ELAC Alchemy Series DPA-2 power amplifier Measurements

Sidebar 3: Measurements

Although the ELAC Alchemy DPA-2 is a stereo amplifier, Jason Victor Serinus auditioned it as a pair of monoblocks. (In mono mode, the signal is fed to either input and the output taken from the left channel's red binding post and the right channel's black binding post.) I therefore performed a complete set of measurements with a DPA-2 in mono mode then repeated some of the tests in stereo mode. As the DPA-2 has an output stage operating in class-D, I inserted an Audio Precision auxiliary AUX-0025 passive low-pass filter between the test load and my Audio Precision SYS2722 system. This filter eliminates RF noise that could drive the SYS2722's input circuitry into slew-rate limiting. I used it for all the tests other than frequency response.

Though the Alchemy DPA-2 has both balanced and single-ended inputs, I performed most of the measurements using the balanced inputs, which the manual says are preferred. With the amplifier in stereo mode, I measured a low voltage gain of 18.7dB into 8 ohms with both types of inputs and the gain set to Normal. High gain mode increased the gain by a useful 5.9dB. In mono mode, the DPA-2's gain into 8 ohms was 24.6dB and 30.4dB on the Normal and High gain settings. The amplifier preserved absolute polarity (ie, was noninverting) with both inputs in both stereo and mono modes. The XLR jack is wired with pin 2 hot. The input impedance is specified as 12k ohms single-ended and 19k ohms, balanced. I measured 11.9k ohms at 20Hz and 1kHz for the single-ended inputs, dropping inconsequentially to 10.7k ohms at 20kHz. The balanced input impedance was 18k ohms from 20Hz to 20kHz, ie, 9k ohms per phase.

The output impedance in stereo mode is specified as 0.03 ohms at 1kHz. I measured a very low 0.078 ohms at 20Hz and 1kHz rising to 0.095 ohms at 20kHz. (These figures include the series impedance of a 6', spaced-pair speaker cable.) The output impedance was a little higher in mono mode, as there are now two output stages in series. I measured 0.105 ohms at 20Hz and 1kHz, and 0.14 ohms at 20kHz. Even so, the modulation of the ELAC's frequency response driving our standard simulated loudspeaker was minuscule, at ±0.05dB (fig.1, gray trace). The small-signal bandwidth was restricted by the low-pass filter between the amplifier's class-D stage and its output terminals. Into 8 ohms (fig.1, blue trace), the ultrasonic rolloff reached –3dB at 50kHz, a little more into lower impedances. The rolloff lengthened the risetimes of a 10kHz squarewave (fig.2). There is a critically damped overshoot on the tops and bottoms of the waveform, but there is no ringing.

521Elacfig01

Fig.1 ELAC Alchemy DPA-2, mono mode, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (blue), 4 ohms (magenta), 2 ohms (red) (1dB/vertical div.).

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Fig.2 ELAC Alchemy DPA-2, mono mode, small-signal, 10kHz squarewave into 8 ohms.

Without the auxiliary low-pass filter, 191mV of ultrasonic noise was present at the DPA-2's output terminals. With the AP filter and the DPA-2 set to stereo mode and normal gain, the unweighted, wideband signal/noise ratio, taken with the single-ended inputs shorted to ground, was an excellent 82dB ref. 1W into 8 ohms. It improved to 95.4dB when I restricted the measurement to the audioband and to 98.1dB with an A-weighting filter in circuit. In high-gain stereo mode, these ratios were all around 3dB lower; they were down another 3dB or so in mono mode. Nevertheless, this is a quiet amplifier. Spectral analysis of the low-frequency noise floor in mono mode (fig.3) revealed some AC-supply–related spuriae at 60Hz and its odd-order harmonics, though these all lie below –100dB ref. 1W into 8 ohms. The red trace in fig.3 is the spectrum with the amplifier set to normal gain. High gain (blue trace) increased the random noise components by around 3dB.

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Fig.3 ELAC Alchemy DPA-2, mono mode, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms with normal gain (red) and +6dB gain (blue) (linear frequency scale).

The Alchemy DPA-2's maximum power is specified at 325Wpc into 4 ohms in stereo mode (22.1dBW ref. 1W into 8 ohms). With clipping defined as when the THD+noise in the output reaches 1%, the ELAC's clipping power in this mode with one channel driven was 339W into 4 ohms (22.3dBW, fig.4). In stereo mode, the amplifier clipped at 202W into 8 ohms (23.05dBW, not shown). As expected, considerably more power was available in mono mode. At 1% THD+N, the DPA-1 delivered 590W into 8 ohms (27.7dBW, fig.5) and 550W into 4 ohms (24.4dBW, fig.6). Though the 8 ohm power is a little lower—0.3dB—than the specified power of 625W (28.0dBW), the AC power line had dropped to 118.5V with the amplifier clipping into 8 ohms.

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Fig.4 ELAC Alchemy DPA-2, stereo mode, distortion (%) vs 1kHz continuous output power into 4 ohms.

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Fig.5 ELAC Alchemy DPA-2, mono mode, distortion (%) vs 1kHz continuous output power into 8 ohms.

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Fig.6 ELAC Alchemy DPA-2, mono mode, distortion (%) vs 1kHz continuous output power into 4 ohms.

