A&R SA200 Power Amp
There's no such animal as a studio power amplifier per se. Any amplifier we choose to hang on to the end of the monitoring chain is made to cater either for the disparate needs of the broader professional market (from the relatively undemanding requirements of broadcast monitoring through to the extreme demands on size, power output and ruggedness made by touring PA users), or, as is the case this month, fashioned for use in the home, as part of a Hi-Fi system.
Either path has its swings and roundabouts, of course. For domestic power amplifiers, one generally expects to encounter defective interconnection standards, and poor reliability when equipment is stressed or abused. At the same time, UK Hi-Fi manufacturers are - for the most part - very conscious of their reputation for making amplifiers that sound as good as they come, and tend to be acutely aware of the design subtleties that govern the sound of an amplifier when hooked up to music and a real - and very demanding - loudspeaker load.
The A&R SA200 isn't the first Hi-Fi amp we've reviewed; the Quad 405II in HSR Sept 83 is a near counterpoint. But here, for the first time, we're reviewing a power amplifier alongside matching A&R speakers.
The SA200 features 'traditional' bi-polar output transistors. Output is at first sight a modest 100 watts into 8 ohms or 160 watts into 4 ohms, but contrasted with most domestic amplifiers, speaker impedances can dip to 2 ohms with a corresponding power rise to 300W, before any protection circuitry leaps in and snarls up the sound. The relevance of this sort of performance is that the complex passive crossovers beloved by certain Hi-Fi speaker manufacturers present unexpectedly low impedances to the amplifier at spot frequencies. In particular, it's not unknown for the impedance on certain monitors to dip four-fold on music signals (eg. an 8 ohm unit becomes 2 ohms), and whenever a vocal or instrumental tone hits the dip-out frequency, the sound may crack up momentarily - that's unless the amp can cope with a 2 ohm load.
Put in perspective, few domestic bi-polar amps can achieve this, and more than a few PA amps with bi-polar output devices will freak-out or distort badly under these conditions. The SA200 can therefore be pigeonholed alongside MOSFET amps - 'Impedance hassles not anticipated'. Having said that, there's no carte blanche to go driving into silly impedances: the A&R's 2 ohm rating is a short-term reserve capability, and should be treated forgetfully, like a reserve petrol tank. To emphasise the precise nature of the SA200's output capabilities, it has cool 17 amp (!) peak short-term current-sourcing capabilities, actually more than many PA amps, but naturally lacks the gargantuan heatsinking necessary to sustain these arc-welder sized currents for very long.
Front panel hardware is in the fashion of minimalist Hi-Fi. The panel itself is a black, Nextel-finished aluminium extrusion, and the only visible control is a pushbutton power switch. But lurking to the left of this are two integrated LEDs, one red and one green, set dead-flush with the panel. The red and green are mutually exclusive: if an 'error' (ie. any difference) is detected between the signal going into and coming out of the amplifier (this will happen if the output clips or is excessively distorted), the red LED lights for a minimum period of about one second (even if the overload is much shorter) to say 'ouch - you are overdriving me'. The green LED, which signifies 'all present and correct' can only come on when (a) the red LED is out, and (b) the amp is switched on. It follows that at all times, the amp's condition is confirmed by one of the two LEDs being lit. This condition is considerably more intelligent than simple 'clip' indicators; in particular, it correctly tracks the reduction in overload level, should the mains voltage be below par.
The rear panel is dominated by a pair of cast-alloy heatsinks no less. Mains enters via an IEC 'Euroconnector' socket, while speaker outlets are 4mm sockets-cum-binding posts. These aren't the easiest ones to grip tightly, but they are spaced a healthy distance from surrounding obstructions, and Mark (whose quality control mark appeared inside our review amplifier!) had clearly taken the trouble to stagger the wire clamping holes for an easy fit.
