I’m experimenting with (unshielded) inductors as filter elements in a phono-preamplifier (shielded inductors are not available for the needed values).
When I had the boards lying “naked” on the shelf next to the turntable, hum was moderate, because I kept the PSU ~30cm from the phono-board.
Now I put everything in a housing, the phono board and the PSU (with trafo) are separated by sheet of 1mm zinc-plated iron: The inductors pick up a lot of hum.
I’m aware that a mu-metal shielding is probably the way to go. But I’m still reluctant to buy some before I tried other options.
Simply putting additional sheets of iron in the space between trafo and phono boards helps very little.
For now I made a second iron sheet, bended, which wraps around the trafo a bit- I’m going to try it out this weekend.
Has someone experience how to effectively shield against the magnetic field from the trafo?
When I use mu-metal: is it better to shield the phono-board or the trafo?
Also: I have a lot of neodymium magnets left. Can they possibly help to kind of counteract the magnetic field from the transformator?
When I had the boards lying “naked” on the shelf next to the turntable, hum was moderate, because I kept the PSU ~30cm from the phono-board.
Now I put everything in a housing, the phono board and the PSU (with trafo) are separated by sheet of 1mm zinc-plated iron: The inductors pick up a lot of hum.
I’m aware that a mu-metal shielding is probably the way to go. But I’m still reluctant to buy some before I tried other options.
Simply putting additional sheets of iron in the space between trafo and phono boards helps very little.
For now I made a second iron sheet, bended, which wraps around the trafo a bit- I’m going to try it out this weekend.
Has someone experience how to effectively shield against the magnetic field from the trafo?
When I use mu-metal: is it better to shield the phono-board or the trafo?
Also: I have a lot of neodymium magnets left. Can they possibly help to kind of counteract the magnetic field from the transformator?
Last edited:
It is not possible to use a "shield" against magnetic fields.
So, now you might ask the question, why do the iron sheets help ? It is because they effectively "short" the field using their high permeabilities, leaving only a very weak field outside of it. It is somewhat similar to having a low resistance in parallel to a high one, where the former carries most of the current.
Yes, by making a closed box, the field lines leaking out of the core would be "contained", due to the field lines being encouraged to take the high permeability path (iron), with almost no lines running through the low permeability air that surrounds.
Wrapping both is best but either would be enough. Remember that inductors can talk to each other the same way they do with transformers.
Hope that helps.
As explained above, just placing a piece of iron between the source or receiver won’t work. You need to form a ‘circuit’ around your circuit so the field lines can cause current to flow in the shield.
A quick way to do this is to get two tin biscuit boxes, one that fits inside the other, and put your circuit inside the smaller one. Connect the tins to each other with a short wire, and then then to the 0 V on your circuit. That normally gives quite a bit of protection from external fields.
A quick way to do this is to get two tin biscuit boxes, one that fits inside the other, and put your circuit inside the smaller one. Connect the tins to each other with a short wire, and then then to the 0 V on your circuit. That normally gives quite a bit of protection from external fields.
If inductors are not aviable in the values you think to need, this will have a simple reason: They don't work for the cause.
With more than 100 years of phono, there are no new things to invent. Anything working has been build and the non working options are known and avoided by experienced people.
Your hum problem will be somehow high impedance related. If you still want to keep your course, use an external enclosure for the transformer. Anyway, if you build an antenna for electromagnetic noise, you will still have all kinds of problems in a normal living room.
With more than 100 years of phono, there are no new things to invent. Anything working has been build and the non working options are known and avoided by experienced people.
Your hum problem will be somehow high impedance related. If you still want to keep your course, use an external enclosure for the transformer. Anyway, if you build an antenna for electromagnetic noise, you will still have all kinds of problems in a normal living room.
If you have some freedom to change the orientation of the transformer, it could be of a great help: if you manage to locate your preamp in a ~null area, it will reduce the hum considerably.
It might not be enough on its own, but combined with the shields, it could buy you silence
It might not be enough on its own, but combined with the shields, it could buy you silence
Thank you all for the hints!
