Re: USB sound cards

[Bob Masta <audio@xxxxxxxxxxx>]

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
 
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