Solid State Power Amp Reviews

To examine the measured behavior of the Chord SPM 650, I primarily used Stereophile's loaner sample of the top-of-the-line Audio Precision SYS2722 system (see the January 2008 "As We See It" and www.ap.com); for some tests, I also used my vintage Audio Precision System One Dual Domain.

My first test of an amplifier is to run it for 60 minutes at one-third its specified power into 8 ohms, which is thermally the worst case for an amplifier with a class-B or -AB output stage. However, the Chord's thermal sensor shut the amplifier down after just six minutes of running at 43Wpc into 8 ohms, with the heatsinks and chassis almost too hot to touch and the front-panel LED dark. The SPM 650 cooled down quickly, however; to bring it back to life, I pressed, held, then released the front-panel pushbutton to put the amplifier in standby, then pressed the button a second time to turn it on. There seemed to be no permanent damage. The Chord turned itself off several times during my testing, as sustained drive at a not unreasonable power level caused it to overheat. Clearly, the SPM 650's attractive-looking heatsinking is inadequate for an amplifier of this power rating. This may not be a factor under normal listening conditions—Michael Fremer specifically mentioned that the review sample was cool-running—but the SPM 650 would not be a good choice for providing music at a party.

The Chord's voltage gain into 8 ohms was a little above the norm but to specification at 29.9dB, and it inverted absolute polarity from both its balanced and unbalanced input jacks. The input impedance was usefully high, at 53k ohms unbalanced and 106k ohms balanced. The output impedance was commendably low at low and midrange frequencies, at 0.065 ohm, rising inconsequentially to 0.11 ohm at 20kHz. As a result, the modification of the Chord's frequency response by the usual Ohm's Law interaction between its source impedance and the impedance of our standard simulated loudspeaker (fig.1, green trace) fell within tight ±0.1dB limits. While the amplifier's response at 20kHz was down 0.35dB (fig.1, black), the response into 8 ohms (fig.1, blue, red) was almost flat to 20kHz, not reaching –3dB until a high 120kHz, significantly higher than specified. Accordingly, the SPM 650's reproduction of a 10kHz squarewave was excellent, with very short risetimes and no sign of overshoot or ringing (fig.2). The 1kHz squarewave (not shown) was superbly square.

Fig.1 Chord SPM 650, frequency response at 2.83V into simulated loudspeaker load (green), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (gray). (1dB/vertical div.)

Fig.2 Chord SPM 650, small-signal 10kHz squarewave into 8 ohms.

Channel separation (not shown) was better than 90dB below 2kHz in the L–R direction, but up to 10dB worse in the R–L direction. The wideband, unweighted signal/noise ratio, ref. 2.83V into 8 ohms (1W), was modest, at 65.8dB. This increased to 76.3dB when the measurement bandwidth was restricted to the audioband, and to 78.8dB when A-weighted. This last is equivalent to 98.4dB referenced to two-thirds maximum power, which is not quite as good as the 103dB quoted by Chord.

Fig.3 plots the THD+noise percentage in the Chord's output against power into 8 and 4 ohms. The SPM 650 just meets its 130Wpc (21.1dBW) specified maximum power at clipping (1% THD), but delivered 160W (19dBW) rather than 170W (19.3dBW) at clipping into 4 ohms. The shortfall is inconsequential. The L–R downward slope of the traces below 20W in this graph suggests that the distortion lies below the noise floor at lower power levels.

Fig.3 Chord SPM 650, distortion (%)vs 1kHz continuous output power into (from bottom to top): 8, 4 ohms.

Plotting the THD+N percentage against frequency at 10V RMS (fig.4) indicates that the distortion is low into higher impedances, though the right channel (red and magenta traces) is slightly more linear/quieter than the left (blue and cyan) and the THD rises slightly above 5kHz, due to the usual decreasing amount of open-loop circuit gain at higher frequencies. The left channel behaves linearly at lower frequencies into 2 ohms (green), but the high-frequency rise in THD is more severe, though it is fair to note that the distortion still lies below 0.1% at the upper-band edge.

Fig.4 Chord SPM 650, THD+N (%)vs frequency at 10V into 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green).

The harmonic content of the SPM 650's distortion is predominantly low-order in nature (fig.5), though both higher harmonics and some 120Hz content appear at high powers into low impedances (figs. 6 and 7). Testing the Chord's limits with an equal mix of 19kHz and 20kHz tones at a power level close to visual clipping on the oscilloscope screen (fig.8) revealed that the 1kHz difference tone was highest in level at –76dB (0.015%), with the higher-order intermodulation products at least 10dB lower in level.

Fig.5 Chord SPM 650, 1kHz waveform at 60W into 4 ohms (top), 0.014% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.6 Chord SPM 650, spectrum of 50Hz sinewave, DC–1kHz, at 60W into 8 ohms (left channel blue, right red; linear frequency scale).

Fig.7 Chord SPM 650, spectrum of 1kHz sinewave, DC–1kHz, at 120W into 4 ohms (left channel blue, right red; linear frequency scale).

Fig.8 Chord SPM 650, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 101W peak into 4 ohms (linear frequency scale).

Its marginal heatsink capacity and higher than usual noisefloor aside, the Chord SPM 650 measures well, offering a good balance of performance. There was not really anything to indicate why it ultimately failed to provide Michael Fremer with sufficient musical satisfaction.—John Atkinson