I performed a complete set of measurements on one of the JMF HQS 7001s (serial number 311732) with my Audio Precision SYS2722 system, then repeated some of the testing with the magazine's APx500 analyzer. I preconditioned the HQS 7001 by following the CEA's recommendation of running it at one-eighth the specified power into 8 ohms for 30 minutes. At the end of that time, the temperature of the top panel was 101.8°F (38.8°C) and that of the heatsink on the rear panel was much higher, at 173.5°F (78.6.1°C). The JMF amplifier has just enough thermal capacity for its rated power.
The JMF amplifier's balanced input preserved absolute polarity, ie, the XLR jack was wired with pin 2 positive. The HQS 7001's input impedance is specified as 20k ohms. I measured 6.8k ohms from 20Hz to 20kHz. Though this is lower than the specified impedance, modern solid state preamplifiers will have no problems driving the JMF. The voltage gain at 1kHz was higher than usual, at 34.4dB into 8 ohms.
The output impedance, including the series impedance of 6' of spaced-pair cable, was a relatively low 0.15 ohms at 20Hz and 1kHz, rising to 0.23 ohms at 20kHz. As a result, the variation in the frequency response with our standard simulated loudspeaker (fig.1, gray trace) was minimal, at ±0.15dB. The response into resistive loads was flat in the audioband, not reaching –3dB until 180kHz into 8 ohms (blue trace) and 123kHz into 4 ohms (magenta). The rise in output impedance with increasing frequency means that into 2 ohms, the output was down by 3dB at 70kHz. With its wide small-signal bandwidth, the JMF's reproduction of a 10kHz squarewave into 8 ohms featured short risetimes in both modes (fig.2), with no overshoot or ringing.
The unweighted, wideband signal/noise ratio (ref. 1W into 8 ohms), taken with the input shorted to ground, was a good 64.6dB. This ratio improved to 80.8dB when the measurement bandwidth was restricted to 22Hz–22kHz, and to 82.7dB when A-weighted. Spectral analysis of the low-frequency noisefloor while the JMF drove a 1kHz tone at 1W into 8 ohms (fig.3) revealed a low level of random noise, but spuriae were present at 60Hz and its odd-order harmonics, each close to –90dB ref. 1W. These will be due to magnetic interference from the power transformer.
JMF specifies the HQS 7001's maximum power as 300W into 8 ohms (24.77dBW), 500W into 4 ohms (24.0dBW), and 850W into 2 ohms (23.27dBW). Stereophile defines an amplifier's clipping power as being when the THD+noise reaches 1%. With that criterion, the HQS 7001 slightly exceeded its specified powers into 8 ohms (310W, 24.91dBW, fig.4), and into 4 ohms (515W, 24.1dBW, fig.5). The HQS 7001 clipped at 760W into 2 ohms (22.8dBW, fig.6), which is lower than the specified maximum power. However, the wall voltage had dropped from 121.4V AC with the amplifier idling to 118.7V AC with it clipping into 2 ohms; this drop was responsible for the shortfall in clipping power.
I examined how the THD+N percentage varied with frequency at 20V, which is equivalent to 50W into 8 ohms, 100W into 4 ohms, and 200W into 2 ohms. The THD+N percentage was low into 8 ohms (fig.7, blue trace) but rose into 4 ohms (magenta trace) and 2 ohms (red trace). The rise in THD+N in the top three audio octaves will be due to the amplifier having relatively limited open-loop gain, which means that less distortion-reducing feedback is available as the frequency rises (footnote 1).
The distortion waveform was predominantly the third harmonic (fig.8), though there are crossover distortion spikes visible at the waveform's zero-crossing points. These correlate with the fifth and seventh harmonics in the output spectrum (fig.9), but these are much lower in level than the magnetic interference from the power transformer.
With the reduction in corrective feedback at high frequencies, high-order intermodulation products with an equal mix of 19 and 20kHz tones at 50W into 8 ohms were present, though they all lay at or below –72dB (fig.10). Commendably, the second-order difference product at 1kHz also lay below the level of the power supply–related spuriae.
The JMF HQS 7001's measured performance is typical of a high-power, solid state design with a class-AB output stage. Distortion will be lowest into 8 ohms, but the amplifier should not have had any problems driving JVS's Wilson Alexia V speakers to acceptably loud levels.—John Atkinson
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COMMENTS
Nice to see a linear amp review back on deck.
JMF specifies 300W-8ohms, 500W-4 ohms, 850W-2 ohms (this says to me bi-polar)
JA measurements: 310W-8ohm, 515W-4ohm, 760W-2 ohms (this suggest mosfet taking an almost 100w dive into 2ohms, regardless of just 3vac line drop)
Cheers George
Looks like Bi Polar not enuff class A bias for MOSFETS ..!
Regards
When was this changed from 33% of rated output to a 1/8 ..?
When was this changed from 33% of rated output to a 1/8?
The 1/3 power pre-conditioning had always been unpopular with manufacturers as it required large heatsinks, so in 2000 the FTC revised the pre-conditioning requirement to cope with the advent of multi-channel class-AB amplifiers - see my report on the FTC's "Amplifier Rule" at www.stereophile.com/content/ftc-proposes-eliminating-its-amplifier-rule.
I continued using the 1/3 power pre-conditioning for many years, but fairly recently adopted the revised FTC test. I now only apply the 1/3 power test when an amplifier remains cool with the 1/8 power testing.
John Atkinson
Technical Editor, Stereophile
... a pair of power amps in order to attain a state of "bliss" when listening to music reproduced by Harbeth 40.3 XD speakers?
Harbeth's US distributor (as well as a retailer) might well want you to believe so.
However, you might want to know which amp Harbeth's owner/designer Alan Shaw uses at UK hi-fi shows to best demonstrate his company's products.
He has bought and uses Hegel H360/H390 integrated amps. The H390 presently sells for a mere $6,600.
Further note that Mr. Shaw has indicated that a peak power output of about 150W (@ 6 ohms) is likely to be sufficient for most home listeners using his speakers. A Hegel H190 is capable of about 225W continuous and about 320W peak (@ 6 ohms), and is now available for only $2,800.
If JVS observed that the -12dB indicator illuminated only briefly, and the -9dB LED never lit, then he was using, at most, about one-tenth of the maximum output power of which the JMF amps were capable - so, well under 100W. While the H190 would meet that requirement, it would struggle with the 1 ohm EPDR of the Alexia V speakers; however, the H390 would handle it with relative ease.
I was expecting your cost analysis the first day this review dropped. Now do the Luxman!
... that Luxman integrated amp was recommended to Alan Shaw, he deemed it to be too expensive, so instead he bought the cheaper Hegel units.
(Plus, he was under the incorrect impression that Hegel products were made in Scandinavia.)
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