Sidebar 4: Measurements
I performed the measurements of the LSA Discovery Warp 1 with my Audio Precision SYS2722 system. The Warp 1 runs cool—after 3 hours of testing, the chassis was still at room temperature. Because the amplifier has a class-D output stage, I inserted an Audio Precision AUX-0025 passive low-pass filter between the test load and the analyzer. This filter mitigates noise above 80kHz and eliminates noise above 200kHz that would otherwise drive the SYS2722's input circuitry into slew-rate limiting. Without the filter, 378mV of ultrasonic noise with a center frequency of 454kHz was present at the amplifier's output terminals. The level of the noise was not affected by the amplifier's gain setting (see later). I used the Audio Precision filter for all the tests other than frequency response.
The LSA Warp 1 has four gain settings, labeled "0dB," "6dB," 14dB," and "20dB," these selected with internal DIP switches. The default setting is "6dB," in which condition the voltage gain at 1kHz into 8 ohms was 27.4dB with both the balanced and single-ended inputs. The "0dB" setting, which TF had found optimal with his Benchmark LA4 preamplifier, reduced the gain by 6.1dB. The other two settings respectively increased the gain by 7.9dB and 13.8dB compared with the "6dB" setting. The Warp 1 preserved absolute polarity, ie, was noninverting, with all the gain values and both input types.
The LSA's single-ended and balanced input impedance is specified as 47k ohms. I measured 47.4k ohms at 20Hz and 1kHz for the single-ended inputs, decreasing to 41.5k ohms at 20kHz. The balanced input impedance was 49k ohms at low and middle frequencies, 44k ohms at the top of the audioband.
Fig.1 LSA Warp-1, 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) (2dB/vertical div.).
Fig.2 LSA Warp-1, small-signal 10kHz squarewave into 8 ohms.
Considering it's a bridge-tied load design, with two output stages in series with the speaker in each channel, the Warp 1's output impedance at low and middle frequencies was relatively low, at 0.11 ohms. However, the impedance rose above 1 ohm at the top of the audioband, which led to the amplifier's small-signal frequency response at 20kHz rolling off by 1dB into 4 ohms (fig.1, cyan and magenta traces) and by 4dB into 2 ohms (fig.1, green trace). Into our standard simulated loudspeaker, the variation in response in the audioband, due to the Ohm's law interaction between the amplifier's source impedance and the impedance of the simulated speaker, was +1/–0.3dB (fig.1, gray trace). The LSA amplifier's response into 8 ohms (fig.1, blue and red traces) peaked sharply between 40kHz and 50kHz, which correlated with some critically damped overshoot in the Warp 1's reproduction of a 10kHz squarewave into 8 ohms (fig.2).
Fig.3 LSA Warp-1, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (left channel blue, right red; linear frequency scale).
The Warp 1's channel separation (not shown) was excellent in both directions in the midrange, at >100dB, though it decreased to 62dB at 20kHz. The unweighted, wideband signal/noise ratio (ref. 1W into 8 ohms with the auxiliary filter and measured with the single-ended inputs shorted to ground) was very good, at 72.1dB (average of the two channels). This ratio improved to an excellent 83.7dB when the measurement bandwidth was restricted to 22Hz–22kHz, and to 88.3dB when A-weighted. Spectral analysis of the low-frequency noisefloor while the LSA amplifier drove a 1kHz tone at 1Wpc into 8 ohms (fig.3) revealed that AC-related spuriae were absent.
Fig.4 LSA Warp-1, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.5 LSA Warp-1, distortion (%) vs 1kHz continuous output power into 4 ohms.
The Warp 1's output power is specified as 150W into 8 ohms (21.8dBW) and 250W into 4 ohms (21dBW). With our usual definition of clipping—when THD+noise reaches 1%—I measured a clipping power of 110Wpc into 8 ohms with both channels driven (20.4dBW, fig.4) and 165Wpc into 4 ohms (19.16dBW, fig.5).
Fig.6 LSA Warp-1, THD+N (%) vs frequency at 20V into: 8 ohms (left channel blue, right red), 4 ohms (left green, right gray).
Fig.6 shows how the percentage of THD+noise varies with frequency into 8 and 4 ohms at 20V (equivalent to 50W into 8 ohms and 100W into 4 ohms). I couldn't examine the THD+N percentage into 2 ohms at this voltage, as the amplifier muted its outputs after just 3 seconds and the red "Amp Fault" LED illuminated. This happened after 3 minutes with the amplifier driving the same voltage into 4 ohms, for an output of 100W. The distortion into the higher impedances was very low in the bass and midrange but rose considerably in the treble.
Fig.7 LSA Warp-1, left channel, 1kHz waveform at 50W into 8 ohms, 0.011% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.8 LSA Warp-1, spectrum of 50Hz sinewave, DC–1kHz, at 50Wpc into 8 ohms (left channel blue, right red; linear frequency scale).
Fig.9 LSA Warp-1, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 100Wpc peak into 4 ohms (left channel blue, right red; linear frequency scale).
With this very low distortion level, the harmonics' contributions to the waveform (fig.7, bottom trace) are obscured by the residual audioband noise. Spectral analysis, however, indicated that the third harmonic was the highest in level, at just –97dB, right (fig.8, red trace) and –101dB, left (blue trace), although this graph indicates that the second harmonic and higher-order harmonics are also present. With the Warp 1 driving an equal mix of 19 and 20kHz tones at 100Wpc peak into 4 ohms (fig.9), the 1kHz difference product lay at –100dB (0.001%), though the higher-order products at 18 and 21kHz were almost 30dB higher in level.
LSA's Discovery Warp 1 offers respectable measured performance and relatively high power in a small, cool-running package.—John Atkinson
