XPost: comp.sys.mac.system   
   From: burns4@nowhere.com   
      
   On 2/10/14, 3:26 AM, Salmon Egg wrote:   
   > In article , J Burns    
   > wrote:   
   >   
   >> If the impedance of my phones varied from 20 to 170 ohms depending on   
   >> frequency, I could say a 220-ohm resistor in series had the effect of   
   >> negative current feedback.  With my resistor network, the amplifier saw   
   >> more than 20 ohms, but the phones saw 1.8 ohms, meaning 99.2% less   
   >> negative current feedback than from the standard resistor.   
   >   
   > I do not fully understand this point.   
   >   
   > The impedance phones present at its terminals is from a combination of   
   > factors. There is the actual impedance of the internal driving network   
   > (voice coil) with the transducer blocked as modified by the loading of   
   > the diaphragm and the acoustic load of the air it drives. Adding large   
   > series resistance R turns a voltage feedback amplifier into a current   
   > source. That is, for an output voltage V the current through your phones   
   > approaches V/R as R increases no matter what the impedance of the phones   
   > are. It is the Norton equivalent resistance of the amplifier and the   
   > attached R. This is not negative current feedback because the current   
   > flowing in the output has no effect on the signal that ends up getting   
   > amplified.   
   >   
   > Depending upon the details of the phones, constant current drive may or   
   > may not end up giving improved performance over constant voltage drive.   
   > One thing the series resistance will do is prevent blowout of a low   
   > impedance voice coil when a loud passage occurs.   
   >   
   > Because a watt or two of sound is pretty loud in confined regions, a   
   > practical tradeoff is to have high power amplifiers with low efficiency   
   > speakers if the dissipation in the speaker circuitry improves (flattens)   
   > frequency response.   
   >   
   I'm not used to the phrase "negative current feedback." I supposed it   
   could be used to protect the power transistors in an amplifier: if more   
   output current were sensed, the signal would be reduced in the preamplifier.   
      
   If you put a resistor in series with a loudspeaker and the signal   
   voltage coming from the amplifier were constant, and the speaker   
   impedance changed, the voltage at the speaker would vary inversely with   
   the current.  That sounds like the effect of negative current feedback.   
      
   I have connected a sine-wave generator to an amp to check loudspeakers   
   and phones.  All were louder at certain frequencies.  Current was   
   minimal at those same frequencies.  In other words, they produced sound   
   more efficiently at resonance.  Negative current feedback would mean   
   increased voltage at resonant points, making them harsher and more   
   mechanical sounding.   
      
   That observation led me to make a low-impedance source for phones, as is   
   normal for loudspeakers.  I was rewarded.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)