PITFALLS IN audiophile systems
Feb 1, 2000 12:00 PM, Bill Whitlock
For most designers, technicians and users, system interfacing and grounding practices are considered a black art. These columns are intended to increase your understanding of these interfaces and offer guidelines for preventing and solving the inevitable real-world problems. Experience tells us that equipment interconnections are an all-important factor in assembling trouble-free A-V systems of all kinds, but especially important in high-performance audiophile sound systems.
What is high performance?
The ultimate goal of sound reproduction is realism, the kind that invokes suspension of disbelief in the listener. To do this, a reproduction system must do two things. First, it must be capable of reproducing naturally loud concert levels. Just how loud, of course, depends on what kind of music or program it is. Classical music may have peak levels ranging from 103 dB SPL (sound pressure level) for solo piano up to 122 dB SPL for percussive material. For amplified country, jazz, or rock, peak levels can easily reach 125 dB SPL to 130 dB SPL.
Second, equipment noise must be inaudible when listening at these natural levels. Nothing breaks the spell like a background buzz or hum during a quiet musical passage or a suspense-filled moment in a movie. Because a reasonably quiet home listening room may have a background noise level of 20 dB SPL to 35 dB SPL, until recently it was widely accepted that any equipment noise below these levels would be masked and, therefore, inaudible. The fact is that, on average, a listener can detect equipment noise some 15 dB to 30 dB below ambient room noise. Our ears and brain, using learned spectral signatures and directional cues, are smart and can easily identify hum, buzz or hiss even in the presence of ordinary household noises.
The dynamic range of a sound reproduction system is defined as the difference between its loudest undistorted output (hopefully concert level) and its residual noise floor (expressed in dB). Therefore, to satisfy our requirements for realism, we need a dynamic range somewhere between 100 dB and 125 dB. There is an excellent paper on this subject [ref 1]. Some modern CD recordings use special digital techniques to extend their dynamic range to around 115 dB, and it is a safe bet that most program sources, especially digital ones, will develop even wider dynamic ranges in the near future. The point is that even for an ordinary CD having 90 dB to 95 dB of dynamic range, realism can be totally lost if it is played on a system whose dynamic range is only 80 dB. The quietest passages would be buried in noise, painfully reminding the listener that he is listening to a mere recording. Consumer expectations are high, and there are more golden ears (real and self-appointed) out there than you might imagine.
Losing it
Signals accumulate noise as they flow through the equipment chain and once contaminated, it is essentially impossible to remove the noise without degrading the original signal. Because the dynamic range of an entire system can be no better than its weakest link, noise and interference must be prevented all along the signal path. In the vast majority of audiophile systems, the worst loss of dynamic range is not caused by the equipment's internal signal processing but by the coupling of power line noise at the signal interfaces.
Most real-world systems consist of at least two pieces of equipment that are also connected to and powered by the AC line. These power-line connections unavoidably cause a significant voltage to exist between the chassis or local ground of any two pieces of equipment, whether safety grounded or not. Because line voltage normally consists of a broad spectrum of undesirable harmonics and noise along with the pure 60 Hz sine wave, the inter-chassis voltage will be noisy, too. We must accept this as a fact of life. This inter-chassis voltage is what causes hum, buzz, pops, and clicks in most systems.
The price alone of high-end audiophile equipment implies state-of-the-art design, and makers often tout impressive measurements of performance. Because these measurements are made in a laboratory setting, however, they reveal little, if anything, about how the equipment will perform in a real-world system. Unfortunately, most of this expensive audiophile gear still uses an audio interface system introduced more than 50 years ago. The ubiquitous RCA cable and connector form an unbalanced interface, which is extremely susceptible to noise coupling from the power line, making it nearly impossible to assemble a noise-free system.
As shown in Figure 1, when two pieces of equipment are connected via an RCA cable (or any unbalanced interface), the noisy voltage between the two chassis grounds causes current flow in the shield conductor of the cable. This causes a small but significant noise voltage to appear across the length of the cable (Ohm's Law). Because the interface is unbalanced, this noise voltage will be directly added to the signal at the receive end of the cable [ref 2]. Note that the noise is not "picked up from the air" as is so widely believed.
Effective, safe solutions
Because interfaces are the danger zone for signals, systems with fewer interfaces will obviously have fewer noise problems. Of course, selecting equipment with balanced inputs and outputs (the only kind worthy of being called "professional" in my opinion) will preclude most noise problems, but most audiophile and home theater systems will consist mostly, if not entirely, of equipment with unbalanced inputs and outputs. In all but the smallest systems with very short cables, noise problems are highly likely.
As shown in Figure 2, when a transformer ground isolator is inserted in the signal path, the connection between devices through the cable's shield is broken. Because power-line noise current cannot flow acrossthe transformer windings or through the cable's shield, no noise is coupled to the signal. A ground isolator is not a magic filter that can remove hum and buzz no matter where it is placed to eliminate noise; it must be installed at the interface responsible for the noise coupling. This can be determined easily with a simple test adapter and procedure [ref 3].
High-quality ground isolators, such as Jensen's ISO-MAX
If you ask equipment manufacturers about a hum and buzz problem, most will deny that their product has anything to do with it and blame bad grounding, whatever that is. Some are so ignorant or reckless that they actually recommend the use of ground-lifting AC adapters to break the ground loop.
If a short develops in a "ground lifted" piece of equipment, the audio or video cables that interconnect equipment will carry lethal voltages throughout the system and/or start a fire. Never defeat the function of the third prong on any equipment's AC plug. It is both illegal and very dangerous.
Beware of marketing hype. There is nothing unexplainable about audible differences among cables. For example, the physical design of a cable affects its coupling of ultrasonic power-line interference. Even low levels of this interference will cause audible spectral contamination in downstream amps. The real solution is to prevent the coupling in the first place with a ground isolator, not agonize over which expensive designer cable makes the most pleasingly subtle improvement. Expensive and exotic cables, whether double or triple shielded, made of 100% pure unobtainium or hand-braided by Peruvian virgins, will have no significant effect on hum and buzz problems.
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