Accustic Arts Audio Mono II monoblock power amplifier Measurements

Sidebar 3: Measurements

I performed a full set of measurements on one of the Accustic Arts Mono IIs, S/N M2-1JB1258, using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). I repeated some of the tests on S/N 1259, the sample that Jason Victor Serinus found had developed a hum toward the end of his auditioning. Before doing any testing, I preconditioned the first sample of the Mono II by running it at one-third power into 8 ohms for an hour. At the end of that time the side-mounted heatsinks were hot, at 120.6°F (49.3°C). The top panel was also hot, at 104.1°F (40.1°C).

The Accustic Arts' voltage gain into 8 ohms is specified as 31dB. I measured 30.8dB into 8 ohms with the single-ended input; unusually, the balanced input's gain was 6dB lower. The amplifier preserved absolute polarity (ie, was non-inverting) for both inputs. The balanced input impedance is specified as 40k ohms, which was confirmed by my measurements. However, I measured 28k ohms at low and middle frequencies for the unbalanced input impedance, dropping to 23k ohms at 20kHz. While this is lower than the specified 100k ohms, the difference is inconsequential.

The Mono II's output impedance was very low, at 0.074 ohm at 20Hz and 1kHz, rising very slightly to 0.082 ohm at 20kHz. As a result, the response with our standard simulated loudspeaker varied by less than ±0.1dB (fig.1, gray trace). The audioband response into 8 ohms (blue trace) is flat up to 20kHz, with an ultrasonic rolloff reaching –3dB at 100kHz. The responses of the two samples matched very closely. The Mono II reproduced a 10kHz squarewave with very short risetimes, and no overshoot or ringing (fig.2).


Fig.1 Accustic Arts Mono II, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (blue), 4 ohms (magenta), 2 ohms (red) (0.5dB/vertical div.).


Fig.2 Accustic Arts Mono II, small-signal, 10kHz squarewave into 8 ohms.

Despite JVS finding that S/N 1259 developed a hum, I didn't find any significant differences in the two samples' signal/noise ratios. The wideband, unweighted S/N ratio, ref. 2.83V and measured with the unbalanced input shorted to ground, was a high 79dB, which improved to 86dB when the measurement bandwidth was restricted to the audioband, and to 89.2dB when A-weighted. Spectral analysis of the Mono IIs' noise floors (fig.3) revealed that while 1259 (blue trace) did have higher levels of odd-order harmonics of the 60Hz power-line frequency than 1258, these spuriae were still very low in level and will not be audible. I don't know why, therefore, JVS found that '1259 developed a hum.


Fig.3 Accustic Arts Mono II, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (S/N 1259, blue, S/N 1258, red; linear frequency scale).

Accustic Arts specifies the Mono II's maximum power as 300W into 8 ohms (24.8dBW), 500W into 4 ohms (24.0dBW), and 700W into 2 ohms (22.4dBW). With "clipping" defined as when the THD+noise reaches 1%, I found that the Mono II failed to meet its specified powers. Fig.4 was taken with 1258 and indicates that the Accustic Arts amplifier clipped at 220W into 8 ohms (23.4dBW). I repeated this test with '1259 and got an identical result. Into 4 ohms (fig.5), the Mono II again failed to meet its specified power, delivering 385W (22.8dBW) at 1% THD+N. Into 2 ohms (fig.6), the Mono II clipped at 575W (21.6dBW).


Fig.4 Accustic Arts Mono II, distortion (%) vs 1kHz continuous output power into 8 ohms.


Fig.5 Accustic Arts Mono II, distortion (%) vs 1kHz continuous output power into 4 ohms.


Fig.6 Accustic Arts Mono II, distortion (%) vs 1kHz continuous output power into 2 ohms.

These measurements were taken with a balanced input signal. I repeated them with a single-ended signal, and while the distortion was higher at low powers than it had been with balanced drive, I got the same clipping powers. This is still a high-powered amplifier, however. Figs. 4 and 5 indicate that the distortion with a balanced 1kHz signal is very low at all powers below clipping. However, when I tried to plot how the THD+N percentage varied as I swept the frequency down from 20kHz to 20Hz at a moderate power level, 50W into 8 ohms, the amplifier's protection operated as soon as it was fed the 20kHz tone. I had to reduce the level of the input signal by more than 6dB before I could perform the test without the Mono II turning off its output. The results at this lower level are shown in fig.7. The THD+N in this graph is very low at low frequencies into 8 ohms (blue), 4 ohms (magenta), and 2 ohms (red). However, as the impedance dropped, the distortion started to rise at an increasingly lower frequency, reaching high levels at the top of the audioband. This behavior suggests that the Mono II has a limited open-loop bandwidth, with less gain margin available for corrective negative feedback to be applied as the frequency rises (footnote 1).


