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Re: USB sound cards

I hate to add even more confusion to this topic, but if there is a low-valued resistance in series with the output of a headphone amp at all, it is often OUTSIDE the feedback loop. Only a few modern headphone amps have such a design, and as far as I can see there seem to be three reasons given:

1) it limits low-frequency current flow and so prevents damage either to the output transistors, or the headphone.

2) it prevents instability in circuits which might otherwise be prone to it.

3) it makes the headphones sound "warmer", since the amplifier behaves more like a valve amplifier. 

There's a brief discussion about this in the "pitfalls" section of this link:


I also found some pertinent statements on Wikipedia:

(see https://en.wikipedia.org/wiki/Headphone_amplifier)

"The 1996 IEC 61938 standard recommended an output impedance of 120 Ohms, but in practice this is rarely used and not recommended with modern headphones."

"... low output impedance can reduce distortion by improving the control that the source has over the transducer."

This Wikipedia article and most other sources I've found seem to suggest that the lower the output impedance the better, both to obtain a flat frequency response and low distortion; but as I mentioned in an earlier posting to this list, historically there has been a strong opinion amongst some audiophiles that a small but significant resistive output impedance IS still desirable. Usually quoted as between 30 and 50 Ohms when driving 600 Ohm headphones.

I have not been able to find any suggestion anywhere that this might make any aspect of the headphone's behaviour closer to an analytical ideal - it seems as though it is simply designed to replicate the sound of valve amplifiers and be more "pleasing to the ears".

Steve Beet

On Fri, 19 Dec 2014 08:03:48 +0000
"Oberfeld-Twistel, Daniel" <oberfeld@xxxxxxxxxxxx> wrote:

> 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@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!