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Electro-Voice Sentry 100A Speakers

Until Shuttlesound took over UK distribution two years ago, American microphone and speaker manufacturer Electro-Voice had kept a low profile in the UK. For readers who are still in the dark, Electro-Voice (or 'EV') is one of the three original professional audio transducer companies, established in 1927, immediately after Tannoy (1925), and Shure (1926). The EV philosophy is apparently along the lines of 'Speak softly, but carry a big stick'; their profile may be low, but judged by what they design and produce, they're in the top league.

Their score card of developments in speaker technology is also impressive (diffraction horn, constant directivity drivers, and Thiele-Small analysis, to name but three); it's all down to 'good old solid technology', rather than the technoflash hyperbole approach adopted by certain other speaker manufacturers. And so the emphasis is on carefully balanced designs, instead of boasting spectacular sensitivity or power-handling figures at the expense of other parameters.


The Sentry 100A's vented enclosure has special significance, for EV was the first company to put the work of Neville Thiele and Richard Small into practice. In 1961, Thiele published a paper giving a means of analysing, and above all, predicting the performance of drive units in sealed and vented enclosures, particularly the bottom end response. Ten years later, Richard Small refined the work. Heavy mathematical analysis rarely cuts much ice with audio people, because academics all too readily become divorced from reality.

What makes the Thiele-Small analysis so valuable is that it really works in practice, and moreover, the pages of mathematical abstractions can be boiled down to simple bench measurements, rules of thumb, and handy tables. Or there's a variety of computer programs, which quickly run through the countless combinations of driver parameters versus cabinet size, versus vent area, versus port length, versus bottom end frequency response. The designer can then rapidly home in on one of several useful combinations. The result of this so called 'computer optimisation' is a highly tuned affair, which can readily be further refined according to listening tests. This contrasts with previous trial-and-error techniques, hence the term 'Thiele loaded', rather than a mere bass reflex.

Today, many speaker manufacturers make use of Thiele parameters (These are figures which define the vital statistics of any particular drive unit, and are used in Thiele's calculations), but Electro-Voice maintain a reputation for the practical application of this technique, being well versed in the finer details of getting Thiele to sound right. So with this background, we can expect the Sentry 100A's 8" bass/mid driver's parameters being matched precisely to the job in hand.

New HF Technology

Moving on to the tweeter, a little history will again help to put things into perspective. To begin with, making a tweeter with a flat response from, say, 2kHz up to 20kHz, with useful power handling capabilities and sensitivity, plus low distortion is extraordinarily difficult. Indeed, the technology for this sort of performance has only just begun to move forwards again.

Conventional Hi-Fi tweeters trade-off bandwidth, power handling and sensitivity, whereas HF drivers for rock 'n' roll PA invariably suffer serious colouration and distortion (with the exception of a handful of units from TAD, JBL and the like), almost in direct proportion to their power handling and sensitivity.

In the past, Electro-Voice have concentrated on striking a midpoint between these extremes. The T35, T350 and ST350A horn tweeters, for example didn't exactly have flat response curves, but these could be easily corrected with a simple, dedicated EQ circuit, whilst highish sensitivities in the 105dB (@ 1W @ 1m) region made up for their rather limited power handling capabilities (PHC), at around 5 watts. At the same time, distortion, and dispersion versus frequency received more attention than was customary in the past for professional HF drivers. The ST350A, the forerunner of the Sentry 100A's tweeter, was an important step in the latter direction, dispersion remaining wide, even at 16kHz. This means (a) the high treble doesn't disappear when you move your head slightly off axis, and (b), the imaging is stable, particularly on percussive sounds. Alas, most HF drivers (including the ST350A) do not work well below 4kHz to 5kHz; if run at too low a crossover frequency, power handling is greatly reduced, and the first octave tends to be highly coloured.

Looking now at the bass/mid-range driver, an unreasonably small cone diameter is necessary to maintain any sort of dispersion above 3kHz. Given that, in a 2-way system, this same driver has to be reasonably sized in order to handle low bass, the constant-directivity tweeter is rather let down by the bass/mid-range frequencies. As a result, unless you keep your head on axis (dead in line with the speaker's centre), there will be a 'hole' in the direct soundfield at around 1-4kHz, with dire consequences for vocal sounds.

The Sentry 100A incorporates a new tweeter which goes 'one better' than most. To begin with, PHC is 25 watts, compared with 5 watts for most (non-PA) units.

This capability is relative to a 2kHz crossover frequency, ie. worst case dissipation. The unusually low crossover point should also mean the speaker as a whole avoids the worst of the bass-mid driver's beaming effects, and the specifications confirm this, giving a truly excellent 158° (+/-32°) dispersion angle across the midrange, between 250Hz and 6k3. With the more usual 4 or 5kHz crossover frequency, dispersion would be at least half the Sentry's minimum figure, ie. circa 60 degrees.

