1200 watt PA system

At the request of a local keyboard player and doctor I manufactured a small (well relatively!) PA system rated at 1200W RMS! Quite a lot of power! It was intended to be a unit mid way between the better type of hifi and a full multi speaker stage PA system. I must admit that, due to limited time it was not possible to design everything so some kit modules had to be used. I did however design and build the power supplies, electronic crossover, interface to the rest of the world and the speakers themselves as well as the metal work and electronics. I can only say that if you like your home hifi loud and clean then you need to go active. There is no comparison with speakers with normal crossovers. Active systems just ozze power, dynamics and control and can be tweaked so easily. Tests were carried out with a Mini Moog and a Yamaha DX9 along with various other keyboards all of which sounded wonderful. Normal music signals from hifi sources worked extremely well to. I would love to build two more modest systems, fully active for home but so far the time has not been on my side!

In case your interested, the bass driver was supplied to me by the customer. It was a JBL 15inch 400 watt driver with a resonance of approximately 35hz. this was loaded into a ported cabinet as shown in the photographs. It took several attempts to get a good match sonically for this driver but I was able to prove good output maintained down to 30hz or so without undue stress. I should point out that this particular driver cost £300 at the time and so I would have expected nothing but a good performance! The original enclosure for the driver was a primitive folded horn arrangement over 5 foot tall with 5 other drivers (2 x Bose 1 ohm midrange!!!!, 8x2 midrange horn and two piezo bullet tweeters) and and a high power 4 way crossover that blew up every 20 gigs or so. Not a good sound at all!
The new system midrange drivers were 75 watt 6 inch paper cone drivers to try to get the original midrange sound of old Fender PA and guitar stacks. This almost worked! The aim was to keep the sound clean without too much colouration so I didn't go too far down this particular route! It was clear that this combination had some of the better aspects of the Fender Musicman sound. Lower crossover was set to 350 hz but later raised to 750 hz approximately to allow the JBL to do more of the work in the critical upper bass lower midrange region.
The high frequency section was made from three 150 watt extended range ferro fluid cooled tweeters from the Audax range. This enabled a move away from the harsh gritty noise so prevalent in PA systems largely caused by Piezo tweeters. Upper crossover was at 4800 hz with a top roll off to kill the harmonics associated with square and pulse waveforms that could potentially over heat the tweeter coils (yes I burnt out the first set of tweeters I brought with the Mini Moog at full output!) which resulted in a top limit of 18.5khz.

The amplifcation was a selection of parts from Maplin and a crossover design from Elektor. The amps were the classic MOSFET 100 watt modules with the bridging module to bring 6 amps into the three way configuration. The crossover settings were at first preset to something that gave a reasonably smooth and flat response but then trimmers were added to the side of the cabinet in a recessed pocket.

Obviously carrying handles were very important. As was the power supply. To get the PSU in a suitable state a 625va transformer was used together with a bank of 6 10,000 uf 80 volt caps just for the power stage with a seperate 15 volt supply for the low voltage sections running the bridge modules and the crossover.

Obviously now the Maplin MOSFET is no longer around, there would have to be some alternative versions made today. But the options are huge!


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