Subject: Re: USB sound cards From: "Oberfeld-Twistel, Daniel" <oberfeld@xxxxxxxx> Date: Fri, 19 Dec 2014 08:03:48 +0000 List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>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? 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 > -----Original Message----- > From: AUDITORY - Research in Auditory Perception > [mailto:AUDITORY@xxxxxxxx On Behalf Of Bob Masta > Sent: Thursday, December 18, 2014 2:57 PM > To: AUDITORY@xxxxxxxx > 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!