It is more than 4 years after Ron Maxson posted this question about his speakers humming. I'm guessing he's given up by now and tossed the speakers in the recycle bin at work somewhere.
I found this exact set of speakers in the electronics recycle bin at work. Usually stuff that ends up in the recycle bin has some kind of issue. I will add that I don't know anyone by the name of Ron Maxon at work, so I am pretty sure the ones I found are not his. :-) Before even trying the speakers, I searched Fixya to see what kind of problems they might have. This was the only issue I found on Google or Fixya. I was kind of hoping the set I picked up had the same issue since I haven't seen any other issues or repairs. When I hooked the speakers up to my PC, there was a very loud and irritating buzz/hum/tone that I could hear whether music was playing or not. And the "hum" seemed to match the description Ron gave.
So I have the same set of speakers (Phillips SPA7351/27) with what I beleive is the same issue (constant "hum"). Often times in electronics, when something was working, but now isn't, or is doing something different, it can be due to a bulging capacitor. My parents had an internet modem once that stopped working. A light on the modem indicated there was some error that would need to be serviced, but that's all the info we could get about the error. Inside the wall-wart I discovered a couple of bulging capacitors and replaced them. The error on the modem went away and worked as normal.
In this speaker set, I suspected it was something along the same lines. The way this set of speakers works starts at the AC plug that goes into the wall socket. This AC voltage feeds a step-down transformer that reduces the home AC to an AC voltage closer to around 15V AC. That 15V AC will then feed into a set of diodes that will full-wave rectify the AC signal so that it becomes more like DC (instead of a sine wave, you get something that looks more like camel humps if you see it plotted against time). Then a smoothing capacitor is used to level off the humps so that a closer to DC voltage is created for any following circuitry. The DC voltage plotted should be a constant value through time. The smoothing capacitor is usually pretty big when AC is converted to DC using a transformer/bridge-rectifier circuit. The value is usually around 1000uF. In the case of audio electronics, this smoothing capacitor is usually something like 4700uF and is about half the size of a AA battery (imagine cutting one in half).
When I opened it up, there was a bulging 4700uF 25V capacitor sitting there next to where the transformer connects into the main PCB. I saw that Amazon had the same value capacitor for sale in a set of 10 for $8.99. I'm sure you can pick one up for cheaper on eBay or Digi-Key, but I wanted to get this fixed pretty quick and already have "prime" 2-day shipping. Plus I can use the extra caps for other projects/repairs.
My capacitors came in today so I de-soldered the bulging cap and installed the new one (got some help pushing the new cap in while I soldered from my 4yo son... what a good helper!). I added some thermal paste to the back of the amplifier chip. By the way, the amplifier chip is an STMicroelectronics TDA7379 chip. You can google TDA7379 and find the data sheet which contains circuit examples including how to configure a circuit for 2 stereo speakers and a single speaker.
I finished the part replacement, screwed everything back together, and WOW! What an improvement. The buzz/hum/tone I was hearing is gone. I have a nice set of PC speakers now for probably about $1-$2 worth of parts and part of an afternoon.
If you have the same set of speakers or even a similar set with these symptoms, you might want to open it up and check for bulging capacitors. They are easy to find/replace for cheap if you have a decent soldering iron or know someone who does. Likely the circuitry is very similar. I am including pictures of my teardown and hopefully can show the bulging cap as well as the new one. The bulging cap is the "KONMO" part while the replacement good one is "ChengX" in the pictures.
One thing they always say is to be careful with large capacitors as they sometimes still hold charge. I've never had an issue with them but I always try to be careful not to short the two wires coming off of the capacitors.
Ron if you still have this set laying around somewhere, you might check for this issue and see if it fixes it.
*Sadly my annotations in the photos were removed. I spent a good amount of time highlighting what I wanted to focus on in the pictures and the annotations showed up before I posted them. I don't see them now. Feel free to ask me if you have a question on one of the photos.
This photo shows the circuit board part number which is similar to the speaker set model number.
This photo shows the location of the bad capacitor. I have removed the circuit board from the rear panel of the subwoofer.
Below you can see that the top of the problem cap is not flat but bulging. If you see a bulging cap, it is no longer working. This cap's job is to make sure you get clean DC power to your amplifier circuitry.
The step down transformer is this big heavy thing screwed to the inside back panel of the subwoofer assembly. It takes wall AC and turns it into lower voltage AC on the blue wires that connect to the circuit board next to the faulty capacitor.
Since I removed the circuit board from the back panel, I thought I would show how the holes and screws line up to the back panel. Notice the white spot on the right side. This is thermal paste used to help transfer heat from the amp chip to the metal of the back panel. You can buy some and add more if you want to make sure it transfers heat well. Also notice that I put the screws back in the holes they came out of on teh circuit board and back panel. I had a couple of days to wait for parts and used this system to make sure I remembered where they went.
Here's the side of the new capacitor. It shows the -40 to + 105C rating which matched the capacitor I removed.
This is the other side of the new capacitor showing the 4700uF value and 25V rating.
Below is the old bulging capacitor before I removed it. Take a picture of it before removing so that you can remember which side the strip (negative side) faces. These can only be installed two ways, but it is important to get it correct.
Here is the new capacitor installed. Notice the white stripe (negative side) matches in orientation with the previous photo of the bad capacitor. Also notice how the top is flat.
Here is a picture of the amplifier chip if you are curious about how it connects up to the speakers and other circuitry.
The "X" on the top of the capacitor is designed to release gasses when a capacitor fails. It lets the gas out in a slow and controlled way so that the capacitor doesn't explode catching your circuit board/speakers/house on fire. When you see a bulging cap, it has let the gas out and no longer works the way it was designed. In the case of these speakers, it was not smoothing out the voltage to the amplifier which caused the "hum".
Here is a link to the amplifier chip if you are curious of the specs and suggested connections: http://www.st.com/content/ccc/resource/technical/document/datasheet/fa/38/9a/71/78/45/44/16/CD00000160.pdf/files/CD00000160.pdf/jcr:content/translations/en.CD00000160.pdf
Here's a link that shows the full-wave rectified graph to give you an idea of what the capacitor is doing. It creates the red line in the graph: https://www.quora.com/Why-do-we-use-capacitor-in-rectifier-circuits
This link also gives some information: Selection and use of Capacitors Circuit Clinic 2
Notice in this link where the auther writes: "Here the smoothing capacitor is 4700uF 50V because the power supply is that of an Amplifier circuit. If the smoothing is not perfect, humming noise will appear in the speaker." Previously he was describing a 1000uF capacitor and mentions the 4700uF value because of the audio amplifier circuit.
Hope this has been helpful! It was a quick and easy repair.