"Jeesh! My Hammond organ is nuclear??"
you might say when you see the word "reactor" used in the X66 article.
Not so at all. A saturable reactor is really in a sense a special type of
transformer. Saturable reactors are used to control the flow of alternating
current in some circuits. Because audio signals are alternating current (AC)
signals, we can use a saturable reactor to control audio AC.
The saturable reactor works by having a winding through which alternating current flows. On the iron core of this special transformer, there is a second coil or winding through which we send direct current, or DC. When DC flows through this coil, it magnetizes the iron core of the saturable reactor. Now, rather than go into a hugely detailed explanation of electromagnetic induction and the effects of inductance in a circuit through with AC flows, all of which you can find in google by searching these terms, I will confine this explanation to the way in which the Hammond engineers used saturable reactors to control certain audio AC signals in the lower end of the X66 drawbar section for the purpose of introducing vibrato.
Anyhow, an inductive device opposes the flow of alternating current. The opposition is called inductive reactance. This simply says that if we send an audio AC signal through a coil wrapped around an iron core (which constitutes an inductive device) the coil will oppose the flow of AC in the circuit. It won't eliminate it entirely, but it will oppose it to at least some degree, the amount depending on both the amount of inductance in the coil and also the frequency of the AC signal. All things being equal, the inductive reactive opposition to the flow of AC INCREASES as the frequency of the current increases. This we can easily prove by showing the formula for calculating inductive reactance in an inductive device which is this:
The saturable reactor works by having a winding through which alternating current flows. On the iron core of this special transformer, there is a second coil or winding through which we send direct current, or DC. When DC flows through this coil, it magnetizes the iron core of the saturable reactor. Now, rather than go into a hugely detailed explanation of electromagnetic induction and the effects of inductance in a circuit through with AC flows, all of which you can find in google by searching these terms, I will confine this explanation to the way in which the Hammond engineers used saturable reactors to control certain audio AC signals in the lower end of the X66 drawbar section for the purpose of introducing vibrato.
Anyhow, an inductive device opposes the flow of alternating current. The opposition is called inductive reactance. This simply says that if we send an audio AC signal through a coil wrapped around an iron core (which constitutes an inductive device) the coil will oppose the flow of AC in the circuit. It won't eliminate it entirely, but it will oppose it to at least some degree, the amount depending on both the amount of inductance in the coil and also the frequency of the AC signal. All things being equal, the inductive reactive opposition to the flow of AC INCREASES as the frequency of the current increases. This we can easily prove by showing the formula for calculating inductive reactance in an inductive device which is this:
In the formula,
XL is the inductive reactance in ohms;
f is the frequency of the AC, and L is the physical inductance of the
device in question. As you can see, Pi is in there too. But without getting
into it any further here, if you know even the fundamentals of algebra,
you'll see that XL must increase with an
increase in frequency.
Now, suppose,
however, that by means of a second coil on this device, we begin to magnetize
the iron core around which we wrapped our coil which is carrying the audio
signal. As the core becomes more and more magnetized, the inductive opposition
begins to decrease. When the core is fully magnetized (saturated) the
inductive opposition has essentially disappeared. So therefore, the simple
variation of a DC magnetizing flow in the control coil of the saturable
reactor will subsequently control the flow of AC in the main coil. In
the X66, as in the Wurlitzer vibrato system, we have to "gate"
or control audio AC signals which differ in phase so that the resultant
signal will change phase smoothly from one to the other, which as we know
from reading the Hammond and Wurlitzer articles in this series results
in a pitch change of the composite signal.
In real life, there are certain other considerations which govern the design of saturable reactors, and while the above describes the theory and the principle, an actual saturable reactor usually has three windings and two separate magnetic cores. The controlling DC winding wraps around part of each core, and there is a signal or AC winding on each of the two cores. The signal windings on the cores are wound so that current flows in the opposite direction through each of the signal windings. In simplified form, the saturable reactor in the X66 bass vibrato system uses a DC voltage which rises and falls at the vibrato rate to control the AC signals flowing through the signal coils on the saturable reactors and thus modulates between two AC signals which are out of phase with each other. How they get out of phase with each other to begin with will be discussed in the part of this series which describes the X66 vibrato system in detail. Nevertheless, alternately gating audio signals that are out of phase with each other creates a continuous changing phase shift, and that results in vibrato. Just like magic! Only thing is, I'm the guy behind the scenes who is explaining the magic. And this particular magic is detailed in the Wurlitzer 4600 series article right here.
In real life, there are certain other considerations which govern the design of saturable reactors, and while the above describes the theory and the principle, an actual saturable reactor usually has three windings and two separate magnetic cores. The controlling DC winding wraps around part of each core, and there is a signal or AC winding on each of the two cores. The signal windings on the cores are wound so that current flows in the opposite direction through each of the signal windings. In simplified form, the saturable reactor in the X66 bass vibrato system uses a DC voltage which rises and falls at the vibrato rate to control the AC signals flowing through the signal coils on the saturable reactors and thus modulates between two AC signals which are out of phase with each other. How they get out of phase with each other to begin with will be discussed in the part of this series which describes the X66 vibrato system in detail. Nevertheless, alternately gating audio signals that are out of phase with each other creates a continuous changing phase shift, and that results in vibrato. Just like magic! Only thing is, I'm the guy behind the scenes who is explaining the magic. And this particular magic is detailed in the Wurlitzer 4600 series article right here.
