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Re: USB sound cards

The series resistor shown in the Analog Devices application note is for stability, as you say. However, since it is within the overall negative feedback loop, the circuit's output impedance is reduced close to  zero.

The possibility of current driving rather than voltage driving transducers depends very much on the transducer's design and the objectives. With moving coil motors connected to near-rigid diaphragms such as in loudspeakers and many headphones (not balanced armature types) a pure voltage drive is generally needed for best results as it is the displacement rather than the force that is the key.

Electrical impedance matching is usually to avoid reflections in long transmission lines and, in practical audio circuits where the source impedance is due to a resistor, ensures that half the power is actually dissipated as heat.


At 04:43 PM 12/18/2014, Oberfeld-Twistel, Daniel wrote:
Series resistors are often put in for reasons of stability, for example when driving capacitive loads. An example is shown in figure 2 of the attached PDF.
I also always believed that a low output impedance is optimal for precisely controlling a "reactive" load like a loudspeaker, although this is of course not the most power-efficient design (-> see impendance -matched transmission lines). At least that is what most texts on amplifier design suggest - but anyway, that might be wrong and hey, I'm only a psychologist, not an electrical engineer ;-)

[Although I actually had to build a low-noise headphone amplifier for my PhD experiments, because back then my lab wanted to save the money for something like a TDT device? Interesting experience.]]
PD Dr. Daniel Oberfeld-Twistel
Johannes Gutenberg - Universitaet Mainz
Department of Psychology
Experimental Psychology
Wallstrasse 3
55122 Mainz
Phone ++49 (0) 6131 39 39274
Fax   ++49 (0) 6131 39 39268
From: AUDITORY - Research in Auditory Perception [ mailto:AUDITORY@xxxxxxxxxxxxxxx] On Behalf Of James Johnston
Sent: Thursday, December 18, 2014 6:16 AM
To: AUDITORY@xxxxxxxxxxxxxxx
Subject: Re: USB sound cards
Not sure of your point, Jont. Of course it is the current through the voice coil that actually moves the diaphragm and thus makes the pressure.
None the less, the back EMF is key, especially for low-frequency system designs, and the back EMF is converted to current by both the internal and external resistance.  This is a key part of the filter design problem that makes the system something approximating flat at low frequencies.
I'm not sure who you think is putting resistors in series, one generally avoids that, except for the obligatory resistance of a crossover if one is using a passive crossover system.
On Wed, Dec 17, 2014 at 3:51 AM, Jont Allen <jontalle@xxxxxxxxxxxx> wrote:
Jim + Auditory-list

While it is true that speakers are "designed" to be driven by the voltage, our detailed research has shown that it is the
current that controls the pressure (i.e., force on the cone). If you want the details I already sent the links, but I'll send them once
more, just in case you missed it.

The theory of how the current drives the force is given by our ASA presentation from last month, at the special session on historic transducers:

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

The full theory is in a Hearing Research article:

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

Ms Kim's (i.e., Noori's) PhD thesis has the most complete story, and is available directly from her (noorimail@xxxxxxxxx).

Because it is the current that determines the pressure, via the electrical input impedance, and because this impedance can be (i.e., is) reactive (have inductance), it can be tricky to figure out what is going on. One must know (i.e., measure) the load impedance, or at least the current. Also the distortion is mostly determined by the nonlinear flux vs current (i.e., B-H curve), as described by Hunt, for example.
Hunt mostly quotes Mott and Minor's BSTJ Jan 1951 article, adding little new material to their story.

In the interesting case of the Etymotic ER-3 (insert headphone), they place a 10 ohm resistor across the input terminals.
So if you try to beat this with a series resistor, you will not be successful in changing anything, because all the current is going into that 10 ohm resistor.
I opened up one of my ER-3 and cut out the resistor, and then I could see what was really going on. It worked better too.

When that ER-3 went back for repairs once, they were shocked to see the resistor was missing. I had to admit I cut it out, and wanted it left that way.

In general, adding a series resistor to your circuit, is working blind. You will likely get a random result, and random is never better.
Thus I dont advise it unless you are willing to measure the input impedance and the resulting frequency response of the headphone. While there
are some interesting things you can do with a series impedance, a series resistor is not one of them.

Jont Allen

On 12/16/2014 06:12 PM, James W. Beauchamp wrote:

I agree with Bob. Most
loudspeakers are designed to be driven

an ideal voltage
source. Under this criterion they attempt to get

the flattest possible
response overall. This is not necessarily

the most efficient
response. In fact, some loudspeaker designs,

like the Small-Thiele
closed-box non-vented design, are very





Bob Masta

From: Bob Masta

Date: Sun, 14 Dec 2014
09:15:19 -0500


Subject: Re: USB sound


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

On Sat, Dec 13, 2014 at
5:52 AM, Bob Masta


It's the other way
around:  Adding resistance in

driving circuit gives
poorer damping.  "Damping Factor"

a power amplifier is
the reciprocal of output


Bob, I wasn't aware of

I was thinking of


rather than


Oops, you are
correct... I should have said it is

*proportional* to the


Is there an
understanding of why high "damping factor" would be

Jont's findings suggest
otherwise (I believe he's saying the current

typically more relevant
than the

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


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. 


loudspeakers are designed to be driven by

voltage sources, not
current sources. The current may be

"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






           D A Q A R T

Data AcQuisition And


Scope, Spectrum,
Spectrogram, Signal

with your sound


James D. (jj) Johnston
Independent Audio and Electroacoustics Consultant