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Fostex SPA-11 Powered Monitors

The SPA 11 is primarily aimed at sound reinforcement, and packs a complete - if minimalist - 100 watt PA into a fortified plastic cabinet, a little bigger than a rival speaker, let's call it the '101'. Because some 40% of the cost of a power amp is accounted for by the packaging and associated hardware, a pair of SPA-11s naturally costs less than any similar separate speakers, plus stereo 100 watt power amp, and this fact alone makes it attractive for those on a tight budget. And like any activated speaker, simplicity of application is its keynote: (1) apply 240 volt juice; (2) plug in line or mic feed, and away you go.


A tiny 4" x 2" recessed back panel crams in a volume control, a speech/music filter switch (which gently EQs the source), line and mic inputs, and a line output, marked 'Aux Out'. All the latter are standard ¼ 'A' gauge jack sockets. The mains lead is a healthy 2 metres in length, and captive; though for mobility, I'd have preferred a Bulgin mains connector - it makes unhooking easier, and would fit in the space available. There's also a panel mounted mains fuse - you'll need a screwdriver to undo it - an on/off rocker, and a red 'juice light'. The rocker works back to front in the US style - pushing the lower half down turns the amp off. The remainder of the back panel is mostly taken over with a recessed heatsink.

Out front reside a pair of 3½" drivers; between them, a port. This is slightly flared, but don't mistake it for an HF horn - it's not. The port's mouth is covered with gauze to prevent nasty metallic objects getting inside, and shorting out the electronics. Meanwhile, the drive units are sheltered behind steel gauze, which is box shaped for rigidity. Result: you can walk over an SPA-11, or drop it face down without denting or other damage.

The plastic enclosure is black - hardly original, but at least the material is butch enough. I wouldn't rate it for dropping out of a second floor window onto the pavement (principally because of the power amp's added weight), but for all the routine Rock 'n' Roll abuse, it should retain its appearance and integrity better than speakers encased in chipboard, veneer or no veneer.

Compact Electronics

A Thiele-loaded speaker utilises the cabinet volume, using it to extend the low frequency performance. So any significant volume of air displaced from inside by power amp parts will throw away valuable bass-end capabilities, either the efficiency, or the extension. Thus space is at a premium, and accordingly, Fostex have pared down the electronics to the bare essentials.

Ordinary laminated mains transformers are built as economically as possible, to satisfy bare safety requirements: they saturate badly, and are generally bigger and run hotter than they ideally should. In short, they waste energy, turning some of it into heat or hum fields, rather than amplifier watts. But Fostex have taken a look at C-core technology. Laminated transformers with C-cores cost more than run-of-the-mill types, but outperform them when it comes to efficiency, size, stray fields and civilised behaviour all round. Accordingly, the SPA-11's mains transformer is smaller than you'd expect, being encircled by a telltale copper band which cuts flux losses, and boosts efficiency. And despite the mic input's sensitivity circuitry being placed only a few inches from the transformer, there's no audible hum pickup, though to be fair, there's a mumetal screen placed between the transformer and the input's circuit board.

The mic input's sensitivity is 2mV, whereas full output is 30 volts, so the overall voltage gain is 30V/2mV = 15,000. This level of amplification warrants two circuit boards: one houses the front end, which brings up line and mic levels to around +8dBU (at full drive). The second card houses the power amp, keeping the heavy output stage currents well away from the sensitive low input circuitry, for stability and also by extrapolation, low distortion. Incredibly for a 100 watt bi-polar amp, the output card is actually smaller than the front-end PCB, being a mere 3" x 2" in size, yet isn't unduly crowded.

A third PCB houses the power supply parts. Here, you'll be relieved to know that the reservoir capacitance is up to high standards, at 10,000μF per side. This augers well for the amp's bass end, but also begs the question "Why is everyone else using such huge capacitors when much smaller ones are available?", for these particular Nipponese capacitors occupy only around 40% of the volume of competing US/European components.

From the angle of maintenance, both sides of the cards are clearly visible as soon as the front baffle is shifted, except the power supply card, but it's easy to unscrew in any event. All screws were tightly secured - an important point with so much vibration going on - but I feel more lock washers wouldn't go amiss, particularly on the mains transformer, which is capable of causing considerable damage if it splits asunder from its moorings.


The SPA-11 is quoted at '200 watts music' in the brochure. 'Music power' ratings have long since been all but discredited (because the figures are so easily cooked to look good), and it's sad that Fostex should have adopted such a rating. However, Bandive, who import the SPA-11, stress that the power rating judged by conventional standards is more like 100 watts. Our tests confirmed this: the SPA-11 drives 120 watts at 1 kHz before it clips, on a sine wave tone. Sensitivity on the line input is around 80mV, enough for the most weedy line sources, whereas the mic input's sensitivity is 2mV. Translated into everyday usage, a standard low impedance mic produced enough signal to drive the SPA-11 at full power when spoken into from a distance of 4", say. The auxiliary output level follows the line input: for 80mV in, 80mV comes out.

