Opera Callas loudspeaker Measurements

Sidebar 2: Measurements

John Marks was impressed by the Callas stand-mounted loudspeaker from Italian manufacturer Opera Loudspeakers. The Callas is unusual in that it has tweeters mounted above and below its single SEAS magnesium-cone woofer. The conventional wisdom is that the higher the frequency, the closer together you need to position drive-units covering the same passband if you want to avoid unwanted off-axis lobing. Not only does the Callas break that rule, it also has three rear-firing tweeters mounted vertically in-line on its rear panel, below a pair of reflex ports.

Nevertheless, JM was impressed by what he heard. Driving the Callases with Unison Research's S6 amplifier, he wrote that he "was pleasantly surprised by both the dynamic capability and the bass extension of the Callas-S6 combo. Unlike with many two-way speakers, I never got the sense during most normal listening (as distinct from playing very loud to impress myself or friends) that there was 'almost' enough bass—there really was enough bass. . . . There was never a sense of too much treble unless the recording itself was too hot; the treble and midrange were very well integrated."

Intrigued, I asked JM to ship the speakers to me when he was done with them, so that I could run them through my test protocol. I used DRA Labs' MLSSA system (www.mlssa.com) and a calibrated DPA 4006 microphone to measure the Opera Callas's frequency response in the farfield, and an Earthworks QTC-40 for the nearfield responses. The Callas is not very sensitive, its estimated voltage sensitivity a low 84.3dB(B)/2.83V/m. However, it is a relatively easy load for the partnering amplifier to drive, its impedance magnitude (fig.1, solid trace) dropping below 6 ohms only in the low midrange and high treble, and reaching a minimum value of 3.7 ohms at 165Hz.

The traces in fig.1 are free from the small wrinkles that would imply the existence of cabinet resonances. However, when I investigated the vibrational behavior of the enclosure, I found a fairly strong resonance at 586Hz on the sidewalls (fig.2). This is probably high enough in frequency to have no negative effect on sound quality. There was also a lower-level mode at 350Hz on the top of the cabinet, but again, this is unlikely to affect the speaker's sound, given the small radiating area affected by the resonance.


Fig.1 Opera Callas, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).


Fig.2 Opera Callas, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of side panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The saddle centered on 42Hz in the impedance-magnitude trace in fig.1 suggests that this is the tuning frequency of the two small ports on the cabinet's rear panel. Measured in the nearfield (fig.3, blue trace), the woofer output's minimum-motion notch (ie, where the back pressure from the port resonance holds the cone stationary) is indeed at 42Hz. The ports' output (red trace) peaks in textbook fashion between 30 and 65Hz, but there is also a vicious-looking peak visible at 700Hz. I could hear this peak as a slight whistle imposed on the sound of pink noise when I stood behind the speaker, but it's fair to note that JM didn't remark on any coloration in the upper midrange that could have resulted from this port behavior.


Fig.3 Opera Callas, anechoic response on woofer axis at 50", averaged across 30° horizontal window and corrected for microphone response, with nearfield responses of woofer (blue) and ports (red) and their complex sum, respectively plotted below 310Hz, 1kHz, 310Hz.

Higher in frequency in fig.3, the Callas's farfield response is impressively flat from the midrange through the mid-treble, though with a vestigial peak at the approximate frequency of the ports' midrange resonance. Though JM didn't feel that the speaker's treble sounded exaggerated, there is actually a significant excess of energy between 8 and 24kHz. To some extent, this will be compensated for by the speaker's increasing directivity in the horizontal plane in this region (fig.4). However, the room's reverberant field will be boosted at high frequencies by the fact that the farfield response at the speaker's rear (fig.5) is also boosted in the top octaves, thanks to the three rear-firing tweeters. An upper-midrange peak can be seen in this graph, though peculiarly, it is a little lower in frequency than the peak in the ports' nearfield output.


Fig.4 Opera Callas, lateral response family at 50", normalized to response on woofer axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.


Fig.5 Opera Callas, anechoic response from behind at 50".

In the vertical plane (fig.6), there is the inevitable pattern of off-axis peaks and dips that result from the spaced tweeters. This speaker definitely needs to be listened to on the woofer axis to get the most even balance treble balance, though moving slightly above or below that axis will pull down the top octave a little.


Fig.6 Opera Callas, vertical response family at 50", normalized to response on woofer axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.

The Opera's step response on the woofer axis (fig.7) indicates that all three drive-units on the front baffle are connected with positive acoustic polarity, and that the outputs of the tweeters lead that of the woofer by a small amount. Other than a very small amount of delayed energy at 3.2kHz, probably due to a residual mode in the woofer's cone, the Callas's cumulative spectral-decay plot is superbly clean (fig.8).


Fig.7 Opera Callas, step response on woofer axis at 50" (5ms time window, 30kHz bandwidth).


