You are reading the older HTML site

Positive Feedback ISSUE 14
july/august 2004



Caliper Signature loudspeakers

as reviewed by Bradley Morrical





Apogee Caliper Signatures and Ribbon/hybrid DIY project.

Audible Illusions M3A w/ John Curl gold phono board, Jolida JD3000b, and PS Audio Phonolink preamplifiers. Sphinx Project 14MkIII, Spectral Acoustics Digital, and  Golden Tube Audio SE100 amplifiers.

Michell Gyrodec SE w/Rega RB300 arm and Ortofon Vero II cartridge, Cambridge Audio Diskmagic Transport w/Monarchy Audio DIP and Monarchy Audio 22Bse DAC. Samsung SPN4235 42" Plasma.

Monster, Kimber, and XLO.

Behringer UltraCurve 8024 with Digital in/out (this is a new one from a friend and works much better this way so I think I will do that review on it after all once I have sorted it out completely), Behringer Ultradrive DEX 9624 digital crossover (for the DIY project), Behringer UltraCurve DEQ 9426. Sennheiser HD540 headphones.


The editors at Positive Feedback Online recently decided that PFO should occasionally take a look at some of the great hi-fi gear of the past. A glance at one of the audiophile chat sites will immediately tell you that this is worthwhile. Every day, people ask advice about gear that is no longer being made. They want to know whether going back to older designs makes financial sense, and whether classic gear can hold its own against the best components of today. If your goal is to build the best-sounding system possible within a budget, you will probably find yourself contemplating the purchase of classic audio gear.

Many audiophiles spend the lion's share of their audio budget on loudspeakers. I do not think this is wrong, as differences among loudspeakers are more profound than those among other components, but my recent trip to High End 2004 in Munich made me painfully aware of how much top-quality loudspeakers cost. For most exhibitors, a "real" speaker system cost at least $10,000. Ouch! If you have a limited budget, purchasing a good pair of loudspeakers will leave precious little money for sources, electronics, cables, and accessories. It is also possible that many fine loudspeakers do not reach their full potential due to inadequate supporting gear. On the other hand, what if you could purchase a pair of cutting edge, fantastic-sounding loudspeakers for a relative song? This could free up your budget enough to allow you great freedom in choosing the equipment to accompany the speakers.

In my opinion, loudspeakers have not seen the kind of technological progress that there has been in source components, and it is debatable that buying new loudspeakers will get you cutting-edge technology.  If you look at the measurements of recent loudspeakers in Stereophile and compare them to measurements from ten or even twenty years ago (if you can find them), you will see the same gross errors in frequency response (+/-3dB is still a common spec), but more troubling is the relatively poor off-axis behaviour, lack of transient correctness due to non-linear phase/driver position, and poor decay spectra from cabinet and driver resonance and diffraction. Today's best designs address many, if not all, of these issues, but at what cost?

Apogee Acoustics is one of the classic audio marques of the last thirty years. What made the brand special was its blend of cutting edge technology, sci-fi looks, and of course, sonic performance. Most Apogee speakers were full-range ribbon designs and, in their earlier incarnations, were a nightmare to drive. (In fact, Apogee helped companies like Krell and Classe make a name for themselves, as their amplifiers were able to drive the early models.) Apogees were the exotic loudspeakers of their day, and they still turn heads like almost no others. The subject of this review, the Caliper Signature, was for many years the company's smallest loudspeaker, but don't call it little. The Caliper Signature is a trapezoidal monolith, and at over 49 inches tall and 30 inches wide (at the bottom), it is, by most standards, a medium to large speaker.

A full-range planar, the Caliper Signature is a two-way design incorporating a 1-inch by 36-inch ribbon covering the range from about 500 to 20,000 cycles and a large, trapezoid-shaped panel covering the range from 500 down to around 30 cycles. There was much debate about whether this bass panel was a ribbon or a planar magnetic driver (the operating principles between the two drivers are similar, but not identical). I will not delve deeply into this, except to say that I consider the bass panel to be planar magnetic. There is no doubt, though, that the midrange/tweeter (MRT) unit is a true ribbon, as it is suspended by only two points and is open both front and back. In addition, has vertically running conductors and is suspended between the poles of two rows of high-energy magnets. Both drivers are made by the same lamination of aluminium foil with Kapton plastic. Exotic stuff to be sure. Few companies have attempted this type of driver, let alone successfully executed it.

