Lowering the SE Amplifier's Output Noise (Part 2)

primary were connected to the power supply rail, no noise would appear at the output.
   Pentodes (and other high internal impedance devices) would not work nearly as well at coupling one end of the transformer to ground and would consequently make for a much quieter amplifier.
    Making triodes as quiet as pentodes is our goal. If the triode output tube could produce a signal at its plate identical to the noise of the power supply connection, then the transformer would function as if both its primary leads were connected to the power supply rail. In essence, the output tube must become a noise amplifier.

   Last month we covered a technique to reduce the noise from the output stage of an SE amplifier. It relied on interjecting a portion of the power supply noise into the cathode of the output tube. This technique, however, only worked for cathode biased output tubes. This month fixed biased output tubes get their turn. (This month's technique will also work with cathode biased output stages.)

A quick review
How does power supply noise make its way into the output signal of an amplifier?  (Hum from the filaments, resistor noise, and tube microphonics are not covered here.) Power supply noise can be introduced from the previous stages into the grid of the output tube or it can be contracted through the output transformer from the power supply connection. This last source of noise is something particular to triodes, as they exhibit a low internal impedance.

Using the Grid
    The two amplifying inputs of a triode are its grid and its cathode. Last month we covered how to use the cathode. Now we will cover how to use the grid as a noise input. The first step is to invert the power supply noise. This is necessary because the grid is the inverting input of the triode when it is used in a Grounded Cathode amplifier. If the noise at the grid were not phase inverted, the noise at the plate would become phase inverted and would thus amplify the noise at the output, not diminish it.
   Inverting the noise's phase would be difficult in a single stage driver circuit: the grid could not easily be used both as an input for the noise signal and the music signal, as the uncertain output impedance of the signal sources feeding the amplifier would make tuning difficult. Two gain stages offer more choices. Phase inversion of the noise signal by cas-cading two inverting gain stages and interjecting the noise into the second grid is much easier, as the first stage then becomes a controllable variable. In fact, the first stage's own limited PSRR will serve as the source of the power supply noise signal for the second stage. The first stage's triode functions as the bottom resistor in a two-resistor voltage divider for the power supply noise. This attenuated noise is then fed into the second stage's grid, which will result in it becoming inverted in phase and amplified at the plate of the

    The triode's low impedance serves effectively to ground one end of the output transformer primary winding and since the winding's other end is connected to the power supply rail, a large portion of the power supply noise will develop across this winding, which means an unwanted signal at the output. Obviously if both ends of the primary were connected to ground, no noise would appear at the output. Equally true, but not as obvious for some, if both ends of the


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