Re: USB sound cards (James Johnston )

Subject: Re: USB sound cards From: James Johnston <audioskeptic@xxxxxxxx> Date: Thu, 18 Dec 2014 11:41:23 -0800 List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY> --90e6ba3fd739f922e3050a82c623 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Actually, some number of us ARE EE's. Please. I was referring to putting a series resistor in a system for the speaker's sake. If you have an amp that can't handle the impedance of what it has to drive without a resistor, or a parallel resistor/inductor combo like one sometimes sees at the output of a power amplifier, that's a different question, and in higher powered systems, it's almost always the parallel combination for obvious reasons. In soundcards, there is most often a large (but not large enough) capacitor because the amplifier has a single supply and thus needs decoupling. There are some amp chips that have on-board charge pumps that can rectify that, but in my experience they haven't actually penetrated the market, allegedly for cost reasons, even though one could reduce part count. (There is an "interesting" story here regarding requirements from one of my previous employers.) On Thu, Dec 18, 2014 at 1:40 AM, Zlatan Ribic <zlatan@xxxxxxxx> wrote: > > I think no one in this discussion (except Bob) is "electrical engineer" > Zlatan > > > *From:* Oberfeld-Twistel, Daniel <oberfeld@xxxxxxxx> > *Sent:* Thursday, December 18, 2014 9:43 AM > *To:* AUDITORY@xxxxxxxx > *Subject:* Re: USB sound cards > > 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 amplifi= er > 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=E2=80=A6 Interesting experience.] > > > > Best > > > > Daniel > > > > > > PD Dr. Daniel Oberfeld-Twistel > > Johannes Gutenberg - Universitaet Mainz > > Department of Psychology > > Experimental Psychology > > Wallstrasse 3 > > 55122 Mainz > > Germany > > > > Phone ++49 (0) 6131 39 39274 > > Fax ++49 (0) 6131 39 39268 > > http://www.staff.uni-mainz.de/oberfeld/ > > https://www.facebook.com/WahrnehmungUndPsychophysikUniMainz > > > > *From:* AUDITORY - Research in Auditory Perception [mailto: > AUDITORY@xxxxxxxx *On Behalf Of *James Johnston > *Sent:* Thursday, December 18, 2014 6:16 AM > *To:* AUDITORY@xxxxxxxx > *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 an= d > external resistance. This is a key part of the filter design problem tha= t > 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 i= s > using a passive crossover system. > > > > On Wed, Dec 17, 2014 at 3:51 AM, Jont Allen <jontalle@xxxxxxxx> 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: > > http://hear.ai.uiuc.edu/public/Allen/NK_ASA_Indianapolis.pdf > * Kim and Allen (2014) *Historic transducers: Balanced Armature Receiver > (BAR),* ASA meeting INDY, IN, Oct 30, 2014 pdf > <http://hear.ai.uiuc.edu/public/Allen/NK_ASA_Indianapolis.pdf> > > > The full theory is in a Hearing Research article: > > http://hear.ai.uiuc.edu/public/KimAllenMar20.13.pdf > * Kim, Noori and Allen, J.B. (2013). "Two-port network Analysis and > Modeling of a Balanced Armature Receiver, "Hearing Research, special MEMR= O > 2013 issue (pdf <http://hear.ai.uiuc.edu/public/KimAllenMar20.13.pdf>) > > Ms Kim's (i.e., Noori's) PhD thesis has the most complete story, and is > available directly from her (noorimail@xxxxxxxx). > > Because it is the current that determines the pressure, via the electrica= l > 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. Al= so > 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 th= at > 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 tha= t > 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 