I am not a fledgling audiophile. In the last fifteen years, I have owned several conventional loudspeakers (Paradigm, Dynaudio, DIY) and horns (Klipsch La Scalas). However, my dissatisfaction with these designs compared to what I heard at live concerts (and occasionally at hi-fi shows) directed me towards planars. I had heard ribbons (I still remember my jaw dropping, along with everyone else's, upon hearing the Apogee Grands at the 1993 CES), planar magnetics (Magnepan), and of course electrostatics (Martin Logans, Quads, Finals, and Audiostatics), but for many years I stuck to more familiar designs. This changed suddenly one day about three years ago, when a friend and I were window shopping at a hi-fi shop near my new home in Zürich. We had spied a pair of Magnepans there (I forget which model), and had come back to hear them. We were told that they had been sold, but the proprietor of the store asked if we would be interested in hearing a pair of Audiostatic ES100s.

I hadn't heard Audiostatics for about eight years, but remembered their surpassing transparency, and jumped at the chance to hear them. I bought the speakers, and began to bask in their overarching strengths—absolute transparency and coherence. I thought I had found the perfect loudspeakers, as Sonny Rollins's saxophone had never sounded more real, but I ultimately found that I couldn't listen to them in stereo. I preferred to sit in front of just one. Why would I possibly want to do this? The high frequencies were phasey, and even the slightest head movement caused cymbals to jump back and forth between the speakers. This might not bother some people, but to me it was a very frustrating experience that led to fatigue, even though some of the other attributes of the speakers were state of the art. Knowing that I had to change speakers, but wanting to keep much of the purity I heard in the Audiostatics, I bought a pair of Infinity IRS Betas. These imposing monsters were extremely neutral, ultra low in harmonic distortion, relatively uncolored (though with an occasional hint of metallic coloration), and dynamic as hell. However, the Betas were too big for my apartment and tended to overload the room rather easily (but oh, the impact with full orchestra!). Still, I knew I was heading the right direction, especially when I compared their sound to that of live music. The Betas' impact and lack of coloration invited me to listen to types of music I had never really considered, and I developed a growing affection for classical music, which I had largely ignored.

Then, as chance would have it, a friend was selling his Apogee Caliper Signatures (for reasons he now doesn't remember), so Anna (my girlfriend, a professional violinist) and I listened to them as possible replacements for the Betas. After a short listening session, it became clear to us that the Calipers had many of the desirable traits that we were looking for, in a more reasonably sized package.

How do the Apogee Caliper Signatures sound? They are simply, from top to bottom, the most coherent loudspeakers I have heard that use more than a single driver per side. The integration between the MRT and the bass panel is seamless. Having owned single-panel loudspeakers (the Audiostatics), and listened frequently to a friend's single-panel Stax ESL F-81s, I can say that the Calipers sound as if the sound is coming from a large, single source and not two panels. Unlike those two speakers, it does not suffer high frequency beaming (Audiostatic) or severe dynamic limitations (Stax). How does it compare to conventional loudspeakers, with their differing driver materials and complex crossovers? In terms of coherence, there simply is no comparison. Only a few conventional speakers I have heard (Thiel and Von Schweikert come to mind) can even come close. My recent attendance at the Munich show (report coming soon!!) confirmed that most loudspeakers simply cannot sing with a single voice. I heard frequent evidence of discontinuity, even in the most expensive designs. A good way to test speaker coherence is with a recording of a grand piano. A coherent speaker will reproduce the lowest notes to the highest without any obvious discontinuity, whereas a speaker that lacks coherence will exhibit subtle tonal shifts or have different dynamics in different parts of the frequency spectrum.

The issue of coherence is important because it is one of the ways that a loudspeaker system destroys the illusion that you are listening to live music. In my experience, coherence has much to do with the colorations of the drivers being used. When drivers are made of more than one material (say paper for the woofer, ceramic or Kevlar for the mid, and titanium for the tweeter), each material will contribute its own distinctive sound that can be heard (if sometimes subtly) as an instrument passes through that driver's frequency range. These colorations are so pervasive that unless you have spent a great deal of time listening to live music, or been heavily exposed to electrostatics—that "other" type of planar loudspeaker—you are unlikely to even note their presence.

