Grounding V- on a SMPS

[kmalone]

I'm making a guitar preamp and I'm powering it from a 12v SMPS (Switched mode power supply) (Meanwell NES-15-12). The ground i'm using in the circuit is the negative terminal of the SMPS. I'm getting a lot of noise when i connect the guitar to the preamp through a coaxial cable even though the strings and shielding are grounded (to the negative terminal of the SMPS). I was wondering if I could connect the negative terminal of the SMPS to earth/ground on the AC line, would this a) work/ not blow up or short and b) reduce the noise?

 

[andre_luis]

The audible noise seems to be AC mains frequency ?
I believe you should to focuse aptemption at pre-amplifier side.

+++

 

[atripathi]

Is this isolated or non isolated type SMPS, if isolated, try connecting negative terminal to earth. For non isolated type, connecting earth may blow up whole thing, so be careful and sure before you do this.
You may want to use ferrite bead (or complete filter circuit is even better) between SMPS O/P and Guitar peramp input.

 

[mtwieg]

You'll have to describe things a bit more clearly.
Is the supply isolated?
What frequency does the supply operate at?
What exactly does the "noise" sound like? Is his a white noise hiss, or is it more of a tone?
Why would you use a SMPS for a preamp? At low power levels it probably doesn't make sense over a battery or linear regulator.

 

[FvM]

FYI: NES-15 is a standard enclosed AC/DC SMPS.

I fear, that the mains interferences coupled through Y-capacitors may be sufficient to make the power supply effectively unusable for a guitar preamp. Connecting the ground to PE will create ground loops and possibly even more severe "noise". Besides interference issues, I won't like of-mains SMPS for stage equipment in terms of electrical safety.

 

[mtwieg]

Yeah the possibility of a ground loop is a major suspect. But it should have an isolation transformer, so the Y caps shouldn't matter (unless you're talking about secondary side Y caps?). The transformer might not be screened properly though. I would try looking at the output of the amp through a good isolation transformer and see if the "noise" appears then.

 

[FvM]

Yes you are right, there are not necessarily Y caps between mains and secondary, because NES-15 has a PE terminal connected to the enclosure and most likely a screen between primary and secondary. Did you connect the PE terminal?

 

[mtwieg]

Well here's the datasheet for his supply:
**broken link removed**

The block diagram does show a secondary side y cap to the chassis, which is connected to the input side FG terminal. Also there's a primary side Y cap in there, but it's probably after the input filter. But in any case, no matter how you use it there is some kind of ground loop in there.

 

[FvM]

The Y capacitors can be expected quite small, about 1.0 to 4.7 nF, so they won't cause much hum, but some interferences at higher mains harmonics may be generated. You would know the source, if you hear it. The noise will be surely bad with unconnected SMPS PE terminal.