From: "Michael R. Kesti"
Subject: Re: Output Impedence
Date: Fri, 25 Oct 2002 16:22:35 -0700
Organization: MK Associates
X-Mailer: Mozilla 4.7 [en] (Win98; I)
John Woodgate wrote:
>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
Yes, I had line outputs in mind. I suppose this is because I can think
of reasons to need to know a line out's impedance but not a speaker's.
Also, I've never owned a soundcard with speaker outputs. Audio gets
in and out of my computer only in the digital domain!
Similar technique is used to determine power amp output impedance,
though. You may not want to let Rl get too low, especially with the
junk that serves as amplifiers in typical computer sound cards, but
the difference between the output's open circuit and loaded voltages
will still yield the output impedance.
Michael Kesti | "And like, one and one don't make
| two, one and one make one."
firstname.lastname@example.org | - The Who, Bargain