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Old 05-20-2017, 08:26 PM   #5
drumphil
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Part 2. Impedance.

Impedance matters for other reasons besides allowing you to calculate current and watts.

The relationship between the output impedance of the amp and the impedance of the headphones affects frequency response, distortion levels and volume in headphones.


http://nwavguy.blogspot.com.au/2011/...impedance.html

Quote:
WHY DOES OUTPUT IMPEDANCE MATTER? It matters for at least three reasons:

The greater the output impedance the greater the voltage drop with lower impedance loads. This drop can be large to enough to prevent driving low impedance headphones to sufficiently loud levels. A real world example is the Behringer UCA202 with a 50 ohm output impedance. It struggles with some 16 - 32 ohm headphones.

Headphone impedance changes with frequency. If the output impedance is much above zero this means the voltage delivered to the headphones will also change with frequency. The greater the output impedance, the greater the frequency response deviations. Different headphones will interact in different, and typically unpredictable, ways with the source. Sometimes these variations can be large and plainly audible.

As output impedance increases electrical damping is reduced. The bass performance of the headphones, as designed by the manufacture, may be audibly compromised if there’s insufficient damping. The bass might become more “boomy” and less controlled. The transient response becomes worse and the deep bass performance is compromised (the headphones will roll off sooner at low frequencies). A few, such as those who like a very warm “tube like” sound, might enjoy this sort of under damped bass. But it’s almost always less accurate compared to using a low impedance source.
The guy who wrote that is the guy who designed the O2 headphone amp, and released the design to the world under a creative commons type license. Anyone can build them and sell them.

One of the best designs around. The guy who designed never it revealed his identity, but it is assumed he is an electrical engineer working for some company, who got sick of the terrible designs going into headphone amps everywhere, and decided to do somethings about it.

If you want a really good headphone amp, get one of those.

Well worth checking out the rest of the stuff on his site. He goes into a lot of detail on the technicalities of building a headphone amp properly.


OK, to your headphones:

32 ohm... That's unfortunate. To not run into the problems mentioned in the quote above, you want at least 8 to 1, or more ideally 10 to 1 or greater ratio between the input impedance of the headphones and the output impedance of the headphone amp.

So, for 32 ohm headphones, you'd want a headphone amp with an output impedance of 3.2 ohms.

The scarlett has about three times that output impedance at 10 ohms. I'd be willing to bet that the mackie and tascam are much worse. Possibly over 100 ohms. 60 if you're lucky.


I wonder how much of your troubles are related to impedance and how much is related to the power of the amps? I mean, the amps are all pretty weak, but the impedance issue isn't helping, and degrades both tone and volume.


The O2 headphone amp would actually drive your headphone properly. It has an output impedance of 0.54 ohms! It can output 613 mW into 32 ohms, 355 mW into 150 ohms, and 88mW into 600 ohms.

But realistically, few 32 ohm headphones are very good, and if you're going bother to get an O2 headphone amp, you're probably going to want better headphones anyway.

Last edited by drumphil; 06-02-2017 at 06:05 AM.
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