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QSC Audio Model 1400 Power Amplifier



QSC Audio hail from Costa Mesa in California, and through their UK distributor, Music Labs, they've recently unleashed a series of innovative state-of-the-art power amps. The 1400 model reviewed here gives us 200 watts of power into 8 ohms, or 300 into 4 ohms in stereo; or, 600 watts and 400W into 16 and 8 ohms respectively in mono, when power-bridged.

Architecture



All 5 QSC models in the range utilise a common circuit, which differs from most amps in being unusually simple and straightforward. Notable features include a basic, yet effective differential input, ie. balanced. Like other US manufacturers, QSC have aimed to cut down component count with the help of IC op-amps, instead of the convolution of many discrete transistors and ancillary parts still found in the majority of UK and Japanese designs.

One reason for this is perhaps the bad name scored by IC op-amps in this role: a dreadful RCA application circuit depopularised the idea, although certain manufacturers copied it (no names mentioned!) without perhaps being aware of how poor the performance was. But QSC have sprung back with an op-amp assisted circuit which really works, and without using the IC as an excuse to pile on extra circuit complexity - the QSC yields the simplicity we'd expect.

A unique feature is the output coupling: the supply rail's reservoir capacitors are placed in series with the output, with the output signal appearing on the supply rails. This trick, in effect, brings in an output coupling capacitor, without any extra parts, because the output caps double as the power supply reservoir.

But what's the advantage of this? In a word, built-in DC protection. Like it or not, most power amplifiers have direct-coupled outputs, and will quickly burn out drive units if a fault develops. But the QSC's outputs are cleverly blocked to the passage of DC - rather like placing a DC protection capacitor in series with a top-end drive unit.

The output stage displays Bi-polar transistors, but the circuit topology is complementary, (that's using symmetrical transistors), and moreover, the devices themselves are fast Japanese types, all of which tends to lessen intrinsic HF (high frequency) distortion.

Facilities



The gain controls are sited on the rear panel. This is likely to prove a real pain if you wish to make adjustments at all frequently. On the plus side, these controls are calibrated in dB. But as they're not stepped attenuators, the scales accelerate, as on the HH V200 amp, for example. However, the pots used here sport the more accurate linear law, with faking resistors being used to even out the change in level vs. rotation.

But how even? In this instance, the knob swings from +4dBu (5 o'clock) at maximum, via -2dBu at the 12 noon setting, and to -24dB at 9 o'clock. Thereafter, the level drops off rapidly. Although not linear vis-a-vis decibels, this represents (subjectively) a good audio taper, and is in fact superior to that offered by most 'log' pots. As to the pots themselves, although ordinary (and cheap) carbon types, they're very smooth, and are above average quality. The pots are PCB mounted, with the knobs partially recessed behind the panel. One advantage here is that the PCB will yield (unlike the panel) if the pot's knob is stuck. This augers well for their survival under road conditions.

Next to the gain controls, there are two octal sockets, one for each channel. Normally, these are linked across by DIL programming switches (accessible from outside with a screwdriver), but if you decide to adopt one of QSC's add-on modules (eg. for power limiting), this is where they plug in. Below the octals come the input terminations, catering for both XLRs and stereo A gauge jacks (large tip). Alternatively, if an unbalanced connection proves acceptable, you can use an ordinary quarter-inch mono jack. Both sockets are special PCB-mounted types, and on the road, this could make replacement difficult if you're unable to obtain spares. The same applies to the pot, incidentally, but the problem is not beyond a resourceful maintenance engineer.

As is usual on US equipment, the jacks have metal bodies, and are mounted directly on the chassis. This would prejudice the more sophisticated grounding practice in UK studios and rigs (all about keeping metalwork divorced from signal grounds) but for the balanced connection. In other words, the QSC's jacks won't cause the grounding headaches that plague other marriages between UK and Japanese/US equipment, provided you always use the jacks (or XLRs) in the balanced mode.

