Tannoy Little Reds
This month, we're assessing Tannoy's legendary SRM12B - or the Little Red Monitor, as it's colloquially known. But wait: if perchance you've never seen an SRM12B, you're forgiven if tempted to gesticulate in aesthetic horror. The 'Red' has nothing to do with the colour scheme (they're actually Walnut-wood brown). No, this allusion to Martian virility derives from the famous Tannoy Gold (an early, classic dual-concentric driver, which had a gold-coloured chassis) and thence from the Super Red, a large prestige model, which is naturally the Little Red's big brother and immediate predecessor.
The Tannoy series is unique for its long history. In 1923, Guy Fountain was engaged as an engineer in the big growth business of the decade - not microprocessors, but wireless receivers. In the early twenties, everything - however simple - consumed power measured in Amps, and ran from hefty accumulator batteries. These needed endless recharging and Guy's problem was to rectify the AC mains - without the aid of power valves and later, bridge rectifiers, that we all take for granted.
A popular rectifier in 1923 was the electrolytic bath; AC mains was converted to DC by passing it through electrodes dipped in a chemical solution. And his electrodes? The best results came when Guy used plates of Tantalum and lead alloy. So he coined the trademark Tannoy for his invention. Three years later, in 1926, he began manufacturing speakers for the fledgling wireless, audio and PA markets. In 1935, he produced his all-time masterpiece, the dual-concentric driver, and not only is Tannoy's continuing success, for the most part, due to this invention, but the basic principle is still with us today - in the Litte Reds for instance!
So, these Little Reds possess dual-concentric drivers (also known as coaxial drivers) but what does that mean? Essentially, the idea is two speakers in the space of one. The tweeter, a separate driver in its own right, is set in a hole down the middle of the bass driver. In fact, it sits down the middle of the voice coil, a space that's wasted in ordinary cone speakers. This speaker-within-a-speaker confers very potent advantages. To begin with, the configuration guarantees that the apparent sound source at low frequencies coincides with its location at the top end of the spectrum.
More significant perhaps, is the physical coincidence between the axis of the two drivers, which helps to keep things phase coherent. With two or more separate drivers, phase shift is contributed not only by the crossover and the drivers themselves, but also by the different path lengths between you (the listener) and the woofer, and you and the tweeter. This path differential, and hence the phase shift, accords not only with the driver's relative spacing, but also depends on the position, direction and distance to the listener. These factors are uncontrolled, and therefore difficult to predict or allow for.
In the coaxial system, only the drivers themselves contribute to the phase error, so the system's characteristics are more under the control of the designer. The correct location of woofer and tweeter spells a time-aligned system ie. woofer and tweeter are coincident in space and synchronised in time. This is a big help in achieving a good stereo panorama.
Coaxial speakers can use any type of tweeter. The designer can locate a dome tweeter in front of the woofer cone, but this is subject to some irritating compromises. In particular, to fully taste the benefits of time synchronisation, the tweeter must be set back deep in the bass/mid driver to be in line with the latter's voice-coil, or as near as we can get it.
This sort of mounting would freak out a direct radiator tweeter; under these conditions they would exhibit rather dubious dispersion properties. Have you noticed the lengths that other speaker makers go to in order to lessen diffraction effects when dome tweeters are mounted on a smooth, plane surface? For this reason, coaxial (dual-concentric) drivers routinely employ horn-loaded drivers because the dispersion pattern is readily defined. The designer can then set up a dispersion pattern that limits the incidence of high frequency energy on the bass cone's surface, thereby avoiding serious diffraction.
Stop! Cheap 'coaxial' speakers don't use true two-way drivers at all. Rather, a single woofer coil directly drives a subsidiary high frequency dome. This drone-cone set-up is, in fact, a mechanical crossover or decoupling network. The resulting 'wide-range' loudspeaker is inexpensive (take a look at any lead guitar stack!) but lacks the low distortion and tightly defined phase and dispersion characteristics of the real McCoy...