At this stage, the one and only 'defect' was noted: one of the terminals wouldn't unscrew fully. This terminal happened to be one of the left channel outputs, and as such, it's well exposed, being at one extreme side of the case. It's therefore likely that it was knocked during our tests. The moral here is to remember that however much a domestic amp boasts studio quality performance, you shouldn't expect to chuck it about with impunity: money that's spent on making PA amplifiers abuse-proof is invested differently in the SA200.
Input signals can enter either through a 5 pin DIN (ugh!), or some ordinary phono sockets. Actually, aside from the complications of actually wiring up a DIN plug, the DIN's contact qualities are superior to the average, well-used phono interface. Besides, A&R are mindful of the horrendous confusion that's rife when it comes to guessing which pins on the DIN to wire up - they've thoughtfully printed and fully identified the exact pinouts on the back panel. In case you were curious, the stereo inputs are on pins 3 and 5; but please be sure to use individually screened cables for each input - otherwise you risk impairing the amp's excellent stereo imaging qualities. Admittedly, fitting two cables into a DIN plug isn't easy: the paralleled phono input sockets are located either side of the DIN, just in case you're tempted.
More good news: the input connectors are mounted on a steel plate subsidiary to the cast backplate. Thus it's readily removed, and replaced with one punched for XLR sockets. A&R plan to make this option available shortly. Input impedance is a comfortable and sensible 20k. All pro-audio gear will drive this happily, and all but the most abysmal Japanese electronics should cope - if only just! Of course, it's as easy as changing a resistor to zap up the input impedance of any amplifier, to 100k say. But the sad, practical results of this idealism are related in detail in the A&R Arcam One speaker review (see elsewhere).
A sign of thoughtful design, the input sockets are isolated from the chassis and the SA200 has independent and permanent groundlift resistors on each channel PCB. These are 100 ohms each and appear in parallel, hence there's 50 ohms all told between signal ground and mains earth, when both modules are in place. This is fine for Hi-Fi, but could be too low in a more complex system. Should a hum loop become evident when the SA200 is hooked up, the remedy is simple - R1 in the middle of the PCB should be clipped out. You can then solder a 1k groundlift resistor either back into the PCB, or between the DIN socket's ground terminal (= chassis ground) and the centre pin (2). If possible, the 100nF RF bypass capacitor (fitted across each of the original groundlift resistors) should be resoldered in place of R1 on each card. Or failing this, link one of the capacitors in parallel with the new groundlift resistor at the input terminations.
I outlined the history of bi-polar Hi-Fi power amp circuitry in an earlier review. In common with many other respectable bi-polar designs, there are necessarily lots of parts to the A&R. These sub-circuits are intended to squeeze as much gain out of the amplifying transistors as possible, but with low distortion. The excess 'loop' gain (that's the gain over the 30dB we actually need) is then used to reduce distortion further.
Unlike oriental Hi-Fi makers, A&R have wisely stuck to those circuit refinements (for which read extra cost and complexity) which actually pay audible dividends - as opposed to spectacular and heavily advertised 'revolutionary' techniques which (a) don't, and (b) aren't even original. Thus, A&R have surreptitiously bypassed electrolytic capacitors with parallel plastic film capacitors. This reduces high frequency dissipation and hence distortion on cymbal sounds and the like. Ditto, the supply rails are generously decoupled, for the, same overall result, and critical wiring is kept to a minimum: there are no looms.
In common with QSC's 1400 amplifier say (reviewed HSR Sept 84), A&R have elegantly and successfully integrated the lightweight power supply parts with the main amplifier card, leaving only the toroidal transformer on the chassis. This is, incidentally, the sole component shared by the two channels, the result being excellent sound separation between channels. The capacity of the effectively independent power supplies is impressive by any standards, but especially so for such a modest power output. To wit, each rail on each channel boasts a pair of paralleled 10,000/μF reservoir capacitors.