To do this, you certainly don't need balanced outputs. Simply wire your (unbalanced) mixing desk outputs to the ring and tip of a stereo jack plug, ignoring the sleeve (outer) connection unless you're using paired cable. If so, you can connect the drain/screen wire to the sleeve, but you must insulate or isolate this wire at the desk, otherwise if it connects to the desk's zero volt line, you'll defeat one object of the balanced input, namely the abolition of hum loops.

The same rubric applies to the paralleled XLR inputs, with the desk's ground and hot going to pins 3 and 2 respectively and the drain/screen wire to pin 1. But wait a second - aren't US manufacturers famous for wiring their XLRs back to front? Well yes, but QSC adopt the European convention (pin 2 = HOT) and this gives the product credibility: for here, one US manufacturer has awoken to what's going on in the world outside!

Next stop is a barrier strip, carrying parallel input terminals, a horrible piece of 1940's hardware beloved by installers doing it on the cheap (crimps are a fraction of the cost of jacks, let alone XLRs). The manufacturer has obviously incorporated it for the sake of completeness, and its one saving grace is that you might use it to meter the inputs without unplugging anything - handy for troubleshooting.

A recessed but easily operated slide switch selects stereo or mono bridge mode. Less favourably, the centre of the rear panel is heavily vented, and the slots are uncommonly broad, and large enough to poke a large object up, though not a finger. However, I'm thinking principally of metallic objects. There's no hazard in the professional sphere, but this amp would be ill-advised in a domestic environment, unless racked away from little fingers, knitting needles and necklaces.

Oddly, the output terminals are about to get stick because they lean in the opposite direction, that of over-protection. To wit, they're binding posts, but they don't accept 4mm plugs, and you can't easily and reliably wrap bare wires around them, unless said wire is unhealthily thin.

To terminate properly, you'll either have to fit a round pin or an insulated spade connector. These are best crimped on. You can probably obtain said connectors from a garage, cost around 10p each. Failing this, you could fit decent binding posts with integral 4mm sockets - a ten minute job with a spanner. One point not made crystal clear on the rear panel is the fact (obvious to many readers) that for bridge-mode operation, the black (or common) terminals are relinquished. The speakers are connected solely across the two hot (red) ones.

Last on the back panel, the mains cable is clamped with a cheap plastic device, and the absence of adequate bushing spells an immediate hazard if the amp is used and/or stored. Many unracked amps are used like this in practice. To exemplify the hazard, even during the process of the review examination, with the amp lying on a soft carpet, the weight of the enclosure on the mains cable was sufficient to visibly crease and weaken the PVC insulation. Pretty soon, this could lead to lethal bare wires. QSC should therefore fit a cable with better quality PVC, plus a support bar adjacent to the cable to obviate the stress outright.

The QSC front panel is very smart, if rather too evidently Americanesque - students of architectural styling please note! It will also stay smart, because the paintwork is tough powdercoat, and more important, the legend is built into a scratchproof plastic. It's also easy to clean, being smooth all over. The LEDs in particular, are placed behind the legend strip, rather like the metering on some mixing consoles. The mains switch sports a 'digitally encoded' legend that's unambiguous whether you speak Urdu or Spanish, viz. 1 (= ON) and 0 (OFF). It's also repeated in Queen's English for people residing in Tunbridge Wells.

Adjacent, there's a resettable circuit-breaker, which replaces the usual mains fuse, and being no hassle to reset, can act to give routine protection. Breakers of this type are ideally limited to tripping-out small excess currents (caused by severe overdriving), but NOT the massive fault currents that can flow in response to a dead short across the mains inside. For this reason, you're strongly advised to fit a normal 7 or 10amp HRC fuse to the mains plug to take care of this contingency.

Illuminations are threefold: in the centre, a green LED confirms that juice is present, while on either side, red LEDs light if either channel clips.

Interior Design



Casting an eye inside, the QSC scores high points for ease of access for maintenance. 98% of the electronics, including the output devices, are mounted on a single PCB. It's very rigid, and all the parts are identified with a printed legend. The design does away with archaic wiring looms, although there may be compromises in other directions. For example, a catastrophic failure is more likely to blow out PCB tracks.