The bass/mid section of the Little Red's dual-concentric driver is a 12" unit, with a standard pulp (paper) cone, and an alloy chassis. But hark: did you spot the horn-loaded tweeter? It's lurking behind a grille, mandala-shaped, like a large dustcap. This is to keep out ferrous debris; otherwise the magnet might suck them up the horn, to cause damage to the diaphragm, or at best, a nasty crunching or scraping noise.
The horn's mouth is 50mm (2") in diameter, which implies a tolerably wide dispersion up to 8 or 16kHz (depending on where you draw the line). At any rate, it's about an octave less than a common 1" dome tweeter. At lower frequencies (= longer wavelengths) meanwhile, the bass cone's much shallower profile is progressively seen by the HF wavefront as an extension of the horn's boundary, or mouth.
The general result is that the location and diameter of the mouth shifts with frequency. This makes synchronisation dicey, because the velocity of sound in a horn is lower than that in free air, therefore the wavefront suffers a greater propagation delay at the lower audible frequencies, where the length of the composite horn (ie. horn plus bass cone) lengthens to around 6".
On the other hand, it's all kudos for directivity. The increasing mouth dimensions as frequency goes down, means that the sound dispersion is apt to remain more regular over the 1kHz—20kHz range - that's more than 4 octaves - in contrast to any 2 or even 3 way direct radiator system. The mildly regulated HF directivity of the dual-concentric is a serendipitous bonus, because it implies more stable stereo imaging, thereby enhancing the benefits of the basic coaxial principle.
One magic quality of dual-concentric speakers that's not immediately obvious is that on their own, there's complete parity: the unit radiates equally into 180° solid-space, meaning you can mount the enclosure on its side, at an angle, or upside down without creating any asymmetry in the radiation pattern. The same is certainly not true for 2 or 3 discrete drivers, which must always lie in a vertical line for a cogent image.
However, don't forget that the Little Red's enclosure is vented for improved bass-end efficiency. Working at low frequencies only, it won't, of course, affect the imaging, but its location vis-a-vis room boundaries needs to be thought about. For example, if we placed the Little Reds in a corner, close to the ceiling, and angled them downwards, it would be worthwhile inverting the enclosure (so the EQ knobs are up top), in order to evaluate the effect on the bass of moving the vent away from the corner's apex.
Let's not take the horn tweeter for granted: horn-loading any driver raises its efficiency by 10 to 25dB. You can then trade off for bandwidth or power handling, and still out-perform a direct radiator. Horn loading also lets us choose any dispersion pattern we wish - in this instance, it's 90° conical, as implied in the discussion above. Trouble is, for a flat response, practical efficiency can never be greater than the bass/mid driver. Using a 12" driver helps boost bass/mid efficiency, but inevitably much of the horn tweeter's potential has been traded off in exchange for high frequency power, which is no bad thing.
Overall then, sensitivity is typically 4 to 5dB above average (at 95dB) in a mildly reverberant environment (ie. non-anechoic). Put in plain language, the Little Reds will sound as loud on a 50 watt amp as a (hypothetical) speaker with (i) the same response, but (ii) a 92dB sensitivity - when it's driven at twice the power, that's 100 watts.
For coaxial drivers, the choice of crossover slope doesn't gravely influence the imaging or coherency of the sound, unlike discrete drive units. In other words, Tannoy could ideally get away with a simple -12dB/octave crossover if they wished. However, the Little Red's crossover ranges across two PCBs no less, and with 27 parts, it's the most complex network we've encountered so far. Part-and-parcel of the high component count is the inclusion of a precision switched passive EQ, for the control of top-end energy.
