Date: Wed Mar 11 19:31:54 2009
Back to Contents
Author: Jerry DiMarco
Subject: Re: stereo speakers
This stuff has been slowly coming back to me. I just remembered that
many mfrs specify amplifier output at different impedances. The example
below was excerpted from an amplifier spec sheet on a webpage:"Power
Rating 60W @ 8 Ohms 75W @ 4 Ohms"The numbers are not what the
uninitiated expect, and other amps might have a different set of numbers,
but hopefully this answers Anthony's questions from this morning. The
part about designing amplifiers with low output impedance is true, but I
think the goal is to minimize the effect of the speaker impedance changing
with frequency. Otherwise the speakers would draw varying amounts of power
depending on the frequency of the output, and would not faithfully
reproduce the sound levels set at the recording studio...
At 3/11/2009 03:31 PM, you wrote:
>"Since there is no way to match the amplifier to the speaker (the speaker
>impedance is partly a function of the mechanical qualities) it gives the
>best sound to approach an amplifier with zero output impedance."
>The power transfer efficiency between an amplifier and a speaker of a
>Hi-Fi audio system is poor, by design.
>The sound would be awful if one was to maximize the electrical power
>efficiency between the amplifier and the speaker. This is because of the
>problems of audio and electrical distortions.
>The audio amplifier source output is intentionally designed to have a
>lower impedance than the speaker load. An ideal audio amplifier would
>have zero output impedance.
>The audio speaker presents a constantly changing load to the source.
>This is due to a range of mechanical and electrical conditions: The
>capacitive reactance changes with the audio frequency; The inductive
>reactance changes with the audio frequency; The motion of the cone
>changes with the audio frequency; the Air load experienced by the cone
>changes with the audio frequency. This is just a sampling of the
>complexities in handling a speaker.....
>You can throw Ohm's Law out the window with the bath water in dealing
>with an audio system. - JZ
From email@example.com Wed Mar 11 19:31:54 2009