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Re: USB sound cards

Steve and list,

My student Noori Kim and I have looked into some of these questions in detail. Here is a paper we gave at the ASA meeting. They put us in the wrong session. There was a special session on Historic transducers, on Tuesday, and they put us in a regular transducer session, on Thurs.
It was an unfortunate mistake, for us.

In this presentation it shows how the current is the drive to the flux, which is the source of the force. Thus it is not the current that drives
the pressure, it is the current.

By placing a resistor in series with the drive-amplifier, you can
reduce the distortion, as you come closer to a current source.

Here is the presentation we gave at the ASA meeting. Noori's PhD theis
is now available, as well as her publication in hearing research, which
is a reference in the ASA talk.

Kim and Allen (2014)
"Historic transducers: Balanced Armature Receiver (BAR),"
ASA meeting INDY, IN, Oct 30, 2014


Kim, Noori and Allen, J.B. (2013). "Two-port network Analysis and Modeling of a Balanced Armature Receiver, "Hearing Research, special MEMRO 2013 issue



FROM: Jont Allen
Sent: Dec 13 5:05AM, 2014


Are these claims about magnetic earphones, or electrostatic earphones?

They must be direct drive, by a voltage, which then creates a current. Right?

Otherwise this makes no sense that it would depend on the source impedance. Right?

Following up on this then, the difference is in the current vs voltage.

If the impedance is nonlinear (it is in a magnetic earphone), then this makes sense.

The flux vs current has a non-linear relationship. This is well documented in the literature actually. It is the flux that creates the force on the diaphragm in magnetic drive.

Driving the inductive impedance with a current source would be an alternative if it were not for the instability that ensues. Have you ever tried to drive an inductor with a current source?
Do you know what happens?


On 12/12/2014 05:36 PM, Steve Beet 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.