On the basis that a generous reservoir capacity broadly influences the bass-end qualities for the better, and pro-rata increases in capacity are necessary to retain performance at higher powers or lower speaker impedances, this puts certain PA amplifiers to shame. Relative to its power output, the SA200's energy storage is 7 times as great. Remember, the SA200's power supply is capable of sourcing 17 amps per channel into hungry loudspeakers for short periods! Consistent with this sort of machismo, the SA200 doesn't feature the pathetic plastic power devices beloved by Nipponese Hi-Fi manufacturers, but instead resorts to the MJ15003. In case you hadn't guessed, this is a rugged metal-can power transistor, a fairly expensive device rarely seen outside of upmarket PA amplifiers. Not content with this, A&R have paralleled two per side, per amplifier, to boost the safe operating area. In plain language, this means you are unlikely to blow up the SA200 in studio use, despite its relative lack of power-limiting protection.
Other positive features include the simple, uncluttered layout - every part is visible, being laid out horizontally. There are only eight wires located about the chassis, and the PCB mounted DC supply and speaker fuseholders are also clearly identified, and accessible, once the lid is removed. And even this part is easy - only three screws need be undone. PCB replacement is also straightforward - like Quad, or Crimson Electrik, or 3rd Generation (etc), the modules are bolted to the heatsink via an L-bracket, most of the connections being made with snap-in Molex style plugs. The one exception is the output connections - A&R solder these, both to the PCB card, and at the output terminals, presumably to score audiophile credibility. You'll also need a hexagonal socket key to undo the modules, but a suitable tool is supplied with the Arcam One speaker stands.
I was impressed with the rigidity of the SA200's assembly - for a domestic amplifier, the enclosure is uncommonly sturdy, and simple too. The PCBs also score top marks for being absolutely rigid. This is achieved with four screws and a pair of tapped spacers, costing all of 14 pence (gasp!). Certain amplifier manufacturers (no names mentioned!) could learn a lesson or two about getting their mechanics together and mounting capacitors so they don't flop around, by investing in an SA200.
Less favourably, the longevity of the power switch is questionable, not in the home so much but in a larger studio or decently wired-up workshop, where a low impedance mains supply can be expected. This will greatly increase the current surge at switch on, which could exceed 100 amps for a fraction of a second, judging by the 80,000/μF worth of reservoir capacitors waiting to be charged up. But if there is a problem, it would probably only become evident in the long term like replacing the mains switch three times in eight years, contrast once in the same period. If so, there's a simple remedy - link across the mains switch, and fire up from a switched 13 amp socket, if necessary.
The SA200 is comprehensively fused - too comprehensively by some people's reckoning. There's mains fusing, of course - we all agree that's necessary on pain of burning down the house. But some will feel that speaker fusing combined with fused supply rails (making 7 fuses in the amp overall) is too much.
As supplied for domestic use, the SA200 comes fitted with a 3.15A speaker fuse, which is inside the amp, and therefore an aggravation if it splits. This is unlikely unless the amplifier is thrashed into low impedance speakers (eg. a 4 ohm model that dips down to 1.8 ohms, say), though a 5 amp Quick Blow fuse in the output would be much more in keeping with the 'no hassle' requirements of the professional user who is unlikely to overdrive anything for long periods. But in this circumstance, A&R's generous guarantee terms are naturally invalidated, although they recognise this as a professional requirement - and there's no reason to believe that a 5A fuse necessarily imperils the SA200. Incidentally, if you're wondering how such a small fuse can possibly pass 17 amp peak currents without rupturing, perhaps no one told you that glass-bodied fuses typically carry nearly twice their rated current for an indefinite period. Also, 17 amps is a peak 'music' rating - and a 3.15A fuse can just manage 17 amps for very brief periods, just as it can pass 6 amps for an indefinitely long period. It's therefore a good idea to establish whether you really need a bigger output fuse in practice; if the originally fitted fuse lives for a few loud sessions, all should be well.
