In the last installment, we examined the difference in behavior between single ended and balanced circuits when exposed to electromagnetic fields (e.g., radio waves, AC fields from power cables and motors, radiation from computers and other digital devices, etc.).
To review: In a single ended circuit, interfering signals from such fields are picked up by the lone signal conductor, but not by the ground wire. The single ended amplifier “sees” and amplifies the difference between ground (zero volts) and the voltage on the signal conductor, which now includes the sum of the music and the interference. Thus, the music is degraded and you hear some of the interfering signal as noise or hum. But in the balanced circuit, with its two closely spaced signal conductors, the interfering signals are picked up in both signal wires in similar phase and amplitude. The differential amplifier in such a circuit amplifies only the difference between the two signal conductors. It therefore ignores (the engineers say “rejects”) the interference because the interfering voltages are very nearly the same on each of the two conductors.
Now we move on from the interference caused by external electromagnetic fields to consider single ended and balanced circuits in relation to another, equally or even more pernicious source of noise and distortion lurking within preamplifiers and amplifiers: the power supply. That supply is, of course, essential to the functioning of any amplifier. Its job is to supply DC current at a constant voltage to the amplification circuits, which use that DC power in amplifying the musical signal. You might think of amplification as the process of using the small input signal to shape the relatively large DC voltage/current from the power supply into an enlarged copy of the input signal.
Ideally, the DC from the power supply is clean. It has no variations in current or voltage, and therefore provides a stable medium out of which to form the large (amplified) copy of the input signal. But, in any real world power supply, some amount of interfering voltages ride along on the DC furnished by the power supply. These include, among other things, a) 60 cycle ripple voltages that are the remnants of the AC voltage from the wall socket, b) radio frequency interference that the power lines picked up on the way into your house, and c) high frequency switching noise from the diodes used in the rectification stage of the power supply. In effect, these varying power supply voltages destabilize (or move) the medium out of which the amplifier is trying to make an exact copy of the input signal.
If you were drawing a picture and the paper you were using was moving back and forth rapidly as you drew, your picture would be distorted. The movements of the surface would show up as wiggles in the lines of your drawing. Similarly, any variations in the voltage from the power supply have the potential to be incorporated into the large copy the amplifier makes of the input signal. These variations will manifest themselves as noise and distortion.
So, how do single ended and balanced circuits deal with varying power supply voltages? They respond in much the same way as they do to interference from external fields. In single ended circuits, the changing power supply voltages show up in the signal conductor, but not in the ground. They are therefore part of the difference between that conductor and ground, and are amplified.
In a balanced circuit, however, both sides of the circuit (i.e., both signal conductors) are typically affected in the same fashion by the power supply disturbances. Those disturbances thus present no difference between the two conductors of the balanced circuit, and therefore are not amplified. Thus, the balanced circuit has the ability to reject disturbances of the power supply voltage. While an ideal balanced circuit would reject 100% of power supply voltage variations, real world circuits are able to eliminate most, but not quite all of those disturbances.
The key point here is that, other factors being equal, a balanced circuit will always pick up significantly less noise and distortion from the power supply than will a single ended circuit.
Given the advantages of the balanced circuit in rejecting interference from external electromagnetic fields and power supply voltage variations, how can it be that so many audiophile quality single ended amps and preamps sound as good as they do? There are several factors involved. First, cable lengths in audiophile home systems tend to be much shorter than those in professional studios, and audiophiles are usually meticulous about routing their cables, so the wires in their systems are often fairly well protected from external fields. Second, much (but not quite all) audiophile gear is well shielded from such interference.
Third, and perhaps most important, a great deal of effort and cost is lavished on the power supplies in audiophile preamps and amps to reduce as much as possible the voltage disturbances that affect single ended circuits. While it is not possible to eliminate these variations entirely in any real world power supply, it may be feasible (although quite expensive) to reduce their level enough to get below the threshold of audibility.
Given these factors benefiting the single ended circuit, is there still reason to go the balanced route? Oh yes. But you will have to wait for the next installment of this Blog to read about it.

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