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Altronics’ new 250W Aussie-made PA Amp A few months ago, Altronics were pretty excited about their brand new 250W PA “Redback Phase 4” mixer/amplifiers and wanted us to have a look at one. In fact, it was so new that we had to wait some time for it . . . W e anxiously awaited its delivery because we always use a transformer to step up their output to a nomihad a major sporting event coming up where we nal 100V level (some older ones are 70V). This is fed into could give it a “baptism of fire” evaluation against the lines going off to the speakers, which themselves have another transformer to step the voltage back down again. equipment we knew and trusted. This results in a much lower line current to the speakAs Murphy’s law always has it, the amplifier (Altronics told us it was the first one off the production line) ers. As line losses are a square of the current times the didn’t quite make it in time (it arrived just two days after line resistance and, by definition, long speaker lines are a the event – of course!) so, while we were able to give it a feature of most PA installations (each with perhaps several preliminary check-out, we couldn’t easily give it a real-life ohms resistance), this can make a very significant difference, especially at high power levels. run until recently. The other advantage of running “100V lines” is that all Testing a high power PA amplifier is normally not quite the problems of speaker impedance as simple as testing a stereo amplifier or matching blissfully disappear! similar. Amplifiers are normally tested Review by Instead of worrying about speaker in two parts – on-bench testing using combinations in series and parallel to dummy loads to see if they meet manuRoss Tester keep to a (say) nominal 8W at the amplifacturer’s specifications, then evaluation by ear using high performance, known speakers to see how fier, all 100V line speakers are run in parallel – all you need do is add up the power ratings of all the speakers in the they sound. True PA amplifiers have an extra “wrinkle”– they’re system and ensure the total is kept at or under the power normally designed to work into 100V (or 70V) lines. This rating of the amplifier. So if you have a situation requiring a couple of 35W is to minimise I2R losses over long speaker lines. Our dummy load setup was never really intended to speakers, four more 25W speakers and eight 10W speakers, you simply run them all in parallel on your 250W cater for 100V-line amplifier measurement. amplifier. 100V lines Need to add a couple more speakers to fill in some Maybe some explanation is needed. PA Amplifiers almost “sound holes”? Most PA speakers have selectable power AUDIO PRECISION SCTHD-W THD+N(%) vs measured 10 LEVEL(W) 20 MAR 103 06:23:28 AUDIO PRECISION SCFREQRE AMPL(dBr) 10.000 vs FREQ(Hz) 20 MAR 103 06:36:00 0.0 1 -10.00 -20.00 0.1 -30.00 -40.00 0.010 -50.00 1 10 100 500 Power output into a 4W load. It didn’t quite make the 250W mark but the difference is too small to be noticed. 80  Silicon Chip 10 100 1k 10k 50k Claimed frequency response is 50Hz-12kHz (-3dB) and this test shows that’s just about spot on! siliconchip.com.au The front of the amplifier has a clean, uncluttered layout – though we did find the “up and down” positioning of the level controls took a bit of getting used to (we’re more used to a straight horizontal layout as per a multi-channel mixer). The LED indicators, bargraph included, all make life easy when you’re busy programming an event! tappings so if you need to, you can reduce the power of But that’s something you could get used to. one or several speakers as required to keep that total under On the right side of the front panel are several LED the 250W figure. indicators, used to show an input signal, signal overload, Having said all that, the new Altronic Redback Phase4 signal peak, excess temperature and power. There’s also a PA amplifier can be run into 100V, 70V or low impedance LED bargraph VU meter showing program level from -24dB (4-16W) loads. So it is one very flexible through 0dB to +3dB, the final LEDs amplifier. Note that we said “or” – you being yellow and red respectively to cannot run combinations of loads (eg show overload. Power output: . . . . . . . . . . . . 250W RMS 100V and 8W) at the same time. A whopping big power switch and So it turns out we could check the Distortion: . . . . . . . . . . . . <0.5% <at> 1kHz Redback/Phase4 logo round out the amplifier using our bench testing setup Speaker output: . 70V, 100V or 4 - 16W centre of the front panel, with airflow and the low impedance outputs. But in- Line output: . . . . . . 600W balanced, 0dBV slots on the left and right sides and of the-field testing would have to wait until Frequency response: course the rack-mounting flanges with Mic inputs: . . . . . . 50Hz -12kHz, -3dB their handles. another opportunity arose where a 100V Aux inputs: . . . . . . 50Hz -15kHz, -3dB line system was in use. Just try picking the amplifier up Recently, that opportunity did arise – Input Sensitivity: from the horizontal using the hanMic inputs: . . . . . . . . . . 3mV balanced dles . . . if you can, you’re a stronger and this is the result! Aux inputs: . . . . . . . . . . 