Most of the connections to the motherboard carry mains, coupled by well insulated lucars. The mains wiring is mostly double insulated, and sundry other connections are insulated as far as possible. But a rubber 'boot' over the mains switch and breaker wouldn't go amiss. The remaining PCB connections go to the outputs; coupling is with a latching 'Molex' plug, a cheap yet reliable connector, provided it's carefully assembled.

The reservoir/output capacitors are PCB mounted, so there are no terminal screws to come loose. The DC supply rails to each channel are fused individually, and access involves removing the cover as usual (moan), but QSC have selected holders with unusually tight spring clips. They're also easy to get at - and clearly labelled.

As to the input stage componentry, two parts are worthy of comment. First, the op-amps are mounted in sockets, so input stage failures can be quickly sorted out. On the minus side, the sockets in the review sample weren't exactly the best, and don't grip the ICs tightly enough. So we recommend that QSC fit something better, perhaps a turned-pin socket, otherwise some of the care and detail that's gone into this design will be thrown away.

Second, turn-on/off muting for both channels is accomplished with a tiny, 2 pole PCB relay. This shorts the voltage amplifier stage to ground at switch-on, so preventing thumps and bangs (these usually arise from the preceding equipment, namely crossovers and consoles). This is good design, in that it's fail safe, because the signal doesn't have to pass through the relay contacts. And because it's not handling large currents, so there's no arcing to eat away the contacts, longevity in the relay is promoted.

The PCB relays in the QSC are in any event smaller and cost less than 20 amp monsters strapped in series with the outputs. But here we should note that QSC are able to get away with this elegant arrangement, simply because the output terminals are AC coupled; in other amplifiers, the relay has to be placed at the output, for positive protection against DC faults.

The QSC comes fitted with an internal fan. The thermal design isn't outstanding - for instance, the fan's main airstream misses out each end of the heatsink. But the ducting is excellent, if the size of the holes on the back panel is anything to go by, and it's fair to say that this amp won't suffer from dust accumulations. The fan runs at two speeds: normally slow and relatively quiet, it speeds up when tripped by a thermal sensor, which comes in when the amp is driven hard and begins to heat up.

Check Out



Table 1 gives the power versus load impedance and associated sensitivity data. Note that the sensitivity is 6dB lower than indicated if you operate unbalanced, going up pins 1 and 2 or on the sleeve and tip (viz. with a mono jack), but if you use pins 1 and 3, or the sleeve and ring, it'll be the same as in the balanced mode ie. as given in Table 1. The overload LEDs don't have a definitive threshold - they become progressively brighter over a 2dB range. Despite this, they're always clearly visible within ½dB of clipping unless the ambient lighting is very intense. Moral: keep spotlights off this amp if you want to see the LEDs in action. In suitably dim lighting, the QSC's overload LEDs are accurate regardless of the speaker loading, and may be read with confidence.

Protection is also positive: we grossly overloaded the amplifier by driving it in bridgemode with sine waves, into a 4 ohm load. After the heatsink temperature had shot up, the breaker tripped out, killing the mains juice. Sadly, this stops the fan - at least until you reset the breaker. If QSC rewired the fan to bypass the circuit-breaker, the big cool-down would be usefully hastened. This is particularly important for PA work. A big plus point however is that the breaker replaces complex electronic protection, which is apt to damage the sound surreptitiously when the amp drives speakers with awkward impedance curves. In this regard, the QSC can perform as well as many MOSFET amps.

Returning to the unique AC output coupling, one disadvantage of this topology is an above-average hum level, around -80dB relative to 100 watts into 8 ohms, or -48dBu. This will only be relevant though if you're using unusually sensitive speakers in a quiet environment: you'll hear a dull buzz if you put your ear hard against the drive unit, otherwise not. Having said that, for very critical listening, don't forget the subliminal effects of hum; even very low level buzzes can make people irritable or fatigued after a few hours monitoring.

