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  1. Last year at about this time Brian Oppegaard at Speakerpower sent me one of his 240V, dual mono, SP2-12000 rack amplifiers so I could try it out and do a comparison against one of my Powersoft K10's. This was after discussing back and forth for a while. Of course a few weeks after that I became very busy and made a move to sell my K10's for K20's with all the bells and whistles which put the project off for a while. So here we are a year later and I've finally gotten around to what I intended to do from the get go. (My apologies for the extreme delay Brian.) I'm more of an objective data driven person rather than subjective so to that end the plan was to actually gather some form of rough comparison data on the amplifiers. Also I do not have an amplifier test bench and need another test regimen and equipment setup like I need a hole in the head, so the plan was to test the amplifiers while powering subwoofers. This should be more representative of a complex real world load on the amplifiers but also has a number of challenges. First is that these amplifiers are among the most powerful on the market and it will take a number of drivers to be able to handle their full output without being the limiting factor. #2 is that it is going to be loud. I already had 8 sealed, dual voice coil, RE Audio XXX 18's in my HT space so I figured that should take care of the speaker load. I never planned to do any testing or listening to either amplifier above the bass range. Really who buys 10kW rated amplifiers and uses them for anything other than bass? I wanted to use a sealed system in order not bandwidth limit the amplifiers and to require them to supply power down deep. I originally planned to do the measurements outdoors to remove room effects and to keep from subjecting my house to the absurd level of bass that would be produced, but after looking at the logistics of transporting the four 260lb dual opposed cabinets, the amplifiers, the wiring change overs, setup time and worrying about a good window in the weather I gave up on that. Indoors was so much easier I decided to take the lazy route. The test regime was to wire up as many XXX drivers in groups of two that would meet nominal loads of 8, 4 and 2 ohms. The amplifiers would be connected to each nominal load and then be run up to maximum output with 6.5 cycle shaped CEA-2010 type bursts in order to look at dynamic output and also with the same 23 second long sine sweeps that I use for testing speakers outdoors in order to look at something with an extremely high duty cycle, much worse than any music or movie content I'm aware of. this would give a good glimpse of long term output and short term. The amplifiers were both run from the same 240V AC line, the output was recorded by an Earthwork's M30 microphone placed at the headrest of the main listening position. The signals used were all software based and the microphone preamp providing the signal to the amplifiers was a Presonus Firebox. The gain for both amplifiers was set to the highest available setting. No DSP or other filtering was used for either amplifier. Originally I only planned to do a set of single channel driven tests since the SP2-12000 is a dual mono design with each internal channel already bridged. However after discussions with Brian and encountering issues with bus pumping with the K20 (Which is not a full bridge design) when running a single channel a set of tests running both channels was also done. For the dual channel tests the SP2-12000 was run in stereo since each channel is already a full bridge design and the K20 was operated in bridged mode. An effort was made to use wiring that was as close to each other as possible for each amplifier. It could not be exact but in general was very, very close. I used my existing 4 runs of 4 pole Speakon cables in the room and put together a wood panel with a set of Speakon jacks that would allow me to use a variety of jumper schemes to switch the load presented to each amp. In general I would always use the same drivers for both amplifiers if all 8 were not used and maintained parallel / series wiring of voice coils and drivers for both amplifiers with any changes needed in the wiring scheme accomplished at the amplifier or at the Speakon switch plate so that the loads were as close as possible. Obviously by using speakers as the load and amplifiers with this much juice thermal and dynamic compression in the drivers is a concern. However the tests using the full compliment of 8 drivers seemed to suffer from this very little and those using 4 drivers much less than I expected. Still it will be noted which amplifier was tested first at each nominal resistance as the second one to be tested may have been dealing with a bit of heat in the drivers. More to come...
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