WhitakerAudio JC Whitaker 20 W Stereo Integrated Audio Amplifier, Manual, Page: 160 / 196

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7.1.2 Phonograph Preamplifier Measurements
In order to fully characterize the performance of the phonograph preamplifier circuits, it would be
necessary to remove the bottom panel and attach a test lead to terminals on the front panel Input
selector switch. This testing procedure would normally only be done if problems are encountered
or the builder wants to document exact performance of the stage. The following tests will assume
that the measurements are taken at the speaker output terminals, not at the phono preamp output.
As a practical matter, if the tone control circuits perform as documented in the previous section,
measurements on the circuit in isolation are not necessary.
As with the bass and treble maximum cut/boost measurements, the output voltage of the
amplifier at the reference level of 1 kHz must be kept low (1.25 V rms) to avoid clipping of the
amplifier output on low frequencies, which are boosted by the RIAA curve.
7.1.2.1 RIAA Equalization
The purpose of pre-emphasis in vinyl disk recording and the complementary de-emphasis in
playback is to boost the amplitude of the recorded waveform at higher frequencies to improve the
signal-to-noise ratio (S/N). The emphasis, adopted as a standard by the Recording Industry
Association of America (RIAA) is known as the RIAA characteristic . The complementary
recording and reproduction curves are illustrated in Figure 7.1. Note the response is not a simple 6
dB per octave filter, but rather a shelved response, which was intended to compensate for certain
Frequency, Hz
20 50 100 200 500 1000 2000 5000 10,000 20,000
20
15
10
5
0
–5
–10
–15
–20
Relative velocity of voltage level, dB
Reproducing characteristics
Recording characteristics
75
μ
s 318
μ
s 3180
μ
s
Figure 7.1 Characteristics for fine-groove records. ( RIAA Dimensional Characteristics for 33-1/3
rpm Records. )