[Elecraft] audio amplifier/speaker/headphone impedance matching

[Rick Shindley]

People sometimes are confused about impedance (Z) matching. They mistake the
need to do so at rf frequencies with the conventions used at audio
frequencies.

At rf, power transfer is maximized when the load impedance matches the
source impedance. In the audio realm, maximum power transfer occurs when the
source impedance is zero! That is what is termed the "dynamic impedance"
characteristic of an audio power amplifier's output.

The coupling medium at audio frequencies is not coax, waveguide, or open
wire feedline.  No need for that.  An old lamp cord works just fine.  (Note:
that's why MFJ doesn't make an audio SWR bridge!)

At audio frequencies, power transfer becomes essentially the same as with
resistance voltage divider circuits. At such low frequencies, the reactance
component of "impedance" is very low and resistance dominates the
mathematics.

The output "impedance" of an audio amplifer is essentially zero Ohms.  All
the output voltage from the amplifier appears across the speaker terminals.
Basically, the amount of power transferred to the speaker from an ideal
amplifier is dependent only upon the ability of the speaker to receive it
(voice coil resistance, magnet strength, cone size, etc). Placing a 16 Ohm
speaker in place of a 4-Ohm speaker won't hurt the amplifier, it just will
result is 75% less audio power from the speaker!  If you want the same power
from the 16 Ohm speaker as before, you would have to double the voltage to
it somehow.

This is what goes on in public address systems.  They use an amplifier with
high current drive capability, and cause it to output audio RMS voltages of
70.7 volts.  Any amount of power can be retrieved off the 70.7 V audio "bus"
by way of a step down transformer at each speaker.  Want more power at a
given speaker?  Just move the voltage tap up more on the local speaker's
transformer secondary!  No "impedance matching" going on there!  It's very
much the same as electrical power distribution in communities.  No Z
matching going on there, either.

The zero Ohm source impedance of most audio amplifiers assures that the
speaker voice coil will be forced into position with authority.  That means
low acoustic distortion. If the amplifer had a non-zero output impedance,
the voice coil would not accurately track the voltage signal and acoustic
distortion would result.

Rick  KC0OV
K2 1360