[Oya?]

Hi guys, I'm trying to case up an amp and power supply in a modest ABS enclosure. This is really technically only my 2nd DIY build so I'm not really sure if I'm laying it out 'right'.

I was thinking of just doing a really simple layout like this:



But there's kind of a lot of wasted space in this pic, and I'm thinking I want to stick a toroid (salvaged from my broken Millet!) in there as well so that it looks like this:



What I really have no clue about is how much distance I should keep between the toroid and AC wires, and the amp, pot and signal wires etc.; In this pic I've moved the toroid as far away from the amp as I could but even with careful wiring I don't know if there's enough of a gap. I'm also not really sure where to put the regulator. Does this look acceptable, or will I end up with a noisy amp?

I also bought an aluminium sheet which I intended to use to mount the toroid and also ground the IEC earth to. If I cut it up, and used a piece as a shield like in the red line I drew, will it do anything to help make it quieter?

Any help would be appreciated!

Thanks.

 

[Mr.Duck]

Aluminium will not shield the transformer. You would need a ferrous metal like steel for that.

I wouldn't use a transformer like that either. A standard, unregulated wall wart will be better since it is much safer and has no grounding issues to deal with. Also keeps the transformer noise away from the audio but with a torroid I doubt you will be able to hear it even if you put it very close.

 

[fault151]

Interms of layout the second pic is probably the best approach. The first one would mean you need the psu in a second box. I would go with version two. Like 'Mr.Duck' says, you probably won't hear any noise anyway. I always try keep any signal wires/components away from the AC inputs and power cables, which you would manage in the 2nd design quite easily.

 

[Oya?]

Not having to use a separate power unit or plugpack would be a big plus. I should be able to play around with the wiring and keep everything neat.

Also I didn't know that aluminium won't shield the toroid, thanks for clearing that up.

Mr.Duck mentioned something about grounding issues; a while ago I posted here asking about how I should be grounding/earthing toroids and IEC sockets in a non-metal enclosure, I never quite figured it out. I saw that a lot of people mount their toroids on aluminium panels installed inside their plastic/wooden enclosures so I figured this is the way to go. Would there be any issues with this approach? Or is it perfectly acceptable to just mount the toroids as they are and leave the IEC earth floating?

 

[linuxworks]

the only REAL way to know is to do a test, using good test gear. wire it up and move the trafo around (safely) and watch for the lowest hum and noise location.

you can only really do it experimentally. and you'll have to watch the analog outputs for noise and hum on a scope so you'll know when you found the 'dip'.

unless you are very experienced, its hard to know just by 'looking'. you have to move the trafo around and see the realtime effect as you move it. then you'll know.

 

[Oya?]

As soon as I wire up the inputs and the headphone jack I'll be sure to play around with the layout. I don't have any test equipment but I'll listen for hum.

Luckily everything plugs and unplugs easily.

(Linuxworks, I stole the idea of using socket strips from one of the PPAs in your sig btw.

Too bad I don't have any thinner wire though so everything's a bit stiff)

 

[Mr.Duck]

Quote:

Originally Posted by Oya? /img/forum/go_quote.gif a while ago I posted here asking about how I should be grounding/earthing toroids and IEC sockets in a non-metal enclosure, I never quite figured it out.

For safety, you always need to connect the metal parts of the chassis to earth (probably not in your case), and the audio circuit's 0v or ground needs to be earthed as well. This is incase the transformer fails and puts mains voltage on it's secondaries. Make sure there is a fuse in there too.

I really very strongly advice against using that transformer. There are no advantages, and only downsides. Wall warts really work well if you get an unredulated one to feed into you DC-DC converter / regulator. Double insulated ones are so ideal for this use. It has this symbol...

So safe and no ground loop issues to deal with. As well as more compact and no transformer noise.

 

[Uncle Erik]

Quote:

Originally Posted by Mr.Duck /img/forum/go_quote.gif Aluminium will not shield the transformer. You would need a ferrous metal like steel for that. I wouldn't use a transformer like that either. A standard, unregulated wall wart will be better since it is much safer and has no grounding issues to deal with. Also keeps the transformer noise away from the audio but with a torroid I doubt you will be able to hear it even if you put it very close.

I disagree.

Ferrous metals do shield RFI, however, aluminum works fine. I've used it myself and you'll find aluminum widely used as shielding.

Further, ferrous metals can sometimes magnetically couple to the circuit, causing other troubles.

Second, wallwarts are often of marginal quality and a good toroid would be an upgrade. I'd rather have a good quality transformer and tackle a grounding issue than use a so-so wallwart to avoid that.

 

[tangent]

Quote:

Originally Posted by MrDuck with a torroid I doubt you will be able to hear it even if you put it very close

I can't find it now, but one of the projects on Headwize had a circuit board layout included which put the toroid and amp right next to each other, and was well known to have audible amounts of AC hum. Some of those who built this project then cut their boards in half to put more space between the amp and power supply, and the problem went away.

It's true that you don't need huge amounts of space with a toroid, but "very close" is a bad idea. The proposed layouts in the pictures at the top of this thread look fine to me, though. They put about as much space between the two as is practical, the best plan IMHO.

Quote:

Originally Posted by linuxworks wire it up and move the trafo around (safely) and watch for the lowest hum and noise location.

That shouldn't be necessary with a toroid. Its field should be nearly symmetrical, changing only near the places the primary and secondary wires come out. Empirical layout like you describe should only be needed with other types of transformers, like EI and split bobbin types, which have much greater fields in some directions than others.

You will want to be careful about the routing of those primary and secondary wires, of course. Keep the AC as far away from the things that run off DC as possible, bringing them together only at the rectification and regulation part of the circuit.

