Hi Dave,
I've written about this many times, but here goes again. :) Your choice
of the words "designed impedance" is a good one.
Audio output stages have an internal impedance MUCH lower than the loads
they are designed to drive. For example, a typical speaker output stage
is designed to drive any load impedance greater than 4 ohms, but it's
own internal impedance (also called "source impedance") is a small
fraction of an ohm. The ratio between those two impedances is called the
"damping factor," and a value of 100 is typical. Line level outputs for
consumer gear are typically in the range of 200-400 ohms, and are
designed to drive 47K. Pro line outs are typically 100 ohms, designed to
drive 5-10K. The impedance of mics is defined as 5X their minimum load
impedance.
Output stages that feed plug-able jacks are usually built with resistors
in series to protect the output devices from damage by a short when a
jack is being inserted or is inserted improperly. Typical resistor
values are in the range of 10-500 ohms. Headphone outputs are designed
to feed headphone impedances between 8 and 500 ohms.
The impedance of headphones is simply a ratio between voltage and
current. Lower impedance headphones need less voltage but more current,
higher impedance headphones need more voltage but less current. That
series protection resistor tends to compensate for those differences.
Engineers who design headphones understand all of this and they want
their headphones to work with anything they are plugged into, so they
tend to design for high voltage sensitivity.
It's a pretty safe assumption that any "brand name" headphones are going
to work with any well-designed headphone amp. The KX3 is a bit of a
special case, because Wayne always designs for low battery drain, which
means that his audio output stages are somewhat "current starved," so
they don't provide a lot of output. As usual, I find his designs to be
very good compromises in that regard.
I find the KX3 plenty loud enough with any of the decent headphones I've
tried, and pretty good in a quiet location with the internal speaker. I
virtually always operate with cans, and use the speaker only for casual
monitoring. I find the KX3 quite useful for chasing RFI or checking out
temporary antennas.
73, Jim K9YC
On Mon,1/23/2017 12:06 PM, w7aqk wrote:
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.
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