Balancing Acts      Part 2

across the impedance of the power supply elements defines a bouncing B+ voltage, which pretty much demands a voltage regulator to be overcome. And if not overcome, the sonic result can be a flat, compressed or distorted sound.
   When the net current does not vary with the applied signal, as in a differential amplifier, both the robustness and the cost of the power supply can be reduced substantially. In a solid state design, where voltage regulators cost fifty cents and 10 KF capacitors are only two inches tall, this may not seem important, but in tube circuits the cost of regulators and size of high voltage capacitors can be decisive.
    Beyond cost and size, there is the issue of the potential sonic cost of active regulation (most regulators employ many tens of dB's of feedback) or of excessive power supply capacitance (the greater the capacitance, the shorter the charge time, and the bigger the current spike). Particularly with tubes, better usually beats more. It is usually better to have only 20 f of polypropylene than 10 KF of electrolytic; better to have a circuit that does not need a regulated power supply than one with the most complex and expensive regulator... well, maybe. This line stage circuit should help settle this issue, as incorporating a simple voltage regulator could be tried out. Maybe, like a very linear amplifier surviving the deleterious effects of feedback because it does not have to work as hard at lowering distortion, a high feedback regulator could sound better because it does not have to iron out a bouncing supply rail.
   Okay, let us agree that a constant current draw is a worthy goal; how do we achieve it? And what about the noise that originates from the power supply itself, not from the signal bouncing of the power supply rail?  Let us start with the last question first. As long as the power supply noise is equally shared between phase legs, the noise will tend to drop out of the signal as it makes its way through balanced components. Remember, balanced designs amplify what is different and ignore what is common. Now for the first question: circuits that are balanced, such as the Differential amplifier, or quasi-balanced, such as the Long Tail phase splitter, are constant current drawing by nature, as each phase leg should have an equal, but opposite phase signal imposed upon it, thus canceling any net current draw variation.

Premature Conclusion
    The conclusion is obvious enough: use a Long Tail phase splitter to provide gain and phase splitting at once. Obvious, but faulty. This circuit needs a negative power supply to work well and its plate resistor values must be tweaked to balance its output signal perfectly. Unfortunately, the tweaking of the signal balance unbalances the amount of noise per phase leg. If feedback were to be wrapped around the circuit, it would in effect become a Paraphrase phase splitter, which would not work well with differing loads on the dual RCA jacks.

    Last month's article briefly explained how balanced audio gear differed from unbalanced equipment. It also reviewed some of the most commonly used phase splitters. This month we will turn a circuit's liability into a feature. (Which is really why this and the previous article were written.) The result is a low gain, zero feedback, unbalanced input, balanced output, line stage amplifier.
    Once again, why balanced? The need for a line stage with balanced outputs, in today's audio scene arises because many new amplifiers sport an XLR input and because of the theoretical advantages, such as noise cancellation, that derive from a balanced configuration.

Line Stage
    Our project then is to design a low gain line stage with unbalanced inputs and a balanced output that can accommodate both XLR and RCA plugs. Functionally, it would work like this: an input selector chooses from unbalanced signal sources, say, CD, phono, tuner, tape, auxiliary. From there, the signal finds an attenuator, then is both amplified and phase split and finally buffered through two cathode followers per phase leg. These anti-phase output signals could feed both an XLR connector and two RCA jacks per channel. Because of the low output impedance, all the output connectors can be used concurrently.
   Added pluses are the uses to which a second pair of output RCA jacks can be put. For example, if you are running a bi-amped system or if you have a separate headphone amplifier or if you have a surround sound setup, then the second outputs will prove handy. Up to three devices could be connected to the outputs, providing one was XLR at the input and the others RCA. Furthermore, this circuit can  serve as a split-phase signal source for strapping a stereo power amplifier for mono block use (70w from one Stereo 70). Additionally, a push-pull amplifier could be built that did not contain a phase splitter, a la the Swiss Nagra VPA 845 amplifier. In other words,  the pair of RCA jacks per channel could prove very handy, indeed.

Internal Advantages of Balanced Circuits
    A balanced stage is usually a constant current draw stage. (Here we are dealing with the net current draw, not necessarily the use of a constant current source for a load.) Most circuits are not constant in their current draw. As the signal moves up and down, so does the power supply draw, regardless whether the circuit is a Cascode, Cathode Follower, SRPP, or a Grounded Cathode amplifier. As these circuits cascade into one another, the net draw may increase or decrease with the signal. This bouncing current draw

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