th= ere > 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 by > > 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 > > inefficient. > > > > Jim > > > > Bob Masta wrote: > > From: Bob Masta <audio@xxxxxxxx> <audio@xxxxxxxx> > > Date: Sun, 14 Dec 2014 09:15:19 -0500 > > To: AUDITORY@xxxxxxxx > > Subject: Re: USB sound cards > > > > On 13 Dec 2014 at 21:59, Richard F. Lyon wrote: > > On Sat, Dec 13, 2014 at 5:52 AM, Bob Masta <audio@xxxxxxxx> <audio@xxxxxxxx= arta.com> 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 > > > > D A Q A R T A > > Data AcQuisition And Real-Time Analysis > > www.daqarta.com > > Scope, Spectrum, Spectrogram, Signal Generator > > Science with your sound card! > > > > > > -- > > James D. (jj) Johnston > > Independent Audio and Electroacoustics Consultant > --=20 James D. (jj) Johnston Independent Audio and Electroacoustics Consultant --90e6ba3fd739f922e3050a82c623 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable <div dir=3D"ltr"><div><div>Actually, some number of us ARE EE's. Please= . I was referring to putting a series resistor in a system for the speaker&= #39;s sake. </div>If you have an amp that can't handle the imped= ance of what it has to drive without a resistor, or a parallel resistor/ind= uctor combo like one sometimes sees at the output of a power amplifier, tha= t's a different question, and in higher powered systems, it's almos= t always the parallel combination for obvious reasons. </div>In soun= dcards, there is most often a large (but not large enough) capacitor becaus= e the amplifier has a single supply and thus needs decoupling. There are so= me amp chips that have on-board charge pumps that can rectify that, but in = my experience they haven't actually penetrated the market, allegedly fo= r cost reasons, even though one could reduce part count. (There is an &quot= ;interesting" story here regarding requirements from one of my previou= s employers.) </div><div class=3D"gmail_extra"> <div class=3D"gmail_q= uote">On Thu, Dec 18, 2014 at 1:40 AM, Zlatan Ribic <<= a href=3D"mailto:zlatan@xxxxxxxx" target=3D"_blank">zlatan@xxxxxxxx</= a>> wrote:<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0= .8ex;border-left:1px #ccc solid;padding-left:1ex"> <div style=3D"PADDING-TOP:15px;PADDING-LEFT:10px;PADDING-RIGHT:10px" link= =3D"blue" vlink=3D"purple" name=3D"Compose message area" lang=3D"DE"> <div>I think no one in this discussion (except Bob) = is=20 "electrical engineer"</div> <div>Zlatan</div> <div>=C2=A0</div> <div style=3D"FONT:10pt Tahoma"> <div> </div> <div style=3D"BACKGROUND:#f5f5f5"> <div>From: <a title=3D"mailto:oberfeld@xxxxxxxx STRG + Klicken, um Verkn=C3=BCpfung zu folgen" href=3D"mailto:oberfeld@xxxxxxxx= MAINZ.DE" target=3D"_blank">Oberfeld-Twistel, Daniel</a> </div> <div>Sent: Thursday, December 18, 2014 9:43 AM</div> <div>To: <a title=3D"mailto:AUDITORY@xxxxxxxx STRG + Klicken, um Verkn=C3=BCpfung zu folgen" href=3D"mailto:AUDITORY@xxxxxxxx= S.MCGILL.CA" target=3D"_blank">AUDITORY@xxxxxxxx</a> </div> <div>Subject: Re: USB sound cards</div></div></div> <div> </div> <div> Series=20 resistors are often put in for reasons of stability, for example when drivi= ng=20 capacitive loads. An example is shown in figure 2 of the attached=20 PDF. =C2=A0 I=20 also always believed that a low output impedance is optimal for precisely= =20 controlling a "reactive" load like a loudspeaker, although this i= s of course not=20 the most power-efficient design (-> see impendance -matched transmission= =20 lines). At least that is what most texts on amplifier design suggest - but= =20 anyway, that might be wrong and hey, I'm only a psychologist, not