Re: laptop supply needs filtering ?
- From: chrisgj198@xxxxxxxxxxxxxx
- Date: Thu, 25 Sep 2008 08:00:29 -0700 (PDT)
On 24 Sep, 02:13, Eeyore <rabbitsfriendsandrelati...@xxxxxxxxxxx>
wrote:
mark krawczuk wrote:
hi, a friend of mine has setup a conference listeneing using VOIP .
he has the pre amp out connected straight up to the laptop .
he has the mains adapter plugged in to the laptop.
but there is a hum coming thru ,, when he disconnects the mains supply for
the laptop , and the laptop runs on its own batteries , the hum goes away
....
would the laptop supply need some sort of filtering ?
thanks,
PLEASE DO NOT MULTIPOST ! You should crosspost items like this to groups that
are all relevant to it.
I gave the correct answer in aus.electronics. Ignore any other specious
explanations, they're WRONG.
It's caused by a cheap way of meeting EMC specs which places a safety capacitor
between the rectified DC primary side reservoir cap and the low voltage DC
outout ground. A typical value is 2n2.
This causes a potential significant leakage current which wants to find its way
to ground
if it can. Poorly designed audio equipment with inadequate attention to good
grounding and EMC issues will amplify the sound of this leakage current
beautifully.
Either get some properly designed audio gear or run the laptop on batteries.
Oh - or get an audio isolation transformer. Commonly called D.I.s in the music
trade. You should be able to get one in a guitar shop.
Graham
Graham, Spehro is right: in the case of some laptops (including mine),
the capacitor is not the problem. If the power adapter has a three
pin mains plug, and if the negative rail of the power supply output is
connected to the mains ground, then the problem occurs as follows:
The laptop draws several amps from the power supply, with pulses of
greater and lesser current as required by the DC-DC convertor that
powers the CPU. Depending on CPU activity, the current drawn through
the negative wire connecting the laptop to the power adapter can vary
rapidly, changing by amps. With even a very small resistance in the
negative wire of the DC cable, the voltage of the ground of the laptop
rises above mains earth (right now, mine was just measured varying
from 60-100mV above mains earth).
If an audio cable connects the laptop to an amplifier which has its
audio input ground connected to the mains earth, then some of the
current drawn by the laptop will return not by the negative wire of
the DC cable, but instead via the audio cable. This will mean that
the voltage is not identical at the two ends of the ground conductor
of the audio cable.
For a consumer, the easiest solution would be to buy an audio
isolation transformer. These are sold for use when adding extra
amplifiers in car audio systems. It would probably introduce
distortion but that may be better than the existing problem. A
floating (class 2) audio amplifier would also work, until some other
grounded device were connected to another input of the the audio
amplifier. An amplifier with differential input ports could be used
but would be uncommon for consumer applications.
Really the laptop manufactueres should make DC adapters that have the
DC output isolated from mains earth, and if it is desired to earth the
laptop to mains earth then there should be a third conductor in the DC
cable which connects the laptop to mains earth but does not carry the
main DC current consumed by the laptop. Probably this costs more so
they don't feel like doing it. They could also do something at the
audio output where they could allow the ground of the audio out socket
to float up to say +/-500mV from the laptop ground by putting anti-
parallel diodes and maybe a parallel 1kOhm resistor from the audio
output socket ground to the laptop ground, and then make a circuit in
the laptop with opamps that sense the voltage on the ground of the
output connector and add this voltage to the left and right audio
signal output voltages, so that the external amplifier would not see
the hash.
Even with a pair of headphones (totally isolated from ground)
connected to my laptop, I hear some (much quieter) hash so it seems
like the laptop makers have a pretty tenuous grasp of Ohm's law
anyway. Probably they just don't care as long as people keep buying
it.
Chris
.
- References:
- laptop supply needs filtering ?
- From: mark krawczuk
- Re: laptop supply needs filtering ?
- From: Eeyore
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