lower nine frequencies were omitted and manual and pedal rewiring made them unnecessary. (!) When the wide version of the 91 frequency generator was introduced, (from the ServiceManual) "...the original twelve two-toothed wheels were replaced with twelve two-toothed ComplexToneWheels, which supply a fundamental tone that is enriched with the odd-number harmonics." Thus these lower twelve frequencies have a different harmonic content from the, allegedly sinusoidal, remaining seventy-nine.

The narrow version of the 91 frequency generator with ComplexToneWheels is the one that is used in the B-3, C-3, RT-3, A-100, D-100, and some of the earlier models. The ServiceManual lists the breakdown of the models that do by serial serial number.

Not surprisingly there is an audible difference between the ComplexToneWheels and the regular ToneWheels.

Foldback in Spinets.

Hammond SpinetOrgans do not have foldback. The harmonics which are folded-back on ConsoleOrgans are just missing on the spinets. This can be remedied: see AddingFoldbackToASpinet.