Maintaining Audio Quality in the Broadcast Facility 2011 - Orban

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Maintaining Audio Quality
duce a certain “sound signature,” the phono preamp is not the place to do it.)
Some of the better preamps have adjustable equalizers to compensate for frequency
response irregularities in phono cartridges. Since critical listeners can detect
deviations of 0.5dB, ultra-accurate equalization of the entire cartridge/preamp
system is most worthwhile.
The load capacitance and resistance should be adjusted according to the cartridge
manufacturer’s recommendations, taking into account the capacitance of
cables. If a separate equalizer control is not available, load capacitance and resistance
may be trimmed to obtain the flattest frequency response. Failure to do
this can result in frequency response errors as great as 10dB in the 10–15 kHz region!
This is very often the reason many phono cartridge evaluations often produce
colored results.
The final step in adjusting the preamp is to accurately set the channel balance
with a test record, and to set gain such that output clipping is avoided on any record.
If you need to operate the preamp close to its maximum output level due
to the system gain structure, then observe the output of the preamp with an oscilloscope,
and play a loud passage. Set the gain so that at least 6dB peak headroom
is left between the loudest part of the record and peak-clipping in the preamp.
Choose a high quality A/D converter. Many computer onboard or internal sound
cards produce excessive noise. Disable any soft clipping or limiting options in the
A/D.
7. Routinely and regularly replace styli.
One of the most significant causes of distorted sound from vinyl disk reproduction
is a worn phono stylus. Styli deteriorate sonically before any visible degradation
can be detected even under a microscope, because the cause of the degradation
is usually deterioration of the mechanical damping and centering system
in the stylus (or actual bending of the stylus shank), rather than diamond wear.
This deterioration is primarily caused by back-cueing, although rough handling
will always make a stylus die before its time.
Styli used in 24-hour service should be changed every two weeks as a matter of
course—whatever the expense! DJs and the engineering staff should listen constantly
for audible deterioration of audio quality, and should be particularly sensitive
to distortion caused by a defective stylus. Immediately replace a stylus
when problems are detected. One engineer we know destroys old styli as soon as
he replaces them so that he is not tempted to keep a stock of old, deteriorated,
but usable-looking styli!
It is important to maintain a stock of new spare styli for emergencies, as well as
for routine periodic replacement. There is no better example of false economy
than waiting until styli fail before ordering new ones, or hanging onto worn-out