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Peavey DECA-700 Power Amp


Long awaited, the Peavey DECA 'digital conversion amp' is claimed to be a completely new type of power amplifier, offering major benefits over traditional designs. In Tune has been testing one of the first samples in Britain.


The story behind Peavey's DECA power amp is long and complicated. It isn't 'just another power amp' - it's a new type of power amp; one which has resulted from Hartley Peavey's considerable investment in research. In the future we can expect to see increasing developments from this side of the Peavey organisation as they begin to apply their high tech research right across the product range. But the first fruit of the new Peavey programme is the DECA 'Digital Energy Conversion' power amp, and IN TUNE was among the lucky few to be able to see and try a U.K. specified sample.

Understanding how the DECA works, and exactly what Digital Energy Conversion is, requires a considerable amount of background in how conventional amps function, something which we haven't the space to cover here and which many (most?) musicians might well feel happier not having to digest. For most of us, what matters is how well or otherwise the DECA lives up to the claims made for it and how good it sounds. It's these areas which we'll concentrate on in this review, although a little preliminary detail wouldn't, of course, go amiss.

The problems which Peavey say they have set out to solve with the DECA are mostly ones of amplifier efficiency and reliability, although they also claim substantial benefits in distortion-free performance, sound quality etc. Going beyond the Class A/B design principles which are used in almost all modern power amps, Peavey say, has enabled them to overcome the huge amounts of wasted energy produced by power amps, the dissipation of which, as heat, calls for huge heat sinks, fans, large heavy construction and so on. Peavey claim 90 per cent efficiency for their DECA amps. In effect they are claiming the benefits of Class A's freedom from transient intermodulation distortion, while overcoming the attendant heat problems which would result from a change back from Class A/B to Class A designs.

While admitting that the following is a gross over-simplification, the DECA has an Analogue to Digital converter at the input stage (sampling at 500kHz) and uses 'Pulse Width Modulation' techniques to enable the output devices to function in an on/off state. Transistors, generally speaking, give off no (or very little) heat when either full on or full off. The heat problem arises when they are neither on nor off and that's where the Pulse Width Modulation technology comes in - it enables the output devices to be in one or other of their optimum cooled states all the time. Hence, in theory, next to no heat.

Unfortunately, compounding the difficulty in understanding how the DECA works is a considerable amount of secrecy surrounding the internals. Peavey say they have applied for six patents on the system and, perhaps understandably, are reluctant to let people pry around inside their amp until they have these through the patenting process. For that reason we were asked to evaluate the Peavey but not disassemble it or let anyone else do so. Not having the test facilities to undertake a full spec/performance check (few, if any magazines do - they nearly all use outside consultants) we were more or less restricted to a performance test: not, as it happens, a bad way of evaluating a product given how much confusion can arise from the various ways different people have of presenting and interpreting measurements! As we said earlier, an attempt to explain the design details is impossible here, so what follows is a look at the physical details, followed by the results of our performance tests.

THE DETAILS



Two models are offered in this new Peavey range - the DECA-700, delivering 350 watts RMS a side into 4 Ohms (bridgeable into mono at 8 Ohms where it will give 800 watts) and the 600 watt stereo DECA-1200 version. These outputs, by the way, are with the 'DDT' Compressor switched in. With this distortion limiting circuitry switched off the DECA-700 will give out around 450 watts per side into 4 Ohms.

Remarkably for such a powerful amp, the Peavey will easily fit into a 2 rack high space in the usual 19" wide format. Its also a lightweight; the 700 weighing just 30lbs - and, what's more, it has no fan cooling (an unusual feature in this power range), nor even anything extraordinary in the way of ventilation. According to Peavey, no special provisions for cooling need be taken other than leaving 1 space high around it in a rack. This benefit we've explained above, with reference to the PWM principle.

The cleanly laid-out rear panel has a sensible range of connection features. In addition to the mains IEC there are both binding posts and jack sockets for speaker connections, jacks for unbalanced inputs, XLRs for balanced feeds, two line-out jacks and recessed sliding switches for on/off on Peavey's DDT 'Distortion Detection Technique' compression circuitry. The DECA's restrained front panel has properly graduated rotary controls marked 'Input Sensitivity' which, in effect, act as master volume controls, a large central on/off switch and a selection of coloured LEDs which show DDT on/off, thermal (overheating) and line faults (including D.C. faults, short circuits, etc). There are also two exceptionally fast-reacting LED chains which show power on, the percentage of power being used, and available headroom, in dBs. Overall it's a tremendously convenient package; small in size, low in weight and very handily laid-out for ease of use.

