Many laptops are pretty well known for having terrible audio interfaces. There are usually noise and grounding issues. This article tries to describe the reasons for those problems.
Grounded power supply issues
A recent post Yet Another Dell Laptop Audio Grounding Problem blog posting documents a well known and horrendous audio problem found on many Dell laptops. I have faced similar noise problems also on other laptops as well.
Starting from Dell laptops. I did some analysis on one DELL INSPIRON laptop that used 3 pin power connector on the laptop. First I measured the resistance between laptop metal parts and mains lead ground and found out that the ground is pretty well connected to laptop case. The strange thing happened when I had the laptop running: I measured a around 200 mV DC voltage between mains ground and computer case! And that voltage difference can cause up to several hundred milliamperes of pretty noisy current through the audio cables that are connected to grounded audio equipment! For more details read my Dell laptop ground loop problems posting.
I think that that this kind of grounded low voltage side is pretty common on laptop power supplied.
Isolated power supply issues
Other popular configuration used is floating output. There are power supplies without ground, for example some Apple laptops and many other brands. The power supplies where there is no ground on connection on the mains side are for sure built in this way. If it didn’t need the ground, the plug would be two-pronged. It is also possible to built a grounding power supply that has the output floating.
This kind of isolated power supply avoids ground loop problems. Could easily think that ideally the laptop is intended to be floating, this means it has no connection to ground. Lack of ground has it’s benefits, but can also cause a whole different problems.
Electric noise on audio recording article tells that ideally the laptop is intended to be floating, this means it has no connection to ground. In practice is pretty impossible to make a poer supply that is completely floating, there is always more or less capacitance between mains side and low voltage side. It’s very common for power supply manufacturers to place a capacitance between the Mains side and ELV (Extra Low Voltage) side of the power adapter to meet Radiated Emissions requirements (EMC). This causes leakage current that can cause problems.
When connecting the laptop to peripheral equipment (such as a microphone, or amplifier output) that is grounded the noise is introduced because the floating laptop ground being pulled down to real ground potential causes a voltage to be induced across the capacitor. This in effect causes the whole laptop to bounce up and down at 50/60Hz, and depending on how the power supply is built, can also introduce the switching noise.
Designing low leakage current power supplies article tells that majority of AC/DC power supplies provide isolation from the high-voltage AC input to the low-voltage DC outputs. Safety standards specify both the strength of the isolation barrier. The maximum leakage current allowed to flow is based on the specific classification of the application. In many applications a low leakage design (considerably lower than allowed maximum) is desirable – especially when the laptop has metal parts and manufacturer prevent users from feeling the “touch current” when they hold the computer when it is connected to mains power supply. One of the biggest contributors to leakage current in AC/DC switching power supplies is the Y-capacitor—a safety agency rated capacitor that can be used to bridge the isolation barrier (typically used to prevent EMI). In general, the larger the value of the Y-capacitor, the lower the magnitude of the EMI and higher leakage current. For a two-wire (without a protective earth connection), universal input power supply with a floating output, rounding down to the next standard capacitor value gives a maximum Y capacitance of approximately 2.2nF. Information technology (IEC60950) standard gives the following maximum leakage current values of 0.25 mA for devices with accessible parts not connected to protective ground. For grounded devices the allowed currents (IEC60950) are larger: Handheld (0.75 mA) and portable (3.5 mA).
Inverters, Laptop Power Supplies and Earth Leakage article tells that commercially approved laptop adapters that are supplied with 2-core mains flex have enhanced insulation, creepage distances and filtering that obviates the need for the earth. 3-core laptop adapters don’t, and are dangerous to use with the earth disconnected.
Resistance grounded power supply output
I have one pretty new ACER 5552G laptop that cam with with Delta Electronics ADP-90CD D8 power supply that has grounding practice different from other laptops I have seen:
The power supply output (otherwise floating) – side seems to be connected to mains ground through 1 kohms resistance on the power supply. This keeps the laptop pretty near ground and avoids ground loop problems. This is kind of “ground lift” circuit built into power supply. I measured 110 mV DC between laptop case and mains ground. When I measured the current that flows from laptop case ground to mains ground I got around 0.1 mA DC and 0.03mA AC. Very low currents that should not cause problems.
How to solve this kind of problems.
Usually the laptop audio interfaces work pretty acceptably well (although there are also exceptions to this) when laptop runs with batteries. Usually the problems start when you plug in the mains adapter. Usually if the ground of the power adapter is connected a lot of noise comes out. Unplug the power adapter from the laptop or mains outler and the noise is gone. This is usually a classical ground loop problem.
In my experience in those cases, using an audio line isolation transformer or a passive DI unit (with ground lift) between the laptop and grounded audio system (for example PA system sound mixing console) can helps. Any well isolated DI works most of the time. But there are case where this trick does not help: Sometimes power supply noise induced in the audio path inside the laptop and there’s no way to remove it. The best thing in those cases is just to do is pull the power lead on the laptop whilst playing back the audio and hope that you have enough battery capacity to run the show.
An unearthed power supply may work but research first as the earth is there for a reason! I would not recommend using a PSU without a ground connection. That would cause a whole different problem.
How to solve grounding related issues
I would suggest there are four options:
1, Buy a new PSU that is designed for your laptop but doesn’t create as much mains bourne interference (you might need to try several different power supplies to find the right one as you can’t typically find reasonable details on this on sales documentation)
2, Buy a laptop with sound in mind (some laptop are just too noisy for any reasonable audio use)
3, Pull the plug! (Not practical for long editing, and running on batteries slows most laptops down)
4, Filter the output from the laptop going to the sound desk either using a 1:1 isolating transformer or a DI box ( not always as successful)
In some cases grounding ungrounded computer to the same ground potential as the rest of the system is grounded can help to get leakage currents out of sensitive audio connection or measurement circuits that are connected to your laptop (note: the point where you connect the ground to laptop can have effect on the overall noise levels).
Usually mains so-called cleaners won’t help here. If problem is caused by the “switched mode” part of the power supply and so no amount of mains interference cleaning equipment would help! If the problem is caused by ground connection, only breaking it will help – and power cleaners typically do not do that. If the problems are due leakage current on power supply and lack of grounding – then you could attempt adding ground or use power isolation transformer for mains power isolation (laboratory safety isolation transformer or maybe balanced power).