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.
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