Fig.8 Opera Callas, cumulative spectral-decay plot on woofer axis at 50" (0.15ms risetime).

Again from JM's review: "To sum up the Opera Callas: luscious midrange, sweet treble, large soundstage, surprising bass, eminently listenable; Class B (Restricted Extreme Low Frequencies)." Other than that resonant mode in the ports' output and the excess of high-treble energy, I would say that its measured performance confirms JM's high opinion of the Opera Callas's sound quality.—John Atkinson

Opera Loudspeakers
US distributor: Colleen Cardas Imports
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harishcs's picture

   The review doesn't mention any significant shortcomings, other than than a "soft-focus" and "Technicolor" tonal quality.  So why not Class A ?  In particular, what are the  reasons for giving  a "Class A" badge to the KEF LS50  and not this one ?  I mention the LS50 only because it is significantly cheaper.     

Long-time listener's picture

In case you haven't noticed, Stereophile isn't in the habit of allowing you to make a truly informed choice about equipment by giving you the full story, if it includes the shortcomings of any product. When I bought the Stereophile Class-B rated Dynaudio Excite X12, I expected great things, since there was absolutely nothing but superlatives in their review. After I bought it, Iistened to a capella music by Orlando de Lassus that was beautifully recorded, and was surprised to find how rough and unpleasant the male voices sounded (compared to my previous Class C speakers). Likewise, despite their claim that it can handle orchestral works,  when I listened to my Phillip Glass Symphony No. 2 (on Nonesuch, with Dennis Russell Davis conducting; beautiful recording), half of the bass line went missing as it dipped below the frequencies the Dynaudio could handle. And so on. It's a nicely balanced little speaker, but far less perfect than their review will lead you to believe. The Monitor Audio Bronze BX2, at about 1/3 the price, seems to do about as well in most respects. It's more detailed and has better bass extension.

Similarly, if you're lucky enough to buy a T+A Power Plant integrated amplifier, you'll find that despite its Class A rating, voices don't sound natural at all (this was not the pairing I used when listening to the above), and despite their claims that it has ample bass and power at 240 watts into 4 ohms, it still can't drive a 4-ohm speaker like the Dynaudio mentioned above well at all.

And so on. Good luck getting the full story out of Stereophile.

Patrick Butler's picture

A more likely explanation for your experience with the Excite X12s is what you are doing with them in your room, rather than some sort of consipracy perpetrated against readers by Stereophile. 

Long-time listener's picture

No, you're wrong. I'm not suggesting a Stereophile conspiracy, but instead a style of reviewing that highlights the positive in exaggerated terms, while very carefully tiptoing around the negative. You can refer to their review of the Excite X12s to see what I'm talking about. That review contains nothing but superlatives, so there is no hint of why it would ultimately end up in Class B, rather than Class A. This is exactly the question posed in the original post above, isn't it? This supports the idea of reviews that consistently omit important negative aspects of performance heard by the reviewer (or reinforces the idea of conspiracy, if that's what you think well-founded criticism is). [flame deleted by John Atkinson]

I get good overall performance from the X12s, and I'm more or less satisfied with them in their particular system. Their performance is in fact surprisingly good given their size, and my room doesn't pose any particular problems that wouldn't have been apparent with other speakers in the same position. But I was dismayed at their distinct shortcomings, which I simply couldn't have imagined based on their review. After the fact of purchase, however, the review's omissions become apparent. The reviewer raves about how they handle female voices--but hardly a word about male voices (the low crossover point to the tweeter means that some of the female vocal range is handled by that driver, rather than the woofer, which has higher distortion). Likewise, piano was disappointing next to the Class C Epos M5i speakers I had before, whose midrange had greater purity and clarity.

The T+A amplifier is a dog, and there's no way it deserves Class A. Thin sounding, poor timbre, and unable to drive even small 4-ohm speakers well despite its claimed power output. It has a thin, bleached-out, colorless midrange with harsh mid-treble. Terribly disappointing. I trust Stereophile to the extent that it includes measurements--and within the limits of what measurements can tell you--but otherwise I don't trust it much any more. I bought the T+A amplifier without seeing any measurements, a mistake I'll not make again.

volvic's picture

I subscribe to that old addage "trust but verify".  While I respect the reviewers opinions on gear they listen to, I have different needs.  Fremer may say how he loves the pinpoint imaging of the MBL's or Meijas loves to hear the synthesizer growl on a particular track he likes.  I have other needs from a hi-fi system, and whether or not I get to hear the individual plucking in the back of a concert hall of a viola or cello isn't as important to me,being able to follow the line that runs through the music is of greater importance to me.  I guess I am quite easy to please then.  But when a reviewer raves or likes something that I may be interested in I go and listen and judge for myself.  Point being I always use the reviewers opinion as a springboard towards making my own decision.  

