[aos]

I did some RMAA 5.1 measurements on my Sony SACD player for no particular reason. Here's the headphone amplifier results, when not loaded and when loaded with 3 different headphones.

http://members.shaw.ca/ifkovic/elect...comparison.htm

Take a look at frequency response. Looks familiar? I went to headroom website and looked at impedance graphs of headphones:

http://www.headphone.com/graphCompar...are+Headphones

Look at the shape of the graphs (where they peak and where they dip, rise and fall etc.). So did I find a tool to measure (at least in relative terms) the impedance of a headphone? Sure looks that way. Note that the volume control knob was not at the same position for all 3 headphones as the level during calibration did not match - that should've been the clue right there, I've never dealt with an amp that has varying output voltage depending on what headphones are plugged in.

Perhaps surprisingly, it still souds good, to me at least.

 

[PeterR]

Quote:

Originally posted by aos Look at the shape of the graphs (where they peak and where they dip, rise and fall etc.).

Yes, the amp's output impedance forms a voltage divider with the headphone impedance - as this is variable over frequency you get the result you're seeing...
Quote:

So did I find a tool to measure (at least in relative terms) the impedance of a headphone?

Yes, placing a known resistance in series is the way this is measured.

Quote:

I've never dealt with an amp that has varying output voltage depending on what headphones are plugged in.

You sure have, you just never noticed because usually it's not that pronounced. That Sennheiser graph looks like output impedance is somewhere around 180 ohms...

 

[aos]

Quote:

You sure have, you just never noticed because usually it's not that pronounced.

I think that's only of academic interest with good amps. In practice it's below the sensitivity level of our "equipment" (soundcards).

 

[Joe Bloggs]

Funny, it's just a CDP with a headphone amplifier--why would the output impedence still be so high? It's as though they put a loudspeaker amp in there and then had to lower the output using an inline resistor.

Quote:

Perhaps surprisingly, it still souds good, to me at least.

If you really like it, you can probably improve it by using the impeder--then the frequency response would be pretty much flattened (or, knowing you, you could open up the player and change the resistances around :O )

Quote:

I've never dealt with an amp that has varying output voltage depending on what headphones are plugged in. --- You sure have, you just never noticed because usually it's not that pronounced. --- I think that's only of academic interest with good amps. In practice it's below the sensitivity level of our "equipment" (soundcards).

Any amp with non-zero output impedance will have varying output voltage depending on what load is put on it.

Can't believe there's something I can teach aos

 

[aos]

Quote:

Any amp with non-zero output impedance will have varying output voltage depending on what load is put on it.

Yeah, I haven't thought of that. Anyhow, a neat way to graph impedance vs frequency for any headphone. I guess one can just add a resistor in series with output of any amp, then measure off that point.

 

[Joe Bloggs]

great, we need to teach somebody to graph the impedance response of the R10!