[TenTec] 565 Low level audio on headphones

[Bob McGraw - K4TAX]

Excellent analysis and explanation!

73
Bob, K4TAX

----- Original Message ----- 
From: "Jim Brown" <k9yc at audiosystemsgroup.com>
To: <tentec at contesting.com>
Sent: Tuesday, March 19, 2013 5:52 PM
Subject: Re: [TenTec] 565 Low level audio on headphones


> 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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