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Operation
Practical Hints for Use of the Test Records.
General Instructions.
2. OPERATION
The test records should be handled with the same care as commercial high
fidelity LP records. The number of playings which can be made without
alteration of the characteristics is greatly dependent on the care exercised in
the use of the records. By conserving one record
si~e
as a reference, it will
be possible to check on the service life of each record in use.
It
is of course of prime importance to check the reproducing
stylus
tip and the
tracking weight before each record playing. The tip radius should be between
12 and 18
p,
(o.0005 and o.0007 inch.) or 26 fl (o.OOl inch.) in the case of
lateral bands, with an included angle of 40° to 50°.
The records are cut in accordance with the Recommendations of the IEC
which are recognized by most countries. All modulations are sinusoidal with
negligible distortion.
Finally, the records should be carefully protected from dust. The use of
"antistatic" cleaners is recommended.
Turn-table.
When reproducing heads are to be tested separately, a good professional turn-
table should be selected, especially with regards to investigations at low
frequencies, where the influence of wow and rumble is predominant. The
rotating speed must be adjusted with great care by stroboscopic comparison
with the mains frequency. Finally, it is recommended to isolate the turn-table
and possibly also the preamplifier from external vibrations. This may be done
very efficiently by suspending the table supporting the two devices from the
ceiling by means of four rubber tubes. {Another possibility is the use of
electrical high-pass filters).
Use of rotating speeds other than nominals.
Since the test frequency and velocity levels are proportional to the rotating
speed, it is possible to extend the frequency range of the tests by using different
rotating speeds. However, it is then necessary to modify the RC equalizing
filters employed accordingly, by means of a proportional change of the
capacitors.
Automatic Frequency Response Plotting.
In the following, an example of operating procedure for automatic frequency
response plotting by means of the B
&
K Instruments Type 2603, 4409 and
2305 is given.
17
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