Manley Laboratories 175 monoblock power amplifier Measurements

Sidebar 3: Measurements

Following its 1/3-power, one-hour preconditioning test, the Manley 175 was no warmer than normal for a tube amplifier. Its input impedance at 1kHz measured 146k ohms; its output impedance was 1.6–1.7 ohms at low and middle frequencies, decreasing slightly to just under 1.5 ohms at 20kHz into 4 ohms. These are middle-of-the-road values for a tube amplifier, in my experience. The Manley's performance—particularly its overall in-system frequency response—will be somewhat dependent on the partnering loudspeaker.

The 175's voltage gain into 8 ohms measured 28.1dB; DC offset was a low 0.7mV; signal/noise (unweighted ref. 1W into 8 ohms) measured 95.7dB. The amplifier was non-inverting in its unbalanced mode.

Fig.1 shows the frequency response of the 175. The differences between its 8 ohm and 4 ohm responses, within the audible range, is insignificant. The 175's 10kHz squarewave (fig.2) indicates one cycle of damped, ultrasonic ringing (possibly due to the mild, 30kHz rise visible in fig.1), also visible as a very narrow overshoot spike in the 1kHz squarewave (fig.3), which is otherwise a textbook squarewave.

Fig.1 Manley 175, frequency response (from bottom to top): into 8 ohms and 4 ohms (0.5dB/vertical div.).

Fig.2 Manley 175, 10kHz squarewave into 8 ohms.

Fig.3 Manley 175, 1kHz squarewave into 8 ohms.

The THD+noise vs frequency curves are shown in fig.4, suitably low other than into the punishing 2 ohm load. The 1kHz distortion waveform in fig.5, for 2W output into 4 ohms, indicates a mainly second-order component. The result for 1W into 4 ohms (not shown) was similar, but for 4W into 2 ohms (fig.6), the distortion becomes third-order, with a higher-order component also present.

Fig.4 Manley 175, THD+noise (%) vs frequency at (from bottom to top): 1W into 8 ohms; 2W into 4 ohms; 4W into 2 ohms.

Fig.5 Manley 175, 1kHz waveform at 2W into 4 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.6 Manley 175, 1kHz waveform at 4W into 2 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

The spectrum of the 175's output reproducing 50Hz into 4 ohms at a level of 113W is shown in fig.7; the largest artifact is the third harmonic at 150Hz at –37dB, or about 1.5%, with the second and fifth harmonics not far behind. Fig.8 shows the output spectrum with the 175 amplifying a combined 19+20kHz signal (the graph shows the intermodulation products resulting from an input signal consisting of an equal combination of these two frequencies). The level was 52W into 8 ohms, with clipping visible just above this power level. The highest artifacts here are at 1kHz (–45.6dB, or about 0.5%) and 21kHz (–44.4dB, or about 0.6%). The artifacts of this input signal at 53W into 4 ohms (not shown) were 5–8dB higher: about 1% at 1kHz, and 1.2% at 21kHz. Both the 50Hz and the 19+20kHz spectral responses are acceptable performance for a high-power tube amplifier, though not at all exceptional compared with the best we've measured from amplifiers in general.

Fig.7 Manley 175, spectrum of 50Hz sinewave, DC–1kHz, at 113W into 4 ohms (linear frequency scale). Note that the third harmonic at 150Hz is the highest in level at –37dB (1.5%).

Fig.8 Manley 175, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 52W into 8 ohms (linear frequency scale).

The 1kHz THD+N vs output power curves are shown in fig.9. The distortion characteristic here is quite different from that of most solid-state amplifiers. The distortion increases unmistakably up to the break-point, or knee. The overall levels of distortion are moderately high—though acceptable—and the amplifier doesn't quite make its power specs. The clipping levels are shown in Table 1 for three levels of THD+N. (The manufacturer's power rating is obtained at the 1.5% distortion level.)

Fig.9 Manley 175, distortion (%) vs output power into (from bottom to top): 8 ohms, 4 ohms, and 2 ohms.

Table 1: Maximum Output Power

Load 1% THD+N1.5% THD+N3% THD+N
ohmsW (dBW)W (dBW)W (dBW)
8126 (21.0)127.5 (21.0)132.6 (21.2)
448 (13.8)87.5 (16.4)*160 (19)
317 (6.3)26.8 (8.3)78.4 (12.9)
All line voltages 115V except * = 114V

Overall, the measurement results for the 175 are good for a high-powered tube amplifier. The 175's distortion at lower impedances restrains its power output for a given level of distortion, but it will put out significant power into 4 ohms if you allow for a 3% THD+N level (1kHz).

As delivered from Dick Olsher, the bias levels of the 175's output tubes had been set about 20% below the recommended values. I rebiased the tubes according to the manufacturer's directions; the above results reflect this. However, before doing this, I made several measurements at the lower bias setting; the differences, while favoring the recommended bias, were not particularly significant. I measured about 10W more output into 8 ohms, and only marginally lower distortion with the correct bias. Apparently, this adjustment isn't overly critical in the 175.—Thomas J. Norton

Manley Laboratories, Inc.
13880 Magnolia Avenue
Chino, CA 91710
(909) 627-4256