That is, the peaks and dips in the responses correspond closely to the peaks and dips in the impedance plots. Note that the Sonic Frontiers' frequency-response deviations when driving these loudspeakers show the same general trends as the impedance magnitudes of each respective loudspeaker. This is not to say that the Sonic Frontiers cannot provide excellent performance, but any amplifier with a high output impedance will be prone to this matching sensitivity.įig.1 Melos 400, frequency response at 10mW into Martin-Logan Aerius from (from top to bottom at 20kHz): 1 ohm tap, 2 ohm tap, 4 ohm tap, and 8 ohm tap (1dB/vertical div.).įig.2 Vandersteen 2Ce, frequency response at 10mW at speaker terminals driven by (from top to bottom at 20kHz): Hafler 9500, Aragon 4004 Mk.II, Melos 400 (4 ohm tap), and Sonic Frontiers SFS-80 amplifiers (1dB/vertical div.).įig.3 Spica Angelus, frequency response at 10mW at speaker terminals driven by (from top to bottom at 20kHz): Hafler 9500, Aragon 4004 Mk.II, Melos 400 (4 ohm tap), and Sonic Frontiers SFS-80 amplifiers (1dB/vertical div.).įig.4 Martin-Logan Aerius, frequency response at 10mW at speaker terminals driven by (from top to bottom at 20kHz): Hafler 9500, Aragon 4004 Mk.II, Melos 400 (4 ohm tap), and Sonic Frontiers SFS-80 amplifiers (1dB/vertical div.).įigs.5-7 show the measured impedances of the 2Ce, Angelus, and Aerius. As expected, this high output impedance results in clear deviations in the frequency response into real loudspeaker loads. Note, however, the results with the Sonic Frontiers SFS-80, an amplifier with a high (above 3 ohms) output impedance. The Melos is also respectably flat (taken from the 4 ohm tap for all of these curves). Note that the two solid-state amplifiers-the Hafler 9500 and the Aragon 4004 Mk.II-produce the flattest responses. Figs.2-4 show the results of four different amplifiers driving each of the loudspeakers-one at a time, of course. There are no significant differences below 10kHz (note the ±5dB scale on figs.1-4). I tested three popular audiophile loudspeakers: the Vandersteen 2Ce, the Spica Angelus, and the Martin-Logan Aerius.įig.1 shows the response of the Melos 400 when driving the Aerius from its 1, 2, 4, and 8 ohm taps, respectively. A very low power output-about 10mW-was used to prevent possible damage to the loudspeakers, especially at high frequencies. All of the loudspeakers were connected with an 8' run of Symo cable shorter lengths of ordinary cable didn't significantly change the results. Since the Melos 400 also had a relatively low output impedance for a tube amplifier (at 0.43 ohms at low and mid frequencies, rising to 1.2 ohms at 20kHz, from its 8 ohm tap), I took that opportunity to run some frequency-response measurements using an actual loudspeaker as the load for the amplifier.įigs.1-3 show the results, all measured at the loudspeaker's input terminals. Some months back-before the CAD-805 arrived-I investigated this phenomenon in conjunction with measurements for a forthcoming review of the Melos 400 monoblock amplifier. The Cary CAD-805, for example, has a lower output impedance than most tube amplifiers, and should be less prone to such interaction. A high output impedance-such as is found in many tube amplifiers-will interact with the loudspeaker's impedance in a way which directly affects the combination's frequency response. On a number of occasions we have commented on the effects of an amplifier's output impedance on a system's performance.
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