Introduction
It is always important to have as good a reference for ones
test measurements as possible. This is why HP (Aligent) and others sell us
nice shiny big boxes with calibration certificates traceable to national and
international standards.
Sometimes it is necessary to use slightly more modest setups
and it is useful to make some base line tests to gain a series of
measurements to relate to when interpreting "live" test results. It is after
all no use chasing amplifier distortions of less than 0.001% THD if the
measurement system itself isn't able to better that performance.
The following measurements were made with my current PC
based soundcard setup. I now test 20 Hz to 96 kHz, although these tests are
from 45 Hz upwards. Below 20 Hz it is better to use 4 million point FFTs,
and 16 million if one wishes to go sub 1 Hz with any fine detail.
Even with the obvious limitations of the PC based
measurement system looking at the BIG PICTURE enables one to see low level
detail that is not apparent in much of the published audio test results.
Typical of a "standard" test is a 100dB range maximum (relative to the
signal peak, not absolute) and a bandwidth from 1kHz to 20 kHz. The
following image illustrates the very small window through which those tests
are looking.
Comparison between 45 Hz to 96 kHz and 1 kHz to 20 kHz range FFT tests.
Although not very apparent in these loop-back tests many if
not most of my amplifier tests have both 50 Hz mains and 20+ kHz EMC/RFI
from switching power supplies/monitors showing. This is the nature of bench
testing and I deliberately leave these in to show the whole story.

Test Setup
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24 bit - 192 kHz - 1 Million Point FFT
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Open circuit or loop-back tests, balanced
output to balanced input.
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Twin + screen low noise microphone cable
with Neutrik XLR connectors.
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Levels and results as per WINaudioMLS
software.
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Images are 957 pixels wide for clarity.
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Left and Right Channel Open, Generator Mute. Display Range
45hz-96khz.

Left and Right Channel Open, Generator ON at 1khz. Display
Range 45hz-96khz.

Left Channel Loopback with Right Channel Open, Generator Mute.
Display Range 45hz-96khz.

Left Channel Loopback with Right Channel Open, +4 dBu Out, +4
dBu In, Display Range 50hz-96khz.

Left Channel Loopback with Right Channel Open -10 dBv Out, +4
dBu In, Display Range 50hz-96khz.

Left Channel Loopback with Right Channel Open -10 dBv Out, +4
dBu In, Signal at +7.2dB = 0.0034% THD

Left Channel Loopback with Right Channel Open -10 dBv Out, +4
dBu In, Signal at -2.8dB = 0.0107% THD

Left Channel Loopback with Right Channel Open -10 dBv Out, +4
dBu In, Signal at -12.8dB = 0.0331% THD

Left Channel Loopback with Right Channel Open -10 dBv Out, +4
dBu In, Signal at -22.8dB = 0.1041% THD

Left Channel Loopback with Right Channel Open -10 dBv Out, +4
dBu In, Signal at -42.8dB = 0.9885thd% THD

Left Channel Loopback with Right Channel Open -10 dBv Out, +4
dBu In, Signal at -62.8dB = 9.5938% THD

Left Channel Loopback with Right Channel Open -10 dBv Out, +4
dBu In, Signal at -83.0dB = 68.36% THD

Loopback Including TPA6120 Line Driver
Left Channel with TPA6120 Loop, Right Channel Open. -10 dBv
Out, +4 dBu In. 1khz +4.3dB, 50hz-96khz, THD = 0_0050%