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HIFIREVIEW Amcron 1200 professional stereo power amplifier How do professional power amplifiers differ from domestic hifi stereo amplifiers? In the case of Amcron the difference is radical, as we found in our review of the Macro-Tech 1200. "Like a battleship". That's the expression most people would use if they had to come to grips with the Amcron Macro-Tech 1200 power amplifier. That's not entirely appropriate though because the battleship term conjures up something extremely rugged but otherwise lacking in technical refinement. So yes, the Amcron is rugged but that is only a small part of the story. We have seen a lot of power amplifiers in our time but not many intended for professional use. By professional use we are thinking of amplifiers used to drive studio monitors or for sound reinforcement in cinemas and theatres. In most of these applications the amplifiers are bolted into place and once commissioned, are never moved for years afterwards. And then there is the other "professional" application which involves sound reinforcement at indoor or outdoor venues that don't have an adequate PA system. In this situation, the amplifier must withstand the rigours of transport, rough handling and less than ideal installations but still deliver high power under often arduous conditions (read: rock concerts). These two separate applications produce a conflict for the designer. For the static installation it does not matter too much if the amplifier is really large and heavy, so long as it does the job very reliably. But the need to have gear that is move.able means that it really can't be too bulky or too heavy. Gear that is really heavy is often mistreated by "roadies" partly because it is too heavy to handle safely and partly because they just don't like it. So Amcron has evidently tried to satisfy two separate camps. On the one hand, it is a rugged amplifier but it is not all that large since it only occupies a standard two-unit high (88mm) 19-inch rack case. Nor is it outstandingly heavy for a 400 watt per channel amplifier (40 FTC rating) with an all-up weight of around 20kg. In the manner of most professional equipment it isn't all that pretty either. "Functional good looks" would better describe it. The lower half of the front panel is the inlet grille for the ventilation fan. The grille has a foam filter behind it to prevent dust and dirt from being sucked in by the fan. There are two level controls, one for each channel and a number of LED indicators which are much brighter than the subtle illumination found on domestic hifi amplifiers. They are meant to be bright; they dim to indicate overdriving or malfunctions. Internal details Inside the steel chassis is quite radically different from hifi amplifiers, as you can see from our photograph. For a start there is a high velocity fan which pulls air from the front and then blows it out through the honeycombed heatsinks at each side. And note the twelve power transistors on each heatsink: that's 24 in all! Two large power transformers are used, which is how Amcron Amcron use forced-air cooling and two separate power transformers to obtain a low profile in their MA-1200. It actually uses two amplifiers in bridge mode in each channel. 16 SILICON CHIP manages to make such a powerful amplifier with a low profile. Each transformer feeds two bridge rectifiers, a main and subsidiary, with the main bridge feeding a 10,000µF lOOV electrolytic capacitor. From there on the circuit becomes very complicated, although that is not immediately apparent. Removing the bottom panel reveals a whole lot more. One large board across the full width of the chassis accommodates the small signal circuitry while a board running down each side accommodates the power amplifier output stages. These latter boards are interesting because all the components, with the exception of the power transistors themselves, are mounted on the copper side. As you look at the amplifier it becomes appararent why they did it this way. The boards are effectively mounted on one side of the flat heatsink while the power transistors are mounted on the topside, with their base and emitter leads passing through and soldered to the copper side of the boards. As mentioned before, each heatsink has a honeycomb section (on the top, next to the power transistors) through which a lot of air is passed. That keeps it compact and cool. The trouble about looking at the inside of the Amcron though, is that it tells you virtually nothing. It is not until you look at the circuit diagram that you realise how unusual it is. Instead of the Amcron being a conventional stereo design, each channel is actually two power amplifiers with the loudspeaker load being driven in bridge mode. This allows the use of a relatively low supply voltage (90 volts total) which means that the bipolar transistors can deliver a great deal more power into low impedance loads. "Come again?", you say. The problem with bipolar transistors is "second breakdown". This severely limits the amount of current that they can deliver when they are running from a high voltage supply, even though they may have a dissipation rating of up to 250 watts. Amcron have got around that problem by halving the supply voltage and using two power amplifiers in bridge mode, in each This is the Amcron MA-1200 with the top cover removed. Immediately visible are the two power transformers, the 12 bipolar power transistors in each channel and the high capacity cooling fan. channel. So that's four power amplifiers in total. Amcron call it "full bridge" technology. We won't even try to to explain how the amplifiers are powered and connected together while still avoiding earth loops. That's complicated enough, but consider that the MA-1200 has three operating modes, selected by a slide switch on the back panel. The three modes are stereo, paralleled mono and bridged mono. Understanding how the two channels are run in parallel is easy enough, but how do you bridge two channels, that each consist of two amplifiers in bridge mode, to get one super-powered mono amplifier? Figure that one out. (There is no switching of the outputs by the way). Another interesting aspect of the MA-1200 is its protection circuitry, referred to as ODEP - Output Device Emulator Protection. This monitors the heatsink