Josephson C725, Руководство пользователя

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JOSEPHSON ENGINEERING • C725 USERS GUIDE
Microphone circuit
The C725 uses a new variant of the cascode circuit that's used with some variation in many of our
microphones. In our other mics, the cascode is implemented using two large-geometry junction
field effect transistors. In the C725 the same JFET is used for the lower or voltage-gain part of the
circuit, while the upper or current-buffer part of the circuit uses a vacuum tube, in this case a
selected EF806S from current production. The drain current of the FET is the cathode current of the
tube. The tube contributes particular sonic characteristics, resulting in a smooth and gradual
compression of the waveform as the limit of voltage swing is reached. Overloads typically do not
cause clipping, only rounding-off of the peak waveform. The differences among brands of tubes
were found to have little or no impact on the sound, in this circuit.
To achieve this characteristic with low distortion, two linearizing circuits are employed. One
applies a preset amount of negative feedback taken from a dedicated output winding of the
custom nickel core output transformer directly back to the source of the FET, setting the overall
gain of the circuit. The other applies a very small amount of negative feedback to the gate of the
cascode FET from the plate circuit of the tube, when the "moon" position is selected through an
electromechanical reed relay, which is activated remotely from the power supply.
In operation the microphone becomes warm, due to heat from the tube and its filament regulator,
which is provided so the tube receives the same voltage with any cable length up to 50 m.
Operation with ambient temperatures up to 45 C is possible with automatic shutdown if
temperature exceeds internal limits.
Power supply
The PS725 power supply uses a custom screened dual-voltage transformer through several stages
of RC filtering to a low noise MOSFET shunt regulator at 150 volts. Regulated -9 volt DC for bias
and vacuum tube filament are provided using a separate supply. Another supply provides positive
and negative 5 volts for the low noise MOSFET regulator circuit. A pattern control switch supplies
0 to 120 volts in 30 volt increments to set the rear diaphragm polarization voltage, which in turn
determines the resulting pattern of the microphone.
Potentially lethal voltages are present in the power supply, and more than 50 Joules of energy are
stored in filter capacitors. No user serviceable components are inside.
Filament and bias values are set inside the microphone by additional regulator circuits so that
cable length has no effect on voltage.
Cable and connector
A dedicated five-pair shielded cable is used to connect the microphone to the power supply.
Traditional large-Tuchel style connectors, now made by Binder, are used. Pin 4 of the male
connector is removed and the corresponding female contact mechanically blocked so that
microphones of other types cannot be connected to the power supply. Extension cables up to 50
meters can be supplied.
Service
If a problem is suspected in operating the microphone, please do not attempt to repair it in the
field but contact the dealer where it was purchased, or Josephson Engineering directly. The tube
should only be replaced if it is known to be faulty. Qualified users should contact Josephson
Engineering for instructions, but the tube may be replaced if necessary with any good quality