Long-time listener's picture

Yes, I agree completely. That's the way to do it. But sometimes you're looking for something, and a certain item seems to fit the bill, but there's little information available on it, and you can't audition it in a meaningful way that would tell you how it will work out in your own system. I guess if that's the case, the answer is simply not to buy. Trust but verify, then buy. Otherwise don't.

Nellomilanese's picture

I'm a newbie in this field but I have been visiting audio dealers obsessively in the last year and experienced everything from 200 $ to half a mil $ system, and I completely agree with the above. 

Here's something interesting: I buy at least 3 Hi-Fi mags / month and I always try to go hear their group test winners....GUESS WHAT...most of the time I come back dissapointed....and I would like another speaker just happened to be on display.

It happened with the B&W CM7&8 series which for me was my dream speaker...until I heard it in an otherwise wonderful system playing a Blu-ray concert with excellent cables, Oppo player, Rotel amp etc.! I know the recording of that concert is very good...so it wasn't the source or the amp.

I didn't like them...I was up on my feet constantly tweaking the system...after 3 minutes I got tired and walked into another demo room. It all sounded to clinical, no warmth, it didn't draw me in....and it only got worse as I drive up the volume! The CM series would've worked for me problably only connected to a record player and valve amp.

Same thing happened at this dealer with the "award winning", 5* Tannoy DC6T...couldn't stand that tweeter....dialed the highs to 0 and even negative values and yes it got a little bit barable but stil no way in hell I would've buy them.

Btw I did an audio test in a showroom with high grade Grado headphones and test file, and I could hear all the way up to 17-18 khz. The 18khz was barely audible, mostly a high-energy "feel".

The saleman said that might be my "issue" LOL

So there you go....what's 5* for someone might be 1* or less for you :D

Long-time listener's picture

Having lived with this amplifier for some time now, my impressions are the same. Break-in has allowed the bass and mids to settle in, and they're acceptable, but there are serious problems in the upper midrange and treble. As often as not, cymbals are just white noise. Trumpets are ear-piercing. Strings are shrill. Pianos seem made out of metal, not wood. 

I cant' imaging how on earth this got a Class A rating. I thought Stereophile was more dependable and more objective than other magazines; it's not. I've lived with this amp for a while now, and my "associated equipment" is good stuff--the Stereophile Class A+ rated NAD M51 DAC, first-rate cables, passive line conditioning, and a room with good acoustics. I don't think it was appropriate to just put on a few electric guitar CDs and pronounce it Class A. Reviewers should be more thorough. They should listen closely with lots of genres, especially if the equipment is not going to undergo a measurement routine. I give the Power Plant Class C. Equipment with such audible flaws, and such clearly distorted highs, should not be in Class A.

rapet's picture

I have heard these Operas and they are far superior than other stuff rated class A. One thing, the Josehp Audio in Class A use the same drivers...I heard them in a show and even though it was not the right environment I do not remember them to be superior than the Operas in any way....yes, rating in Stereophile is quite subjective....

John Marks's picture

For the most part, when I decide which class I will NOMINATE a loudspeaker to be placed in (because the final decision always is John Atkinson's), for the most part I intentionally DISREGARD whether there are any inclusions in a class that I might disagree with. I act like an Originalist, or, at least, I go back to the text.

There can be no argument that the official rubric of Stereophile RCL Class A is:

The best of the best, regardless of price or practicality.

Or, other words to that effect.

As much as enjoyed my time with the Opera Callases, and as much as they reminded me of one of my subjective-favorite loudspeakers (ASA Pro Monitors), I found them to depart from "accuracy" in rather noticeable ways, and also to lack the resolving power of a speaker not in but certainly near their price tier--the Vivid V-1.5.

There have been other speakers, such as the Vivid V-1.5 that I enjoyed as much as the Opera Callas, and which also did not depart from accuracy in any important or even noticeable way. Therefore, I made the decision that as far as I was concerned, the Opera Callas was not The best of the best, regardless of price of practicality, but rather, "The next best thing," and that is Class B.

For Heaven's sake, Class B is where Harbeth's P3ESR is. Class B is not a slum.

I am sure that JA did not merely rubberstamp my suggestion. He measured the Callas and I believe he listened to the pair. Had he disagreed with my suggestion, he could have put the Callas in Class C...

Of course, I speak only for myself, and that also means that I am not going to defend any RCL placement I had nothing to do with. And in that regard, I think that it is a good thing that not all the writers toe the same company line. So, expect some differences of opinion.

I respect the feelings and the listening skills of people who think the Callas should have been put in Class A. However, I try to stick to the literal wording of the gatekeeper clause, and the Callas did not in my view make it into Class A. Perhaps if some other writer had covered the product (Hi, Art!), the end result would have been different.

But I again restate that Class B is not Santa's Island of the Broken Toys.

Class B means this is a great product, and nearly the very best you can get.

John Marks

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