In the last Blog, I explained that single ended circuits can sound very, very good because, among other reasons, designers of state-of-the-art high end gear lavish great attention, effort and resources on making the power supplies as nearly free of noise and voltage variations, even minute ones, as possible. That left us with the question, “Does it nonetheless make sense to employ balanced circuitry?” The answer is yes, for two quite different reasons.
First, for many, high end audio is about the pursuit of perfection. We strive to improve our systems and listening environments so that the music we hear is as close as possible to some, admittedly unobtainable, ideal. That ideal might be Harry Pearson’s “absolute sound” of unamplified acoustic music, the signal encoded on the master tape, or one’s own view of what is “musical.” But the quest for this ultimately unobtainable goal is for many what this hobby is about.
In light of this quest for perfect sound, it only makes sense that each piece of gear should impose the minimum possible unintended alterations on the musical signal that passes through it.[1] No matter how good a power supply is, how much care is used in its design, how many large capacitors are employed, how elaborate the discrete regulator, how many filtering stages smooth the voltage, it will still have some residual noise and ripple in its output. No physical system is perfect. A single ended amplifier circuit will pick up and amplify that noise and ripple. If the power supply is good enough, the resulting alteration of the musical signal may be perceived by some as unobtrusive, musical, or below the threshold of audibility. But it will be there and very often can be measured with a standard audio analyzer. Some audiophiles may hear its effects as detracting from the goal of perfection.
A balanced circuit, in contrast, will block most of the residual noise and ripple from the power supply, and amplify only a tiny portion. This very useful characteristic is called Power Supply Rejection and is measured by the Power Supply Rejection Ratio (PSRR), which is usually expressed in decibels. PSRRs in the range 80 dB are commonly achieved in balanced circuits; a circuit with a PSRR of 80dB will amplify only 1 part in 10,000 of the residual noise and ripple. Obviously, we get closer to the goal of perfection if we amplify only one ten thousandth of the residual “junk” from even the best power supply than if we amplify most or all of it. Will the improvement be audible if the power supply is already excellent? That is for your ears to decide.
Second, of greater practical significance for most of us who do not have unlimited money to spend on audio gear, is the cost effectiveness of balanced circuitry. There is added cost to building gear in balanced mode. But once you have a reasonably good power supply, it can be less expensive to negate the effects of the residual power supply voltage variations with balanced circuitry than by adding large, exotic capacitors, bigger transformers, discrete regulators and low noise diodes to the supply. In a perfectionist, cost-no-object design, one would do both on the theory that less (at least when dealing with ripple and noise on the power supply rails) is always more. But in a piece of audio gear the rest of us can afford, a modest power supply and balanced circuitry will often get you very close to the cost no object design for significantly less money.
A confession: In my designs for LKV, I do not wholly follow my own advice. LKV’s 2-SB phono stage and our upcoming line preamp are designed to accept and output both balanced and single ended signals. Internally, all amplification circuitry is balanced using differential amplifiers. But when the single ended input and output are used, the signal is subject to some of the power supply degradations from which any single ended device suffers. The degradation is reduced by the fact that all internal amplification is in balanced mode. But, it is still a potential problem. To deal with it, I admit to overbuilding my power supplies in the LKV gear, because I want the audiophile using our products in single ended mode to have nearly as good an experience as the one running balanced. When everyone has converted to balanced systems, I will happily make the power supplies simpler and reduce our prices commensurately.