Switching mode PS causing buzz

[SBob]

Hi,

I have just finished building my first gainclone. I was happy with the sound, but there was an annoying buzz when the pre-amp's volume was high. After a couple of hours troubleshooting I discovered that it came from my Soundbridge (a multimedia player). It has a switching mode wall-wart supply, when I instead used a lab PS the buzz was gone! I tried with another switching mode PS and the buzz was back. It is a nasty buzz with harmonics all over the frequency range.

My question is: is there a general problem with some parts of your system using switching mode power supplies, or is it just my GC that has a problem? (I have a bi-amped system, the buzz was there before I put in my GC, but became MUCH worse as soon as I plugged it in)

/Robert

[Leolabs]

Try to put a RF input filter.

[v-bro]

Do you use a tuner? I had a problem like this a while ago and it came from the antenna (cable radio).... it went away when I connected a filter between the antenna. (Monacor RFT-2)

[SBob]

There is a tuner in the NAD 7240 that I am using as preamp and woofer poweramp, but there is no antenna connected to it. (I am using the Internet radio in the Soundbridge)

I assume the "RF input filter" should be applied to the DC coming out of the wall wart, any suggestions on how it should be designed?

[v-bro]

You can try a small toroid coil...(like the ones in D-amps from the link below) one in series should filter RF. But maybe it can be solved by adding extra buffercaps against ripple...(large electrolytes with voltage rating well above the dc voltage of the output of the wallmart...

[SBob]

Thanks, I will try adding a toroid. How many turns and what thickness of wire should I use? I.e, how many mH?

[v-bro]

To filter well: the larger the better..(larger ones will start filtering at a lower frequency).

To be able to handle a lot of power it can be wiser to keep it small and use thick wire...

The ones used in many class D amps are 10uH, but you don't need a very precise value, just try something like fifty windings with thick wire...see if it helps...

[SBob]

I have tried using 1.1 mH inductances in series and also added a capacitor across. No effect, the buzz is still there

Since no-one alse seems to have this problem with switched power supplies I am assuming that the Soundbridge is badly designed. I tried building a prototype "normal" PS and there was no buzz. So; I will finalise that normal PS and use it instead.

Thanks for your help!

[v-bro]

Very odd, I've used five different SMPS on all kinds of (class D) amps and never had such problems...

But linear supply's are nice and easy to design, tough will have large voltage drop when not regulated, but regulating costs some power...
Keep in mind that the voltage of the non-loaded supply is not higher than the absolute max. rating of the amp!