A TUNE-UP FOR THE DUAL TURNTABLE - ThaiHDbox

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-