From: firstname.lastname@example.org (carltons)
Subject: Re: Output Impedence
User-Agent: NewsWatcher-X 2.2.3b2
NNTP-Posting-Date: Sat, 26 Oct 2002 16:00:58 GMT
Date: Sat, 26 Oct 2002 16:00:58 GMT
In article , John Woodgate
> I read in sci.electronics.design that James Meyer
> wrote (in email@example.com>) about 'Output Impedence', on Fri, 25 Oct 2002:
> >On Fri, 25 Oct 2002 20:16:06 +0100, John Woodgate
> > wroth:
> >>I read in sci.electronics.design that Michael R. Kesti
> >>wrote (in <3DB97CF0.1682A5C0@gv.net>) about 'Output Impedence', on Fri,
> >>25 Oct 2002:
> >>> Then load the output with
> >>>about 1000 ohms (Rl) and again measure the output voltage (Vl). Maximum
> >>>accuracy will occur when Vl is about half of Vo. If Vl and Vo are almost
> >>>equal, then the value of Rl is too high and you should try again with a
> >>>smaller value. Conversely, if Vl is small compared to Vo, try again
> >>>using a larger value for Rl. Doubling or halving the value will quickly
> >>>get you to an appropriate value.
> >>>Another approach is to use a potentiometer for Rl and adjust it until
> >>>Vl is exactly half of Vo. Rl and Ro are equal under this condition
> >>>and May be determined by measuring the resistance across the potentiometer.
> >>DON'T do this with the outputs for loudspeakers! You will zap the output
> > The output devices will zap about 0.001% of the time.
> > If you solder the connections to the output, DON'T pick up the
> >iron with the pointy end.
> James, the output source impedance of an output for a loudspeaker is
> typically less than one-tenth of the minimum rated load impedance. So
> searching for the output voltage to drop to half involves applying loads
> far less than that minimum. While many audio amplifier ICs are short-
> circuit protected, they are not necessarily protected against loads of
> intermediate impedance. This is particularly the case if you put a pot
> across the output and wind the pot down.
A good, modern audio amp is designed as a voltage source. Its output
impedance is extremely low and is exactly where it should be. This is
achieved through very high gain in the amp with no feedback and a
controlled gain with feedback. Conjugate matching of the output is
downright silly in most applications. Modern amps drive their speakers in
complete sync with the incoming signal, that is, the output SPL out of the
speaker matches the incoming signal level. There is no "boinga boinga"
under-damping like you get with a tube amp. If you want under-damping to
colour your music, don't use a good amplifier, get a 6L6 and pay through
the nose for something that you can do with a DSP. BTW, I was designing
circuits using 12AX7's and 6L6's in 1962, so don't give me the flame for
Peace, love, dope,