Re: USB sound cards (Neil Adams )


Subject: Re: USB sound cards
From:    Neil Adams  <adamstcs@xxxxxxxx>
Date:    Thu, 18 Dec 2014 19:44:11 +0800
List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>

<html> <body> 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&nbsp; zero.<br><br> 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. <br><br> 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. <br><br> NeilA <br><br> At 04:43 PM 12/18/2014, Oberfeld-Twistel, Daniel wrote:<br> <blockquote type=3Dcite class=3Dcite cite=3D""> <font face=3D"Calibri" size=3D2 color=3D"#1F497D">Series resistors are of= ten put in for reasons of stability, for example when driving capacitive loads. An example is shown in figure 2 of the attached PDF.<br> &nbsp;<br> I also always believed that a low output impedance is optimal for precisely controlling a &quot;reactive&quot; load like a loudspeaker, although this is of course not the most power-efficient design (-&gt; 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 ;-) </font><br> <font face=3D"Calibri" size=3D2 color=3D"#1F497D">&nbsp;<br> [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=85 Interesting experience.]]<br> &nbsp;<br> Best<br> &nbsp;<br> Daniel<br> &nbsp;<br> &nbsp;<br> PD Dr. Daniel Oberfeld-Twistel<br> Johannes Gutenberg - Universitaet Mainz<br> Department of Psychology<br> Experimental Psychology<br> Wallstrasse 3<br> 55122 Mainz<br> Germany<br> &nbsp;<br> Phone ++49 (0) 6131 39 39274 <br> Fax&nbsp;&nbsp; ++49 (0) 6131 39 39268<br> <a href=3D"http://www.staff.uni-mainz.de/oberfeld/" eudora=3D"autourl"> http://www.staff.uni-mainz.de/oberfeld/</a><br> <a href=3D"https://www.facebook.com/WahrnehmungUndPsychophysikUniMainz" e= udora=3D"autourl"> https://www.facebook.com/WahrnehmungUndPsychophysikUniMainz</a><br> &nbsp;<br> </font><font size=3D2><b>From:</b> AUDITORY - Research in Auditory Perception [<a href=3D"mailto:AUDITORY@xxxxxxxx" eudora=3D"autourl"> mailto:AUDITORY@xxxxxxxx</a>] <b>On Behalf Of </b>James Johnston<br> <b>Sent:</b> Thursday, December 18, 2014 6:16 AM<br> <b>To:</b> AUDITORY@xxxxxxxx<br> <b>Subject:</b> Re: USB sound cards<br> </font><font face=3D"Times New Roman, Times">&nbsp;<br> 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.<br> 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.&nbsp; This is a key part of the filter design problem that makes the system something approximating flat at low frequencies.<br> 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.<br> &nbsp;<br> On Wed, Dec 17, 2014 at 3:51 AM, Jont Allen &lt;<a href=3D"mailto:jontalle@xxxxxxxx">jontalle@xxxxxxxx</a>&gt= ; wrote:<br> Jim + Auditory-list<br><br> While it is true that speakers are &quot;designed&quot; to be driven by the voltage, our detailed research has shown that it is the<br> 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<br> more, just in case you missed it.<br><br> 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:<br><br> <a href=3D"http://hear.ai.uiuc.edu/public/Allen/NK_ASA_Indianapolis.pdf"> http://hear.ai.uiuc.edu/public/Allen/NK_ASA_Indianapolis.pdf</a><br> * Kim and Allen (2014) <i>Historic transducers: Balanced Armature Receiver (BAR),</i> ASA meeting INDY, IN, Oct 30, 2014 <a href=3D"http://hear.ai.uiuc.edu/public/Allen/NK_ASA_Indianapolis.pdf"> pdf</a><br><br> The full theory is in a Hearing Research article:<br><br> <a href=3D"http://hear.ai.uiuc.edu/public/KimAllenMar20.13.pdf"> http://hear.ai.uiuc.edu/public/KimAllenMar20.13.pdf</a><br> * Kim, Noori and Allen, J.B. (2013). &quot;Two-port network Analysis and Modeling of a Balanced Armature Receiver, &quot;Hearing Research, special MEMRO 2013 issue (<a href=3D"http://hear.ai.uiuc.edu/public/KimAllenMar20.13.pdf">pdf</a>) <br><br> Ms Kim's (i.e., Noori's) PhD thesis has the most complete story, and is available directly from her (<a href=3D"mailto:noorimail@xxxxxxxx">noorimail@xxxxxxxx</a>).<br> <br> 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.<br> Hunt mostly quotes Mott and Minor's BSTJ Jan 1951 article, adding little new material to their story.<br><br> In the interesting case of the Etymotic ER-3 (insert headphone), they place a 10 ohm resistor across the input terminals. <br> 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.<br> 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.<br><br> 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.<br><br> In general, adding a series resistor to your circuit, is working blind. You will likely get a random result, and random is never better.<br> Thus I dont advise it unless you are willing to measure the input impedance and the resulting frequency response of the headphone. While there<br> are some interesting things you can do with a series impedance, a series resistor is not one of them.<br><br> Jont Allen<br><br> <br> On 12/16/2014 06:12 PM, James W. Beauchamp wrote:<br> </font> <dl><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>I agree with Bob. M= ost loudspeakers are designed to be driven by</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>an ideal voltage source. Under this criterion they attempt to get </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>the flattest possib= le response overall. This is not necessarily </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>the most efficient response. In fact, some loudspeaker designs, </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>like the Small-Thie= le closed-box non-vented design, are very </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>inefficient.