[TenTec] 565 Low level audio on headphones

[Jim Brown]

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