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From: "Phil Allison"
References: <email@example.com> <firstname.lastname@example.org>
Subject: Re: Audio noise in diff amps
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Wed, 18 Dec 2002 22:47:57 +1100
NNTP-Posting-Date: Wed, 18 Dec 2002 22:36:11 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Don Pearce" wrote in message
> > ** Rubbish. Unless you believe in perpetual motion - maybe you
> No perpetual motion involved here - this is simply energy from the
> environment. Why do you have a problem with this?
** Conservation of energy is being violated if one resistor can
deliver power to another at the same temp.
> >> OK - given the whole audio band (which you didn't actually specify)
> > ** I damn well did - see the words "in the audio band" ??
** Mr Pearce gives no response - since he cannot face his errors.
> >> then 2.4uV is an OK (if rather high) figure. Pure thermal noise in a
> >> 20kHz bandwidth is actually 1.3uV for a 22kohm resistor, so this
> >> represents more than 5dB excess noise. That is really quite a lot.
> > ** The formula is Vn (rms) = sq rt ( 4KBTR)
> > I think you missed the "4".
> I did the calculation purely in terms of power (ktb) so there was no
> way to lose the "4". Here is the calculation
> Pure thermal noise = -204dBW in 1Hz (kTB)
> Expressed in Watts = 3.98 e-21 W
> As a voltage across 22kohms = sqrt(power x resistance)
> which equals 9.36 nanovolts in 1Hz
> Multiply by the sqrt of the 20kHz bandwidth (141)
> Gives a noise voltage = 1.32 microvolts
> This is 5dB below your figure - hence my comment about excess noise.
** The formula for resistor noise voltage I quoted is correct - go
look it up.
Your figure is out by exactly a factor of 2 - could it be due to
(wrongly) using the power matching theorem ??
> >> As for the power, you calculation is wrong because you are comparing
> >> noise in a 22000 ohm domain to the voltage that corresponds to 0dBm in
> >> a 600 ohm domain (0.775V) - that is why you arrived at an erroneous
> >> power figure in dBm.
> > ** Bullshit. The OP measued a voltage on his Fluke referenced to
> >volts. The use of noise voltage for the resistor is therefore appropiate
> >puts the resistor noise contribution in perspective to his very much
> >measured figure.
> > BTW How many more dumb errors are you intending to make Mr
> > ................... Phil
> dBm is dBm. It is power compared to 1 milliwatt.
** More pedantic rot. The OP was clearly using 0.775 volts so I
> My calculation is correct.
** So is mine - and based on the right assumptions.
If, between you and the OP you have somehow got things in a
> twist over reference settings on a multimeter, that is hardly my
** Well *screw* you Mr Pearce.
So the OP and I must **both** be wrong so you can be right.
BTW You still have given no justification for your crazy and dangerous
advice re mains grounding.
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