Classé Delta Mono monoblock power amplifier Measurements

Sidebar 4: Measurements

I tested the Classé Delta Mono with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). I preconditioned the amplifier by operating it at one-third the specified power into 8 ohms for an hour. At the end of that time, the chassis was cool, at 76.9°F (24.9°C), though the grille over the rear-mounted fan vent was warmer, at 105.0°F (40.6°C).

The Classé's voltage gain into 8 ohms measured 28.7dB from both the balanced and unbalanced inputs and the amplifier preserved absolute polarity (ie, was noninverting) with both input signals. At 86k ohms at 20Hz and 1kHz, the balanced input impedance was both usefully high and slightly higher than the specified value of 82k ohms. Though the impedance dropped to 70k ohms at 20kHz, this is still high. The unbalanced input impedance was 79k ohms at low and middle frequencies, 35k ohms at the top of the audioband.

The Classé's output impedance was a low 0.16 ohm at 20Hz and 1kHz, increasing slightly to 0.1875 ohm at 20kHz. (Though these values are higher than specified, they include the series impedance of 6' of spaced-pair loudspeaker cable.) The modulation of the amplifier's frequency response, due to the Ohm's law interaction between this source impedance and the impedance of our standard simulated loudspeaker, will be negligible, at ±0.15dB (fig.1, gray trace). The response into an 8 ohm resistive load (fig.1, blue trace) was flat to 20kHz and down by 1.5dB at 200kHz, which correlates with the Delta Mono's superb reproduction of a 10kHz squarewave (fig.2). Commendably, there was no overshoot or ringing with the squarewave response. Into 2 ohms (fig.1, red trace), the response was –0.7dB at 20kHz and –3dB at 100kHz.


Fig.1 Classé Delta Mono, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (blue), 4 ohms (magenta), 2 ohms (gray) (0.25dB/vertical div.).


Fig.2 Classé Delta Mono, small-signal 10kHz squarewave into 8 ohms.

Measured with the unbalanced inputs shorted to ground, the amplifier's unweighted, wideband signal/noise ratio was an excellent 80.2dB ref. 1W into 8 ohms, this ratio improving to 95dB when the measurement was A-weighted and to 119.8dB when referenced to the amplifier's specified maximum power. Low-level spuriae at the 60Hz power-supply frequency and its harmonics were present in the Classé's noise floor (fig.3), though at –110dB or below (ref. 2.83V), these are negligible.


Fig.3 Classé Delta Mono, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (linear frequency scale).

The Delta Mono is specified as being able to deliver 300W into 8 ohms and 600W into 4 ohms (both equivalent to 24.8dBW), and 1000W into 2 ohms (24dBW) with the line voltage held constant, all at 0.1% distortion. Using our definition of clipping, which is when the output's percentage of THD+noise reaches 1%, the Classé exceeded its specified powers into 8 ohms, clipping with a 1kHz signal at 380W into 8 ohms (25.8dBW, fig.4) and at 610W into 4 ohms (24.8dBW, fig.5). The Delta Mono clipped at 950W into 2 ohms (23.75dBW, fig.6); however, it is relevant to note that I don't hold the AC wall voltage constant when I test an amplifier's clipping power. With the Delta Mono idling, the wall voltage measured 121.3V. With the amplifier clipping into 2 ohms, the supply voltage had dropped to 117.5V, which is why I found a slight shortfall in maximum power into this load.


Fig.4 Classé Delta Mono, distortion (%) vs 1kHz continuous output power into 8 ohms.


Fig.5 Classé Delta Mono, distortion (%) vs 1kHz continuous output power into 4 ohms.


Fig.6 Classé Delta Mono, distortion (%) vs 1kHz continuous output power into 2 ohms.

I measured how the THD+N percentage changed with frequency at 20V output, which is equivalent to 50W into 8 ohms, 100W into 4 ohms, and 200W into 2 ohms—high enough to shift the Delta Mono's output stage from class-A into class-AB. The THD+N was very low in the midrange into 8 ohms (fig.7, blue trace) but rose into lower impedances (magenta and red traces). However, as Sergiu Ignat had said when I interviewed him, the usual increase in THD+N in the top audio octave is absent, confirming that the amplifier's open-loop bandwidth is commendably wide.


Fig.7 Classé Delta Mono, THD+N (%) vs frequency at 20V into: 8 ohms (blue), 4 ohms (magenta), 2 ohms (red).

The Delta Mono's distortion was predominantly the subjectively benign third harmonic (fig.8). This lay at –94dB (0.002%) at 50Hz into 8 ohms (fig.9), and while it rose to –76dB (0.015%) at the same voltage into 4 ohms (fig.10), the higher harmonics are all extremely low in level. When the amplifier drove an equal mix of 19 and 20kHz tones at 50W into 8 ohms (fig.11), the second-order difference product at 1kHz is missing in action and higher-order intermodulation products lay at an extremely low –99dB (0.001%).


