a non linear gain that acts as a pre-distortion device which compensates for some of the non
linearities in the VCA sections.
The very first Oakley ring modulator module built to replace a dead ARP-4014 module in an ARP2600
synthesiser. The Oakley RM-4014 module borrows heavily from the design shown here.
U3, a dual op-amp, is configured as two voltage followers, one for each input. The signal level
to each one is controlled via its own level pot. The source of the signal can come from either a
direct coupled input via a single resistor, or AC coupled via a capacitor. The capacitor in
conjunction with the resistor following it and the pot itself will act as a high pass filter. This
will attenuate very low frequency signals with a -3dB point of around 1Hz. It is possible to use
both the AC and DC inputs simultaneously thus creating a crude two channel mixer. However,
although no harm to the unit will come about, it does cut down the overall signal level
presented to the input level pot.
Both U3 halves act as protective buffers for the relatively high output impedance of the level
pots. The lowish input resistances of the ring modulator input stages would otherwise
detrimentally affect the pots' apparent taper, the cut-off frequency of the AC coupled input,
and perhaps also the output stage of the connected modules.
The ring modulator core is built from two transconductance amplifiers that are complementary
to one another. A transconductance amplifier is basically a 'voltage in, current out' device
whose gain, in this case, is controlled by another modulating current. And complementary in
this definition means identically opposite, mirrored if you like. In other words they behave
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