JBL SG520 - Hum and Noise

2.2.1.3 Hum and Noise
Step 1. Connect the HP distortion analyzer or an equivalent (HP 400H) AC voltmeter to the
OUTPUT TO POWER AMPLIFIER jack on the SG520.
Step 2. Terminate the desired high level input (AUX or TUNER) with a phono plug containing
a 1000-ohm resistor. The resistor should have short leads to the phone plug and should be shield-
ed by the plug body. A Switchcraft 3502 phono plug provides a convenient means to contain and
shield a 1,000 ohm 14-watt resistor.
Step 3. Select the correct input with the SG520 SOURCE switch, slide the VOLUME control to
maximum and measure the noise output on the AC voltmeter. Express the noise reading in dec-
ibels below full output (3 volts across 10,000 ohms).
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FIGURE 2-3 HUM AND NOISE
2.2.2 TESTING LOW LEVEL INPUTS
2.2.2.1 Distortion
Step 1. Use a floating oscillator and analyzer as described in the high-level distortion measur-
ing procedures.
Step 2. If it is desired to make measurements with both channels drive simultaneously, the os-
cillator must be connected to the PHONO input jacks by equal length shielded cables twisted to-
gether for their full length from the oscillator to the input jacks. If this is not done, stray 60-cy-
cle magnetic fields induce in the cable shields. The input hum signal thus generated can easily
be greater than the amplifier distortion products.
2.2.2.2 Intermodulation Distortion.—The SMPTE* measurement technique specifies that a 4-1 ratio ex-
ists between low and high frequency test signals and is used for test of amplifiers which have uni-
form frequency response. The RIAA equalization used to compensate for magnetic phono cartridge
and recording characteristics prevents accurate interpretation made under SMPTE test conditions.
*Society of Motion Picture and Television Engineers
2.2.2.3 Hum and Noise
Step 1. Terminate the PHONO 1 (or 2) input jack with a 1000-ohm resistor as noted previously.
Step 2. Set the SOURCE switch at PHONO 1 (or 2).
Step 3. Slide the VOLUME CONTROL all the way up and measure the noise output with the AC
voltmeter. The reading may be expressed as decibels below full output or as ‘‘microvolts referred
to the input.’’ To obtain the latter figure, the output noise reading must be divided by the gain
of the SG520 at 1000 CPS. The nominal low-level gain of the amplifier at this frequency is 510.
The exact gain can be obtained by feeding a one millivolt signal at 1000 CPS into the PHONO in-
put and then measuring the output with an AC voltmeter.