Sidebar 3: Measurements
I measured the Ayre Acoustics KX-5 Twenty's electrical performance with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). The volume control operated in 1.5dB steps, the maximum gain obtained when it was set to "46." The maximum gain for both balanced and unbalanced inputs to balanced outputs was just 0.2dB. This is lower than the norm, but it's something I welcome because preamplifiers in general are used to attenuate, not amplify, the levels of source signals. The maximum gain from unbalanced input to unbalanced input was –5.8dB; ie, an input of 1V results in an output of 511mV.
The Ayre's input impedance was very high, at >400k ohms unbalanced and 1 megohm balanced. I can't give more specific values, as the voltage-drop method I use to measure input impedance becomes increasingly inaccurate with the increase in impedance of the device under test. The KX-5 Twenty preserved absolute polarity (ie, was non-inverting) with both balanced and unbalanced inputs. Its XLR jacks are wired with pin 2 hot, the AES convention. The preamplifier's unbalanced output impedance was a low 78 ohms from 20Hz to 20kHz; the balanced impedance was twice that figure, as expected.
The KX-5 Twenty's frequency range is specified as "DC–250kHz," and the preamplifier's response was indeed flat from 10Hz to 200kHz for balanced and unbalanced inputs and outputs at all settings of the volume control (fig.1). Channel separation was superb, at >120dB in both directions below 2kHz, and still 100dB at 20kHz. The KX-5 Twenty is also a very quiet preamplifier: the audioband signal/noise ratio ref. 1V output, measured with the input shorted to ground but the volume control set to its maximum, was 92.5dB (average of both channels). Switching an A-weighting filter into circuit slightly improved this ratio, to 94.6dB, and the wideband, unweighted ratio was a still-good 77.5dB. Spectral analysis of the KX-5's low-frequency noise floor revealed a complete absence of power-supply–related spuriae, other than a minuscule blip at 60Hz (fig.2).
Fig.1 Ayre KX-5 Twenty, balanced frequency response with volume control set to "46" at 1V, into: 100k ohms (left channel blue, right red), 600 ohms (left cyan, right magenta) (1dB/vertical div.).
Fig.2 Ayre KX-5 Twenty, balanced spectrum of 1kHz sinewave, DC–1kHz, at: 2V (left channel blue, right red), 0V into 100k ohms (left cyan, right magenta) (linear frequency scale).
Fig.3 plots the percentage of THD+noise in the KX-5 Twenty's balanced output into 100k ohms. The THD+N rises below 2V output due to the fixed level of noise becoming an increasing percentage of the signal level. The actual distortion rises above the noise floor above 2V, when it is just 0.0018%, and is still only 0.03% at 15.45V output, the maximum I could obtain with the Audio Precision's signal generator. The picture hardly changed when I reduced the load impedance to the current-hungry 600 ohms. For unbalanced input to unbalanced output (fig.4), which is how Art Dudley auditioned the preamp (in my own auditioning I used balanced operation), the distortion was a little higher, and the preamplifier clipped (defined as when the THD+N reaches 1%) at just under 5V, confirming the specified maximum unbalanced input of 8V.
Fig.3 Ayre KX-5 Twenty, balanced distortion (%) vs 1kHz output voltage into 100k ohms.
Fig.4 Ayre KX-5 Twenty, unbalanced distortion (%) vs 1kHz output voltage into 100k ohms.
Distortion remained constant throughout the audioband (fig.5), which is remarkable considering the circuit's lack of loop negative feedback. The spectrum of the distortion comprised only the second and third harmonics (fig.6), which would still be subjectively innocuous at 100 times the level they are actually present in the Ayre's output. As expected, these harmonics were around 20dB higher in the unbalanced output (fig.7), but this is still nothing to be concerned about. Intermodulation distortion was very low, even into 600 ohms (fig.8).
Fig.5 Ayre KX-5 Twenty, balanced THD+N (%) vs frequency at 2V into 100k ohms (left channel blue, right red) and 600 ohms (left cyan, right magenta).
Fig.6 Ayre KX-5 Twenty, balanced spectrum of 50Hz sinewave, DC–1kHz, at 2V into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.7 Ayre KX-5 Twenty, unbalanced spectrum of 50Hz sinewave, DC–1kHz, at 2V into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.8 Ayre KX-5 Twenty, balanced HF intermodulation spectrum, DC–30kHz, 19+20kHz at 2V into 600 ohms (left channel blue, right red; linear frequency scale).
Ayre's KX-5 Twenty is a superbly well-engineered line preamplifier. I just wish I could have shared these results with its designer, the late Charley Hansen.—John Atkinson