Returning to power handling, the tweeter's sensitivity is also relatively high (though being a soft dome unit, it's well down on any horn-loaded tweeter), matching the sensitivity of the efficient Thiele-loaded bass/mid unit. The overall result, then, is a tweeter that will handle at least 5 times the power of most. When any tweeter is overheated, its output falls by 4dB or more, leaving you wondering why the top end has disappeared, and perhaps boosting the desk EQ heavily to finish off the hapless tweeter. So, the benefit accruing from a higher power rating is not merely fewer expensive accidents, but also that of not loosing top-end when working at high SPLs.

The Package

The Sentry's enclosure, in semi-matt black plasticised finish is highly attractive, yet distinctly utilitarian, a point reinforced by the slide-on steel grille, which is cloth covered. This is very sensibly provided to prevent people putting the boot in, should you decide to use your minitors for parties, or on stage, where aesthetics are largely irrelevant. However, the cloth and steel mesh inevitably degrade the sound quality, albeit subtly, so it's a good idea to put the grille to one side for all normal studio operations.

Eagle-eyed readers may have noticed that studio monitor profiles tend to be wide, and shallow, in direct contrast to domestic Hi-Fi designs, which are typically narrow and deep. The reason for this is primarily down to diffraction effects versus the tweeter loading: a direct radiating dome (as used in the Sentry 100A, and most domestic speakers) reacts strongly with surrounding surfaces, so the baffle (the speakers front panel) should be kept as narrow - indeed, as small - as possible.

Classic heavyweight studio monitors utilise horn-loaded tweeters, which aren't particularly sensitive to the baffle surface around them. The trouble starts when you want to use a dome tweeter, whilst retaining that hunky studio monitor profile; we need a means of controlling the inevitable diffraction effects. The EV solution, also adopted by UREI, is to place foam around the tweeter to attenuate peripheral sound energy before it infringes on the baffle, and gets involved in any diffraction and reflection nasties.

Returning to the cabinet per se, the Sentry can be rackmounted, on its side, of course. To do this, you'll need the SRB-7 mounting kit (not included in the basic price); this assemblage of parts is also suitable for flush and angle mounting the Sentries on your studio walls. A top-end attenuator is provided on the front panel, scaled in 1dB steps, from +2 to -4dB, the 0dB setting being the nominally flat setting in the so called 'halfspace environment' which is American parlance for speakers radiating into 180° solid angle, or a hemisphere viz in a normal room, with the speakers hard against the wall. The purpose of the control is two-fold. First, it allows us to compensate a little for excess absorption in a heavily treated room. Second, personal top-end tastes can be accommodated, depending on what your ears are acclimatised to.

Amplifier connection is via bare metal screw terminals, recessed at the rear. Doubtless these are functional, but they're also very inconsiderate when you've got bunch-pin or banana plugs, or XLRs. In view of fibre-glass XLR sockets now being available at under 50 pence to OEM (quantity) users, there is no longer any excuse for failing to fit sensible connectors to £300 + speakers. After all, AKG LSM50s, at a fraction of the price incorporate a relatively expensive Neutrik XLR socket! Failing this, insulated terminals/binding posts would be an improvement, enabling 4mm plugs to be used alongside the 'bared wires' option.


In common with other EV products, the Sentry 100As come with a cogent, yet exhaustive 'no nonsense' information sheet, illustrated with graphs, diagrams and tables, plus a very readable survey of ideas involved in the design, and some guidance on achieving best results:

1) Use the Sentries in a ¼- or ½-space environment. This means mounting the speakers either flush with the wall, or in a corner, and also near to the floor or ceiling, if convenient.

2) Mount the Sentries so the distance between them is slightly greater than the distance between your listening position and the imaginary line between your listening position and the imaginary line between the speakers. For instance, if the distance between speakers, measured from the centre of each is 8 feet, then you should sit about 7 feet away, angling the speakers accordingly. The result, an equilateral triangle, means the on-axis sound reaches the ears at an overall 60 degree angle, which is generally agreed to be optimum for stereo.

Initial listening confirmed the exceptionally smooth, (if slightly 'hot') bass sound that EV are renowned for. The bass also goes deep, the mean bass SPL being only 3½ dB down at 50Hz (in ½-space), or 3dB down at 40Hz if you take a different average SPL. Cone bottoming (a flop or crack sound) when driving hard below 100Hz is a natural tradeoff with small drivers in vented enclosures, and the Sentries are well above average in this region, as proved by their ability to handle very low notes on DX7 synth, without any distress at high levels.

The dynamic characteristics of the Sentry 100As were equally interesting. It's perhaps not generally realised that many speaker parameters alter radically and unpredictably as the voice coil approaches its maximum excursion and temperature. These effects were manifest in the Sentry 100As to a more stylised degree than is usual, the variation in this instance being potentially very useful.