Reliability



To a skilled eye, the QSC 1400 heeds all the rules-of-thumb relating to reliability in a rock'n'roll environment. But there's one area that begs answers: it's worth remembering that the amp's outputs come off the 70 volt supply rails. In other words, the usual direct-coupled output, which is normally at zero volts (but sometimes 'goes DC') is replaced here by an output which is held at zero volts solely by virtue of the series output capacitors. If one of these should go short, bang..! In mitigation, QSC have chosen high-spec Nipponese capacitors designed specifically to cope with the rigours of high temperatures, and massive peak currents. But to play safe, it might be a good idea to have the output capacitors checked out for leakage or other degradation after 12 or 24 months of use.

Having said that, producer Doug Hall tells me that a major UK act he works with has just taken delivery of 120kW of QSCs for PA across 7 continents on a 16 month, 220 date tour. They chose the QSC on the basis of grapevine reputation and of listening tests, while another colleague, John Newsham reported just half an amp 'down' after touring with 100kW of QSCs with US band Styx for six months. To let you into a secret, PA amp fatalities are often much, much higher - say 5% - so on this basis, the QSCs live up to expectations. To sum up, this amp is definitely at home on the road.

In Conclusion



Homing in on studio specifics, the QSC may be likened to a MOSFET design when it comes to driving difficult monitors. If the speaker's impedance dips, the amp won't limit or protect to the same extent as most Bipolar designs. Table 1 bears this out - look at the 4 ohm stereo and 2 ohm bridge-mode figures. From this, we can deduce that the amp will be happy with nominally 4 ohm speakers, under all conditions.

Enclosure size is well below average at 10½ inches, including the space occupied by your plugs, and also just under 3U high, for those of us with rack-space in short supply. Regarding connections, you may need to rewire your feeds to take advantage of the balanced input, and do away with a potential hum loop. Otherwise, it's plug in and go - at least until you get around to replacing the output terminals with something more practical.

There are just two niggles for studio use. One, the gain controls. True, once set they may not need adjustment for a long time, but sometimes they're handy for muting purposes. One ruse you could try here is simply to use the on/off switch for muting; the relay will prevent the commonly encountered cacophony of thumps and bangs. Two, the fan. It's certainly quieter than most, especially since it runs at half speed under normal studio conditions. But with the amp just ticking over, and no signal, fan noise is noticeable and even mildly irritating in a quiet room. To attune this amp fully to studio usage, QSC could consider incorporating a second thermal sensor, using this to switch out the fan when the heatsinking is close to its quiescent temperature. In this way, the fan noise will usually only correlate with the amp being driven hard, and will therefore be obscured by music.

From the baseline up, the QSC stresses elegance, simplicity and roadworthiness, without peril to accurate performance. And despite the Reagan Dollar sinking wildly, today's price of £676 (VAT inclusive) works out at just over £1 per watt. This is a very good figure indeed, particularly with the amp incorporating all the key features - from calibrated gain controls through to fans and overload indicators which tell the truth. The manual also does this: with your QSC, you get a complete guide to rigging power amplifiers, totalling 32 pages of small print, from how to wire your jacks through to protecting HF compression drivers. It's evidently written by an engineer, in clear English prose; there's no attempt to gloss over anything, and the lack of hyperbole must serve to make this amp doubly credible.

Further information from: Music Labs, (Contact Details).

Table 1: QSC 1400 power output and sensitivity
Stereo mode, one channel driven:
Input sensitivity at clipping Power output at clipping Load impedance

volts dBu mean watts ohms
1.5 +3½ 150 15
1.05 +2½ 250 8
0.95 +1½ 400 4
0.9 +1½ 475 2.5**

Bridge-mode:
1.05 +2½ 550 16
0.95 +1½ 825 8
0.77 0 1000 4**

** The QSC 1400 isn't specified for these impedances, and prolonged operation will cause the circuit breaker to trip out. However, the purpose of these figures is to show that the amp will drive speakers' impedance dips without power limiting.

All measurements taken on a 240V mains supply.
Sensitivity refers to balanced input connection.



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Rebis Effects Rack

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Rolling Stones' Mobile Studio


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

 

Home & Studio Recording - Sep 1984

Donated & scanned by: Mike Gorman

Gear in this article:

Amplifier > QSC Audio > 1400

Review by Ben Duncan

Previous article in this issue:

> Rebis Effects Rack

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