Sidebar 3: Measurements
Because the Gryphon Apex Stereo is big and heavy, I drove to Michael Fremer's place in deepest, darkest New Jersey with my Audio Precision SYS2722 system, digital and analog oscilloscopes, and test loads. I preconditioned the amplifier by operating it at 1/8 the specified power into 8 ohms for 20 minutes. At the end of that time, the temperature of the heatsinks was 113.6°F/45.4°C and that of the top panel 105.3°F/40.8°C. The bias was set at High for all testing.
The Gryphon Apex Stereo has only balanced inputs. The voltage gain at 1kHz was 30.85dB into 8 ohms, very close to the specified gain of 31dB. The amplifier preserved absolute polarity, ie, was noninverting. The input impedance is specified as 20k ohms. I measured 19.9k ohms at low and middle frequencies, decreasing inconsequentially to 19.5k ohms at the top of the audioband.
Gryphon specifies the Apex's output impedance as 0.015 ohms. My measurement was slightly higher in the right channel, at close to 0.05 ohms (including the series resistance of 6' of speaker cable), and significantly higher in the left channel, at 0.17 ohms. I checked the wiring and connections and found nothing amiss. I am at a loss, therefore, to explain this discrepancy. The variation in the small-signal frequency response of the Apex's right channel into our standard simulated loudspeaker (fig.1, gray trace) was inconsequential. Into resistive loads (blue, red, cyan, magenta, and green traces), the amplifier gently rolled off well above the audioband, with the –3dB frequency dependent on the load and channel. The left channel's response into 2 ohms (green trace) was down by 3dB at 130kHz. The Apex's reproduction of a 10kHz squarewave into 8 ohms (fig.2) featured very short risetimes and no overshoot or ringing.
The Gryphon Apex Stereo's maximum continuous power with both channels driven is specified as 210Wpc into 8 ohms (23.2dBW), 420Wpc into 4 ohms (23.2dBW), and 800Wpc into 2 ohms (23dBW). The Gryphon amplifier exceeded its specified powers into the higher impedances. With our usual definition of clipping as being when the THD+noise reaches 1%, I measured a clipping power with both channels driven of 240Wpc into 8 ohms (23.8dBW, fig.4) and 450Wpc into 4 ohms (23.5dBW, fig.5). With one channel driven, the Apex clipped at 750W into 2 ohms (22.7dBW, fig.6), even though the wall voltage had only dropped slightly at that power, from 120.1V to 119.5V.
The left channel's distortion signature into 8 ohms predominantly comprised the second and third harmonics (fig.8). Spectral analysis at the same power (fig.9) revealed that while the third harmonic lay at similar levels in both channels, the second harmonic was 16dB higher in the left channel (blue trace) than the right (red), lying at –73dB compared with –89dB. Even with the left channel's higher level of harmonic distortion than the right, the Apex still did well driving an equal mix of 19 and 20kHz tones at 100Wpc into 4 ohms (fig.10). The 1kHz difference product lay 83dB (left) and 87dB (right) below the peak signal level, and while the higher-order products at 18 and 21kHz were a little higher in both channels, they still lay at or below –72dB.
Fig.1 Gryphon Apex, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), and 2 ohms (green) (1dB/vertical div.).
Fig.2 Gryphon Apex, small-signal 10kHz squarewave into 8 ohms.
As expected from its dual-mono construction, the Apex's channel separation (not shown) was superb, at >100dB in both directions below 2kHz and still 80dB at the top of the audioband. The unweighted, wideband signal/noise ratio (ref. 1W into 8 ohms and measured with the balanced input shorted to ground) was very good, measuring 73.4dB (average of the two channels). This ratio improved to an excellent 85dB when the measurement bandwidth was restricted to 22Hz–22kHz, and to 90.6dB, left, and 94.1dB, right, when A-weighted. Spectral analysis of the low-frequency noisefloor while the Gryphon Apex Stereo drove a 1kHz tone at 1Wpc into 8 ohms (fig.3) revealed that although some AC-related spuriae were present, these all lay at or below –100dB in the left channel (blue trace) and –110dB in the right channel (red trace). The Apex has individual AC power cords for its two channels. To avoid a ground loop, I plugged my test equipment into the same wall outlet as the left channel. However, I don't believe this was the reason for the higher level of noise and spuriae in the left channel, as the spectrum was the same whether I grounded the XLR input's pin 1 or left it floating. In any case, though higher than in the left channel, the level is still very low.
Fig.3 Gryphon Apex, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (left channel blue, right red; linear frequency scale).
Fig.4 Gryphon Apex, left channel, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.5 Gryphon Apex, left channel, distortion (%) vs 1kHz continuous output power into 4 ohms.
Fig.6 Gryphon Apex, left channel, distortion (%) vs 1kHz continuous output power into 2 ohms.
Figs.4, 5, and 6 were taken from the left channel's output. Fig.7 shows how the percentage of THD+noise varies with frequency into 8, 4, and 2 ohms at 20V (equivalent to 50W into 8 ohms, 100W into 4 ohms, and 200W into 2 ohms). The THD+N was significantly lower in the right channel (red, magenta, and gray traces) than the left (blue, cyan, and green traces), though still low into the higher impedances.
Fig.7 Gryphon Apex, THD+N (%) vs frequency at 20V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), and 2 ohms (left green, right gray).
Fig.8 Gryphon Apex, left channel, 1kHz waveform at 50W into 8 ohms, 0.0278% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.9 Gryphon Apex, spectrum of 50Hz sinewave, DC–1kHz, at 50Wpc into 8 ohms (left channel blue, right red; linear frequency scale).
Fig.10 Gryphon Apex, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 100Wpc peak into 4 ohms (left channel blue, right red; linear frequency scale).
Presumably, the left channel's higher levels of distortion and noise and its higher output impedance than the right channel's are sample-specific. But even so, the Gryphon Apex Stereo offers high power coupled with a wide bandwidth and primarily low levels of low-order distortion. I wish I could have stayed longer at Mikey's to listen to it driving his Wilson Chronosonic XVX speakers, but I had to beat the rush-hour traffic back to Brooklyn.—John Atkinson
