Solid State Power Amp Reviews

I performed a complete set of measurements on one of the Boulder 1151 amplifiers (serial number 14142) with my Audio Precision SYS2722, then repeated some of the tests with the magazine's higher-resolution Audio Precision APx555. I preconditioned the 1151 by following the CEA's recommendation of running it at one-eighth the specified power into 8 ohms for 30 minutes. At the end of that time, the temperature of the top panel was just 89.8°F (32.1°C) and that of the side-mounted heatsinks 97.3°F (36.3°C).

As the balanced input is wired with pin 2 positive, the AES standard, the amplifier preserved absolute polarity. The 1151's balanced input impedance is specified as 200k ohms. I measured 184k ohms at 20Hz and 1kHz, 90k ohms at 20kHz. The output impedance was a low 0.035 ohms at 20Hz and 1kHz, rising very slightly to 0.038 ohms at 20kHz. As a result, the variation in the frequency response with our standard simulated loudspeaker (fig.1, gray trace) is negligible. The response into resistive loads was flat in the audioband, not reaching –3dB until 130kHz. With its wide small-signal bandwidth, the Boulder's reproduction of a 10kHz squarewave into 8 ohms featured very short risetimes (fig.2) with no overshoot or ringing.

The unweighted, wideband signal/noise ratio (ref. 1W into 8 ohms), taken with the input shorted to ground, was a superb 86.3dB. This ratio improved to 95.2dB when the measurement bandwidth was restricted to 22Hz–22kHz, and to 96.8dB when A-weighted. Spectral analysis of the low-frequency noisefloor as the Boulder drove a 1kHz tone at 1W into 8 ohms revealed a low random noisefloor, and while odd-order harmonics of 60Hz were measurable, these all lay at or below –110dB (fig.3).

Boulder specifies the 1151's maximum continuous power as 250W into 8 ohms, which is equivalent to 24dBW. (The maximum continuous power isn't specified into 4 ohms and 2 ohms.) Using our usual definition of clipping (THD+N of 1%), the 1151 exceeded its specified power, clipping at 310W into 8 ohms with both a 1kHz signal (24.9dBW, fig.4) and a 20kHz signal. The amplifier clipped at 500W into 4 ohms (21dBW, fig.5), but when I measured the maximum power into 2 ohms, the 1151 appeared to go into protection at 356W (19.5dBW). As soon as I turned off the input signal, the amplifier resumed correct operation.

I examined how the percentage of THD+N varied with frequency at 20V, which is equivalent to 50W into 8 ohms, 100W into 4 ohms, and 200W into 2 ohms (fig.6). The THD+N percentage was extremely low into 8 ohms (blue trace), rising into 4 ohms (magenta trace) and 2 ohms (red trace). It was still low in absolute terms, however.

The distortion waveform was predominantly the third harmonic (fig.7), which lay just above –100dB (0.001%, fig.8). While the second and fifth harmonics are present, these are 20dB lower in level than the third. While no even-order supply-related spuriae are present in the low-frequency, low-power noisefloor shown in fig.3, a lot of these tones can be seen in fig.8 along with other tones of unknown origin 20Hz above the supply-related sidebands that surround the fundamental tone and its harmonics. All these tones lie at very low levels, however. Intermodulation distortion with an equal mix of 19 and 20kHz tones was extremely low in level, even at 100W into 4 ohms (fig.9).

The Boulder 1151 features superb measured performance.—John Atkinson