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Music servers and other computer based digital audio technologies.

Linear supplies etc.

Hi Abe,
I DID do a lot of checking of all kinds of equipment with scopes, spectrum analyzers etc. In particular how the power supply in one piece of equipment can travel through the mains and affect the power supplies in other components, and yes get through the supplies in other boxes.

The frequency range that was most problematic was 50KHz to 300KHz or so. A lot of the boxes I looked at do NOT attenuate these frequencies well. Many have networks that work well for 1MHz and up, and many have networks that work well in blocking noise in the audio range, it's the range between these that seems to be "wide open" in many boxes.

Unfortunately this is the range that many switchers run at and it's also the range where diode "switching noise" occurs. MANY boxes, both linear and switching have resonances in this range which can be excited from noise in other devices. Unfortunately very few voltage regulators have any attenuation in these frequency ranges either. So if it gets through the main PS, the local regulators are not going to be much help.

I did find that there is a definite distance affect here, noise generated in this region injected into the mains at the other end of the house does seem to get attenuated quite a bit so noise generated by devices outside of the audio system does need to be fairly close by to have an affect on the audio system. Devices "in the rack" are going to cause more problems than devices in a different room or on the opposite side of the house.

Of course I did not test every piece of audio equipment on the planet, just what I had in my house, which ranged from inexepnsive "consumer" gear to very expensive audiophile equipment. It wasn't just audio but things like TVs, satelite tuners, microwave ovens, routers etc. Every single one of these devices injected noise in this range into the AC mains, how much vaired a lot from box to box. ALL of the audio boxes let some of this noise through, none of them did a very good job of blocking it.

The issue of internal switching converters is an interesting one. I did several experiments building one of my liner supplies to drive different digital devices (small computers, routers, switches etc) and found that the amount of noise in this region injected into the mains went WAY down, even though the device in question had internal switching DC-DC converters.

Overall conclusion: power supplies which inject very little noise in this region into the mains are very useful, IF they are near your audio system. Doing this to stuff a room away or more may have a slight affect, but it's probably not worth it.

For example in my main system the squeezebox Touch and the netgear switch are run off linear supplies, and the DAC has one (well actually 5) built in. The preamp and amp both have supplies using these techniques (I can do that because I build it all myself). The result is the AC in my rack is very clean in this region. All this equipment has damped supplies that do not resonate in this region so noise from outside sources which do make it into the system has relatively small effect. I haven't bothered with any of these techniques for computers in other rooms, TVs etc. The listening room does not have a TV, cable box etc and there is no computer in the room other than the Touch.

It's obvious I have not testsed everything in existance, there certainly may be some companies that do produce boxes with low noise injection in this region and supplies that do a good job of attenuating what is on the line. If you have a system that's composed of these, great, you are lucky. But there is a LOT of equipment out there that is not.

On the comment that it is just a PI netowrk, which have been around for a long time, why is that important? I'm not claiming I have created anything new or mysterious, any new physics or anything. The issue is that a properly designed PI network, properly tuned with the right value caps etc will deliver very low noise into the AC mains and will deliver DC to the regulator whose residual ripple has almost no high frequencies. Sure the concept has been around for a long time, but how much equipment uses it? The probability is pretty high that not a single piece of electronics in your house has a PI filter in it, let alone one that has been designed to meet the criteria of mine.

So if a PI network is so good, how come everybody doesn't use them? Price and tradition. It costs more money, space and wieght to do it. The other issue is tradition. PI networks were designed for high voltage tube circuits, and most electronics engineers today if they even know of the existance of such a circuit, assume its only good for high voltage supplies. Since they have never seen one used for low voltage circuits they never even think about using one, it doesn't even occur to them as a possibility.

John S.


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