The two knobs on the front panel allow us to choose either a top-end roll-off slope, or a shelving response, or both, or neither! The roll-off has most effect, either cutting or boosting at 20kHz; whereas a shelf response lifts or drops the entire top-end equally, rather like turning up (or down) the top-end attenuator on an active crossover. The roll-off control governs the slope, which can be +½, —1½ or -3dB per octave, whereas the shelf control has two boost and two cut settings, which are symmetrical at ±1½ and ±3dB. The central position on each switch returns the response to nominally flat, all depending on the playback room's acoustics, of course.
With these Tannoys, passive component technology doesn't stop at elaborate EQ circuitry. A second PCB carries what looks at first sight like a simple crossover network, but is in fact an all-pass filter or 'delay' circuit, comprising four inductors (coils) and four capacitors.
The idea is to bring the top-end driver into sync with the bass/mid, bearing in mind their displacement ie. the bass/mid driver's voice coil is slightly nearer to you. In Tannoy's own words, "Sync-source is a passive time delay... to align the apparent HF source with the apparent source of low frequencies along the single sound axis of the Dual-Concentric driver. The result is a single point source of sound at all frequencies".
The result isn't under dispute, but the explanation is, in fact, slightly misleading, because it's actually the bass driver's voice-coil that's nearest to you, so correction by delay techniques alone is possible only by delaying not the top, but the bass/mid signal. You can't incidentally, 'time advance' a signal; at least not without getting a licence from Dr. Who!
At this juncture, we must glimpse into the bizarre world of phase vs. the time domain. Given that it's best to leave the bass/mid alone (because an all-pass filter for this would demand bulky and more costly parts), the trick is to balance the phase shift of the bass/mid crossover against a lesser phase shift in the top-end crossover.
To this end, the two filters have different damping factors (which is necessary anyhow, for the sake of a flat response), and the allpass network simply tidies up the phase relationships, to give a 30μs time difference (=11° phase shift) between the drivers at the 1kHz crossover point. This corresponds to about 1/30th of the wavelength of sound at 1kHz, or about ½" (12mm) which also happens to correspond to the distance between the two voice-coils, hence the compensation.
Looking behind the advertising, this technique is not original: Siegfried Linkwitz, Tim Isaacs and Tony Andrews are all names connected with similar time-alignment systems devised six or more years ago. Nevertheless, the fact remains that a coaxial driver is most set to benefit from time-alignment in the speaker's nearfield, and the Little Red is the only monitor in this price bracket with effective time-alignment (barring other Tannoy models, of course).
The passive electronics are naturally convoluted from the standpoint of mechanics. The delay card is screwed down onto a bituminous damping panel, thence to the cabinet, and has eight connections in all, four of which go to the driver where they're hooked up with a snap-on Molex connector. Ditto, the other four which are linked at the delay card on another Molex, and thence back to the main crossover-cum-EQ card, which is screwed down behind the front panel.
This is fine, but raises two points. First, if you're going to have a spare crossover kit, make sure you have the complete assembly, otherwise you may be caught short on the Molex! Second, the tweeter's wires are connected at the delay card end by Lucar connectors, so there's the possibility of connecting them back to front, which would wreck the time-alignment. In fact, there should be no need to remove these (instead, you unplug the Molex at the drive-unit end), but beware if you do it in a blind panic - because the wires are coloured green and yellow, and you may be left wondering which is positive (hot). In fact, +ve goes to the yellow wire, which traces to a brown wire on the driver chassis.
The standard of construction and PCB design is good, but the lead-in wires should be clamped more tightly, and the rotary switches used for the EQ setting need to be treated carefully.
Consistent with the legend, the Little Red's sound possesses many fine qualities, but is at the same time variable and unpredictable. Beginning with tonality, there's a distinct excess presence in the 2.4kHz region, the tweeter's resonance not being as damped down as it could be. Put candidly, the tweeter has a mild honk. This is sufficient to make you sit up and pay attention, but it's not unpleasant, and their intimate portrayal may be thought of like the magnification of a face in a photo, helping us to analyse the region carrying the most detailed features of an instrument's (person's) timbre (character).
