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From: "Phil Allison"
Subject: Re: Audio noise in diff amps
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Fri, 20 Dec 2002 10:02:33 +1100
NNTP-Posting-Date: Fri, 20 Dec 2002 09:50:57 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"John Woodgate" wrote in message
> I read in sci.electronics.design that Phil Allison
> wrote (in ) about 'Audio
> noise in diff amps', on Thu, 19 Dec 2002:
> >"John Woodgate" wrote in message
> >> I read in sci.electronics.design that Phil Allison
> >> wrote (in ) about 'Audio
> >> noise in diff amps', on Thu, 19 Dec 2002:
> >> > ** For decades the term "dBm" was used in audio for reference signal
> >> >levels. It only rarely meant a true power level but rather a voltage
> >> >referred to 0.775 volts rms or 1mW at 600 ohms.
> >> >
> >> > Seems this confused some outside the club so the "dBu" was
> >> >invented to remove the ambiguity.
> >> No, Phil, it confused people INSIDE the club, and caused spats like you
> >> are indulging in at present.
> > ** Only a pom could say that.
> That's just daft.
** My remark applied to your ..."indulging in " put down.
IIRC, Neville Thiele was one of the people involved in
> coining and standardizing 'dBu'. If not Neville, one of his colleagues.
** Nev is cool.
> > Here I am being abused and attacked by
> >Mr Pearce who cannot see HE is in error.
> Well, you're not turning the other cheek, are you?
** The one on my face or somewhere a bit lower down ??
Have you woken up there is a hidden agenda here John. Mr Pearce
posted dangerous advice re de-earthing and will not discuss it despite my
complaints and challenges.
The whole "dBm" thing is a pay back for that.
> > Nor can he see his error with the
> >dangerous de-earthing advice either.
> It's not wise to bring up old issues; it only gets people mad and
> doesn't alter the old issue at all.
** OLD!!! It was only a few days ago. It is a serious safey issue -
Mr Pearce was very wrong to have posted what he did.
> >(Do you really mean power, or just level
> >> relative to 0.775 V?)
> > ** Of course the figure is voltage level.
> That question in brackets was an illustration of the ambiguity of 'dBm',
> not a question for you to answer.
> > 2.4uV is a voltage level
> >isn't it ?
> Well, no, it's a voltage. -110 dBu is a voltage level.
** Lost me. The level of a voltage is its magnitude.
> > The "noise power" of all value resistors is the same - why would
> >ANYONE post that figure and then say it was for a 22 kohm one ??
> Well, since it IS, it's not a very grievous fault, is it?
** It would be an idiocy in the context of the topic of then post.
Mr Pearce is abusing me here for *not* wanting to change my post to a
> >This why 'dBu' was adopted by CCIR (later ITU-R)
> >> and by AES in due course, even though it doesn't gel with the rules for
> >> symbols in IEC 600027.
> >> The r.f. people sill use 'dBm' (which should be 'dB(mW)' according to
> >> IEC 600627, and is then *obviously* a power level). Some of them still
> >> have to cope with reconciling levels in dBm with both 50 ohm and 75 ohm
> >> circuits.
> > ** If only *you* could convince Mr Pearce that "dBm" has another
> >in audio.
> > Because I cannot.
> I've explained it here for him. I can't *understand* it for him. But
> please, EVERYONE, give up using 'dBm', even if an OP does. Unless you
> REALLY mean power level in 600 ohms or 50 ohms or 75 ohms or 300 ohms
> AND you clearly say so! (;-)
** Is this going to be the Don Pearce rule ???
What about all the AC signal meters that have "dBm" on the
scale and all the published specs that refers to it ?? Does the word have
to change to suit one dopey, myopic pommy RF engineer ??
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