Switching PSU filtering

[rp3703]

I built an 8 channel API-312 clone with a linear PSU inside the same chassis. It worked great but I’m starting to notice EMI in the channels closest to the PSU. I would like to swap out the toroidal for one of these:

3-Rail Switch Mode Power Supply +/-16v and +48v

I have read that switching PSU’s require high frequency noise filtering versus the low frequency filtering required for a linear PSU. Does anyone know what methods are commonly used to do this and does this apply only to the PSU or should filtering also be applied to the individual channels as well?

 

[Flatpicker]

At the bottom of the description in the link you posted it has a link to a schematic of the filter you would need: "We recommend filtering the power supply's output for lowest noise. Please see our Power Supply Filtering Schematic (PDF) for a recommended filter circuit." You only need to use one of these filters between the PSU and your preamp circuits.

Alternately, if the toroidal is parallel to the preamp circuits you can try just rotating it some until the noise goes away, or otherwise add some shielding. Lastly, you could mount the toroidal in a separate smaller enclosure with the 120V/240V going in and the toroidal secondary going out to the preamp chassis. That is the method I use and works great.

 

[rp3703]

So did you just put the toroidal in the box or did you have to add any extra safety fusing to the box as well? How far away from the preamps would it need to be to eliminate EMI? I'd like to be able to keep it all packed into an 8 space rack.

 

[Flatpicker]

I use a 1590D aluminum enclosure with a power entry module (w/switch and fuse holder built in) on one side connected to the transformer's primary, and a 3-pin XLR male connector on the other side connected to the transformer's secondary. Be sure to connect the ground tab from the entry module to the 1590D aluminum enclosure via a bolt or some secure means.

I make my own cables for connecting the secondary voltage to the preamp using 3-4ft of 22AWG 3-conductor cable with female XLR connectors on both ends to go from the transformer box to a male XLR on the preamp enclosure. I got the cable from Digi-Key but I don't remember the P/N.

I just set them in the bottom of my rack cabinet (just like you would with a PS brick), but I don't think they would need to be very far away if you have limited space.

 

[Bo Deadly]

I built an 8 channel API-312 clone with a linear PSU inside the same chassis. It worked great but I’m starting to notice EMI in the channels closest to the PSU. I would like to swap out the toroidal for one of these:

3-Rail Switch Mode Power Supply +/-16v and +48v

I have read that switching PSU’s require high frequency noise filtering versus the low frequency filtering required for a linear PSU. Does anyone know what methods are commonly used to do this and does this apply only to the PSU or should filtering also be applied to the individual channels as well?

See the following:

Rules for SMPS w/ Audio

That external supply looks like it might work well if the load match is right. Not sure about the LC filter. I'm a little surprised that it can startup ok with 2200uF caps. I would get one and fiddle with it a bit to make sure it's good.

One of those cylindrical torroid shields might also be just enough. Maybe even just a piece of steel might deflect EMF enough to bring the noise down.

But a properly implemented SMPS would give you the best results.

 

[Ricardus]

I don't suppose anyone has done a board of that SMPS filtering circuit?

 

[dandeurloo]

I did, well it was my own schematic after some testing. Works really good.


https://collectivecases.com/?product=dc-filtering-pcb

I don't suppose anyone has done a board of that SMPS filtering circuit?

 

[Ricardus]

I know you had this, but I was hoping for a community project with Gerbers.

 

[rp3703]

See the following:

Rules for SMPS w/ Audio

That external supply looks like it might work well if the load match is right. Not sure about the LC filter. I'm a little surprised that it can startup ok with 2200uF caps. I would get one and fiddle with it a bit to make sure it's good.

One of those cylindrical torroid shields might also be just enough. Maybe even just a piece of steel might deflect EMF enough to bring the noise down.

But a properly implemented SMPS would give you the best results.

It's been so long since I figured up the load for each channel. I want to say it was around 100ma each but when I look at the specs on the API 312 500 series cards, they list 55ma. Do you think 440-800 ma is not enough load for a 2 amp PSU?

 

[Bo Deadly]

It's been so long since I figured up the load for each channel. I want to say it was around 100ma each but when I look at the specs on the API 312 500 series cards, they list 55ma. Do you think 440-800 ma is not enough load for a 2 amp PSU?

0.44/2 = 22% which is not ideal but it's not clear what the 2A refers to. Is that 2A / 32V or 2 x 2A / 16V? What might be more of an issue is 48V / 150mA. If you have only one 48V on, that's maybe 0.01/0.15 = 7% so you might need to add a dummy load. Or maybe it doesn't modulate outputs at all when underloaded (although that would probably be a bad thing since it would indicate that it's not using a sophisticated SMPS IC which is probably required to achive really good low noise output). Are there any LEDs? With some LEDs and maybe some loads it could be fine. You would really need to just try it and see. It could be that the thing is just noisy regardless of what you do. But it's a nice form factor and connector and it's not unnecessarily expensive so it's probably worth getting one and just trying it.

 

[FIX]

I typically use common mode chokes, right out of the supply, then one for + and one for -, then some .1uf across, then a large cap, then I do it again. I went as high as I could with the inductance factored by the current drop. For the 48, I use one in series.

 

[NOON]

Here's the schem and gerber files for a quick switchmode filter board I made to use with the Meanwell +5/+15/-15 supplies and a 48V module. The common mode chokes are a common one I was getting from Element14 and/or Mouser, let me know if you need an exact part number. If anyone sees any issues with the design or layout happy to modify and repost, but I haven't noticed any issues when using them. I got a batch made up at Elecrow. Sometimes I just bridge the 'output' inductor spot as it's not really required, make sure whatever inductor you use is rated for the current, I made that mistake once and smoke came out from under the board! Can also post the original Diptrace files if anyone wants to modify for their own use.

 

[Ricardus]

Here's the schem and gerber files for a quick switchmode filter board I made to use with the Meanwell +5/+15/-15 supplies and a 48V module. The common mode chokes are a common one I was getting from Element14 and/or Mouser, let me know if you need an exact part number. If anyone sees any issues with the design or layout happy to modify and repost, but I haven't noticed any issues when using them. I got a batch made up at Elecrow. Sometimes I just bridge the 'output' inductor spot as it's not really required, make sure whatever inductor you use is rated for the current, I made that mistake once and smoke came out from under the board! Can also post the original Diptrace files if anyone wants to modify for their own use.

Your part numbers would be cool.

 

[NOON]

Your part numbers would be cool.

Actually it's on the schematic 51305c
https://au.element14.com/murata/51305c/common-mode-choke-3mh-1-9a/dp/3760547
Other parts are pretty standard, whatever you have to hand that fits will do.

 

[Ricardus]

Actually it's on the schematic 51305c
https://au.element14.com/murata/51305c/common-mode-choke-3mh-1-9a/dp/3760547
Other parts are pretty standard, whatever you have to hand that fits will do.

L1, L2, L3, and L4 in the schematic don't seem to be on the board. Yeah, I already found U1 - U4.

Thanks for this upload!

 

[NOON]

There's some components on the underside.

 

[Ricardus]

There's some components on the underside.

Oh! Crikey! I didn't even look.

What are those part numbers?

 

[Ricardus]

Is that a ferrite bead?

 

[NOON]

I had some kind of smd inductor there for low loads, a bigger coil for larger loads when required but they're the one's I often just bridge with a link. Not a critical component for a good quality switcher like the Meanwell.

 

[Ricardus]

I had some kind of smd inductor there for low loads, a bigger coil for larger loads when required but they're the one's I often just bridge with a link. Not a critical component for a good quality switcher like the Meanwell.

Oh, OK, thanks again!