WhitakerAudio 60 W Stereo Power Amplifier, User And Assembly Manual

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Note that no measurements are specified at maximum power output (30 W per channel). Any
amplifier has a “sweet spot” operating range; for this particular design, operation above 25 W is
outside of that sweet spot. While the amplifier will produce 30 W power output per channel, it
does so at the edge of the capabilities of the tubes and the power supply. As such, measurements
above about 25 W per channel do not provide reliable, repeatable results. In any event, the extra
power is available if needed for heavy bass or other transient waveforms.
There are two major items that determine the repeatability of performance measurements of
this amplifier at 30 W. First is the ac line voltage. The power transformer is rated to 120 V
nominal. If the line voltage sags below 120 V, the maximum output of the amplifier is reduced.
The second item is the power capability of the output tubes. When matched tubes are supplied by
a vendor, they are typically given a rating based on measured performance; e.g., a rating of “18”.
A tube set with a different rating (such as “22”) may perform identically in all respects except
maximum power output, where the “22” devices may produce slightly more power than the “18”
devices for the same power supply input. This small difference is inconsequential in all respects
for normal operation of the product. For test measurements, however, widely differing results
may be recorded between the tube sets when operated at the edge of the amplifier capabilities.
Measurements taken at 15 W
power output
(10.9 V rms) Input voltage 0.58 V rms 0.58 V rms
Frequency response, ±1 dB
THD
IMD (@ 8.9 V rms output)
Noise, unweighted, shorted
input terminals
Measurements taken at 20 W
power output
(12.6 V rms) Input voltage 0.74 V rms 0.74 V rms
Frequency response, ±1 dB
THD
IMD (@ 10.2 V rms output)
Noise, unweighted, shorted
input terminals
Measurements taken at 25 W
power output
(14.1 V rms) Input voltage 0.82 V rms 0.82 V rms
Frequency response, ±1 dB
THD
IMD (@ 11.4 V rms output)
Noise, unweighted, shorted
input terminals
Table 7.2 Measured Performance of the 60 W Stereo Power Amplifier