Fig.7 Accustic Arts Mono II, THD+N (%) vs frequency at 8.975V into: 8 ohms (blue), 4 ohms (magenta), and 2 ohms (red).

The waveform of the residual distortion and noise with a 1kHz signal (fig.8) suggests that the subjectively benign second harmonic is dominant, which was confirmed by spectral analysis (fig.9). The second harmonic is still very low in level, however, at –110dB (0.0003%) and rose by just 6dB when I reduced the load impedance to 4 ohms. When I tried to examine the Accustic Arts' intermodulation distortion with an equal mix of 19kHz and 20kHz tones, I again found that the amplifier's protection circuitry limited the power at which I could perform the test. At the highest power I could test the Mono II with this signal, equivalent to 40W peak into 4 ohms, the second-order difference product lay at a very low –90dB (0.003%) and higher-order intermodulation products were even lower in level (fig.10).


Fig.8 Accustic Arts Mono II, 1kHz waveform at 20W into 8 ohms, 0.0053% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).


Fig.9 Accustic Arts Mono II, spectrum of 50Hz sinewave, DC–1kHz, at 50W into 8 ohms (linear frequency scale).


Fig.10 Accustic Arts Mono II, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 40W peak into 4 ohms (linear frequency scale).

I was puzzled by the Accustic Arts Mono II's measured performance. Some aspects are excellent—the low levels of noise; the very low levels of distortion at low and middle frequencies with balanced drive; the dominance of the second harmonic in the distortion signature; the reproduction of squarewaves; and the generally excellent matching between the two samples. But the Mono II's inability to meet its specified powers and the dramatic rise in distortion at high frequencies into low impedances are disappointing.

The Accustic Arts Mono II is capable of much higher powers at low and middle frequencies than it is at high frequencies, especially into low impedances. This may not be an issue with classical orchestral music, chamber music, acoustic jazz, or bluegrass, but it could lead to problems with highly compressed rock that has high levels of energy in the top octaves. I was also surprised by the apparently limited open-loop bandwidth (footnote 1), which is uncommon in modern solid-state amplifiers.—John Atkinson

Footnote 1: See fig.3 here.
Accustic Arts Audio GmbH
US representative: Finest Fidelity
3 Sagebrook Drive
Bluffton, SC 29910
(386) 341-9103

JRT's picture

I would like to see Stereophile subjectively and objectively review and test a ~$3k Rotel RB-1590 stereo amplifier, claimed to be two separate monoblocks in one enclosure. Rotel rates it at 350W/channel into 8_Ohms, 4_Ohm min load, 120_dB s/n A-weighted (presumably referenced to full power), 0.03%_THD over the range of 20_Hz to 20_kHz, frequency response 10_Hz to 100_kHz ±0.5dB.

It would have been interesting to know how that $3k Rotel subjectively and objectively compares to this $25k/pair Accustic Arts Audio Mono II monoblock power amplifier, after being rebuilt with proper power supply transformer (these big mistakes are not confidence building). More than 8x the price should buy some nontrivial improvement, else the opportunity costs need further consideration.

Bogolu Haranath's picture

For $3,000, we can also get PS Audio Stellar mono-blocks (Stereophile Class-A) 350 WPC 8 Ohms and 700 WPC 4 Ohms :-) ...........

Ortofan's picture

... following are a couple of other subjective and objective reviews to tide you over:

JRT's picture

I had not seen those.

Ortofan's picture

... in the Mono II's price range, you know what to do next."

Permit me to finish that thought.
Buy a pair McIntosh MC1.25KW or a pair of Pass Labs X600.8 amps.
Or, if you can live with a two-channel amp, then a Pass Labs X350.8 or a D'Agostino Classic 2 will save you about $10K.
Better yet, save another $10K with a Parasound JC5.

