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PAB-415 PAB-415 – Upgrading a PA speaker Most PA speakers are designed to be lightweight, play as loud as possible and not cost too much. This results in poor cabinet construction, crossover tuned for quantity instead of quality and low cost crossover components. In this exercise I chose one of the better models from the IMG Stage Line brand, a two-way monitor of which the drivers at least were of above average quality. The PAB-415 to be more exact.
The drivers The Monacor MRD-140 is a 1,33” aluminium compression driver with a standard 1” mouth. With a rated efficiency of 106dB 1W/1m and a low resonance frequency of 500Hz it shouldn’t be too difficult to implement the driver. The Monacor SP-38A/300PA is a 15” paper cone woofer with a very smooth response up to 4kHz, is rated at 102dB 1W/1m and has a nice low loss suspension system. Power handling is 300 watts RMS with an Xmax of +/- 4,8mm so it should be able to produce reasonably high levels of sound with good quality.
The cabinet modifications The raw cabinets are made from cheap 18mm particle board (I wouldn’t call it mdf – even though it is trying to pretend to be so). The outside is nicely covered with black felt, has recessed metal handles and a black metal protective grille on the front. Connections are professional Neutrik Speakon and in the base plate there is a well-mounted insertion point for stand mounting the speakers. Inside they come standard with a small piece of polyester fibre dampening wool stuck to the bottom – not very effective. The rest of the cabinet is empty. First thing to do, was to give the cabinet some extra strength: I mounted a solid piece of wood between the baffle and the rear wall of the cabinet. Due to the cut-outs for the woofer and the tweeter in the baffle, the baffle is otherwise rather flimsy. Secondly, I strengthened the sidewalls with 18mm mdf plates covering a large area of the internal walls. Furthermore the inside of the metal recessed handles is covered with bitumen sheets; otherwise they ring like a bell. Damping is done with a single large piece of Pritex wedge moulded foam that covers the lower half of the side and rear walls. The original piece of wadding is finally placed loosely throughout the cabinet.
Crossover and listening. The midwoofer / tweeter network is a classic second order parallel crossover – yes you read it correct: a parallel crossover! Maybe it was due to the fact that the tweeter needed lots of padding to match it to the system SPL of the woofer, but I couldn’t get a series crossover to sound right. I tried several configurations but the parallel crossover everything fell into place. Capacitor C1 and inductor L1 form the high-pass for the tweeter and L2+C2 the low-pass for the woofer. Furthermore the woofer has a parallel Zobel-network that compensates the natural impedance rise due to the voice-coil inductance of the woofer. Resistors R1+R2 form an L-pad that lowers the tweeter output level by over 8dB! The LCR-network parallel the tweeter cuts-out the resonance peak in the tweeter output at 4kHz, without this network the tweeter can sound a bit rough. This speaker is mainly designed as a PA speaker and most of the time during its life it will be used for keyboard and vocal amplification. But I also designed it with hifi qualities in mind so how does it perform when connected to a normal hifi-system? Very well actually, it has a dynamic approach to music not normally found in so called hifi speakers, lots of attack and bite but never tiring or edgy. Overall response is quite even. Deep bass is limited due to the relatively small size of the enclosure but bass is still powerful with enough body. Out in the field it performs above average with very good resolution, low distortion and nicely expressive, that makes vocals easy to follow. Crossover components: L1 = 0,33 mH copper foil inductor Mundorf CFC-10, R = 0,07 ohms L2 = 1,50 mH copper foil inductor Mundorf CFC-10, R = 0,16 ohms Lz1 = 1,50 mH iron-core inductor Intertechnik Ferrobar, R = 0,22 ohms C1 = 6uF Hovland Musicap or 2,7uF +3,3uF Mundorf M-Cap Supreme or Intertechnik Audyn Cap Plus C2 = 4,7uF MKP polypropylene foil capacitor Cz1 = 0,90uF MKP polypropylene foil capacitor R1 = 4,7 ohms, 10 watts metal film resistor R2 = 3,9 ohms, 10 watts metal film resistor Rz1 = 3,3 ohms, 10 watts metal film resistor
Measurements Overall balance may look a little rougher than other designs, but it is still within +/2dB – efficiency is about 97dB/2,83V/1m.
NOTE: This design is strictly for the home DIY enthusiast and not to be used professionally without my permission! Tony Gee, The Netherlands March 2006 |
The original.
