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What are the fundamental differences between speaker amps and headphone amps?

post #1 of 4
11/6/07 at 7:36pm
Thread Starter 
What I'd like to know about are the technical differences between amps that are designed to drive speakers and those that are designed to drive headphones. Someone suggested building this amplifier instead of a beta22 for driving headphones, but after seeing the following graph of the amplifier's THD+N with respect to output power, it would seem pretty pointless to build it with the intention of using it to drive headphones.



The amp's distortion bottoms out to nearly immeasurable levels at around 40W, but is several orders of magnitude higher at 10mW, which is still about 10 times more power than I'll need. This is with the amp biased for a maximum output of 100W. Is adapting this design for driving headphones (specifically the UE9 with 30 ohm and 96db) as simple as reducing the bias voltage, or are there fundamental differences between amplifiers meant for outputting higher wattages and/or driving the low impedance load presented by speakers, and amps intended to drive headphones, which are of generally higher impedance and very high sensitivity?

As you can tell I haven't read too much into this amp's design yet. Though I am particularly interested in this design, I'm mainly hoping that answers I get will help me know what kinds of designs would be best suited to my application.

Thanks, guys.
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post #2 of 4
11/6/07 at 8:01pm
Nevermind.
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post #3 of 4
11/7/07 at 12:58am
A 100W-class speaker power amp would have way too much gain for most dynamic headphones, especially low-Z ones. The THD vs. power curve posted above is very typical of many amplifiers, and is the result of several factors, the dominant one being noise floor. As the power output is decreased, the noise voltage becomes a larger percentage of the signal. Since the curve actually shows THD+Noise vs. power output, it leads to the shape of curve you see.

Increasing the bias won't make things better as that amp is already class A, and decreasing the bias will most likely increase distortion.

Reducing the gain would decrease the noise floor and move the curve downward. But, as you said, what is the point of using a 100W amp to drive headphones? All you're doing would be increasing the chance of frying the headphones.
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post #4 of 4
11/7/07 at 4:26am
You can use the front end of the amplifier (top part in the schematic) and create your own output buffers.
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