Power supply noise in audio

Question

I have what is surely a classic problem with regard to power supply switching noise and audio but I am unable to sort myth from reality with regard to what I found so far on the topic.

Setup:

1. I have a notebook with an external power suppy and/or battery
2. A radio receiver which has its own power supply (i.e. not fed by notebook SMPS)
3. the radio receiver feeds an audio signal into line-in of the notebook
4. the radio receiver is controlled by the notebook via RS232 (tuning, etc.)

Problem:

• If I unplug the notebook from its power supply and run it from battery everything works perfectly
• But if I use the notebook SMPS, I hear a tremendous amount of noise in the audio

Can anybody tell me where the problem is likely to lie? There is a lot of talk about ground loops but I have difficulty believing they really exist in such a small-scale installation.

Am I right to assume that it is likely a problem of a varying ground level in the notebook and the fact that the line-in input of the notebook is non-differential? Or is there a more likely explanation?

What is the best solution? Use an opamp to construct a differential input amplifier and feed its output to the line in? What do I use as ground reference for the opamp?

Proposed solutions in the comments and answers

From the answers it would appear that there are two possible problems: 1. ground loops and 2. RF pickup from the external SMPS in the audio wire.

Suggested solutions are:

1. Differential amplifier solution. Advantages/Disadvantages?
2. Kortuk: Combat RF pickup from the SMPS in the audio link with a grounded shield. Advantage: invisible solution; Disadvantage? Question: does not help with any ground loops?
3. Russell McMahon: Audio transformer in the audio-line. Advantage: Simple; Disadvantage: not easy to source, expensive or poor frequency response. Question: does this help with RF pickup in the audio line?
4. Russell McMahon: clamp EMC ferrites on the audio line to combat RF pickup. Does not help against ground loops. Question: does this help with noise in the audible range? It was my understanding that ferrites only help to filter very high frequencies.
5. David Kessner & Mary: Grounding the notebook. This shunts CM noise to ground. Advantage: cheap, simple; Disadvantage: additional wire to handle. Question: combats both RF pickup (if audio-ground is shunted) and avoids ground loops?
6. Mary: ferrite absorber around the DC line to the notebook and RF CM chokes in the audio line and RS232 lines. Disadvantage: high component count & effort with the RF CM chokes. Does not prevent ground loops.

Answer 1

The problem is common to this type of audio system. I would bet if you looked at the noise spectrum you would see 60 Hz plus many of the harmonic frequencies (120 Hz, 180 Hz, 240 Hz, etc.). The fact that it is more than just 60 Hz, or 50 Hz in some countries, is an indicator that it is not just simple ground loops.

I would also bet that your laptop power supply has only a 2-prong AC plug-- lacking the third ground plug.

In this type of power supply, the output is electrically isolated from the AC input. But it is not perfectly isolated. There is a small amount of current that flows between the isolation barrier. This is called the "leakage current". It is not a lot of current, but it doesn't have to be.

Some laptop users report getting shocked or having a tingling sensation in the legs when using the laptop while wearing shorts! The reason for this is that leakage current is going through the screws in the bottom of the laptop and into their legs. It sounds dangerous, but the amount of current is well below the safety limit. It is more startling than anything else. If you are wearing pants then you're insulated.

Laptop chargers that have the 3rd prong on the AC plug do not have this problem because that third plug is connecting the laptop chassis shield to ground-- forcing that leakage current to go to ground instead of into your leg. Of course, there is no leakage if you are running off of batteries.

In your case, the leakage current is not just going into your leg, but into your radio receiver. The solution to this is to properly ground your laptop.

You will have to experiment with this a little bit to find the best solution. Getting a power supply with a 3-prong AC plug is the best, but not always possible. The next option is to find something on your laptop that you can ground. Make an adapter from that 3rd prong to "something". That something could be the signal-ground on the output cable of your power supply. It could be a screw on the laptop. Or a shield on an unused laptop connector. Or the ground/shield on your audio cable.

Make that 3rd prong adapter, but leave the other end bare for the moment. Then start poking it around to see if or where you can connect it and have the noise go away. Once you have found a place or two, then finish up the adapter so it is easy to use.

Two warnings when doing this: Make sure that whatever you are grounding is actually ground! On the power supply output, make sure you ground the negative or gnd conductor. And when poking around, understand that you might actually have to poke a little hard. Both the bare wire and whatever you are poking will likely have a thin layer of non-conductive stuff on it, and you need to apply enough force to poke through it. Rubbing sometimes helps too. The non-conductive layer is sometimes paint on screws, or an oxide (rust) on the metals.

Oops, here is a 3rd warning: Be super careful when making that 3rd prong adapter. You're messing with potentially lethal voltages and we don't want you to die. Build the adapter in a way that there is no possibility of it failing and shorting out against either one of the other two conductors in the AC plug.

Give it a try and report back what you found!

Answer 2

Several possibilities, and possibly several at once.

Kortuk is looking to SMPS noise, which is a valid thing to look at.

A ground loop and an RF pickup can both very happily exist in a system of that size. When you look at the size of the physical loop that may have been formed you'll see it is large compared to the dimensions that would be used for a radiated signal pickup coil at RF and even at smps frequencies. And a ground loop does not need to be very large physically - its "size" is measured not in physical dimensions but in the magnitude of the common impedance in the common ground that two subsystems are sharing and in the signal current passing through the resistance. V = I x R.
V_ground_loop ~= Signal_current x R_common_in_shared_ground_lead

You can insert ground loop breakers - they are simple to DIY and consist of the electromagnetic equivalent of the differential amplifier that you correctly identified as one possible solution.

To "break the loop" insert a 1:1 audio transformer in the audio signal circuit that is closing the loop for grounds - In this case = radio receiver to notebook. I say 1:1 if the circuit exists and you do not wish to affect levels, but you could usefully use 1:N or N:1 depending on overall circuit. You can get "transistor radio" audio transformers of various turns ratios from disposal houses or amateur market component sellers. These will often be low cost. Failing that you can sacrifice a suitable old transistor radio. If you only have 1:N transformers and have several "in the junk box" you can wire these 1:N into N:1 giving 1:1 overall.

As well as Kortuk's foil screen you can try a clamp on or loop through EMC ferrites which are often supplied with new appliances with RF and audio functions.