The fact that both of the Calipers' drivers are made from the same materials (aluminium and Kapton) means that both will have the same sonic characteristics. You might be thinking, "I have heard that other planar magnetic speakers can have a metallic edge in the lower treble, and that electrostatics can have a ‘plasticky' sound." You would be right, as I have experienced this with older Infinity loudspeakers that use EMIT and EMIM drivers, and I heard a hint of a plastic "quack" sound from my Audiostatics. Martin Logans sometimes exhibit this as well. Somehow, the Apogee Caliper Signatures manage to avoid this problem to a degree that I have not heard in other speakers. Perhaps it is because the MRT ribbon is fairly loosely tensioned, with a very low resonant frequency, and when played at normal levels (i.e., under 90dB), it barely moves. The bass panel is tightly tensioned and, at least in theory, could give this effect. That it isn't noticeable can be explained by the fact that it is rolled off at around 400-500Hz, and thus does not get excited in a way that creates this coloration. Most other planar magnetic loudspeakers I have heard (Magnepan, Infinity, Eminent Technology, and, to a much smaller extent, Bohlender Graebener) use highly tensioned midrange drivers. The pure metal ribbon of the Magnepans can also sometimes have a bite that is missing in the Calipers.

In addition to low coloration, Calipers have exceptionally low harmonic distortion, which is caused by non-linear drive effects in an electromagnetic motor. In 1988, Martin Colloms measured the harmonic distortion of an Apogee Duetta Signature, and found that even at levels of 96dB the distortion on the bass panel and MRT was under 1% above 50Hz. At more normal levels (around 86dB), he found the distortion to be less than 0.1% over nearly the whole frequency range. While the Calipers are not as big and powerful as the Duettas, their design concept is the same, and they should produce similar distortion (or lack thereof) at similar output levels.

Another way in which loudspeakers destroy the illusion of reality is in the compression of dynamic peaks, usually due to thermal compression. When a driver has a low sensitivity, these effects can be pronounced, as a lot of current has to flow through the driver's voice coil to produce a decent volume level. The resultant heating of the coil reduces current flow, resulting in less cone motion at peaks. The subjective result is that when the music gets loud, there is a marked loss in transparency, an increase in distortion, and dynamic compression. The fact that many moderate-sensitivity speakers lack low-level resolution due to a high resistance to initial motion means that many have a very narrow range in which they are performing in a truly linear fashion.

I have heard many speakers that don't sound good at low levels. You turn them up a bit and say, "Ah that's better," but when a really rocking track comes on and you crank them, the sound gets hard and glassy. Some newer, more sensitive conventional designs have improved greatly in this respect, but at low levels they still cannot reproduce scale properly. As far as I can tell, the Calipers do not suffer from the effects of thermal compression. When the volume increases, the sound does not turn hard and glassy, nor do the dynamics become constricted. Instead, the peaks just soar. Likewise, at low volumes, there is a better balance between the low, mid, and high frequencies, which allows the music to sound as if it is coming from high end speakers rather than a table radio.

One of my favorite jazz albums is a 1967 recording on Verve of Stan Getz with a young Chick Corea. This album has superb dynamic contrasts, with Getz's sax rather startlingly going from light to soft to an all-out jam. Because I frequent jazz clubs in Zürich, I am quite familiar with the sound of live saxophone, and love its rich but biting sound. What I hear from my Calipers is very close in terms of dynamic range to what I hear live. This is especially true of the instrument's bloom, something I believe a speaker system can only do correctly if its dynamics are unrestricted. What do I mean by "bloom"? When a saxophone is playing at a relatively low level, the sound seems to be coming from within the instrument, but as the sound level increases, so does the apparent size of the sound image. It not only appears to get wider and taller, but to expand toward you. I hear this effect live when I am sitting close to the stage, but only on very few loudspeakers, and with very good recordings. I hear it with the Calipers on the Stan Getz/Chick Corea record.