Naturally enough, for a monitor with a horn-loaded top-end, the Little Red's rendition of brass instruments (horns!) is unerringly vital. Likewise plucked objects, for example, the shimming acoustic in Robin Millar's production 'Each and Everyone' and vocals were also distinguished, but some mild pluminess was diagnosed in the lower mid region, emphasising any thick, nasal sounds in a voice.
Bass qualities were on the whole satisfactory, and at no point was it possible to bottom the cones. Yet there was more variation than is usual, and the sound of some bass instruments seemed lacking whilst possessing no obvious defects. After shuffling the Little Reds around a lot, we were satisfied that the room played only a small part in the effect.
One symptom was a lack of 'snap' on some kick drums, and this points to a lack of articulation in the high bass/low mid, more so at moderate levels, say 95dB (that's only 1 watt of input power!). But this is readily corrected by turning up the gain, and perhaps moving backwards to compensate, if working in the nearfield. Besides, if the sound lacks articulation, it will make you strive harder to achieve it, which is no bad thing.
The depth of bass was disappointing for a vented enclosure of this size, not to mention a 12" driver; the A&R Arcams reviewed last month could go deeper with a much smaller driver, and the same sort of cabinet volume.
Another peculiar quality is the tendency for occasional hardness and harshness. This undoubtedly relates to the very high Q (sharp) resonances that are characteristic of many compression drivers, especially those with aluminium diaphragms. Tannoy's phase correction plug helps smooth out the worst of these aberrations, but the fact remains that harmonics at certain spot frequencies, like 6.3kHz, will excite hot spots, whilst others will be absent, having been sucked-out.
If this sounds unappetising, just consider how lucky you are: with Little Reds, there's no great thermal compression to contend with and the HF response is consistent over an unusually wide range of SPLs. So your task is to decide whether the Tannoy horn's swings are better than the roundabouts of the common (or garden!) dome tweeter.
As you'd expect from our past discussions on the coaxial principle, the Little Reds stereo image quality is nothing short of exemplary. Like Celestion's SL6, it opens out the room boundaries and unleashes a big space; what you put into the stereo picture is limited only by your imagination and your appetite for outboard space processors.
An excellent demonstration of the capabilities of time-aligned, coaxial speakers came our way, thanks to Adam Asiz and Chris Warwick, in the format of a compact disc (CD) called 'World Record' (DDZ 10129). Produced by an independent London label, it contains 60 minutes of acoustic ethnic music, from Salsa to Soka, recorded wholly digitally with Soundfield and Holophonic mics, and transferred direct from Sony PCM-F1 to CD without intervening processes. Its clarity is therefore in advance of most analogue masters, and the appearance of Mike Skeet's name in the sleeve notes is evidence that this is a good disc to listen to if you want a 'prime cut' to contrast your own recordings against.
If you want to squeeze the best image from your Little Reds, you'll need to mount them away from the room boundaries. The time-honoured way is with a swivel bracket hung from the ceiling, but any movement away from the equally time-honoured corner position will entail an apparent trade-off in the low bass output. But it's really for the better, because you'll excite few room modes as you come off the wall/out of the corner, which means an altogether smoother bass response in an acoustically untreated room.
The good fact is that nothing really bad can happen to the sound, however careless you are when positioning. That means that unlike some Hi-Fi speakers, you can dump a pair of Tannoys in any old room and simply get on with it. Short of forgetting to point them in the general direction of your ears, there aren't many ways you can fail to achieve good results. And this, in a nutshell, is one of the Tannoy's golden qualities: professional users working under pressure don't want to waste time messing about, and more than most competitors, the Little Reds are ultimately dependable in panic situations; better the devil you know....
Returning to the stereo image, one magic quality of dual-concentric speakers is that you can listen really close up without the sound disintegrating into bass (down there) and top (over there). Witness many IBA broadcast studios, where the producer may choose to sit with her/his ears only inches away from Tannoy dual-concentrics (usually in the format of Lockwood monitors). This degree of proximity boosts SPL enabling us to make do with a low-powered amp, like a Quad 405(1), say.
