Re: USB sound cards (Steve Beet )

Subject: Re: USB sound cards
From:    Steve Beet  <stevebeet@xxxxxxxx>
Date:    Fri, 12 Dec 2014 23:36:00 -0000

This may be a red herring, but I've seen some self-proclaimed "audiophile" publications which claim that when headphones are driven from a resistive source impedance of a few tens of Ohms, they "sound better" than when they are driven from an ideal (very low impedance) voltage source. As far as I recall, these statements were referring to listening tests of professional-quality headphones with nominal impedances of 200 Ohms. These publications didn't present any analytical measurements to suggest why this might be the case, but the output resistance added to many headphone amps might not be there solely to prevent damage or distortion - it might also be to persuade audiophiles that they're getting the best sound quality. For music produced and mixed to be listened to via loudspeakers, it may be that adding a series resistor might indeed make the headphones sound more like the original mixing engineer or producer intended, but for scientific perceptual experiments I can't see any advantage in artificially increasing the resistance. Steve Beet -----Original Message----- From: AUDITORY - Research in Auditory Perception [mailto:AUDITORY@xxxxxxxx On Behalf Of Bob Masta Sent: 11 December 2014 17:03 To: AUDITORY@xxxxxxxx Subject: Re: USB sound cards However, there *is* a problem getting low output impedance as well. The native design of modern amplifier stages has essentially zero output impedance due to negative feedback (milliohms or less). That means that if you connect such an amp to a low-impedance load, the current draw can be high... high enough to damage the output stages, or at least cause massive distortion as they go into protective current limiting. Since these are for consumer use, where anyone can plug in most anything that fits the jack, manufacturer's typically add some output impedance.

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Electrical Engineering Dept., Columbia University