The Calipers do exhibit a bit of dynamic compression at high levels (above 100dB). The limiting factor in these circumstances is driver excursion, not the thermal compression effects that afflict many speakers, which means that the limiting is more abrupt than the insidious compression that most speakers apply long before their ultimate limits are reached. The levels at which this limiting occurs, moreover, are much louder than those at which most people listen. At more normal listening levels, I have not heard any dynamic compression. Since the Calipers do not suffer from thermal compression, they can—within their ultimate limits—freely express all of the dynamic nuances in the music. Bigger Apogees should be even better in this regard than the Calipers, as reflected in their peak loudness specifications, but they will also need much more room to be at their best.

The result of the Calipers' combination of coherence, low coloration, excellent micro- and macrodynamics, superb transient response, and low harmonic distortion is high resolution with little or no smearing. This means that, even in the most complex orchestral music or electronica, the Calipers keep everything properly sorted out and easy to hear. Take Schubert's "Great" symphony. In the third movement, there is strong interplay between the bass, cellos, and violins and the flute and oboes in the back of the orchestra.  The wind instruments can be heard clearly over the other parts of the orchestra, even when the strings are going full tilt. Also, the bowing of the lower string instruments comes through cleanly, along with the powerful resonance of the bodies of these instruments that is making up the foundation of the music. Even at low listening levels (with peaks around 75dbC), the tonal balance of the Calipers doesn't change significantly, and musically relevant details pour forth. I have heard the Calipers' low-level response bettered only by a few full-range electrostatic speakers and horns.

The Calipers' transparency is superb, and you can hear all the way into a recording. Although here again the Calipers can perhaps be bettered by the best electrostatic designs, they outperform nearly every cone-driven system I have heard. Some horns can be exceptionally transparent, but colorations interfere and give a distinct signature. Instead, the Calipers invite you deeper into the music, bathing you in the experience. Resolution comes effortlessly, without ruthlessness, as the Calipers tread that fine line like acrobats, revealing everything on the recording without beating you over the head with detail. There is a calmness between the notes, and between instruments, that allows fine details to emerge without sounding overly busy. The effect is hard to describe, and could be taken as boring if it weren't for the fact that, hours later, you are still enjoying the music.

My Infinity IRS Betas were the most revealing loudspeakers I have owned with regard to their ability to tell you what a new piece of gear was doing in the system. However, the Betas presented information in a relentless fashion, and were not the least bit forgiving to associated gear. Anna, with her superior high-frequency sensitivity, found them fatiguing. The Calipers are just as revealing, but manage to impart the information without the pain. Changes in associated gear are easily identified, but not in the ruthless, "Get it the hell out of there" manner of the Betas.

You might be thinking that the Apogees are nearly perfect loudspeakers, but I have not yet mentioned frequency response, and this is where the Caliper story gets a bit more complicated. In my room, they have a smooth response from about 200Hz and up, with a gentle roll-off in the upper frequencies starting around 8kHz. This high-frequency response mandates that they be used in a relatively live room, with little or no damping behind the speakers. The major frequency response problems are in the lower midrange through the bass. Due to the dipole nature of the speaker, there is some cancellation in the region from 200Hz down to about 80Hz that will shift depending upon the distance of the speaker from the rear wall. (Due to constraints in my room, about one meter from the back wall gives the best balance.) Because of the way the Caliper is tuned, there is a broad hump extending from about 70Hz down to 30Hz. This has two effects: it gives the Calipers quite powerful and extended bass, but it can make them sound overwhelming and boomy unless great care is taken in setup. The tilt angle of the speakers also plays a large role in the frequency response at the listening position, as the ribbon is quite directional in the vertical axis. I advise much experimentation in this regard, as both position and tilt will profoundly affect the speakers' tonal balance. Following the owner's manual (if you have one) is your best bet for getting the tilt correct, but there are no hard and fast rules, so it pays to experiment.

Ironically, one of the most serious criticisms that I can make about the Calipers, and Apogees in general, is not nearly as much of a problem now as it was fifteen years ago. Of all the parameters I have discussed so far, frequency response is the easiest to correct with external processing. Even with careful placement, the frequency response of the Calipers is not perfect, but I think I have found a relatively inexpensive cure—DSP digital correction. I am not yet ready to spill the beans about the new device in my possession, but it appears to be transparent, and it makes in-room correction possible for a fraction of the cost of similar systems. With the frequency response properly balanced, I am finding that the other strengths of the Caliper Signatures are allowed to shine more brightly.