an el= ectrical=20 engineer ;-) =C2=A0 [Although=20 I actually had to build a low-noise headphone amplifier for my PhD experime= nts,=20 because back then my lab wanted to save the money for something like a TDT= =20 device=E2=80=A6 Interesting experience.] =C2=A0 Best =C2=A0 Daniel =C2=A0 <div style=3D"BORDER-TOP:medium none;BORDER-RIGHT:medium none;BORDER-BOTTOM= :windowtext 1pt solid;PADDING-BOTTOM:1pt;PADDING-TOP:0cm;PADDING-LEFT:0cm;B= ORDER-LEFT:medium none;PADDING-RIGHT:0cm"> <p class=3D"MsoNormal" style=3D"BORDER-TOP:medium none;BORDER-RIGHT:medium = none;BORDER-BOTTOM:medium none;PADDING-BOTTOM:0cm;PADDING-TOP:0cm;PADDING-L= EFT:0cm;BORDER-LEFT:medium none;PADDING-RIGHT:0cm">=C2=A0</= u></div> PD=20 Dr. Daniel Oberfeld-Twistel Johannes=20 Gutenberg - Universitaet Mainz Department=20 of Psychology Experimental=20 Psychology Wallstrasse=20 3 55122=20 Mainz Germany =C2=A0 Phone=20 ++49 (0) 6131 39 39274 Fax=C2=A0=C2=A0=20 ++49 (0) 6131 39 39268 <a href=3D"http://www.staff.uni-mainz.de/oberf= eld/" target=3D"_blank">http://www.staff.uni-mainz.de/oberfeld/</a><= u> <a href=3D"https://www.facebook.com/WahrnehmungUndPsychophysi= kUniMainz" target=3D"_blank">https://www.facebook.com/WahrnehmungUndPsychop= hysikUniMainz</a> =C2=A0 <div style=3D"BORDER-TOP:medium none;BORDER-RIGHT:medium none;BORDER= -BOTTOM:medium none;PADDING-BOTTOM:0cm;PADDING-TOP:0cm;PADDING-LEFT:4pt;BOR= DER-LEFT:blue 1.5pt solid;PADDING-RIGHT:0cm"> <div> <div style=3D"BORDER-TOP:#b5c4df 1pt solid;BORDER-RIGHT:medium none;BORDER-= BOTTOM:medium none;PADDING-BOTTOM:0cm;PADDING-TOP:3pt;PADDING-LEFT:0cm;BORD= ER-LEFT:medium none;PADDING-RIGHT:0cm"> = From: AUDITORY - Research= =20 in Auditory Perception [mailto:<a href=3D"mailto:AUDITORY@xxxxxxxx" = target=3D"_blank">AUDITORY@xxxxxxxx</a>] On Behalf Of James Johnston <spa= n style=3D"FONT-WEIGHT:bold">Sent: Thursday, December 18, 2014 6= :16=20 AM To:</span= >=20 <a href=3D"mailto:AUDITORY@xxxxxxxx" target=3D"_blank">AUDITORY@xxxxxxxx= S.MCGILL.CA</a> Subject: = Re: USB sound=20 cards</div></div> =C2=A0 <div> <div> <div> Not sure of your point,= =20 Jont. Of course it is the current through the voice coil that actually move= s the=20 diaphragm and thus makes the pressure.</div= > None the less, the back= EMF=20 is key, especially for low-frequency system designs, and the back EMF is=20 converted to current by both the internal and external resistance.=C2=A0 Th= is is=20 a key part of the filter design problem that makes the system something=20 approximating flat at low frequencies.</spa= n></div> I'm not sure who you think is putting resistors in= =20 series, one generally avoids that, except for the obligatory resistance of = a=20 crossover if one is using a passive crossover=20 system.</div> <div> =C2=A0 <div> On Wed, Dec 17, 2014 at 3:51 AM, Jont Allen <<a hre= f=3D"mailto:jontalle@xxxxxxxx" target=3D"_blank">jontalle@xxxxxxxx<= /a>>=20 wrote: <div><div class=3D"h5"><div> Jim + Auditory-list While it is true that=20 speakers are "designed" to be driven by the voltage, our detailed= research has=20 shown that it is the current that controls the pressure (i.e., force on = the=20 cone). If you want the details I already sent the links, but I'll send = them=20 once more, just in case you missed it. The theory of how the curr= ent=20 drives the force is given by our ASA presentation from last month, at the= =20 special session on historic transducers: <a href=3D"http://hear.ai.u= iuc.edu/public/Allen/NK_ASA_Indianapolis.pdf" target=3D"_blank">http://hear= .ai.uiuc.edu/public/Allen/NK_ASA_Indianapolis.pdf</a> *=20 Kim and Allen (2014) Historic transdu= cers:=20 Balanced Armature Receiver (BAR), ASA meeting INDY, IN, Oct= 30,=20 2014 <a href=3D"http://hear.ai.uiuc.edu/public/Allen/NK_ASA_Indianapolis.pd= f" target=3D"_blank">pdf</a> <div> The full theory is in a Hearing Research=20 article: <a href=3D"http://hear.ai.uiuc.edu/public/KimAllenMar20.13.