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is needed is a few precision resistors and capacitors. In fact, the Miller effect of the second stage, which is normally a liability in most tube circuits, is here used to add to the needed shunting capacitance of the RIAA network.
If perfectly matched triodes are used in the first stage, then the coupling capacitor and bias resistor on one leg of preamp will not be needed. But as tubes vary not only from each other but themselves over time, it is best to retain these components, as even a 1 volt difference between plate voltages on the first stage will greatly unbalance the second stage.
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Yes, a DC balancing potentiometer could be wired up in between the two cathodes of the first Differential amplifier, but I cannot recommend it, as it may contribute too much noise, and the thought of those few microvolts of music signal having to jump across the scraper's contact with the resistive element should scare all of us.
Balanced Moving Coil Preamp
Moving coil phono cartridges put out about one tenth of the voltage of a moving magnet cartridge. This means we will need ten times more gain, i.e. 60 dB or 1 : 1000, after the RIAA equalization network insertion loss (-20 dB). This increase in gain would be possible with just one additional amplification stage. In fact, because we have three cascading stages, each stage need only provide 26 dB of
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gain, 1 : 22, to realize the final gain of 60 dB:
(22 x 22 x 22) / 10 = 1064.
The RIAA equalization is a little differently arranged from the equalization network used in 40 dB version. The passive RIAA equalization is split into two sections between gain stages. The first half of the equalization network provides the 50 Hz and 500 Hz poles; the second half, the 2121 Hz pole. It also differs in that the only one capacitor is used to set first half of the equalization.
Because the two phase legs are in opposition from each other, bringing the two together will result in a null; or we can partially and frequency dependently bringing them together though the capacitor and resistor network which will achieve the same frequency contouring with only half the capacitance value and twice the resistor value as in the
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