When monitoring in the nearfield, be sure to angle the speakers correctly; they ideally need to be only a little further apart than your own distance from the line drawn between them. At 24", for example, I'd recommend a spacing of no more than 32" between the central axis. Even more important, the drive axis should cross immediately in front of your nose. When these conditions are satisfied, the soundfield will gain a new depth and forward spaciousness. This advice applies to any stereo speaker set-up of course, but it's most awkward to comply with when in the nearfield.
Listening tests began in the normal passive mode, but later, we activated the units. A wiring loom is supplied for this purpose. First you remove the driver (Tannoy have sensibly incorporated a suitable Allen Key), then detach the Molex on the smaller time alignment PCB, and pull the existing Lucars off the input terminals. These are best tucked safely out of harms way, or better, taped up. You can then drop in the new loom, and snap on the Molex (at the delay card) and four Lucars. Two of these connect to the input terminals for the HF driver, which are unoccupied in the normal passive role. Polarity is preserved provided you study the colour codes next to each input terminal, and match these to the wires.
We used a Rauch Q22/41 2-way crossover kindly loaned by MST and set up for the Tannoys. Power on bass/mid came from a Yamaha PC2002 amp, whereas for top-end, we used a Bose F1. Both power amps are capable of 250 watts plus into 8 ohms but the power needed for a satisfactory level on the Tannoys was much less than usual, at around 50 to 100 watts. This confirms the well-above-average sensitivity rating. It's also good for the professional who's in a hurry and doesn't want to have to hunt out a 1kW power amp (i) to get audible results, and (ii) to avoid amplifier overload on high level passages. In the less than ideal situations that are rock'n'roll's hurdles, even a 40 watt Hi-Fi amp will rock when Little Reds are hung on the end.
As is to be expected, we found we had to lower the top-end attenuator on the crossover by 10dB or so, otherwise the top is way too hot. This isn't throwing anything away though, because in an active system, any amplifier power we don't need is turned into useful headroom. So, the nett effect is like having a dome tweeter capable of handling 26 to 40 watts of continuous tone power. It also correlates with fewer blown tweeters, or blown ears, for that matter.
Activation cleans up the sound at high levels and increases apparent efficiency. It also gives you useful extra control over tonal balance, and even polarity. For example, by phase-reversing the bass/mid drivers, the bass sound and articulation were improved, but at the expense of something else. "Which polarity is best" is an irrelevant question; what's important is that you know the effect exists, know how to avoid it, and know how to use it when you truly need it.
Frankly, the active system's benefits are less marked when contrasted with other, more conventional speakers, and this reinforces the "Can't go far wrong with Tannoys" attitude. For should your top-end amplifier blow up, you can revert to passive operation without any great ado. Alas, for other speakers, the reversion could prove a humiliating experience.
Trusty, bewildering; magic, analytical; hard, soft; smooth, coloured - the whole gamut of qualitative adjectives can be applied to Tannoy's Little Reds. Their essence is a mixture of great strengths and some frustrating little weaknesses.
Tannoy's dual-concentric monitors have often been described along the lines of a 'monstrous compromise' - but one which has paid off in practice. To outsiders (read American studio engineers!) they are a curious English idiosyncrasy. If invented afresh today, 50 years later, the dual-concentric driver would receive a hostile response, and you'd probably need a licence to own one, if the cartel of "Buddha-is-a-2-driver-direct-radiator-speaker" (drone) people had any say in the matter. Nevertheless, the fact remains that over the past quarter century, most of the UK's recorded music has been balanced and mixed on Tannoys, and the Little Reds are one big chapter in this testament.
Tannoy Little Reds kindly loaned by main UK distributors F.W.O. Bauch Ltd., (Contact Details). Recommended retail price is £364 (each).
Review by Ben Duncan
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