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Resistive load testing, how can the measurements match the headphone response?

post #1 of 5
Thread Starter 

How can they model headphones as load?  Headphones are not only resistive, but I guess resistors can be reactive at very high frequencies, but higher than audible, so it should not apply when testing the ouputs of device sources with resistors. Can it match noise?  Distortion?  Resistors respond differently to noise than headphones which are inductive loads, magnetic.  I would think Distortion measurments would differ from resistors.

 

When you see FR graphs of devices going all crazy with resistive loads, that can't be accurate.


Edited by SilverEars - 3/16/14 at 8:09pm
post #2 of 5
You don't have to model anything. You could just use a headphone for the load. Though I doubt it would change any of the measurements appreciably for most any competently designed amp.

se
post #3 of 5
I think the point in using a resistive load is that a precise resistance is easily obtained and the test is repeatable.

Different headphones have different frequency-dependent impedance behaviors, even among the same headphone model. This reduces the general applicability and repeatability of the test if it is specifically tailored to one headphone's impedance.

A lot can be learned about an amplifier using a simple resistance load and it is even an excellent model for orthodynamic designs.

Cheers
post #4 of 5

Most headphones, despite the back emf, resonances, quirks in mechanical and acoustic properties (e.g. electrical impedance can measure differently when clamped on head vs. not), the fact that you're dealing with a coil, etc. actually still are pretty resistive throughout the audio range. At higher frequencies than audio, I guess it looks more inductive than resistive, but not as much under 20 kHz. You can see the impedance sloping up at high frequencies at some headphones.

 

I'm not sure what you mean by this:

[quote]When you see FR graphs of devices going all crazy with resistive loads, that can't be accurate.[/quote]

 

Maybe some amps are marginally stable and misbehave with some non-resistors, like maybe headphones connected with a very long (capacitive) cable.

 

The biggest thing to look out for is the output impedance, which can cause easily non-trivial differences in frequency response. On some sets it results in a little higher nonlinear distortion as well.

 

But anyway, the reasons for using resistors are outlined in the first two replies. Anyhow, it's been suggested that some amps measure great into resistors but don't look as good with actual headphones, but when pressed to demonstrate that (hook up headphones as load instead of resistors and measure that), people don't seem to be able to demonstrate that happening.

post #5 of 5

Don't forget the cabling.  Depending on the cable geometry, the cabling can look capacitive or inductive structure in series with the headphone impedance.

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