Table 1
Frequency Music* Speech*
(Hz) input = line input = mic
30 +1 -6
100 +5 -1½
300 +2 0
1k 0dB 0dB
2k 0 0
4k +1 0
8k +2½ 0
16k +2½ -1

Fostex SPA-11, electrical frequency response
*Refers to the EQ switch position

The electrical response is tweaked by EQ circuitry on the front-end board - looking at Table 1, there's a moderate bass and treble boost when the filter switch is set to 'music'. With the switch in the 'speech' position, the output balance is inverted, with the low frequencies in particular being rolled off, for the sake of cutting through in the midband. A British Telecom response, if you like.

The SPA-11 isn't just a box of electronics though; its performance is tied in with the speaker enclosure and the two drivers. The nominal sensitivity is 92dB SPL @ 1 watt @ 1m, but that's taken across the midband alone. With the switch in the music position for the flattest overall response, sensitivity drops to around 88dB. This is on the low side for a PA speaker, comparable with a refined domestic hi-fi speaker, in fact. Incidentally, although the SPA-11 is active in that it has an integral amp, it's not bi-amped - both the speakers are driven full range in parallel.


Each driver is angled outwards. This imbues the sound from a single SPA-11 with a surprising sense of depth. But before commenting on the sound at length, the SPA-11 needs to be put into perspective. It's not designed as a studio monitor, so one salient question might be "If I need one for some other purpose, like gigging say - could it perhaps come in handy for monitoring?"

A consequence of the inbuilt EQ is an acceptably flat response at moderate levels, but by the same token, the bass EQ inevitably leads to premature overload, thus the maximum SPL is strictly a little less than one would expect whenever clean music reproduction is sought. Of course, for PA use, and with the EQ button set to the 'speech' position, the bass is rolled off, making harder driving feasible before the amp distorts excessively. Despite this, the SPA-11 is incredibly loud, bearing in mind its size and its bass response.

Overall, the balance is a trifle on the bright side, and a lot of bass energy lurks around the enclosure. This suggests the vent fails to project the low frequencies, a regular shortcoming on mini, vented enclosures. As usual, placing the SPA-11 against a wall, on the floor, or better still in a corner, will go a long way to improving the bass projection. When driven hard, the driver's mechanical excursion is gently limited, while the midband sounds maintain some of their clarity, instead of being grossly cut up by intermodulation effects, as is more often the case.

Used at more moderate levels, vocal, horn and percussion sounds were rendered with above average clarity and detail. This signature is akin to a horn-loaded driver, and apparently correlates with the large magnets attached to the drive units. Typically, the result of using over-powerful magnets is to augment the leading edge of transients. This puts the 'bits that went missing' back in the recording chain, at least for incisive sounds.

Enough speculation: the SPA-11 exhibits potent, if vague, imaging, the natural consequence of having two full range drivers per side. This lends a fake spaciousness to the sound which some will find very appealing. In practice, it could cause you to over-estimate the stereo's depth and breadth in relation to a more conventional stereo set-up. Even listening to one SPA-11 alone (in mono, of course) generates a soundfield about 4 feet wide from a distance of 6 feet. This is with the cabinet resting horizontally on a stand. But what if we turn the cabinet on its end, so the drivers are in vertical alignment? Our ears are less sensitive to displacements in the vertical plane, so the anticipated 4 foot high image fails to materialise. Instead, image depth is enhanced, a useful result deriving from the drivers being angled. This positioning is recommended for stereo listening, but please experiment.


Like the '101' of a rival manufacturer, this Fostex speaker comes with a wide range of intelligent mounting accessories, such as clamps, brackets and stands. Cabinets can be stacked; they mount flush with one another, and this will usefully augment the low-end response. Interconnection is simply a case of plugging the line output on No. 1 into No. 2's line input, and so on. This is called 'daisy chaining'. Back to the fixings: threaded inserts are available on the base, top and side panels.

Unlike various competing products, Fostex's SPA-11 costs more, but packs in a high power amp sensitive enough to be driven by virtually any likely source. And to judge their value, contrast their price with that of a decent 100 watt stereo amp and a pair of (...) 101 s, plus the speaker leads. Also, if there's no EQ on your mixing console's monitoring section, how much will it cost to tailor a competing pair of speakers to the same judicious cut?

In the past, powered speakers from the Orient have received the thumbs down from serious users, but Fostex's SPA-11 is set to challenge this. In particular, don't be way-laid by the 'PA' label. You can certainly thrash the SPA-11s with impunity, but equally, at more restrained levels, the clarity and balance is outstanding.

Returning to the question we posed earlier, these Fostex speakers reconcile the seemingly disparate requirements of monitoring and sound-reinforcement to a degree hitherto unprecedented. So what impossible feat (gasp) will Fostex pull out of the bag in 1985?

The SPA-11 retail for £180 inc VAT (each).

Available from UK importers: Bandive Ltd. (Contact Details).

Previous Article in this issue

DeltaLab Effectron Digital Delays

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


Home & Studio Recording - Dec 1984

Donated & scanned by: Mike Gorman

Gear in this article:

Monitors/Speakers > Fostex > SPA-11 Powered Monitors

Gear Tags:

PA Speaker

Review by Ben Duncan

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