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Guest
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 Posted:
Sat Dec 03, 2005 3:43 pm Post subject:
Power - brainteaser |
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Hi,
I got the following brainteaser I can't handle with: what power we'll
get on a resistor powered from 100W amplifier set on the level -40dB,
when the source is a CD-player producing white noise at level -36 dB ?
What current flows thru that resistor ? We assume, that all the
circuits are linear.
TIA
jvereker
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CWatters
Guest
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| Posted:
Sat Dec 03, 2005 5:35 pm Post subject:
Re: Power - brainteaser |
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<jvereker@wp.pl> wrote in message
news:1133603022.647250.144540@g44g2000cwa.googlegroups.com...
| Quote: | Hi,
I got the following brainteaser I can't handle with: what power we'll
get on a resistor powered from 100W amplifier set on the level -40dB,
when the source is a CD-player producing white noise at level -36 dB ?
What current flows thru that resistor ? We assume, that all the
circuits are linear.
|
If I remember correctly...
0dB would be 100W and you have -76dB
so the output is
= 100 x antilog (-76/10)
= 2.5uW
which sounds a bit low. |
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Charles Schuler
Guest
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| Posted:
Sun Dec 04, 2005 1:35 am Post subject:
Re: Power - brainteaser |
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"CWatters" <colin.watters@pandoraBOX.be> wrote in message
news:sAikf.63465$mf2.3599417@phobos.telenet-ops.be...
| Quote: |
jvereker@wp.pl> wrote in message
news:1133603022.647250.144540@g44g2000cwa.googlegroups.com...
Hi,
I got the following brainteaser I can't handle with: what power we'll
get on a resistor powered from 100W amplifier set on the level -40dB,
when the source is a CD-player producing white noise at level -36 dB ?
What current flows thru that resistor ? We assume, that all the
circuits are linear.
If I remember correctly...
0dB would be 100W and you have -76dB
so the output is
= 100 x antilog (-76/10)
= 2.5uW
which sounds a bit low.
|
0 dB is meaningless as stated here. 0 dBm would mean 1 mW is the reference
level.
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CWatters
Guest
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| Posted:
Sun Dec 04, 2005 1:35 am Post subject:
Re: Power - brainteaser |
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"Charles Schuler" <charleschuler@comcast.net> wrote in message
news:qO-dnR07T43Ejw_eRVn-tQ@comcast.com...
| Quote: |
0 dB is meaningless as stated here. 0 dBm would mean 1 mW is the
reference
level.
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So the solution is...??? |
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Tom MacIntyre
Guest
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| Posted:
Sun Dec 04, 2005 1:35 am Post subject:
Re: Power - brainteaser |
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On Sat, 3 Dec 2005 16:47:02 -0500, "Charles Schuler"
<charleschuler@comcast.net> wrote:
| Quote: |
"CWatters" <colin.watters@pandoraBOX.be> wrote in message
news:sAikf.63465$mf2.3599417@phobos.telenet-ops.be...
jvereker@wp.pl> wrote in message
news:1133603022.647250.144540@g44g2000cwa.googlegroups.com...
Hi,
I got the following brainteaser I can't handle with: what power we'll
get on a resistor powered from 100W amplifier set on the level -40dB,
when the source is a CD-player producing white noise at level -36 dB ?
What current flows thru that resistor ? We assume, that all the
circuits are linear.
If I remember correctly...
0dB would be 100W and you have -76dB
so the output is
= 100 x antilog (-76/10)
= 2.5uW
which sounds a bit low.
0 dB is meaningless as stated here. 0 dBm would mean 1 mW is the reference
level.
|
Why? The 0 dB on the meter would correspond to the full output of the
amplifier...this sounds like an exam/homework question anyway.
How can we calculate the current without the resistor value anyway?
Tom |
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Charles Schuler
Guest
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| Posted:
Mon Dec 05, 2005 1:35 am Post subject:
Re: Power - brainteaser |
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"CWatters" <colin.watters@pandoraBOX.be> wrote in message
news:fCpkf.63877$Kh2.3945594@phobos.telenet-ops.be...
| Quote: |
"Charles Schuler" <charleschuler@comcast.net> wrote in message
news:qO-dnR07T43Ejw_eRVn-tQ@comcast.com...
0 dB is meaningless as stated here. 0 dBm would mean 1 mW is the
reference
level.
So the solution is...???
|
What is the resistor value?
If this is homework, it is more like stupid busywork.
I suppose one could assume 0 Db = full output, but that would need to be
stated up front. The dB system is often abused and misused. Without a
reference level, (i.e. dBmV, dBU, dBA, dBSPL, dBW, dBm, dBV, dBf, dBu, dBk,
dBd, ad nauseum) the dB is often meaningless since it is not an SI unit. |
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CWatters
Guest
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| Posted:
Tue Dec 06, 2005 1:36 am Post subject:
Re: Power - brainteaser |
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"Tom MacIntyre" <tom__macintyre@hotmail.com> wrote in message
news:2ue4p11k7t2vrc2mv3mi5srebummc6nkrh@4ax.com...
| Quote: | How can we calculate the current without the resistor value anyway?
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The way I read it the OP added that bit about current. The question says
"power". |
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Charles Schuler
Guest
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| Posted:
Tue Dec 06, 2005 1:36 am Post subject:
Re: Power - brainteaser |
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"CWatters" <colin.watters@pandoraBOX.be> wrote in message
news:6R3lf.65339$vJ7.3975226@phobos.telenet-ops.be...
| Quote: |
"Tom MacIntyre" <tom__macintyre@hotmail.com> wrote in message
news:2ue4p11k7t2vrc2mv3mi5srebummc6nkrh@4ax.com...
How can we calculate the current without the resistor value anyway?
The way I read it the OP added that bit about current. The question says
"power".
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And, as is the norm, forum members dance around each other and the OP never
reappears. Sigh!
I'm having more and more difficulty these days with providing any measured,
and thought-out response to these queries. I'm starting to understand the
frequent nasty responses found on UseNet. |
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