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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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In Reply to: Here's what happens... posted by Al Sekela on August 19, 2006 at 15:56:29:
HiGood post but you say something that isn’t quite right
"You will notice that in this ideal state of affairs, the feedback amplifier dissipates zero energy from the Back-EMF in its output stage, since energy is power integrated over time, and instantaneous power is voltage times current. Zero times anything is zero. All the energy represented by the Back-EMF, which comes from the mechanical energy stored in the speaker and room air, must be dissipated in the speaker motor and speaker cable resistances."While the feedback loop does keep the output voltage at ideal minus the loss across its output impedance, the power dissipated in the output devices is where the EMF is dissipated.
For example a woofers mass appears as capacitive reactance, in the "worst case", the current leads the voltage by 90 degrees.
For the power transistor in a conventional amplifer, this means that its maximum collector current is at the zero voltage output point in the wave form (where a resistive load’s current is zero).
If you examine an output stages safe area curve you can see why the device dissipation is higher and why switched mode outputs don’t have this issue.
While linear stages must be protected from exceeding load angle E - I, switched systems are essentially immune (why they have been used in motor controls for 20+ years).
Switched output stages don’t have a low intrinsic output impedance however, it is the feed back loop which lowers it.
Best,Tom
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Follow Ups
- Re: Here's what happens... - tomservo 20:49:04 08/19/06 (1)
- Attempting to be concise, I left out - Al Sekela 15:37:27 08/20/06 (0)
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