= pdf" target=3D"_blank">http://hear.ai.uiuc.edu/public/KimAllenMar20.13.pdf<= /a> * Kim,=20 Noori and Allen, J.B. (2013). "Two-port network Analysis and Modeling = of a=20 Balanced Armature Receiver, "Hearing Research, special MEMRO 2013 issu= e (<a href=3D"http://hear.ai.uiuc.edu/public/KimAllenMar20.13.pdf" target= =3D"_blank">pdf</a>) Ms Kim's (i.e., Noori's) PhD thesis ha= s the most=20 complete story, and is available directly from her (<a href=3D"mailto:noori= mail@xxxxxxxx" target=3D"_blank">noorimail@xxxxxxxx</a>). Because = it is the current that=20 determines the pressure, via the electrical input impedance, and because th= is=20 impedance can be (i.e., is) reactive (have inductance), it can be tricky to= =20 figure out what is going on. One must know (i.e., measure) the load impedan= ce,=20 or at least the current. Also the distortion is mostly determined by the=20 nonlinear flux vs current (i.e., B-H curve), as described by Hunt, for=20 example. Hunt mostly quotes Mott and Minor's BSTJ Jan 1951 article, = adding=20 little new material to their story. In the interesting case of the= =20 Etymotic ER-3 (insert headphone), they place a 10 ohm resistor across the i= nput=20 terminals. So if you try to beat this with a series resistor, you will = not=20 be successful in changing anything, because all the current is going into t= hat=20 10 ohm resistor. I opened up one of my ER-3 and cut out the resistor, an= d=20 then I could see what was really going on. It worked better too. Whe= n=20 that ER-3 went back for repairs once, they were shocked to see the resistor= was=20 missing. I had to admit I cut it out, and wanted it left that way. I= n=20 general, adding a series resistor to your circuit, is working blind. You wi= ll=20 likely get a random result, and random is never better. Thus I dont advi= se it=20 unless you are willing to measure the input impedance and the resulting=20 frequency response of the headphone. While there are some interesting th= ings=20 you can do with a series impedance, a series resistor is not one of=20 them. Jont Allen <div> <div> On 12/16/2014 06:12 PM, James W. Beauchamp=20 wrote:</div></div></div> <blockquote style=3D"MARGIN-BOTTOM:5pt;MARGIN-TOP:5pt"> <div> <div><pre>I agr= ee with Bob. Most loudspeakers are designed to be driven by</= span></pre><pre>an ideal voltage source. Under this criterion they attempt to get = </pre><pre>the flattest possible response overall. This is not nec= essarily </pre><pre>= the most efficient response. In fact, some l= oudspeaker designs, </pre><pre>like the Small-Thiele closed-box = non-vented design, are very </pre><pre>inefficient.</u= ></pre><pre>=C2=A0</pre><pre>Jim</pre><= pre>=C2=A0= </pre><pre>Bob Masta wrote:</pre></div></di= v> <blockquote style=3D"MARGIN-BOTTOM:5pt;MARGIN-TOP:5pt"> <div> <div><pre>Fro= m: Bob Masta <a href=3D"mailto:audio@xxxxxxxx" target=3D"_blank"><aud= io@xxxxxxxx></a></pre><pre>Date: Sun, 14 Dec 2014 09:15:19= -0500</pre><pre><sp= an style=3D"FONT-SIZE:10pt">To: <a href=3D"mailto:AUDITORY@xxxxxxxx"= target=3D"_blank">AUDITORY@xxxxxxxx</a>= </pre><pre>Subjec= t: Re: USB sound cards</pre><pre>=C2=A0</fo= nt></pre><pre>On = 13 Dec 2014 at 21:59, Richard F. Lyon wrote:</p= re> <blockquote style=3D"MARGIN-BOTTOM:5pt;MARGIN-TOP:5pt"><pre>On Sat, Dec 13, 2014 at 5:5= 2 AM, Bob Masta <a href=3D"mailto:audio@xxxxxxxx" target=3D"_blank"><= audio@xxxxxxxx></a> wrote:</pre><pre><fon= t face=3D"Courier New">=C2=A0<= /span></pre> <blockquote style=3D"MARGIN-BOTTOM:5pt;MARGIN-TOP:5pt"><pre>It's the other way aro= und:=C2=A0 Adding resistance in the</pre><pre><= font face=3D"Courier New">driving circuit gi= ves poorer damping.