Re: USB sound cards

[Bob Masta <audio@xxxxxxxxxxx>]

On 13 Dec 2014 at 21:59, Richard F. Lyon wrote:

> On Sat, Dec 13, 2014 at 5:52 AM, Bob Masta <audio@xxxxxxxxxxx> wrote:
> 
> > It's the other way around:  Adding resistance in the
> > driving circuit gives poorer damping.  "Damping Factor" for
> > a power amplifier is the reciprocal of output impedance.
> >
> 
> Bob, I wasn't aware of that definition.
> I was thinking of http://en.wikipedia.org/wiki/Damping_ratio
> rather than http://en.wikipedia.org/wiki/Damping_factor

Oops, you are correct... I should have said it is 
*proportional* to the reciprocal.
 
> Is there an understanding of why high "damping factor" would be good?
> Jont's findings suggest otherwise (I believe he's saying the current is
> typically more relevant than the volage).
> 

The second reference you cited covers it under 
"Explanation".  Basically, a conventional electrodynamic 
speaker is both a motor and a generator.  Imagine that the 
speaker receives a momentary voltage pulse, after which is 
is instantly disconnected from the source.  The speaker 
would ring at its resonant frequency, damped only by 
friction.  The generator would be creating a voltage, but 
no current, so no load to add damping.

However, if instead of disconnection the leads were 
*shorted* after the pulse, the generator would be driving 
all its current into the zero-ohm load, giving a maximum 
damping effect.  

Conventional loudspeakers are designed to be driven by 
voltage sources, not current sources. The current may be 
more "relevant" (in the sense of force generation), but not 
for getting a flat frequency response from a conventional 
speaker design.  (Although there have been occasional 
attempts at current drive, the ones I recall required 
special dedicated amplifiers.)

Best regards,

Bob Masta
 
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