Re: USB sound cards (Bob Masta )

Subject: Re: USB sound cards
From:    Bob Masta  <audio@xxxxxxxx>
Date:    Thu, 18 Dec 2014 08:56:34 -0500

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 Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!

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