WhitakerAudio 60 W Stereo Power Amplifier, User And Assembly Manual

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101
60 W Stereo Power Amplifier
The performance targets listed in Table 7.1 are just that—targets. Any given amplifier may
perform better than the target or worse than the target. It is not uncommon for one channel to
perform slightly better than the other with regard to THD and/or IMD. The most likely
explanation is differences in tube performance, although other items may factor in as well. In any
event, the amplifier should be able to come close to the numbers listed in most cases without the
need to adjust the bias operating point beyond the default value of –40 V.
7.1 IMD Measurement
When measuring intermodulation distortion (IMD) it is important to remember that the two-tone
signal used alters the generally accepted relationship between the rms, average, and peak values,
which only hold true for single sine waves. In the case of a single-frequency signal, the meter will
indicate the rms value, while responding to the average value of the signal. In the case of the
complex waveform of a two-tone signal, the meter will respond to the average value of that
signal, but the altered relationship between the average and rms values will cause a discrepancy in
the indication. Such discrepancies are generally not large; however, the difference in peak value
of single and two-tone signals that give equal meter indications may be substantial. A correction
factor, therefore, is often applied; e.g., 81.5%.
For example, since (where P = power in Watts and R = load resistance in Ohms),
for an IMD measurement at 25 W power output, the input signal would be adjusted to yield a
measured value at the speaker terminals on an rms-reading audio voltmeter of 11.5 V rms (14.1 V
Measurements taken at 30 W
power output
(15.5 V rms)
Note 3 Input voltage 0.9 V rms Volume control fully
clockwise
Frequency response ±1 dB, 20 Hz to 30 kHz
THD 1.5% Note 1
IMD (at 12.5 V rms output) 1.8% Note 2
Noise, unweighted (shorted
input terminals) –90 dB Volume control fully
clockwise
Notes:
1 THD measured at 30 Hz, 100 Hz, 1 kHz, 10 kHz, and 20 kHz. The highest distortion reading is recorded.
2. 4:1 mix ratio. See the text for an explanation of the voltage output values given for IMD measurements.
3. To achieve 30 W output, the ac line input voltage must be 120 V.
Table 7.1 Target Performance of Each Channel of the 60 W Stereo Power
Amplifier
E P R
×
=