Making a switching supply quieter?

[phrarod]

I heavily modified a very stout switching supply and housed it in a good metal box with an IEC and handmade DC wire to unit. It does sound good but I do have some upper frequencies anomalies which could be considered noise on some tracks. Its at the very upper end which I've been told is an issue with switching supplies.

Rather than going through the whole deal of building a linear supply is there a way to quiet a switching supply? Maybe a cap across the output?

[Mooly]

I take it you mean noise on the dc output and not physical noise from the magnetic components and ferrites.
You need to view the output of the PSU on a 'scope to identify if there is a noise issue and, if so, what the character of the noise is with regard to amplitude/frequency etc.

Grounding of an amp using a smpsu could be very critical if large HF ripple currents are circulating.

[phrarod]

All I have is a DMM. No scope or know anyone locally with a scope. This is the supply I modified to provide better performance and lower noise with changing caps and wiring.

http://www.alliedelec.com/Images/Pro...A/800-0325.PDF

At this point it is a theory as I don't have a linear supply to test it against.

[Mooly]

All these are low voltage PSU's. Would the amp run off a lead acid battery (12v) to test ?

Without at least a 'scope you are just guessing really. Even if there is HF ripple there that in itself isn't necessarily the main problem. Just as with low frequency line ripple wiring and ground is crucial. It's impossible to guess on this one.

[phrarod]

This is a 12V supply. I did run it off LiPo batteries and the top end doesn't have that noise to it although I loose dynamics and low level detail.

I figured it was a stretch. So just putting a small cap across the DC out would be a useless exercise?

[Mooly]

HF noise from SMPS is hard to eliminate just by adding caps. By all means try but the real answer is perhaps an L-C filter, but that adds impedance to the supply.

[kevinkr]

Common mode filtering using a CM choke and capacitors with very low impedance at the switching supply ripple frequency may help. I'd recommend organic polymer electrolytics or film caps for this chore - keep leads short!

Doing this sort of work really does require a decent scope which isn't necessarily going to run you much money if you shop carefully at local hamfests or on eBay. This will allow you to see how much of an issue you have and how much it improves as you experiment with filtering. It is possible to make very quiet switching supplies with the proper external filtering.

Caps by themselves are of little benefit, and aluminum electrolytics in particular can perform very poorly for this task.

[phrarod]

I've been dreaming about scopes for years and almost snagged one at a hamfest but didn't know if it was the right model.

I would naturally use a film cap.

If anyone is in the greater Los Angeles area with a scope and wouldn't mind a visit that would be awesome.