On 3/19/2013 1:31 PM, Barry N1EU wrote:
Yeah, my mileage does vary. My 200-ohm Heil Pros are significantly more
sensitive/efficient than my 50-ohm Beyer DT109s
Maybe, maybe not. There's also the matter of the voltage divider formed
by the series protection resistor and the load impedance of the
headphones. There will be more voltage dropped (and thus power lost) in
the series resistor when 50 ohm phones are plugged in than when 200 ohm
phones are plugged in.
Efficiency is POWER -- how much power in for how much acoustic power
out. Sensitivity is usually defined as a VOLTAGE sensitivity -- how
much VOLTAGE does it take across the headphones for a given amount of
acoustic power out. With the resistor in the circuit, part of the power
produced by the output stage is dropped across the resistor, so there is
less across the headphones.
Looking at it another way, let's say the headphone amp puts out a 1V RMS
sine wave at clip. With a 200 ohm series protection resistor and 200 ohm
phones, half of the voltage is dropped in the resistor (0.5 volts) and
there's 0.5 volts across the phones. That's 6dB. With 50 ohm phones, 80%
of the voltage is across the resistor, only 0.2 volts across the phones.
That's 13dB, so 50 ohm phones are going to burn 7 dB more in the 200 ohm
resistor than 200 ohm phones. This voltage divider action from the
protection resistor is in addition to the EFFICIENCY of the headphones
themselves, which depends entirely on their physical construction.
This is very different from a loudspeaker power amp, where there is NO
protection resistor. Let's say the power amp is rated for 100W into 8
ohms. That same amp will do 200W into a 4 ohm load (if it has a beefy
power supply), but only 50 watts into a 16 ohm load. Heck -- many power
amps are happy driving 2 ohms, although the output voltage may drop
because the power supply isn't beefy enough. Why? Because the output
impedance is a small fraction of an ohm, so the same voltage is across
the speaker no matter what the load impedance (until the power supply
runs out of gas or protection circuitry kicks in).
73, Jim K9YC
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