Sidebar 3: Measurements
I measured one of the Amphion Krypton3X loudspeakers, serial number 1035, in Tom Fine's listening room, driving it with his Benchmark AHB2 amplifier. I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone with an Earthworks microphone preamplifier to measure the Amphion Krypton3X's farfield frequency behavior and dispersion. I used an Earthworks QTC-40 mike, which has a small, ¼"-diameter capsule, for the nearfield responses. Although TF and his friend Mike Barney were able to lift the 154lb speaker onto a small dolly, the first reflection from the ground still occurred earlier than is usually the case with my testing. I therefore measured the response and dispersion with the microphone at 1m rather than my usual 50". It wasn't possible to measure the off-axis response more than 60° to each side of the tweeter axis.
Footnote 1: See EPDR is the resistive load that gives rise to the same peak dissipation in an amplifier's output devices as the loudspeaker. See "Audio Power Amplifiers for Loudspeaker Loads," JAES, Vol.42 No.9, September 1994, and stereophile.com/reference/707heavy/index.html.
Fig.1 Amphion Krypton 3X, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
Amphion specifies the Krypton3X's anechoic sensitivity as 89dB (no units given). My B-weighted estimate was 85.5dB(B)/2.83V/1m. I measured the Krypton3X's impedance magnitude and electrical phase angle with Dayton Audio's DATS V2 system. The impedance, which is specified as 4 ohms, was higher than 4 ohms for almost the entire audioband (fig.1, solid trace), with a minimum value of 3.175 ohms at 100Hz. However, as the electrical phase angle (dashed trace) is mostly high, the effective resistance, or EPDR (footnote 1), drops below 3 ohms between 46Hz and 724Hz, and below 4 ohms above 10kHz. The minimum EPDR values are 1.51 ohms at 62Hz and 1.7 ohms between 221Hz and 248Hz. As music can have high energy in these regions, the Krypton3X is a demanding amplifier load.
Fig.2 Amphion Krypton 3X, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of front baffle midway between the lower midrange unit and the speaker's base (measurement bandwidth, 2kHz).
The traces in fig.1 are free from the small discontinuities in the midrange that would imply the existence of cabinet resonances of various kinds, and the large enclosure did seem inert when I rapped its walls with my knuckles. When I investigated the Amphion's vibrational behavior with a plastic-tape accelerometer, the highest-level resonant modes lay at 188Hz and 207Hz on the front baffle midway between the lower midrange unit and the speaker's base (fig.2). However, the modes are very low in level and have a high Q (Quality Factor), which means they won't have audible consequences.
Fig.3 Amphion Krypton 3X, farfield response on tweeter axis at 1m, corrected for microphone response, with the nearfield responses of the midrange units (black), woofer (blue), and ports (red), respectively plotted below 350Hz, 830Hz, and 125Hz.
The saddle centered at 24Hz in the magnitude trace in fig.1 indicates that the tuning frequency of the two ports on the rear panel lies in this region. The red trace in fig.3 shows the summed nearfield response of the ports. Their output reaches its maximum at the tuning frequency, and the upper-frequency rollout is very clean. The woofer's nearfield response (fig.3, blue trace) has the expected minimum-motion notch at the port tuning frequency. Unusually, the Amphion uses a series crossover topology rather than the ubiquitous parallel topology. The woofer crosses over to the two midrange units (black trace below 350Hz) close to the specified 160Hz. The crossover slopes in fig.3 appear to be first-order. Both midrange units behaved identically, but their output, with that of the tweeter, has a significant lack of energy in the octave above 750Hz. (This may be why my estimate of the Krypton3X's voltage sensitivity was lower than the specified 89dB.) The farfield response is even throughout the treble, but the tweeter's titanium dome has a 24dB-high resonant peak at 25kHz.
Fig.4 Amphion Krypton 3X, anechoic response on tweeter axis at 1m, averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the nearfield responses plotted below 300Hz.
The complex sum of the woofer and port responses (black trace below 300Hz in fig.4) has the usual rise in the midbass response; this is due to the nearfield measurement technique, which assumes that the drive units are mounted in a true infinite baffle. The Krypton3X's anechoic woofer alignment is maximally flat, and the speaker offers extended low frequencies, with the summed nearfield output down by 6dB at 30Hz.
The black trace above 300Hz in fig.4 shows the Krypton3X's quasi-anechoic farfield response, averaged across a 30° horizontal window centered on the tweeter axis. The suckout in the upper midrange seen in fig.3 is still present, as is the large peak at 25kHz. The response is otherwise even, though the tweeter appears to be balanced 2–3dB lower in level than the midrange units.
Fig.5 Amphion Krypton 3X, lateral response family at 1m, normalized to response on tweeter axis, from back to front: differences in response 60–5° off axis, reference response, differences in response 5–60° off axis.
Fig.6 Amphion Krypton 3X, vertical response family at 1m, normalized to response on tweeter axis, from back to front: differences in response 20–5° above axis, reference response, differences in response 5–10° below axis.
Fig.5 shows the Krypton3X's horizontal dispersion, normalized to the response on the tweeter axis, which thus appears as a straight line. The radiation pattern doesn't start to narrow until the top two audioband octaves, and the contour lines in this graph are even and well-controlled. The suckout centered on 14kHz in the on-axis response tends to fill in to the speaker's sides. The Amphion's radiation pattern in the vertical plane, again normalized to the tweeter-axis response, is shown in fig.6. The loudspeaker's response doesn't change appreciably over a wide ±10° angle, but a suckout close to the upper crossover frequency of 1.6kHz develops 20° above the tweeter axis.
Fig.7 Amphion Krypton 3X, step response on tweeter axis at 1m (5ms time window, 30kHz bandwidth).
Fig.8 Amphion Krypton 3X, cumulative spectral-decay plot on tweeter axis at 1m (0.15ms risetime).
In the time domain, the Krypton3X's step response (fig.7) indicates that the tweeter and midrange units are both connected in inverted acoustic polarity, the woofer in positive polarity. The tweeter's output arrives first at the microphone, followed by that of the midrange units, then that of the woofer. The decay of each drive unit's step smoothly blends with the start of that of the next lower in frequency, which implies an optimal crossover topology. The Krypton3X's cumulative spectral-decay plot (fig.8) is dominated by the ultrasonic tweeter dome resonance. Other than that, and some delayed energy at the top of the midrange units' passband, the overall decay is clean.
In most respects, the Amphion Krypton3X offers good measured performance, but I am bothered by the upper midrange suckout in the farfield response, and it has been years since I encountered a speaker that had such a high-level ultrasonic tweeter dome resonance.—John Atkinson
Footnote 1: See EPDR is the resistive load that gives rise to the same peak dissipation in an amplifier's output devices as the loudspeaker. See "Audio Power Amplifiers for Loudspeaker Loads," JAES, Vol.42 No.9, September 1994, and stereophile.com/reference/707heavy/index.html.