Next on the agenda are the supply line fuses. If the negative rail fuse blows alone, all is well, ditto if both the rail fuses blow simultaneously. But woe betide the poor speaker, should the positive (+) rail blow, because the output then 'swings down' to the negative (-) supply rail, that's a whopping -47 volts! This is an example of the infamous DC fault, a frequent death blow for speaker drive units. Putting this into perspective, most bi-polar amps suffer this foible - whereas most MOSFET amps don't. Also, whilst the supply rail fuses should ideally blow only if the amplifier develops a terminal fault, fuses do age, and can decease spontaneously, usually when the amp is switched on by the bleary-eyed! It follows that you should religiously replace aged supply rail fuses in bi-polar amplifiers once every year, say. It's also wise to crimp the fuseholder clips together, to tighten up the contact.
Another precaution is to uprate the fuses - again from 3.15A (as supplied) up to 4 or 5 amps. Overall, and provided you're confident that the amp won't be misused (eg. driven hard into 2 ohm loads), I'd uprate both output and supply rail fuses to 5A. But where abuse can't be fended off, I'd still uprate the supply rail fusing (to 5A), but cautiously leave the speaker fuse at 3.15A. Another idea is to do away with the internal fuse, and wire the output in line with an ancillary panel fuseholder, readily accessible from the outside world. But A&R's fusing is in line with the amplifier's (internal) feedback loop. This prevents its small but significant resistance from messing up the bass sound, but precludes panel-mounted fuseholders if you want to retain the same performance. If in doubt about optimal fusing, A&R's Chris Evans can offer judicious advice ((Contact Details)).
Like the six others, the (single) mains fuse is located inside, and is amply shrouded with a neat insulating boot. Full marks, too, for the remaining mains terminals, which are all generously protected from unsuspecting fingers.
Table 1 gives the usual data on power output versus input sensitivity for different load impedances. These tests are made with a continuous sinewave (tone), and to avoid the fitted fuses blowing in the time taken to read and record the test instrumentation, we had to uprate all of them to 5 amps. However, driving a 3kHz tone into a 2.5 ohm load for 60 seconds is well in excess of the demands placed on the SA200 in normal music monitoring! Nevertheless, we've included the 2.5 ohm output data to remind you of this.
The error LED was found to track the onset of clipping accurately regardless of supply voltage and speaker impedance albeit a regular ½dB after the wicked event. This misdemeanour won't be significant in practice though; any speaker can cope with the occasional clipped signal peak. The main thing is to recognise that if the error LED stays lit up continuously, or nearly all the time, the monitors will be in grave danger of expiring.
The SA200 comes with an 11 page manual. Circuit diagrams with test voltages are also available to professional users, as are a bridging module and a rack-mounting kit. The optional bridging card fits inside, and can be retro-fitted by anyone with basic confidence in their electronics skills. Detailed instructions are provided or you can ask an A&R dealer to fit it. When bridged, power output rises nearly four times, to 400 watts into 8 ohms. This is particularly relevant to tri-amped monitoring, for where a stereo amp drives the mid and top-end, a (mono) bridged amp is tidier and can offer useful extra headroom for 8 ohm bass drivers.
Serendipity led me to first encounter A&R some months before undertaking this review: a source 'close to the grapevine' recently installed an elaborate bi-amped monitoring system in Imagination Studios, with a rack full of SA200s driving a pair of Rogers custom monitors.
It follows that A&R are excited, and keen to talk to professional users; at HSR, we've already outlined to A&R some additional facilities and revised interconnection standards that wouldn't go amiss. The result may well be a 'pro' version of the SA200. But should this fail to emerge, the present SA200 is nevertheless firmly in the upper-class of domestic audio gear, the sort that's worthy of the expense of a customised professional upgrade - even if you go no further than replacing that horrid DIN socket with a pair of flying line Cannons.
The SA200 is available from A&R Cambridge, (Contact Details), or from good Hi-Fi dealers. A dealer list is available from A&R. Price is £379 inc VAT.
Review by Ben Duncan
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