IN USE



No thumps or bangs attend the Peavey's turn-on. The LEDs flash on and the red 'Fault' LED lights while the initial checkout takes place. There's a noticeable hum from the amp but not a bothersome one, and certainly less intrusive than many power amp fans which can sometimes cause problems in small studio control rooms. This, we're almost certain, comes from the 700's toroidal mains transformer and isn't unusual in large power amps. We gather that the big brother model the 1200, will have a switched power supply system and (without the need for this transformer) could be even lighter, almost certainly silent - perhaps the 700 will get this system one day? Till then it's still not a problem.

One of Peavey's major claims for the DECA is a total absence of transient intermodulation distortion - a problem which can sometimes be detected when fast musical transients (usually slapped basses, drums or other sources with a rapid 'rise time') catch an amp's circuitry out and result in an audible degradation of the signal. To be honest this hasn't seemed much of a problem on the best power amps for a few years now, but in any case it certainly wasn't detectable here. Moreover, the actual sound of the transients was superb. Drums had a splendid attack, as did basses, and when we tried the DECA with recorded sources the reproduction of fast signals confirmed our 'live' impression.

The bass response overall was pretty majestic. Describing it with the usual word 'tight' isn't really good enough, because (again well demonstrated when tested with recorded sources) the bass wasn't just tight and extremely well controlled, it was noticeably more prominent. Indeed, from a listening/user test this was one of the major audible differences between the DECA and some of the other power amps we tried it against. This may well have had something to do with how well the Peavey controlled the speakers it was driving; another benefit of the output system.

Unfortunately we were a bit too restricted in the total volume we could use to be able to tell how well Peavey's DDT circuit worked. This system is supposed to compress the overall output so that you can use the amp at full power and still avoid clipping when it' s overdriven. The principle is certainly sound enough, and could be particularly useful when this amp is being used on stage or in PA rigs. Letting it rip without the DDT switched in will give a lot more power, and it was interesting to note Peavey's claim here that under overdrive conditions the amp is said to sound better than most and to 'recover' far quicker than average. It was a pity that we weren't able to test this claim, but Peavey's figurework looks convincing. To be thorough, we tested the Peavey on short-circuit and, as expected, the 'Line' fault LED glowed instantly. One other comment we'd like to make is that the LED output meters seem to respond really fast making them very practically useful, not mere decorations as meters so often are.

Trying to describe a power amp's sound is never easy (it's hard enough with instrument amps!) and it's inevitably a very subjective opinion. Nonetheless, we tried to reach some sort of general agreement among the listeners and came up with the following. Firstly, the DECA sounds remarkably true and clean but not sharp or even painfully clean like some typical big name US-produced power amps. It's definitely got a warmer sound than one is used to with the majority of American high power amps (the Hafler range excepted), and doesn't have that shrillness at high output levels which can be distinctly unpleasant at times. You could, perhaps use the word 'musical' to describe it. Transients, as we've said before, sounded superb and the bass response was extremely impressive. On the negative side, however, one of our testers in particular felt that the amp had what they could only describe as a 'clouded' sound - not muffled as such, but lacking a certain sparkle, especially in the middle. It may be that what was being referred to here was the Total Harmonic Distortion which is, it must be said, somewhat higher than can be achieved with the best conventional power amps. This is a moot point, though, because the THD figures quoted by Peavey still show a figure better than 0.1 per cent at 350 watts RMS per channel at 1kHz. The question, inevitably, is just how much distortion can the human ear detect? Either way (a distortion component or a tonal balance characteristic) our 'bats' ears' tester still wasn't too happy. However, as we've stressed, all these opinions and considerations about sound are intensely personal and must be decided on by the individual customer.

CONCLUSION



Any power amp delivering so much power from such a small package, and especially one not relying on fan cooling or special provisions, must be an interesting prospect to PA users and hire companies, not to mention bass rig owner/builders and studios. Our tests revealed the Peavey DECA to be an excellent performer with a sound which will appeal to very many users, even over and above its physical qualities. The price too is extremely good for a U.S. product - especially one where the makers could normally be expected to try to claw back every last cent to offset against their development costs.

On account of its splendid bass and transient performance we'd be very interested in using one of these amps to drive a bass stack, notwithstanding its obvious application in PA racks where space is at a premium. On technical terms and on value for money it looks like a winner to us. On sound? Well, that's where the potential buyer comes in. It's a different sound from what you might be used to - but you'd be crazy not to try it for yourself, given all the factors in its favour.

RRP £699 Inc. VAT

More details from Peavey Electronics (U.K.) Ltd., (Contact Details).



Previous Article in this issue

Yamaha Compressor & Graphic EQ

Next article in this issue

Boss CE-300 Super Chorus


In Tune - Copyright: Moving Music Ltd.

 

In Tune - Jul/Aug 1985

Donated by: Gordon Reid

Gear in this article:

Amplifier > Peavey > DECA-700

Review

Previous article in this issue:

> Yamaha Compressor & Graphic ...

Next article in this issue:

> Boss CE-300 Super Chorus


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