Bogolu Haranath's picture

You could also consider the new Krell Duo 300-XD stereo-amp ($10,500), 300 WPC into 8 Ohms :-) ........

Ortofan's picture

... four fans. No thank you.

Bogolu Haranath's picture

For most of the listening levels those fans probably won't turn on ........ Even if they did, the noise levels would be low enough, not to interfere with the listening to music ......... See Stereophile review of albeit more powerful older model Krell Solo 575 mono-blocks .......... MF did not mention any thing about fan noise interfering with his listening :-) ..........

Bogolu Haranath's picture

Or, you could also consider McIntosh MC462 stereo-amp ($9,000, Stereophile Class-A), 450 WPC 2, 4 or 8 Ohms :-) ..........

Ortofan's picture

... high enough into lower impedances. Good try, though.

Bogolu Haranath's picture

450 WPC 2, 4 or 8 Ohms ....... For most of the music, for most listeners, for most of the listening levels, with sensitive speakers, we probably don't need more than 450 WPC :-) .........

Bogolu Haranath's picture

McIntosh MC462 can deliver more power than specified 450 WPC ........ See Stereophile measurements :-) ..........

Bogolu Haranath's picture

Or, you could also consider Bel Canto REF600M mono-blocks ($4,990, Stereophile Class-A), 300 WPC into 8 Ohms :-) ..........

Ortofan's picture

... emits ultrasonic switching noise. Strike three.

Bogolu Haranath's picture

No human being can hear that high ultrasonic switching noise ......... No tweeter can reproduce that high ultrasonic switching noise ........ No music extends into that high ultrasonic switching noise ........ See Stereophile measurements of Bel Canto REF600M :-) .........

Ortofan's picture

... what we can or cannot hear. No audio amp should be radiating RFI.

Bogolu Haranath's picture

Ok ..... Don't use your smartphone or AM/FM radio or turn on your TV, while listening to music :-) .......

Bogolu Haranath's picture

If you insist on high power, in addition to the amplifiers you mentioned above, we can also consider the Bryston 28-B/3 ......... 1000 Watts into 8 Ohms and 1800 Watts into 4 Ohms ....... $24,000/pair ....... See Stereophile review of older model Bryston 28-B :-) ..........

Ortofan's picture

... suggestion. Even the 7B³ would be sufficient to far exceed the power ratings of the Accustic Arts amp, plus the Bryston 20-year warranty is not to be sneezed at, unless you're afflicted with upgrade-itis.

Bogolu Haranath's picture

Thank you ....... I could finally appease you :-) .......

Bogolu Haranath's picture

I think that 20 year warranty is also transferable :-) ........

georgehifi's picture

"Manufacturer's Comment
Editor: The lack of performance of the Accustic Arts Mono II power amplifiers was caused by the fact that we unfortunately installed wrong transformers."

When sending a product sample to be reviewed and thoroughly bench tested to Stereophile, one would think that a company would give it's very best sample, (even "cough" better spec'd) than a production one, to be showcased in a magazine that will make or break that product.

this sort of answer happens way to much for my liking
Cheers "cynical" George

georgehifi's picture

"Bel Canto REF600M .........
Submitted by Bogolu Haranath on October 4, 2019 - 2:26am
No human being can hear that high ultrasonic switching noise ......... No tweeter can reproduce that high ultrasonic switching noise ........"

You are correct, but you can hear the effects of the Class-D's output filter that has to get rid of that switching noise, with phase shifts that are large (up to 70 degrees!!!) that reach down to 1khz. That's why many complain about the upper mids and highs with class-D.

(copy and paste this link to your browser)

Cheers George

Bogolu Haranath's picture

There are/were several Class-D amps and integrated amps included in the Stereophile recommended component lists ....... Many of of them earned Class-A distinction in sound quality and in measurements ....... Bel Canto models are one among them ....... Why is that? ........ Heaven forbid, you are not questioning the hearing ability of the Stereophile reviewers, are you? :-) .......

georgehifi's picture

Reviewers have subjective outlooks, each to his own, but surely no one would turn a blind eye to the phase graph linked in my last post above?

Cheers George

Bogolu Haranath's picture

That one is above my pay-grade ........ May be JA1 with his technical expertise could shed some light on that particular topic :-) ..........