Re: USB sound cards (Bob Masta )

Subject: Re: USB sound cards
From:    Bob Masta  <audio@xxxxxxxx>
Date:    Fri, 19 Dec 2014 09:24:15 -0500

On 19 Dec 2014 at 8:03, Oberfeld-Twistel, Daniel wrote: > Dear Bob, > > thanks for clarifying this! So I overlooked that the R > is inside the feedback loop, embarrassing... That's why > I said don't trust my opinion as a psychologist, I > wished to have a better understanding of these EE design > issues... > > But what is the bottom line - is low output impendance > better, or do Jont's analyses indicate the opposite? As far as I can see from a quick read-through, Jont's analysis is for a specific type of transducer (balanced armature), which is very different from the standard moving coil "electrodynamic" movement used in almost all speakers and widely used in headphones. The theory of electrodynamic transducers has been pretty well reduced to practice for 50+ years. These must be driven by a voltage source in order to have any appreciable flat frequency response range. The idea is not to get maximum driving force at any given frequency, it's to balance out drive and loading curves to get a useful flat region. In general, for ordinary loudspeakers, output impedance should be as low as possible... otherwise you are wasting power and (more importantly) reducing peak output level. You are also reducing control of the diaphraghm, especially important on transients. With headphones, however, the power involved is much lower, as is the mass of the diaphraghm you are trying to control. So I can easily imagine that there may be cases where the benefits of low output impedance are outweighed by other design considerations. For on-the-ear and in-the-ear types especially, the overall system is quite different from the standard "piston driving a free-space air load" that is the basis of standard electrodynamic speaker theory. So it seems at least conceivable that some designs could be improved by non-zero amplifier impedance. However, since most amplifiers have either very low or unknown output impedance, it's not clear how headphone manufacturers could deal with this except by selling the headphone and amp as a set. Might be a good marketing ploy for audiophiles (more exotic is always better!), but probably not for mass-market phones. Best regards, Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!

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Electrical Engineering Dept., Columbia University