From: John Woodgate
Subject: Re: Audio noise in diff amps
Date: Thu, 19 Dec 2002 17:49:51 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Thu, 19 Dec 2002 19:34:20 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
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. IIRC, Neville Thiele was one of the people involved in
coining and standardizing 'dBu'. If not Neville, one of his colleagues.
> 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?
> 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.
>(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.
> 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?
>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
> ** If only *you* could convince Mr Pearce that "dBm" has another meaning
> 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! (;-)
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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