The Mellotron Group

Dear Mark

At 08:46 26/06/2010, you wrote:
>
>
>I use a laptop to record the mellotron and if the laptop is plugged 
>into the mains, I get terrible interference in the recorded and 
>monitored sound but if the laptop is on battery power it's fine. 
>Unfortunately it's an old laptop and doesn't run on batteries for 
>very long. Anybody know how to fix this problem?
>
>Cheers

Firstly I have no idea how much you know about the electronic side of 
audio so please excuse me if I am teaching grandmother to suck eggs - 
that is not the intention.

Theres quite a few different things that could cause this. If the 
laptop has an earthed PSU (most are double insulated and don't have a 
mains earth) it could be an earth loop. This is where a small voltage 
(usually an ac signal at the frequency of your mains supply - 50Hz in 
Europe 60Hz in US) is present in the earthing loop of the house earth 
- laptop earth - gnd of audio card - gnd of instrument audio - 
instrument earth - house earth. Ideally these should all be at 0v but 
often some parts are not and a small voltage sits somewhere on the 
loop picking up the mains frequency. That can be enough to interfere 
with audio signals. Grounding the chassis of the PC and instrumet 
together can often work but is easier said than done. Another is to 
isolate the pc and decouple the earth.

Another could be the proximity of the audio cable to the laptop 
supply. Those lump in the line PSUs radiate like mad and even with a 
well screened audio cable the orientation of the audio cable can make 
it an effective antenna for receiving the buzz from the PSU. This may 
be at mains frequency or if at a higher frequency it could well be 
one of the switching components in the PSU as most of these PSUs are 
'switched mode' supplies meaning they have transistors switching the 
mains voltage rather than a big chunky transformer. These switching 
transistors can radiate anywhere from 50Khz to 1MHz and sadly many 
laptop supplies are not well filtered or screened.

A third reason could be the proximity of the laptop psu to the audio 
output stage of the instrument. This has a similar effect to that 
above, but instead of the interference being picked up by the audio 
cable it is picked up by the output circuitry of the instrument.

Another similar effect could be the PSU of the instrument being 
picked up by the audio circuitry but I would expect this interference 
to be present regardless of the laptop being on mains power or 
battery so I think that can be discounted.

I assume you have checked the screening of the audio cables and that 
all connectors are in good shape.

There may well be other causes that have escaped my attention but 
theres a few things to look at. The solution will depend on what is 
causing it and the first step is to try and establish exactly what is 
causing it (OK we know it's the laptop PSU,  but time to look a bit 
deeper!) . I would start by trying to keep the PSU as far from the 
audio cable as possible and the pc as far away from the tron as 
possible. I know that is not easy but with radio interference, which 
is what all the possible reasons above apart from the first are, just 
a few inches can make a big difference and sometimes the orientation 
make a big difference so try not to run audio cables parallel to power ones.

I hope this gives some guidance and I do hope I am not preaching to 
the converted!

rgds
Nic