Loudness Robbing

In his patent, 1,956,350, LaurensHammond went to great lengths to describe the concept of "robbing". He defined this term to mean a drop in output from a tone generator as more and more keying circuits are connected to it. Think of it this way: Each ToneWheel in the ToneGenerator is connected to multiple key contacts (in both ?Manuals and, perhaps, the PedalClavier, too) For example, generator 49 is used 9 times in each manual. If every key which used generator 49 were closed one at a time each additional circuit might "rob" a little of the generator output as each contact is made.

In the patent, Hammond stated that "robbing" was undesirable. As he notes in page 13, line 107 of the patent, "...even though a plurality of circuits to a single generator are closed at the same time, the current flow through each of the different circuits will be substantially the same as if but one circuit had been completed to that generator. In other words, the completion of additional circuits to a single generator does not tend appreciable to diminish the current flow through the circuit originally completed. It may be said that the circuits do not "rob" one another."

In the Series A and Series M organs (and perhaps others), the tone generator coils and the wire connecting them to each key contact is a fixed resistance. In the patent, Hammond lists the tone generator coil impedance at 4 ohms and the resistance wire to each key contact is fixed at 15 ohms. So each additional contact closure adds another 15 ohms in parallel to those contacts which may already be closed. Reistances added in parallel combine as 1/R1 + 1/R2 = 1/Rt. So for two closed circuits from a common generator, the total key contact resistance drops by half to 7.5 ohms. The output voltage of the generator coil is fixed by the number of turns in the coil and the fixed speed at which the magnetic flux changes by the rotating tone wheel. The current is dependant on the resistance of the circuit. So when the contact resistance drops by half, the output current doubles.

But, as it turns out, a little Loudness Robbing is desirable! The Model A organs are noticably brighter and louder than later organs. To correct this, Hammond began using resistance wires of different values and adjusting the output voltage of the generator coils to "voice" the organ so that as additional key contacts are closed, that the increase in output current the increase is not perfectly linear.

To be continued and revised...