Without correction, some might find the Calipers' treble balance a little soft. However, those with good high-frequency hearing—like younger women—will probably find the balance to be just about right, assuming you haven't over-damped the room. Given my hearing ability above 10kHz, I find that using a DSP boost to give a bit more sparkle is closer to what I hear when I am sitting quite close at jazz concerts. Bass can be a problem in many rooms, as the Calipers' bass level is generally a few decibels higher than the rest of the frequency spectrum. I have positioned mine as well as I can in my smallish room, so if your room is of similar size (about 5 x 4 x 2.7 meters), you may not be able to get flat bass response without some equalization. This is not as bad as it sounds because, on most recordings, the extra bass makes the sound fuller. Bass-heavy recordings like Moby's 18, and Art Davis' A Time to Remember (on Reson LP) are the exception rather than the rule, as most recordings sound natural this way.

It is a pleasure to hear the Calipers create a large and lifelike soundstage, particularly with classical symphonies and small ensembles that have been recorded with minimalist techniques. Their ability to do this depends on the amount of absorbing material behind or between the speakers. If this is minimized or eliminated, sounds can be heard coming from beyond the edges of the speakers as well as from in front of and behind them. Imaging is very good, but not razor-edged, as it is with accurate point-source speakers like the Thiel CS3.6s or Totem Model 1s. I am not entirely sure which is correct, but the Calipers' imaging is closer to what I experience at live concerts. With studio recordings, a sharper image is probably more accurate.

Should you buy a pair of Calipers? In considering any classic gear, reliability and repairability are important issues. Most Calipers date from the late 1980s to the early 1990s, so replacing drivers and/or refoaming may be necessary. When the foam that surrounds the bass panels has gone bad, the panels will buzz on high-level peaks. This foam can be redone if you are handy, and there are instructions on the Apogee user's website ( In addition, panels and ribbons that have been exposed to too much sunlight and/or heavy air pollution can deteriorate. Fear not, as it has recently become possible to obtain replacement drivers from Graz in Australia ( The word on the web is that these drivers are superior to the originals in nearly every way. I hope to try these drivers in the near future, and will report the results.

I hope that I have conveyed the numerous strengths and relatively few weaknesses of these truly superb loudspeakers, which can be purchased in good condition for around $1000. At this price, they have no serious competition, and this includes used Magnepans, Martin Logans, B&Ws, Dynaudios, Vandersteens, and many others (although I recommend that you listen to the Thiel CS3.6s, which at less than $2000 used, are also excellent). The combination of strengths possessed by the Caliper Signatures means that they will reveal the nature of the source material and the gear powering them better than most loudspeakers, whether currently produced models or other classics. The better the source materials, source components, and amplifiers, the better these speakers will sound.

Like all loudspeakers, the Caliper Signatures are not perfect. They have some deficiencies in frequency response, but with careful attention to placement they will sound well balanced. They are a bit rolled off at the highest octave, but they don't lack air or delicacy. Bass can be a bit too full on some recordings, and this is where careful placement (or a digital equalizer) can pay off the most. While the Calipers are excellent at low-level detail, they do not quite have the otherworldly low-level resolution of the best electrostats. Finally, the Calipers benefit from ample current delivery. Although I believe that in a moderately sized room, tube amps that can deliver decent power (at least 50 watts) into lower impedances will work well, I have yet to try this. I have driven them successfully with a 200-watt solid state hybrid, a 100-watt tube amp, and a 125-watt pair of Class D monoblocks. Since the Calipers behave like nearly flat 3-ohm resistors, they don't exhibit the problem that most tube amps have with low-impedance speakers, most of which have vicious phase angles and impedance loads that vary wildly with frequency.

I believe that ALL models of used Apogees present the audiophile with an unprecedented opportunity to own near-state-of-the-art loudspeakers for the price of most companies' mid-market garbage. The availability of new replacement drivers means that repairs and upgrades are possible, negating the reason most people dumped their Apogees in the first place. Provided you have the space, the electronics, and the requisite patience for setting them up, the Apogee Caliper Signatures will continue to surprise and please you for life. Bradley Morrical