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Re: USB sound cards

Dear Bob,

thanks for clarifying this! So I overlooked that the R is inside the feedback loop, embarrassing... That's why I said don't trust my opinion as a psychologist, I wished to have a better understanding of these EE design issues...

But what is the bottom line - is low output impendance better, or do Jont's analyses indicate the opposite?


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

> -----Original Message-----
> From: AUDITORY - Research in Auditory Perception
> [mailto:AUDITORY@xxxxxxxxxxxxxxx] On Behalf Of Bob Masta
> Sent: Thursday, December 18, 2014 2:57 PM
> To: AUDITORY@xxxxxxxxxxxxxxx
> Subject: Re: USB sound cards
> On 18 Dec 2014 at 9:43 Daniel Oberfeld-Twistel 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.
> Please note that while resistor Rx is technically "in series" with the load , it is
> *inside* the feedback loop of the amp (due to Rf).  That forces the output
> impedance to near zero, just as if Rx wasn't there.  You can't detect Rx from
> outside the circuit via normal methods of output impedance measurement
> (change in voltage drop when you apply a load).  It's only effect (other than
> allowing the amp to drive high capacitance) is that it does produce a voltage
> drop inside the feedback loop, so the overall amp will not be able to deliver
> quite as high an output voltage before clipping onset.
> >
> > 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 ;-)
> Those texts talking about matched loads and power efficiency are referring to a
> case where you have a fixed driving impedance and want to maximize the
> power transfer to the load.  That hasn't really applied to audio since the
> vacuum tube and output transformer days.  Solid state amps with "zero"
> output impedance are vastly better in this respect, where the power transfer is
> essentially determined strictly by the load.  (Assuming that the amp can handle
> it without letting the magic smoke out... a separate issue.)
> This has made life a whole lot easier in the lab. In the Olden Days the entire
> signal chain was standardized at
> (typically) 600 ohms, which meant you needed special matching "pads"
> between items that had different input or output impedances.  Adjustable
> attenuators (Daven, for
> example) were elaborate affairs of switched resistor networks, to keep the
> impedance constant at all attenuation positions.  If you tried to drive the wrong
> impedance, your attenuation wasn't what was marked on the knob. (For those
> of us old enough to remember equipment with real knobs!)
> None of that nonsense is needed now... and good riddance!
> 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!