Bumping up headphone impedance with resistors (to improve cmoy sound)
Mar 9, 2006 at 9:35 AM Thread Starter Post #1 of 3

gaplessophile

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In a cmoy amp I recently built, I tried putting a resistor (per channel) in series with the output jack, after seeing that idea mentioned on this forum. With my low impedance (16 ohm) Shure e2c's, this effectively bumps up the impedance that the opamp sees the headphones as, making it easier for it to supply enough current. The result is a fantastic improvement in sound quality on my e2c's, and a fair improvement on my MS-1's (32 ohm).

I was wondering, how would this technique compare to a buffered amp design? And are there any negative effects of putting a fairly high value resistor here? I'm using 470 ohm ones, I picked such a high value to also make the output of the amp quieter with my low impedance phones. I suppose since bumping up the impedance means you have to turn your amp's volume up higher to get the same volume in your headphones, the amount of voltage your amp requires increases, but I've found a 9V battery to still be more than sufficient.

Any thoughts, anyone?
 
Mar 9, 2006 at 12:31 PM Post #2 of 3

Garbz

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Yes this has an averse effect on the sound. It's to do with the dampening factor. There was a standard designed that headphones should be driven from a 120ohm source. In reality few companies adhere to it, and normally the lower the impedance the more the headphone performs to the intended spec. Opamps put out <1ohm typically, same with buffers. But as you observed they can run out of juice.

In this case all you can do is test it. If you think that they sound better now try the buffered design. You may like it much better. You may hate it. The point is it'll sound different because instead of dampening the output you are supplying enough power to drive it.

This has a REALLY nasty effect on speakers. They are 8ohm typically, and have many reactive components in the crossover. A 0.5ohm change in output impedance can dramatically change the frequency response of a speaker which is why all speaker manufacturers recommend the lowest possible impedance on speaker cables. But as headphones are less reactive and higher impedance the results are less accented.
 
Mar 10, 2006 at 6:08 AM Post #3 of 3

gaplessophile

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Ah thanks, that's very helpful. Damn, now I need to try a buffered design
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