A TUNE-UP FOR THE DUAL TURNTABLE - ThaiHDbox

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profit from selling exotic wires”; wire never fails, so there is no warranty concern; “the performance claims for exotic wires are boundless, and explanations stretch the laws of physics”; and “if the customer believes, he or she will hear, so satisfaction is almost guaranteed.” He said that “almost all power amps can reveal audible differences between speakers, but others can cause audible differences that should not be there.” He dealt with the damping factor and amplifier power requirement calculations. Part three expanded on room acoustics, choosing loudspeakers, subwoofer number and placement, and other related topics, including problems with center-channel horizontal MTM designs, the origin of the dipole surround speaker design, and how to minimize seat-to-seat bass variations. He showed a graph of “an audio journalist/reviewer’s room” (not mine!) that with a single subwoofer exhibited wildly rough frequency response over 20-80Hz, with seat-to-seat variations among the five seats exceeding 10dB. With four corner-located subwoofers and sound field management, the seat-to-seat variations dropped to a maximum of about 3dB, and the much smaller frequency response anomalies could then easily be corrected with relatively simple equalization. A point he emphasized: “a sad statement about a mature [audio] industry [is that] there is more useful and reliable information on the side of a tire than in most loudspeaker specifications.” This course is so extensive and thorough that attendees should be given an exam and earn college credits! GROUND LOOPS Bill Whitlock’s (Jensen Transformers’ president) “Understanding, Finding & Eliminating Ground Loops” course is even better with his latest revisions and added content. From his course handout: “A cable is a source of potential trouble connecting two other sources of potential trouble. This joke among electronic system engineers is worth keeping in mind. Any signal accumulates noise as it flows through the equipment and cables in a system. Once noise contaminates a signal, it’s essentially impossible to remove it without altering or degrading the original signal. For this reason, no system can be quieter than its noisiest link. Noise and interference must be prevented along the entire signal path. Delivering a signal from one box to another may seem simple, but when it comes to noise, the signal interface is usually the danger zone, not the equipment’s internal signal processing.” From this concise introduction, Whitlock walked attendees through the ways and paths noise can take to get into a system. He explained how to diagnose the source of each problem, and how to prevent it. As in all his classes, he emphasized that bypassing the third-wire ground in a power cord or outlet violates NEC rules and increases the safety risk to technicians and consumers, plus it will void insurance coverage if a fire results. He debunked myths related to wiring, grounding, and noise sources, explaining that all grounds are not equal (even two terra firma grounds several feet apart will have a measurable voltage between them) and that a heavier ground wire will not solve the problem. Another myth is the idea of impedancematching for audio interconnect cables, showing that the goal is to transfer voltage, not power, and that termination for audio is not necessary until cable lengths exceed about 4,000�. He pointed out that 10� of #12 speaker wire will exhibit substantial impedance above the audio band, and an induced current can cause audible noise voltage to be fed back through the amplifier’s feedback circuit, getting amplified and delivered to the speak- E8 audioXpress 12/10 www.audioXpress.com ers. He showed a spectrum of leakage noise-current flow in a 3nF capacitor fed by a typical AC outlet, with the level jagged but overall rather flat out to beyond 1MHz. Whitlock described the noise problems in an unbalanced interface and how a balanced interface, which he clearly and properly defined as equal impedance from each leg to circuit ground (including the source-output and the load-input circuitry), eliminates the cause of the common-mode interference. (The often-used definition that a “balanced interface” is one in which each conductor is equal in voltage but opposite in polarity to the other is absolutely incorrect.) He explained that the resistor tolerances in line drivers seriously degrade the potential common-mode rejection ratio (CMRR); for example: “the CMRR of the widely used [Analog Devices’] SSM- 2141 will degrade some 25dB with only a 1� imbalance,” which can happen using even 1%-tolerance resistors. He also discussed the XLR pin-1 problem’s causes and methods of prevention. In discussing RFI, Whitlock explained the appropriate use of ferrite “clamshell” cores, which provide little help below about 20MHz. Also covered was a straightforward procedure for stepping back through an equipment chain to find the source of noise seen or heard at the system’s output. He talked about power isolation, filters, and “balanced power,” explaining that in his experience treatments to “cleanse or purify” the AC power feed offer marginal improvement, and that “even balanced power proponents admit that actual noise reduction is usually under 10dB and rarely exceeds 15dB.” Whitlock is a proponent of series power-line noise suppression, such as in products by Surge-X (Photo 3) (www. SurgeEx.com). With parallel powerline suppression devices (which typical-
ly are MOVs between the hot, neutral, and ground lines), during a power-line surge or spike very high current will be induced in the safety ground wire, substantially raising the ground voltage at the equipment (leading to high ground-voltage differences between interconnected equipment plugged into different outlets) and potentially causing equipment damage. White papers and additional information on these and related topics are available on the Jensen Transformers website (www. Jensen-Transformers.com). VIDEO, AUDIO AND SYSTEM CONTROL The CEDIA Technology Council and Harman sponsored the Future Technology Pavilion, which was “to showcase and highlight emerging trends and cutting-edge technologies in the home from the bedroom to the garage” (from the CEDIA show directory). In addition to home theater, the pavilion included areas devoted to home health monitoring/care and energy management. The future of displays was represented by NanoLumens’s (www. NanoLumens.com) 112� (diagonal; 8� wide) flexible display (LEDs on polymer substrate,
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A TUNE-UP FOR THE DUAL TURNTABLE - ThaiHDbox

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