Fig.8 Classé Delta Mono, 1kHz waveform at 50W into 8 ohms, 0.0013% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).


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


Fig.10 Classé Delta Mono, spectrum of 50Hz sinewave, DC–1kHz, at 100W into 4 ohms (linear frequency scale).


Fig.11 Classé Delta Mono, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 50W peak into 8 ohms (linear frequency scale).

The Classé Delta Mono performed extremely well on the test bench, a testament to its audio engineering.—John Atkinson

Classé, a division of Sound United, LLC
380, rue McArthur Saint-Laurent
Québec H4T 1X8

Bogolu Haranath's picture

Are you ready for a brand new beat? ...... Summer's here and the time is right ...... For dancing in the 'seat' (street) :-) ......

supamark's picture

"Fortunately, Sound United, whose brands include Denon, Marantz, Polk, Definitive Technology, and Boston Acoustics, then acquired Classé."

Sound United has shuttered Boston Acoustics, the brand effectively no longer exists. Per Wiki, "Currently they maintain the brand name, but no engineering is budgeted and there is no future production planned for Boston Acoustics." Y'all should probably stop mentioning Boston Acoustics when talking about Sound United, except to note that they liquidate some of the brands they aquire. Maybe they'll sell the brand name to someone who wants to make (sealed box) speakers... but doubtful.

Ortofan's picture

... active cooling solution, then try the Rotel Michi S5 or M8.

John Atkinson's picture
Ortofan wrote:
If want a power amp with an active cooling solution, then try the Rotel Michi S5 or M8.

Stereophile will be reviewing the Rotel Michi S5 in a fall issue.

John Atkinson
Technical Editor, Stereophile

Bogolu Haranath's picture

It would be interesting to see MF review the Rotel Michi M8 (1080W/8 Ohms and 1800W/4 Ohms) with his Wilson Alexx speakers :-) ........

rom661's picture

Right. I'm auditioning the Classe stereo amp right now. You're right of course. The fact that the Rotel and the Classe have cooling fans makes them equivalent. Also, please note, a Hyundai and a Ferrari both run on petrol so, of course, they are the same. It's nice that you apparently like Rotel. I sold them for years. Your comment is nonsensical.

Ortofan's picture

... chi-chi Mundorf capacitors.

With Rotel, you have to settle for British patented slit-foil, high-efficiency, low-ESR bulk storage capacitors.

Mikk's picture

I always enjoy gear reviews, but can't help feeling that using a few other speakers with this amp would provide greater insight into how it performs, overall.
Given the amps price, and the apparent engineering that's gone into it, i imagine Classe would have hoped it got a more thorough workout too?
Sorry to nitpick an otherwise detailed review!
(Edit- apologies if recent events within Stereophile made further tests not do-able).

John Atkinson's picture
Mikk wrote:
I always enjoy gear reviews, but can't help feeling that using a few other speakers with this amp would provide greater insight into how it performs, overall.

That's a valid point, but it wasn't possible during the time I had the Classe amplifiers in my system.

John Atkinson
Technical Editor, Stereophile

Bogolu Haranath's picture

MF or JA2 could do a follow-up review of the Classe amps with the full-range speakers they have :-) ......

georgehifi's picture

Classic con yet again, manufacturers understating the 8ohm wattage by 27% to make the 4ohm look as though it doubles. and the 2ohm becomes even more of a joke. When will this industry con stop?
Would have had a chance if top and bottom were both N channel fets.

Cheers George

Jim Austin's picture

I would call this "conservatively rated." You call it a con?

There is a common misunderstanding that doubling into half the impedance is a crucial metric. It only matters if you'll be operating the amp at or near its maximum power. What matters is having enough power to drive the load. For power-hungry 4 ohm speakers, it's much better to have a 380W amplifier that outputs 610W into 4 ohms than to have a 200W amplifier that doubles its output into 4 ohms and so provides 400W. For a low-sensitivity 8 ohm speaker, 380 ohms is better than 300.

If I were a manufacturer, and I knew about this widespread misconception--that doubling is important regardless of the output power--then I'd be tempted to under-specify the 8 ohm impedance, too. In any case, underspecifying the power is not a con.

Jim Austin, Editor

Ortofan's picture

... power measurements, is there any plan to include in the reviews some form of transient peak power (aka dynamic headroom) testing, perhaps using a 200 ms burst signal, as was discussed in the following article?

The article also suggests that equipment would be measured prior to sending it out for listening tests, in part to cull under-performing (let alone defective) samples. Was this procedure ever implemented?

michelesurdi's picture

could you tell us exactly how to go about level matching?

Bogolu Haranath's picture

They place both components side by side and place a 'spirit level' on top of them and then level match ..... Just kidding :-) .......

John Atkinson's picture
michelesurdi wrote:
could you tell us exactly how to go about level matching?

I use the 1kHz, -20dBFS warble tone on Stereophile's Editor's Choice CD and equalize the RMS output voltages at the loudspeaker terminals to within 0.1dB.

