Circuit description
A ring modulator is sometimes described as a four quadrant multiplier, and while this certainly
sounds more grand, it also explains the action of the device in more detail. The four quadrants
refer to the four quarters of a standard XY graph with the origin in the middle. The idea being
that this device allows two inputs, we will call them X and Y, of either positive or negative
voltage, to be multiplied together. The output is therefore either positive when both X and Y
are the same polarity, or negative when they are different.
A standard voltage controlled amplifier (VCA) can be considered as two quadrant multiplier
since one of its inputs can only be of a positive value. The two quadrants where Y is negative
are not computed, and the output normally remains at zero when Y is not positive.
In audio when an input signal is processed so that it changes from a positive value to a
negative one we say it has been inverted, or that it is 180 degrees out of phase with the
original. In this way, a ring modulator may be considered to change the phase of one of its
inputs depending on the polarity of the other.
In a perfect ring modulator the inputs X and Y can be considered as having the same functions
as each other.
ie. X x Y = output = Y x X
In our real life ring modulator this is not true. Imperfections in the circuit topology means that
the output will sound different, or behave differently, depending on which signal you insert
into X and which into Y. Generally, we will use X as our input signal (sometimes called the
carrier), and Y as our modulator. That is, Y will shape X. In many applications it doesn't make
too much difference which is which, so experimentation is the key here.
The Oakley Ring modulator is based around the old ARP module 4014. This was built on an
incredibly tightly cramped circuit board which was then securely potted, with a very hard
epoxy compound, into a small plastic box. The various interconnections were made to some
thin gold plated legs that stuck out from the hard epoxy on the underside. It is almost
impossible to remove the circuitry from its hard plastic shell and all encompassing resin. I
suppose this was mostly down to Alan Pearlman's previous history in making op-amps for the
aerospace industry – enclosing circuits like this would make them less likely to drift with
temperature and be affected by surrounding humidity. However, one might also think that they
did it to keep out the prying eyes of their competitors.
Later versions of the 4014 used soft and rubbery silicone to pot the circuitry into its shell with
only a thin layer of epoxy on the underside to seal it all in. These are easier to repair since it is
now possible, with great care, to remove the delicate circuitry from its shell and potting
compound. Various folk have since reverse engineered this circuit and it can be found in
various places on the internet.
The basic function of the 4014 is of two complementary VCA circuits, both controlled by a
single driver, and each feeding a common current to voltage output stage. The driver features
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