temperature and the power output from moment to moment and produces a signal which is determined by the always- · changing safe operating area of the transistor. If the margin is being ex- ceeded, the drive signal is reduced until the transistor junctions cool back to safe conditions. Power ratings In stereo mode, the MA-1200 will deliver 320 watts into 80, 465 watts · into 40 and 600 watts into 20 (per channel, at 0.1 % harmonic distortion). In parallel mono mode, it will deliver 630 watts into 40, 1000 watts into 20 and 1200 watts into rn. Yep, that's not a mistake and they are continuous ratings too. In bridged mono mode, it delivers 935 watts into 80 and 1200 watts into 40. Quoting the very comprehensive specifications would fill several pages of this magazine. How many manufacturers specify their amplifiers for rn loads? Amcron's manual also details some of the torture tests they do to their amplifiers to check output current capacity, their comprehensive protection circuitry, and their behaviour with inductive loads. We had trouble enough providing loads which could cope with the prodigious power output into 80 continued on page 95 JULY 1988 17 is operating as it should. The 3.8mV DC voltage you've measured at the output is within (ie, less than) the figure of ± 30mV we specified in the article. Nor is the orientation of the output choke critical so you have nothing to worry about as far as the amplifier's performance is concerned. It is possible that your guitar preamp was intended to drive a power amplifier with more sensitivity, which could explain why your new guitar amplifier does not sound quite as loud as your old one. This can be fixed by increasing the gain of the 100W module. To do this, reduce the lkO resistor connected to the base of Q3 to 5600. This will almost double the module's sensitivity. Don't reduce the resistor any more though otherwise amplifier stability might be a problem. Wants polycone loudspeaker system I'd like to see a speaker project using the cheaper polycone speakers which seem to be readily available. Those from Europe are getting a bit expensive now. Also a few photography projects please. I intend building the Studio 200 stereo power amplifier in the near future. Is there any errata on it? Keep up the good work. (P.G., Orient Point, NSW). • We have checked out a number of polypropylene speakers since we started and have found them generally disappointing and certainly not good enough to make a good speaker system. At the moment we have a good woofer and quite a good midrange and tweeter in the workshop. The only problem is that the woofer is 6dB more efficient than the midrange and tweeter. That's a pretty insurmountable problem. When we do come up with the right combination, we will publish the results. Your request for photographic projects is noted. Design info for dedicated ZBOA I wish to build a dedicated microcomputer system based on the Z80A but I can't find the necessary information on it and the Z80 PIO, SIO, CTC, DMA and DART. I won't be using all these chips but I would like information on them all. If you have the data could you send me copies of it or could you tell me where to find it? I also need to know how to program EPROMs. I would also like a course that helped teach the required information to gain an amateur radio licence. (A.R., St George, Qld). • Obtaining all the data you will need for your Z80A project will not be easy - we are certainly not in a position to supply it. The Australian distributors for Zilog are the George Brown Group and they would certainly be able to supply comprehensive data on the Z80 and Amcron power amplifier - ctd from page 17 and 40; never mind the 10 condition. We did confirm enough of the amplifier's specs though to be able to state that this amplifier is a real performer. Its typical harmonic distortion, at just before the onset of clipping, is just .002 % . And the signal to noise ratio was + 104dB Aweighted with respect to 300 watts into 80. These are very respectable figures for any amplifier. What we can't say is how well it performs under gruelling " on road" conditions. But based on our examination of the construction, its low voltage bridge-connected amplifiers and our limited benchtesting, it must be one of the most conservatively rated amplifiers around. Amcron must be confident in its reliablity as they give an unlimited transferable 5-year warranty on all performance specs; and that even covers freight costs. Recommended retail price is $2802. At that price, it must be just about unbeatable for value too. In short, it's a winner. For further information, contact Bose Australia Inc, 11 Muriel Avenue, Rydalmere NSW 2116. Phone (02) 684 1255. (R.F.& L.S). ~ related products. Their phone number in Sydney is (02) 519 5855. However, just having the data might not be enough info for the project you are considering. We suggest that you also consider buying a number of texts on the subject, such as the Z80 Microcomputer Handbook or Z80 Microcomputer Design Projects. Both these texts are available from Jaycar at $24.95 and $25.95 respectively. These texts will also tell you something of EPROMs. You can also obtain a handy chart on the Z80 containing the full instruction set, ASCII, hex and decimal conversions, interrupts, , disassembly table etc from Jaycar for $12.50. Obtaining info on becoming an amateur radio operator is easier. Contact the Wireless Institute of Australia, PO Box 300, Caulfield South, Vic. 3162. Notes & Errata Remote Switch for Car Alarms, March 1988: the inductor marked 12 in the receiver should have an F29 slug, not F16 as specified. PRODUCT SAFETY RECALL DICK SMITH PTY. LTD. ■ L■ CTRONICS NOTICE TO CUSTOMERS BEAT TRIGGERED STROBE KITS Cat No. K-3153 sold In component form for customer construction have been sold in all states and may, If proper safety procedures are ignored, constitute a serious risk when constructed, as the power switch supplied with some kits is incompatible with the wiring diagram. Customers are asked to return this product, whether or not assembled, as a matter of urgency to the nearest Dick Smith Electronics store for Immediate attention to the problem. We apologise for any inconvenience caused. Further enquiries may be made by contacting Rex Callaghan, Technical Service Division (02) 888 3200 or your nearest Dick Smith store. JULY 1988 95