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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: What happens to back EMF with a feedback amplifier? posted by morricab on August 17, 2006 at 05:01:34:
The Back-EMF in the speaker motor causes a current to flow through the motor, speaker cable, and amplifier output stage. In practical situations, most of the energy represented by the Back-EMF is dissipated in the voice coil resistance, as the amplifier endeavors to keep its output voltage at some multiple of the input voltage.For simplicity, visualize for the moment that the input is presently zero while the speaker is still moving from some previous finite signal and generating a Back-EMF and a resultant speaker current. The output voltage of the amplifier is the gain factor times the input, or zero. A feedback amplifier achieves this state by applying a correction voltage to the input so that the amplifier generates a voltage equal in magnitude and opposite in sign to the voltage caused by speaker current flowing through the output stage. An amplifier without feedback does no such thing, and simply allows the actual output voltage to deviate from zero by the amount of the effective output impedance times the speaker current.
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.
Feedback in amplifiers causes problems when the feedback signal is delayed in time from the input signal associated with the error. Feedback amplifiers are stable or unstable, depending on how much forward gain they have at the frequency where this time delay causes the feedback to go from negative to positive. A stable amplifier has unity gain or less at this frequency. Amplifiers that are marginally stable may not blow up on a bench test, but will cause severe treble phase errors that most good audio setups can reveal. As with all other things in audio, how much of this is audible depends on who is doing the listening, and whether or not they have already made up their minds.
There are two kinds of amplifiers without feedback. The purist, low-power SET types have fairly high output impedance and poor damping factor. They do not control modern woofers well (however, mighty woofers from the golden age of audio were actually designed to work with such amplifiers).
The switching amps have very low output impedance because their output transistors are commanded to short the output to either the rail or ground at all times, and their output impedance depends only on the transistor complement and power supply. The Gilmore Raptors have milliohm-level output impedance and do not require feedback to achieve this. They exhibit exquisite control of my Magnepan MG-20s.
Thank you for posting this topic. It has revealed how little the unpleasant inmates actually know about electricity and audio.
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Follow Ups
- Here's what happens... - Al Sekela 15:56:29 08/19/06 (2)
- 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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