Figs.4–6 show that the Alchemy DPA-2 had very low distortion at powers below a few tens of watts. I examined how the ELAC's THD+N varied with frequency at 20V (equivalent to 50W into 8 ohms, 100W into 4 ohms, and 200W into 2 ohms).3 The distortion into 8 ohms was close to 0.01% (fig.7, blue trace), though it rose at high frequencies and into 4 ohms (magenta trace) and 2 ohms (red trace).

521Elacfig07

Fig.7 ELAC Alchemy DPA-2, mono mode, THD+N (%) vs frequency at 20V into: 8 ohms (blue), 4 ohms (magenta), 2 ohms (red).

The ELAC amplifier's distortion signature in mono mode into 8 ohms was primarily third harmonic in nature (fig.8). Though some second, fifth, and seventh harmonics are present (fig.9), these are all at or below –90dB (0.003%). At the same output voltage into 4 ohms, which is equivalent to 200W, the second and higher odd-order harmonics rose in level (fig.10), though the third harmonic dropped a little. With an equal mix of 19 and 20kHz tones and the signal peaking at 200W into 4 ohms (fig.11), the decrease in the circuit's linearity at very high frequencies resulted in some higher-order products, the highest in level of which, at 21kHz and 18kHz, each lay at –70dB (0.03%). The difference product at 1kHz, even at this very high power, lay at –96dB (0.0015%).

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Fig.8 ELAC Alchemy DPA-2, mono mode, 1kHz waveform at 80W into 8 ohms, 0.0187% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

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Fig.9 ELAC Alchemy DPA-2, mono mode, spectrum of 50Hz sinewave, DC–1kHz, at 100W into 8 ohms (linear frequency scale).

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Fig.10 ELAC Alchemy DPA-2, mono mode, spectrum of 50Hz sinewave, DC–1kHz, at 200W into 4 ohms (linear frequency scale).

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Fig.11 ELAC Alchemy DPA-2, mono mode, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 200W peak into 4 ohms (linear frequency scale).

The ELAC Alchemy DPA-2 amplifier is a powerhouse, especially in mono mode.—John Atkinson


Footnote 1: There was one oddity I noted when I measured the DPA-2. I have a portable FM radio in the test lab that I keep tuned to WNYC, New York City's NPR station. When I powered up the DPA-2 and fed it an audio signal, I could hear a tone at the frequency of the test signal, overlaid with fizzy noise, reproduced by the radio's speaker. This was very noticeable when I swept the test signal down from 20kHz to 20Hz to generate the traces in fig.8. I have only encountered this RF interference with one other modern class-D amplifier, the Spec RPA-W7EX that Ken Micallef reviewed in March 2016.
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COMMENTS
Glotz's picture

That relationship (in time) seems to be the reason I keep away from Class D amplification.

I know it can be corrected with additional product investment, but isn't that the real question? Is this a product of all Class D or is this a design/cost issue?

I am in the market for a stereo amp or monoblocks in this price range, and yes, I obviously need to hear these first.

Kudos to Peter here... to impress Jason takes a lot!

What about the Odyssey Stratos Mono Amps for a future review? They seem to still be going strong as well...

cgh's picture

Compared to old fashioned linear amps it's probably impossible to talk about class D without knowing what's inside the box. Slew rate limiting, negative feedback, filtering, signal/power bandwidth, ..., you-name-it and that leading edge has slope on the output. I am biased. All my time with playing and building guitars and violins we focus on that onset transient and the subsequent coloration in the decay. Don't mess with that onset! Flamenco is percussive and classical is not.

tnargs's picture

It’s hard to imagine why you wouldn’t use Class A for the input stage.

Maybe you just mis-reported it.

John Atkinson's picture
tnargs wrote:
It’s hard to imagine why you wouldn’t use Class A for the input stage. Maybe you just mis-reported it.

Yes, it was a typo, now corrected.

John Atkinson
Technical Editor, Stereophile

tonykaz's picture

Hello Jason,

I thought that I'd let you know that China is doing a Big Bicycle Show, just now.

I've owned Bicycle Manufacturing so I have some involvement in Bicycle Accessories.

As far as I can tell, the Shanghai attendance is low.

I'm not seeing much in the way of USA Shows waking up from their Rip Van Winkle naps, same for Europe.

The World still needs about 6 Billion more Vaccinations before any 'Herd-immunity' comfort levels make Big Public Gatherings seem safe.

I had to delay a trip to Port Townsend for the Tally Ho project because of the darn plague. ( I have a Covid travel passport )

The entire World is probably jealous of your lovely Natural Setting, ( I'm hearing that ) parts of NY City are a Hell-hole just now, and that NY'rs are just starting to escape to restaurants for a rare outing.

This Class D probably isn't for the upper Caste folks and may never be but it's a useful solution for folks reaching up with lesser buying abilities, it probably deserves to belong here with us.

Anyway, it's nice reading you again.

Wishing you well,

Tony in Venice Florida

a.wayne's picture

I have always found class D to have timbre issues when ran below 8ohm , measurements here makes it pretty obvious this one is a solid 8 ohm load guy , with plenty power for the Clipsters....!

Regards

PS: Careful John ,brain damage from NPR signal is irreversible ...! :)

Dpl53's picture

I really enjoyed this review, the fact that you were able to favorably compare this amp to your reference amp at a much higher cost is impressive. I would also like to note that I checked out Brian Bromberg after reading this review and really like his music. Always nice to discover a new jazz artist!!

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