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11-09-2008 06:46 AM
The 900A was significantly different than the 600 in that the 900 is a 2 tier class H amp that switches between Vlow and Vhigh as required to increase efficiency and reduce waste heat. It's a great amp, heavy duty with a 3 rack space chassis but quite shallow. It's chief drawback for heavy duty cycle and 2 ohm loads is the location of the air inlet and the air circulation pattern (side to back).
The 1100A is essentially the same thing, except an added tier switch that switches between 0volts, Vlow and Vhigh. This does nothing for resistive load efficiency but improves reactive (speaker) load efficiency. All of the AB amps were designed to drive reactive loads well.
Most of the 900's and 1100's were shipped with the fan kit (notice the slot below the heatsink for air flow) but also require a small diverter that screws to the bottom of the chassis. It looks like a little "V" and insures the air passes up through the heatsink. There will also be a piece of sheet metal across the back of the heatsink that helps with the chimney effect.
The fan mounts to the left side of the chassis and blows IN. There is a fan header marked J6 on the power supply PCB, but the fan control components (Q1-Q3, R1-R7, D1 and the fan thermal switch) must be installed for the fan to work.
At one time, I owned several dozen 1100's and 900's. Both are excellent amps when racked with an understanding of the thermal management required. The 400 and 600 were class AB versions of the 900 and 1100. I was also involved with the original AB Systems & AB International.
If you are interested, I have a quantity of 710C's in average condition (biamp w/ 600 watts on the LF and 100 watts on the HF w/ crossover cards) and a quantity of 900A's in dead-mint condition.
There were about 4 versions of the 900's. Most of the differences were due to different output and driver transistors. I stock most of the parts. Don't be put off that there's only 6 outputs (plus 2 drivers and 2 rail switches) per channel. The class H circuitry reduces the requirement for SOA hence a smaller number of transistors are required. The 1100 also has 6 outputs per channel, (plus 2 drivers, 4 rail switches) but the 3 tier design reduces the requirements even more, making up for the higher operating voltages.
Older versions used 2SB554/2SD424's, later versions used MJ15022 and 23's, and even later units used MJL21193 & 94's. I also thought they used some MJ15024's and 25's, and MJ21193 and 94's but that's not shown on my docs.
Hope this helps.
11-09-2008 09:14 AM
12-23-2008 09:48 AM
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12-23-2008 10:12 AM
Funny how things change. I'm sure that those old ABs were sine wave rated as was just about every "professional" power amp.
I submit that a proper burst test is more real world that sine/RMS/average rated amps because music IS a series of bursts and almost never continuous. And a burst test would be closer to describing the "sound" of a power amp.
I am assuming you do not need to drive a shaker table
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