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The one failing of the 7370, however, is its low mu (17) which results in a potential gain no larger than 17 for the stage. In other words, the burden of increased gain will be shifted to the previous piece of equipment, as it will have to supply at least 3 volts of output swing to drive the hybrid amplifier to full output. For an active preamp this will not be hard; for the passive line stage, impossible, as some inputs only provide half a volt of signal. Consequently, more gain is needed. Using the same 7370 tube in a modified common cathode configuration, the output is taken at the added triode's plate. This circuit is more linear than the simple grounded cathode amplifier, as its cathode resistor is effectively the other triode's rp divided by its mu. In other words, it uses the other triode's nonlinearity to cancel much of its own nonlinearity. Like the grounded cathode amplifier, it does invert the phase at its plate, which will make the entire amplifier non-phase inverting. And feedback (about 10 dB) is taken at the second triode's grid. |
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Above is the power supply for the amplifier. The high voltage are derived from the low voltage windings by using voltage doubler circuits. |
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Pg.
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