100mV & 1V man than I, Gunga Din! (with humble Appearance Signal to noise ratio (limiting bypassed): apologies to Rudyard Kipling). Mic inputs: . . >75dB below rated output The Redback Phase4 250W PA AmpliAux inputs: . . >81dB below rated output The back panel fier is housed in a black, 2U rack-mounting cases. Overall dimensions are 483w x Tone controls: Here’s where all of the inputs and Bass: . . . . . . . . . . . . . ±10dB <at> 100Hz outputs are located along with con330d x 88h and it weighs in at a fairly Treble: . . . . . . . . . . . . . ±10dB <at> 10kHz trols to set the amplifier up the way hefty 19.5kg. Having two big transformers (a power transformer and 100/70V line you want it. output transformer), at least partly explains this. Each of the input channels can be configured for microLevel controls for the six inputs, a master level control phone (3-pin XLR) or line (2x RCA) via its own 4-way DIP and separate bass/treble controls are all on the left side of switch. The other switches in the DIP set select line input the front panel. While clearly labelled, we found the verti- sensitivity (100mV or 1V), VOX priority (for inputs 1-3) cal arrangement of controls at least somewhat confusing, and whether phantom power is on or off. being used to channel-1 to channel-x arranged left to right. As the above implies, with an optional board, channels Specifications Here’s the business end of the amplifier – the inputs and outputs. Each of the DIP switches change various input functions. They’re a tad tiny for fat fingers but most users would set them once and forget them. At lower left are the three different outputs (100V, 70V and low impedance); above that connections for remote volume control, PTT and switched 24V DC. siliconchip.com.au July 2005  81 Inside the beast, looking from back to front. Dominant are the power (left) and output (centre) transformers. Interestingly, the output transformer is not a toroidal type. The fan-cooled heatsink is clearly visible at the right of this photo. 1-3 can also operate in VOX mode (voice operated switch), meaning that the amplifier outputs when there is an input signal. Another option is an internal alert/evacuation tone/siren board, which when activated takes priority for emergency use. To be picky, I found the dip switches fiddly with my fat fingers but this is hardly a criticism of note because in most installations, the amplifier would be “set and forget”. It’s only people like me who want to move an amplifier between various events who might find this a problem. Also on the back panel are the output terminals: 100V line, 70V line and low impedance (4-16W), all screw terminals protected by a clip-on plastic cover which I proceeded to lose within a few minutes (sorry, Mr Altronics). As well as another XLR connector offering preamp output there is also a pre-out/main-in provision so you can add effects if you wish. Another DIP switch allows the preamp output to be pre- or post- the master volume control. A pair of RCA terminals also gives a tape output. The amplifier is normally powered from the mains (via an IEC cable, supplied) but can be powered from 24V DC when in a portable situation, or an emergency application if the mains fails. There is even inbuilt provision for keeping a 24V battery trickle charged (300mA). Finally, one other handy option is provision for a remote volume control – a standard 500W pot can be wired into the system via ordinary figure-8 (ie, unshielded) cable to give volume control up to 100m away from the amplifier. A pair of screw terminals on the back make this simple. anyone on the northern beaches who wondered where that very loud Beach Boys music was coming from on the long weekend, sorry . . .) This amplifier compared more than favourably with another 250W brand I regularly use. Distortion, by the way, is quoted at <0.5% <at> 1kHz – that’s pretty good for a PA amp. . . actually it’s very good! I have collected quite a variety of microphones over the years – some good, some oh well and as you might imagine they have sensitivities all over the place. However, all performed as expected on the microphone inputs (3mV sensitivity). The better microphones sounded very good indeed. Signal to noise ratio is better than 75dB below rated output on microphone and 81dB on line inputs. Using PA horns it was difficult to judge the adequacy of the bass control – no matter how much boost you apply you cannot get much bass out of those babies – but the treble control certainly showed there was plenty of boost and cut available. Claimed control for both is ±10dB. By the way, this was never intended to be a hifi amplifier – it is a PA amplifier. So the frequency response of 50Hz-12kHz (-3dB) might not appear to be too spectacular in these days of DC to daylight – but for its intended use, it’s more than adequate. In fact, too low a frequency response in a PA amp is an invitation for hum and other noises to rear their ugly heads. There are several levels of protection built in. First of all there is automatic output limiting, preventing the voltage exceeding 110V at any time to prevent damage to speakers. There is also overload protection, reducing the output current (instead of blowing a fuse, which my other amplifiers tend to do – always at the worst possible moment). A front panel LED flashes to warn of the problem. There is of course fuse protection on both the AC and DC circuits but these are a “worst case” scenario, protecting the amplifier when the other methods have failed. The amplifier is also fan-cooled, the fans automatically switching in as required. Price, availability The Redback Phase4 250W PA Amplifier (Cat A4085) is available from Altronic Distributors, 174 Roe Street Perth WA 6000 (PO Box 8350 Perth Business Centre WA 6849). Phone (08) 9428 2188. It has a recommended retail price of $1065, which compares well to imported amplifiers of similar capability. There is a 125W version available (Cat A4075) which sells for $875. It looks pretty much identical but is about 5kg lighter. The warranty on both systems was five years but in the last few days has been increased to a whopping ten years. We’ve never heard of anything better! In use Recommended? I tested the amplifier using both microphone and line level inputs, the latter with a variety of program material ranging from tape and CD players through to a portable MP3 player. I also tested both 100V line and low impedance outputs. I was very impressed with the “crispness” of the output, even when the wick was well and truly wound up. (To Now we know what Altronics were getting excited about! If you need a high power PA you’d go a long way to better this one. Most users would rate our few criticisms as nit-picking; it performed flawlessy and did everything we asked of it. The fact that it is designed and assembled in Australia is a bonus! SC 82  Silicon Chip siliconchip.com.au