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 organs, the tone generator coils and the wire connecting them to each key contact is a fixed resistance. In the patent. the resistance wire from the coil to each key contact connected to that coil is 15 ohms. So each additional contact closure adds another 15 ohms in parallel to those contacts which may already be closed. Resistances 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 distance between the magnet and the tone wheel
 * the number of turns in the coil
 * the fixed speed at which the magnetic flux changes by the rotating tone wheel.

The number of turns of wire is set at the factory and the speed of the tonewheel is set by the SynchronousMotor. Only the distance between the magnet and the tonewheel can be adjusted by a technician, ToneWheelGeneratorOutputLevels. The closer the magnet is moved towards the tonewheel, the greater the output voltage (although this does affect the tone quality as well). While the organ is being played, these things never change so the output voltage from each coil is constant.

While the voltage is fixed, the output current is inversely proportional to the total resistance of the parallel keying circuits. So when the contact resistance drops by half when two key contacts are closed, the output current doubles. This doubling of current is evenly split between the two key contacts so, as Hammond noted, "...the current flow through each of the different circuits wil be substantially the same..."

But the ear is not a linear device! When the Model A output coils were adjusted to provide the same output voltage at each coil, the organ was considered too bright. This is because the ear responds better to higher frequencies than to lower frequencies. To correct this, Hammond began adjusting the output voltage of the generator coils so that higher frequencies produce lower voltages. The early Model B organs have a very pronounced bass. The later organs, including the B3, have a noticably lower bass output.

In later organs, Hammond also added different values of resistance wire between the generators and the key contacts to taper or voice the organ so that as additional key contacts are closed, the increase in output current is non-linear. Resistance wire values ranged from 10 to 100 ohms and depended on the frequency of the generator and the harmonic to be used. In effect, Loudness Robbing was intentionally created to compress the range of loudness between settings with few and lots of harmonics. This allowed the organ to have a consistent volume as more harmonics are added.

But tapering is not provided in the Model M and other spinet organs since the extra effort to taper the manuals added to the production cost. In the spinet organs, the resistance is fixed at 16 ohms. As upper harmonics are added by the drawbars, the player must adjust the volume pedal to keep the preceived loudness the same. This is one reason why classic B3 drawbar settings sound different on the spinet organs.