David,
Let me try to respond and perhaps Jim will chime in a well.
Modern audio amplifier output stages are very low output impedance devices.
As such, they can produce as much power as the device connected will
allow. In a practical design of course there is a limit to how much
current the attached device (speaker or headphones) can draw from the
amplifier.
With that being said, the lower the impedance, the more current that
will be drawn. Speakers are generally in the 4 ohm to 8 ohm region and
a 4 ohm speaker will draw more current and develop more power than an 8
ohm speaker. The efficiency of a speaker makes a big difference in the
amount of sound it produces.
Headphones are a slightly different matter (but only slightly) in that
they are typically higher impedance and the current will be lower, and
the voltage across it can be higher, but they are more efficient than
speakers so it usually does not matter. The efficiency of headphones is
usually expressed in SPL - the higher the number, the greater the
sensitivity. Impedance does not matter much except in some low
efficiency Hi-Fi audiophile headphones.
In an analogy, consider the AC power line - that is a very low impedance
source of a fixed voltage. How much power is taken from the line
depends on the impedance of the device plugged into it. Within limits,
an audio amplifier shares some of the same characteristics even though
its voltage and power are much more limited.
73,
Don W3FPR
On 1/23/2017 3:06 PM, w7aqk wrote:
Jim,
Maybe you could expand on that somewhat as I'm still confused a bit.
Perhaps I'm not understanding what I hear, or why, but it almost always
seems that when speakers (or headphones) are not closely matched to the
designed impedance I notice some deterioration in output. Some mismatch
doesn't seem too serious, but when the difference is high, it seems much
more noticeable.
On the other hand, I've also experienced that using headphones with
higher sensitivity ratings (like most Sony ear buds) tends to provide
much higher output. Most ear buds tend to be 32 ohm (16 at lest), but I
find Sony ear buds usually work best as many of them have sensitivity
ratings up around -108 dbm. I could very well be fooling myself!
This topic was covered somewhat similarly back when folks, including me,
were complaining about poor audio output from Heil headphones. The
headphone portion of my Yamaha CM500 works a lot better than my Heil,
but neither works as well as some ear buds. I don't really have another
good pair of headphones around here to try.
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