=C2=A0 "Damping Factor" for</spa= n></pre><pre>a power amplifier is the reciprocal of output impedance.</= span></pre><pre>=C2=A0</pre></blockquote><pre>Bob, I wasn't aware of = that definition.</pre><pre>I was thinking of <a href=3D"http://e= n.wikipedia.org/wiki/Damping_ratio" target=3D"_blank">http://en.wikipedia.o= rg/wiki/Damping_ratio</a></pre><pre>rather than <a href=3D"http= ://en.wikipedia.org/wiki/Damping_factor" target=3D"_blank">http://en.wikipe= dia.org/wiki/Damping_factor</a></pre></blockquo= te><pre>Oops, you= are correct... I should have said it is </pre>= <pre>*proportiona= l* to the reciprocal.</pre><pre>=C2=A0</fon= t></pre> <blockquote style=3D"MARGIN-BOTTOM:5pt;MARGIN-TOP:5pt"><pre>Is there an understanding o= f why high "damping factor" would be good?</= font></pre><pre>J= ont's findings suggest otherwise (I believe he's saying the current= is</pre><pre>typically more relevant than the volage).<u= ></pre></blockquote></div></div><pre>The second reference you cited covers= it under </pre><pre>"Explanation".=C2=A0 Basically, a= conventional electrodynamic </pre><pre>speaker is both a motor = and a generator.=C2=A0 Imagine that the </pre><= pre>speaker recei= ves a momentary voltage pulse, after which is <= /pre><pre>is inst= antly disconnected from the source.=C2=A0 The speaker = </pre><pre>would ring at its resonant frequency, damped only by = </pre><pre>friction.=C2=A0 The generator would be creating a voltage, but <= /u></pre><pre>no current, so no load to add damping.</fon= t></pre><pre><= /u>=C2=A0</pre><pre>However, if instead of disconnection the leads were= </pre><pre>*shorted* after the pulse, the generator would be dr= iving </pre><pre><sp= an style=3D"FONT-SIZE:10pt">all its current into the zero-ohm load, giving = a maximum </pre><pre>damping effect.=C2=A0 = </pre><pre>=C2=A0</pre><pre><s= pan style=3D"FONT-SIZE:10pt">Conventional loudspeakers are designed to be d= riven by </pre><pre>= voltage sources, not current sources. The cu= rrent may be </pre><pre>more "relevant" (in the sense = of force generation), but not </pre><pre>for getting a flat freq= uency response from a conventional </pre><pre><= font face=3D"Courier New">speaker design.=C2= =A0 (Although there have been occasional </pre>= <pre>attempts at = current drive, the ones I recall required </pre= ><pre>special ded= icated amplifiers.)</pre><pre>=C2=A0= </pre><pre>Best r= egards,</pre><pre><s= pan style=3D"FONT-SIZE:10pt">=C2=A0</pre><pre><= font face=3D"Courier New">Bob Masta<u= ></pre><pre>=C2=A0</pre><pre>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0 D A Q A R T A</pre>= <pre>Data AcQuisi= tion And Real-Time Analysis</pre><pre>=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 <a href=3D"http://www.daqarta.com" target=3D= "_blank">www.daqarta.com</a></pre><pre>Scope, Spectrum, Spectrog= ram, Signal Generator</pre><pre>=C2=A0=C2=A0 Science with your s= ound card!</pre></blockquote></blockquote> =C2=A0</div></div></di= v></div> -- <= /p> <div> <div> James D. (jj)=20 Johnston</div> <div> Independent Audio and Electroacoustics=20 Consultant</div></div></div></div></= div></div> </blockquote></div> -- <div class=3D"gmail_signatu= re"><div>James D. (jj) Johnston</div><div>Independent Audio and Electroacou= stics Consultant</div></div> </div> --90e6ba3fd739f922e3050a82c623--

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