Quote:

Originally Posted by Uncle Erik /img/forum/go_quote.gif Ferrous metals do shield RFI, however, aluminum works fine. I've used it myself and you'll find aluminum widely used as shielding.

Yes, aluminum is an effective shield against RFI, as is any metal, but we're not concerned about radio frequency interference here. Only ferrous metals (iron, steel, nickel, and cobalt are the main ones) are effective against EMI, electro-magnetic interference.

EMI and RFI are related, but for practical purposes are different things.

 

[Mr.Duck]

Quote:

Originally Posted by Uncle Erik /img/forum/go_quote.gif Ferrous metals do shield RFI, however, aluminum works fine. I've used it myself and you'll find aluminum widely used as shielding.

That statement is true. But to shield the magnetic field around a transformer, you need a ferrous metal. I expect Aluminium will work no better than a plastic shield.

BTW, radio frequency interference is a type of electro magnetic wave. In other words RFI = EMI. I'm not sure what the difference is if there even is a difference. A Magnetic field is somthing else. Also, a magnetic field can be static.


Quote:

Originally Posted by Uncle Erik /img/forum/go_quote.gif Second, wallwarts are often of marginal quality and a good toroid would be an upgrade. I'd rather have a good quality transformer and tackle a grounding issue than use a so-so wallwart to avoid that.

I strongly disagree here. A standard unregulated wall wart provides the means to get power into your equipment where is can be tightly regulated if required. It's easy to get wall warts rated for say, 3 Amps at 12v, so there should be no issues with over loading them for a project like this.

SMPS wall warts should be avoided most the time because it's too much of a pain to try and filter the switching noise away when you can just start off without it.

 

[tangent]

Quote:

Originally Posted by Mr.Duck /img/forum/go_quote.gif RFI = EMI. I'm not sure what the difference is if there even is a difference.

Yes, radio is a kind of electromagnetic emanation, but that doesn't mean there is no useful distinction. Light is EM radiation, too, but it's rare for audio circuits to need to worry about light shielding. Why? Frequency and the type of field both matter.

There is a tiny little EM field around a wire carrying an audio line-level signal. For that reason, we want many thousandths of an inch space between it and another audio line-level signal. The EM field around a straight wire carrying AC wall current is much bigger, so we want the spacing to be even larger. And the EM field around a transformer -- a pair of coils configured for mutual induction, thus changing the way the field operates -- is even larger, even though the frequency and current haven't changed, so we want even more spacing between it and anything it could affect detrimentally.

As for the RFI side of things, the "radio frequency" part of the acronym is the clue to the useful practical distinction. Wikipedia says RF is 3 Hz to 300 GHz, but when we speak of RFI, we're talking about anything well above audio frequency, up to the low microwave range. Why limit it? This is the range of unwanted frequencies that our circuits respond to. Anything near or in the AF range we can't do anything about because we can't go filtering the AF range, and anything above our circuit's bandwidth limit is irrelevant. You can send 300 GHz radiation at a PIMETA all day and it isn't going to do anything to it, other than warm the board a bit.

That same audio line level trace we talked about above can be affected by RFI, but only at frequencies we can predict from its length. The AC wall power wiring we talked about above can be affected by this, too, and probably has been, since there are AC lines of all lengths near you, so you have a broadband antenna. That RFI goes through the toroid pretty well, too, which is why it's good practice to install AC line filters -- RFI suppressors -- in toroid-based power supplies. But none of this has anything to do with ferrous metal shielding, so we make a distinction between this RFI and the EMI directly produced by transformer action.

All of this is described by the same set of equations, but it's still useful to make these practical distinctions.

 

[linuxworks]

Quote:

Originally Posted by tangent /img/forum/go_quote.gif That shouldn't be necessary with a toroid. Its field should be nearly symmetrical, changing only near the places the primary and secondary wires come out.

not sure where I read or heard it, but even toroids are positional (winding ingress/egress points, like you say).

I don't like touching the trafos while they're powered on, but it seems that you may want to verify that you've found its best null point. 'install once, listen many'; so the install is worth a little bit of experimentation before locking it in.

on my b22, I had to move whole BOARDS around to find least amount of hum.

 

[luvdunhill]

Quote:

Originally Posted by tangent /img/forum/go_quote.gif Light is EM radiation, too, but it's rare for audio circuits to need to worry about light shielding. Why?

I routinely double-blind test the photoelectric effect on LEDs used in CCS in audio amplifiers. I prefer to black out the LEDs with paint, as I find it sounds better.

 

[n_maher]

Quote:

Originally Posted by tangent /img/forum/go_quote.gif That shouldn't be necessary with a toroid. Its field should be nearly symmetrical, changing only near the places the primary and secondary wires come out.

Not to disagree buy my empirical evidence is 100% to the contrary. I found with one of the beta22's that I build that simply rotating the torroid had a very audible effect on the amount of hum. And it was not symmetrical at all, there was only one low point. Based on this I've tried partially and fully shielded transformers (using mu metal) and honestly the only truly silent builds that I've been able to achieve with the beta22 have ones using an externally mounted transformer. But even then if the amp and signal wires are in close proximity with other gear that uses unshielded transformers you're just as likely to pick up noise there too. It's a tough situation and some amps seem more prone to picking this up than others.

So my advice is to try it but be prepared to fight it if it's an unacceptable level to you.

 

[nattonrice]

Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif I routinely double-blind test the photoelectric effect on LEDs used in CCS in audio amplifiers. I prefer to black out the LEDs with paint, as I find it sounds better.

Do you use silver impregnated paint?