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Old 15th May 2008, 09:00 PM   #21
wg_ski is offline wg_ski  United States
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Join Date: Oct 2007
The peak currents may very well be the rail voltage divided by maybe one to one and a half ohms. It will be pure resistive at the
minimum, but you've still gotta supply the current. For 80 volt swings that's 53A. Use five and you're over 10A, where even a
sustained-beta type takes a dive. Your beta will drop to about 15 or 20. Better than older 2N types, but not much. That will take around 3A from the driver, or 30mA from the VAS with no predriver. VAS'es are usually biased with less, and that's even on the hairy edge of what a 5532 op amp will do (even the QSC RMX5050 does this, wonder why it doesn't sound very good at 2 ohms). If you're going to skip the predrivers, you better bias the stage at some 50mA or more, which is gonna be hot, take a long time to stabilize, and won't be as linear as one that's properly designed. Add that one extra current gain stage ahead of all this and drive requirements are around a milliamp or two, even without sustained-beta outputs *or* drivers.
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