29-06-2011, 08:12 PM
Complicated problem with a number of possible root causes.
First, the advice Jamie received about hum was about earth loops. To be honest, the fact it's a laptop is probably not an issue; any source that is earthed could be prone to causing hum in a system. But, it's completely impossible to generalise because so much depends on the design of the following equipment, plus any other equipment that might be connected to the system! Ask any audio pro: hum loops are a real problem, and it's not always restricted to consumer-grade kit... One way to prove that it's an earth loop is to turn off the source equipment, but leave it connected - the hum will still be there.
One possible solution to prevent an earth loop is a 1:1 isolating transformer. These can be very good indeed, although the very cheap ones that CPC sell aren't so nice. Working where I do, it's normally easy to find good quality "rep coils", as they're called, and building them into a box is an easy task. Use a diecast aluminium box, but make sure at least one set of phono sockets are isolated from the case (at the risk of stating the obvious).
As for the noise at higher frequencies, the causes are normally power supply system design. This doesn't just include the external PSU, but can also include the multitude of on-board DC-DC converters that take the single DC rail from the PSU or battery and convert it to the various voltages required (5V, 3V3, 1V5 or whatever the CPU core requires). Also, perhaps bizarrely, PCB design can have an effect; not just the layout (in terms of how close the audio DAC and amplifier stages are to "noisy" things, but how supply and earth returns are routed. The PCB is always a component in any design). So basically, if your system components are taking current in "spikes", according to say CPU load, these currents can get turned into voltages as they travel along PCB tracks (which aren't zero Ohms in reality), and can get added to analogue signals in the audio department.
An external USB soundcard can be an effective cure, and they can be very cheap as well. Though note the use of the word "can". Perversely, these two effects can combine, so that although you've got the soundcard CODEC outside of the noisy environment of the laptop itself, you haven't got isolation between the laptop and the audio system. So, stray currents from the PSU systems can flow in the earth, causing voltages to be developed and added to the audio signal. If your original problem was an earth loop, an external sound card won't help...
To put this into context, if the maximum signal from your soundcard is 1V at 0dBFS, at -60dBFS, the signal is 1mV. Very good vinyl might manage a signal-to-noise ratio approaching that, perfect FM might also be in the same ballpark. So if you're used to the background noise from records and decent FM broadcasts like Radio 3, you've got an idea of the harm just 1mV might do...
It's worth running the laptop as loud as you can without causing clipping (use a 'scope to check) to maximise the signal to noise ratio...
All the best,
Mark
First, the advice Jamie received about hum was about earth loops. To be honest, the fact it's a laptop is probably not an issue; any source that is earthed could be prone to causing hum in a system. But, it's completely impossible to generalise because so much depends on the design of the following equipment, plus any other equipment that might be connected to the system! Ask any audio pro: hum loops are a real problem, and it's not always restricted to consumer-grade kit... One way to prove that it's an earth loop is to turn off the source equipment, but leave it connected - the hum will still be there.
One possible solution to prevent an earth loop is a 1:1 isolating transformer. These can be very good indeed, although the very cheap ones that CPC sell aren't so nice. Working where I do, it's normally easy to find good quality "rep coils", as they're called, and building them into a box is an easy task. Use a diecast aluminium box, but make sure at least one set of phono sockets are isolated from the case (at the risk of stating the obvious).
As for the noise at higher frequencies, the causes are normally power supply system design. This doesn't just include the external PSU, but can also include the multitude of on-board DC-DC converters that take the single DC rail from the PSU or battery and convert it to the various voltages required (5V, 3V3, 1V5 or whatever the CPU core requires). Also, perhaps bizarrely, PCB design can have an effect; not just the layout (in terms of how close the audio DAC and amplifier stages are to "noisy" things, but how supply and earth returns are routed. The PCB is always a component in any design). So basically, if your system components are taking current in "spikes", according to say CPU load, these currents can get turned into voltages as they travel along PCB tracks (which aren't zero Ohms in reality), and can get added to analogue signals in the audio department.
An external USB soundcard can be an effective cure, and they can be very cheap as well. Though note the use of the word "can". Perversely, these two effects can combine, so that although you've got the soundcard CODEC outside of the noisy environment of the laptop itself, you haven't got isolation between the laptop and the audio system. So, stray currents from the PSU systems can flow in the earth, causing voltages to be developed and added to the audio signal. If your original problem was an earth loop, an external sound card won't help...
To put this into context, if the maximum signal from your soundcard is 1V at 0dBFS, at -60dBFS, the signal is 1mV. Very good vinyl might manage a signal-to-noise ratio approaching that, perfect FM might also be in the same ballpark. So if you're used to the background noise from records and decent FM broadcasts like Radio 3, you've got an idea of the harm just 1mV might do...
It's worth running the laptop as loud as you can without causing clipping (use a 'scope to check) to maximise the signal to noise ratio...
All the best,
Mark