John Atkinson
Technical Editor, Stereophile

michelesurdi's picture

hooboy,what a scream

georgehifi's picture

"I would call this "conservatively rated." You call it a con?"

Ok Jim, then why wasn't the 4ohm figure conservatively rated as well by the manufacturer?? It's almost spot on, out by just 2% on what JA measured?? Not 27%!!!! like their 8ohm figure

A blind man can see what the manufacturers are doing, and it just keeps getting done in their specs, "it's a con", and it's about time you guys called them out on it being reviewers, at least JA puts up the measurements, but his silence is a little muted about it.

You guys used to get yourselves and us sooo excited with those big Krell like amps, because when you tested them they did almost double down to 1ohm!!! let alone 2ohms, with words like you can weld off these speaker outputs!!!! And now with speakers like the Alexia ect ect, this spec is even more important.

I'm seeing even more exaggerated examples of "this con" with Class-D, when/if the 2ohm wattage specs are mentioned

Cheers George

Jim Austin's picture

It is possible that some companies understate their 8 ohm power output in order to make it appear that the output doubles into half the load. But if they do so, it is a marketing decision aimed at people who do not understand what matters and what doesn't. As I wrote above, what matters (among much else) is not doubling but the maximum power an amplifier can deliver to a particular load. If a company understates its power output into 8 ohms in order to appear to double into 4 ohms, then, to people who understand, they are making the product look worse, not better. If it is an attempt at a con, then it is a failed attempt. To those with a correct understanding, they would be making themselves look worse, not better.

Jim Austin, Editor

Bogolu Haranath's picture

Some companies like PS Audio specify their power amp output as minimum power outputs ....... For example, minimum power output into 8 Ohms .... Minimum power output into 4 Ohms etc . .....

Hi-Fi News measures dynamic power outputs in addition to continuous power outputs for the amplifiers ...... In many cases, lot of amplifiers put out more power during dynamic output than during continuous output :-) ......

georgehifi's picture

Whatever you say to protect them Jim, doesn't get around the fact that their own specified 8ohm wattage is grossly underrated and in the same breath their own specified 4ohm wattage is correct. And to me it's done just to make it "look like" it can double down.
And the only way it's exposed is in JA's bench tests which 90% of your readers have no idea what their looking at, they just like to read the reviews.


Cheers George

Jim Austin's picture

I'm protecting logic and good sense.

Is power in an amplifier bad or good? Because if power is good, then perpetrating a "con" by "grossly understating" it makes no sense. I'm not expecting you to change your mind, but I do want to make sure that other people reading these comments are not misled.

Jim Austin, Editor

Bogolu Haranath's picture

George is making much ado about nothing :-) ..........

Lars Bo's picture

It seems to me, that EPDRs of 1-2 ohms are not at all uncommon for speakers typically matched with big, drive-it-all power amps (or vice versa).

Bogolu Haranath's picture

Speakers with EPDRs of 1-2 Ohms sound better with amps like the Classe Delta monos :-) .......

georgehifi's picture

"grossly understating" it makes no sense.

Yes understate by 27%!!! on the 8ohm figure. Then by only 2% on the 4ohm figure. (You never see them or others overstate the 8ohm as then will make the 4ohm will look real sad indeed)

I consider that a gross understatement, for a means to baffle/confuse/con/deceive call it what you wish.
It's not right that so many manufactures are doing this now, this is not an isolated indecent, it's only serves one purpose.
I've been calling on this now for a couple of years, it has crept into the manufacturers specs and it's not right.

Cheers George

RoryB's picture

Whether chicken or egg, it is a marketing decision. That's why they are called "nominal" specifications. You're only making yourself look silly.

You got 27% more than you paid for? This is an OUTRAGE! (27% more is right around 2 dB, based on 20 log 1.27. Not enough extra to smoke a tweeter.)

Edit: So my math was wrong. Because power comes from the square of voltage, the actual formula is 20 log 1.127, which comes out to 1.03 dB. Cha-ching. Here, let me turn that down 1 dB for you.

a.wayne's picture

Great measurements there Classe , fantastic except for the 2 ohm current limiting , these measurements puts this Classe into a similiar league to the big Boulder ...


Lee S.'s picture

JA, A few years ago, you reviewed the Classe CA-M600 mono amps and gave them high marks. You said they were the best sounding amplifiers you had reviewed at the time. Your review immensely influenced my decision to purchase a pair. Best sounding amps I have ever owned. I know it’s been several years, but could you comment on the sound attributes between the CA-M600 and the new Delta monos to the best of your recollection. Thanks, Lee

georgehifi's picture

Once again, and your silly if you can't see it.

"Yes understate by 27%!!! on the 8ohm figure. Then by only 2% on the 4ohm figure to make it look better."

Cheers George

Mike-48's picture

JA, I auditioned a previous Classe amp (CA-2300), and while I thought the sound was wonderful, I could occasionally hear the fan running during quiet passages. Are the fans in these monoblocks quieter?