</font> </pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>&nbsp;</font></pre> <font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>Jim</font></pre> <font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>&nbsp;</font></pre> <font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>Bob Masta wrote:</font></pre><font face=3D"Courier New, Courier"></font> <dl><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>From: Bob Masta <a href=3D"mailto:audio@xxxxxxxx">&lt;audio@xxxxxxxx&gt;</a></font> </pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>Date: Sun, 14 Dec 2= 014 09:15:19 -0500</font></pre><font face=3D"Courier New, Courier"></font><br= > <dd><pre><font face=3D"Courier New, Courier" size=3D2>To: <a href=3D"mailto:AUDITORY@xxxxxxxx">AUDITORY@xxxxxxxx</a> </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>Subject: Re: USB so= und cards</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>&nbsp;</font></pre> <font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>On 13 Dec 2014 at 21:59, Richard F. Lyon wrote:</font></pre><font face=3D"Courier New, Courier"></font> <dl><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>On Sat, Dec 13, 201= 4 at 5:52 AM, Bob Masta <a href=3D"mailto:audio@xxxxxxxx">&lt;audio@xxxxxxxx&gt;</a> wrote:</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>&nbsp;</font></pre> <font face=3D"Courier New, Courier"></font> <dl><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>It's the other way around:&nbsp; Adding resistance in the</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>driving circuit giv= es poorer damping.&nbsp; &quot;Damping Factor&quot; for</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>a power amplifier i= s the reciprocal of output impedance.</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>&nbsp;</font></pre> <font face=3D"Courier New, Courier"></font><br> </dl><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>Bob, I wasn't aware= of that definition.</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>I was thinking of <a href=3D"http://en.wikipedia.org/wiki/Damping_ratio"> http://en.wikipedia.org/wiki/Damping_ratio</a></font></pre> <font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>rather than <a href=3D"http://en.wikipedia.org/wiki/Damping_factor"> http://en.wikipedia.org/wiki/Damping_factor</a></font></pre> <font face=3D"Courier New, Courier"></font><br> </dl><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>Oops, you are correct... I should have said it is </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>*proportional* to t= he reciprocal.</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>&nbsp;</font></pre> <font face=3D"Courier New, Courier"></font> <dl><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>Is there an understanding of why high &quot;damping factor&quot; would be good?</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>Jont's findings sug= gest otherwise (I believe he's saying the current is</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>typically more rele= vant than the volage).</font></pre><font face=3D"Courier New, Courier"></font><br> </dl><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>The second referenc= e you cited covers it under </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2> &quot;Explanation&quot;.&nbsp; Basically, a conventional electrodynamic </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>speaker is both a m= otor and a generator.&nbsp; Imagine that the </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>speaker receives a momentary voltage pulse, after which is </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>is instantly disconnected from the source.&nbsp; The speaker </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>would ring at its resonant frequency, damped only by </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>friction.&nbsp; The generator would be creating a voltage, but </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>no current, so no l= oad to add damping.</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>&nbsp;</font></pre> <font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>However, if instead= of disconnection the leads were </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>*shorted* after the pulse, the generator would be driving </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>all its current int= o the zero-ohm load, giving a maximum </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>damping effect.&nbs= p; </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>&nbsp;</font></pre> <font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>Conventional loudspeakers are designed to be driven by </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>voltage sources, no= t current sources. The current may be </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>more &quot;relevant&quot; (in the sense of force generation), but not </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>for getting a flat frequency response from a conventional </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>speaker design.&nbs= p; (Although there have been occasional </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>attempts at current drive, the ones I recall required </font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>special dedicated amplifiers.)</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>&nbsp;</font></pre> <font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>Best regards,</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>&nbsp;</font></pre> <font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>Bob Masta</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>&nbsp;</font></pre> <font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; D A Q A R T A</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>Data AcQuisition An= d Real-Time Analysis</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <a href=3D"http://www.daqarta.com">www.daqarta.com</a></font></pre> <font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>Scope, Spectrum, Spectrogram, Signal Generator</font></pre><font face=3D"Courier New, Courier"></font><br> <dd><pre><font face=3D"Courier New, Courier" size=3D2>&nbsp;&nbsp; Scienc= e with your sound card!</font></pre><font face=3D"Courier New, Courier"></font><br> </dl> </dl><font face=3D"Times New Roman, Times">&nbsp;<br><br> <br> -- <br> James D. (jj) Johnston<br> Independent Audio and Electroacoustics Consultant<br> </font></blockquote></body> </html>


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