From: Winfield Hill
Subject: Re: Output Impedence
Date: 26 Oct 2002 09:39:55 -0700
Organization: Rowland Institute
X-Newsreader: Direct Read News 2.98
John Woodgate wrote...
> Michael R. Kesti wrote...
>> ... 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 devices.
John may be right, following Michael's advice could result in a
destroyed audio-output power amplifier IC. That's because power
amplifiers use internal feedback to reduce distortion, and this
results in a very low output impedance. Therefore the amplifier
will deliver as much current as it's able to maintain the voltage
where it thinks it should be. This amplifier may be a surface-
mount part located on the computer's motherboard, and not easily
replaced by the casual computer owner, so be careful.
One approach for the seriously motivated would be to examine the
mobo by tracing back from the audio connector. Study the circuit
and identify the output-amplifier chip, then look up its specs.
I examined my wife's computer mobo, and found an AD1885 SoundMAX®
There's a TL072, that must be for the microphone input. I see a
small chip labelled L1087, nope sorry, that's a linear regulator.
- - - - Hey, I can't find the onboard audio power amplifier!
Could it be that the puny 1V rms headphone/line out included in
the AD1885 AC'97 codec chip is all that my wife's computer has?
Sheesh! That's not like the old days, they must now assume that
everyone uses powered speakers? Is that what Microsoft and the
Intel Audio Codec '97 (AC '97) Specification specifies?
OK, for the AD1885 output specs: it can deliver 2.83V p-p and
its output impedance is 800 ohms max. That's pretty wimpy!
But for this case, Michael's advice will work just fine. Go
ahead and try a 470-ohm load and see if the output drops much.
But don't use less than say 27 ohms, to avoid hurting anything.
Let us know what you find.