At low levels, the overall effect is smooth, even soft, and yet not unlike a Tannoy sound, whereas at higher levels, a harder sound, classically attributed to JBL monitors develops. So, in effect, you get three monitoring sounds in one package (the most natural, neutral sound occurring at about 10dB below full power) which is helpful for the purpose of making approximate crosschecks along the lines of "I wonder how this would sound on Tannoys?"

These dynamic effects revolve largely around slight changes in the low midrange (ie. understatement at higher levels), also a slight harshness arising in the low treble when driven hard. However, we found the latter effect could be largely circumvented by turning down the top-end control slightly. Despite this setback, the tweeter is one of the best rock 'n' roll dome designs yet, with perhaps only the Scanspeak (which coincidentally appeared in December HSR's Kord speaker review), the Dynaudio, and a handful of others as potential rivals. There was no evidence of thermal compression (viz. top end balance shift at high SPLs) and regardless of the 2kHz crossover point, neither was there any evidence of the 'honk' or 'rasp' which commonly signifies a tweeter being driven below its favourite band of frequencies.

Stereo imaging was stable, though at high SPLs, it was best to move backwards to, say, 10 or 12 feet, with 6 to 7 feet being optimum at lower levels in an average room. In fact, positioning for optimum stereo was critical, but once you find the right place, the sound stage 'clicks' into focus and becomes very spacial.

Anechoic sensitivity is average for a Thiele design of this size, at 91dB SPL @ 1w @ 1m, but power handling for musically significant periods (over 10 milliseconds) is sensibly quoted at 300 watts (more on this in a moment), so peak SPLs of 116dB at 1 metre are available. However, this relates to anechoic space, and in the ½-space of a real control room, you can add 6dB, giving 116dB SPL short-term at 6 feet.

Power Ratings

Speaker power ratings are a highly confusing area, and the topic deserves further exploration in our 'Control Room' series. For the moment, we'll stick to the observation that EV rate their drivers according to precisely defined, if rather obscure tests. The upshot of all this is that the data sheet tells us '30 watts long-term average PHC' (power handling capacity), and '300 watts short-term (10ms) PHC', both ratings being for 40Hz and above (which incidentally confirms the near impossibility of bottoming the cone at all but the lowest frequencies). Neither figure relates directly to ratings given by other manufacturers - they all differ! However, for practical purposes, the long term (30 watt) rating means the speaker will withstand clipping and overdrive at this power level.

Rated according to the methods of other manufacturers, the PHC would be quoted at between 50 and 150 watts, but what really matters is the short-term PHC, for this broadly indicates the optimum amplifier power, in this instance 300 watts, so, the amp can't readily run out of steam, and go into harmful clipping, before the speaker itself is pushed to its limits, and begins to distort and compress. Put simply, you will get best results from the Sentry 100As by using an amp rated at around 300 watts.

The nominal impedance of the Sentries is a rather unusual (if honest) 6 ohms, which means that on an amplifier rated into 8 ohms, you'll achieve about 30% more power ie. a 300 watts into 8 ohm amp will give around 400 watts into 6 ohms. All this is nothing to worry about, providing your amplifier can drive down to 4½ ohms. This is the minimum impedance, occurring at 45Hz and 150Hz.

Thiele enclosures generally involve a very sharp rolloff at the bottom end, and akin to active crossovers, the curve, or Thiele alignment is characterised by the same terminology: It's a B4, rolling off at -24dB/octave below 40Hz. Set any higher, this sort of response can sound unnatural, but as it stands, the bass sound passes unchallenged; EV are one of a handful of manufacturers who consistently achieve vital qualities in the bass that other people's speakers consistently lack.


To conclude, the Sentry 100As excel in their directional properties versus frequency, and the top and bottom-end performance at high levels. In other respects, individual parameters may be unspectacular, but they're certainly balanced, thus the sum of the ingredients is altogether of a higher order.

These are the speakers to use when you're prepared to sacrifice a little frequency response flatness in return for a slight, but audible increase in the maximum SPL. Rack-mounted, they're also ready in an instant to go out on the road - just slip on the protective grilles. To some users, the changes in the sound at various levels could prove a nuisance, but listen for yourself before buying - we think you'll find the dynamic effects valuable.

Last but not least the 25 watt tweeter is one of the few (nearly!) blowout-proof units around, so if you're having problems in this area, consider the Sentries, but do be sure to treat them to a suitably powerful amplifier.

Recommended retail price for the Sentry 100A is £226 (each) excluding VAT.

More details from Shuttlesound, (Contact Details).

Previous Article in this issue

Audio-Technica Microphones

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Aiwa WX110 Dual Cassette Deck

Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.


Home & Studio Recording - Jan 1984

Donated & scanned by: Mike Gorman

Gear in this article:

Monitors/Speakers > Electro-Voice > Sentry 100A

Gear Tags:

Monitor Speakers

Review by Ben Duncan

Previous article in this issue:

> Audio-Technica Microphones

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