Re: USB sound cards

[Bob Masta <audio@xxxxxxxxxxx>]

On 12 Dec 2014 at 22:09, Richard F. Lyon wrote:

> It seems not unlikely that headphones (or speakers) might have some
> reactive elements and resonances, such that some added resistance in the
> driving circuit would lead to higher damping and a more even response.
> Someone should do a test, starting with impedance measurements of various
> headphones to see if they have identifiable resonances.  It might turn out
> that adding resistance is a good thing, for scientific perceptual
> experiments or otherwise.  I wouldn't be surprised either way.
> 
> Dick

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.  
Good power amps should have factors in the 100s (0.01 ohm) 
or even 1000s (0.001 ohm).  But that's for driving 
conventional speakers, not headphones.

However, you are certainly correct about reactive elements 
and resonances in headphones, just as there are in 
speakers.  Manufacturers would want to design their phones 
to work best with the expected driving impedances.  (This 
is different for speakers, where manufacturers expect them 
to be driven with near-zero impedances.)  So it would be 
perfectly reasonable for any given headphone to have a 
preferred driving impedance.

Another factor that has a huge effect on low frequency 
response is the seal to the ear canal, relative to what the 
phone was designed for.  In the olden days when all phones 
had big "surround" ear cushions, a good seal was absolutely 
imperative for low frequencies.  The basic idea was that 
with a sealed system, there is no lower limit on frequency 
response:  When the piston moves in, the pressure goes up, 
period... response all the way to DC (in theory).  Low-
frequency resonance was not a big deal, since it was 
driving a relatively stiff load. But once you break the 
seal, all bets are off.

The introduction of modern on-the-ear and in-the-ear phones 
changed the game.  These are designed more like free-field 
or near-field loudspeakers, where resonance is a critical 
part of the design process.  But (as everyone who uses them 
knows) the bass response still changes when you move them 
farther from the ear, or push them deeper into the canal.  
Some of that change is simply due to the sound "leaking 
out", but some is undoubtedly due to changing the acoustic 
mass seen by the driver, relative to what it was designed 
for.  That would surely change the system resonant 
frequency.

Best regards,

Bob Masta

==================
 
> 
> On Fri, Dec 12, 2014 at 3:36 PM, Steve Beet <stevebeet@xxxxxxxxx> wrote:
> 
> > 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@xxxxxxxxxxxxxxx] On Behalf Of Bob Masta
> > Sent: 11 December 2014 17:03
> > To: AUDITORY@xxxxxxxxxxxxxxx
> > 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.
> >
> 

Bob Masta
 
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