Re: USB sound cards

[Bob Masta <audio@xxxxxxxxxxx>]

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
 
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