So it is how I expected and I buy some mu-metal and wrap amplifier and psu in a separate housing inside the housing…
@Turbowatch: the whole circuitry is pretty low impedance. It is really only the magnetic field from the trafo, coupling into the inductors
@Elvee: I will try this as well. As I had the bare boards on the shelf, just changing the orientation of the trafo did a lot
So it is how I expected and I buy some mu-metal and wrap amplifier and psu in a separate housing inside the housing…
@Turbowatch: the whole circuitry is pretty low impedance. It is really only the magnetic field from the trafo, coupling into the inductors
@Elvee: I will try this as well. As I had the bare boards on the shelf, just changing the orientation of the trafo did a lot
I have a Heath IM Analyzer that uses 2 LC filters, the 2 black thingies on the left.
Even though they have their own shields, possibly mu-metal, they still show extreme sensitivities to their orientations: simply rotating them changes the residual IMD by a sizable amount.
In your case, I'd suggest you remove the power transformer to its own separate box, and as far away from the rest as you can,
Even though they have their own shields, possibly mu-metal, they still show extreme sensitivities to their orientations: simply rotating them changes the residual IMD by a sizable amount.
In your case, I'd suggest you remove the power transformer to its own separate box, and as far away from the rest as you can,
Mu-metal is better than soft iron, but not as much as you might think. Try soft iron first because it's cheaper. You have to close the form or make a box. Another idea, crazy to those who don't understand it, is to put a heavy, say 3/4" or thicker, piece of aluminum between the inductors and transformer. If it makes a difference, I'll tell you about my friend Eddy. 
Another trick is to buck the trend to put everything square. Mount your transformer at an angle so the circuit is in a null area of the field. IMO, toroids aren't as wonderful as everybody thinks- they still put out a field and there's something to be gained by experimenting with the exact mounting orientation.
What's useful about the aluminum block is you don't have to close the form. A simple plate will provide useful shielding. On the downside, I've found that anything under about 5/8" isn't useful.
What's useful about the aluminum block is you don't have to close the form. A simple plate will provide useful shielding. On the downside, I've found that anything under about 5/8" isn't useful.
@Bonsai
Hi
I'm just considering a Pearl 3 phono stage into a handy shortbread biscuit box. (It's just a thought...)
The plan would be to have the power supply in a separate box connected by an umbilical. It seemed to me that the box (or boxes) would be best grounded at the bridge safety ground in the PSU, no?
I wondered what the purpose of the second box is?
On a related question, a number of years ago Tom Evans sold a phono preamp that was praised for its lack of noise. It had a strange arrangement of copper boards on either side of the PCBs all inside an acrylic case. See photo.
Any idea how that worked?
Any idea how that worked?
Hi
I'm just considering a Pearl 3 phono stage into a handy shortbread biscuit box. (It's just a thought...)
The plan would be to have the power supply in a separate box connected by an umbilical. It seemed to me that the box (or boxes) would be best grounded at the bridge safety ground in the PSU, no?
I wondered what the purpose of the second box is?
On a related question, a number of years ago Tom Evans sold a phono preamp that was praised for its lack of noise. It had a strange arrangement of copper boards on either side of the PCBs all inside an acrylic case. See photo.
Any idea how that worked?
Any idea how that worked?
The two tins inside each other offer better shielding than a single layer of tin. The connection to the tin boxes from the PSU 0V will work well.
Separating the PSU and amplifier section is a good idea. This will keep mag fields well away from the sensitive amplifier electronics.
Re the Tom Evans copper thing. In short it didn’t work one bit if we are talking about radiated mag fields. It could possibly help with electric field radiation ie capacitively coupled noise which would be at very high frequencies.
Separating the PSU and amplifier section is a good idea. This will keep mag fields well away from the sensitive amplifier electronics.
Re the Tom Evans copper thing. In short it didn’t work one bit if we are talking about radiated mag fields. It could possibly help with electric field radiation ie capacitively coupled noise which would be at very high frequencies.
Last edited:
It’s a good idea to rotate the toroid by +-60 degrees because you can usually find a null point that will be 10 dB or more lower noise than other positions. Unfortunately you can’t do that with PCB mount components.
Bending, drilling, or otherwise 'working' mu-metal tends to harden it magnetically. The proper way to anneal it after working is in a hydrogen atmosphere oven, which is special equipment most people don't have.
Also, sometimes a piece of mild-steel sheet can help even if it isn't a full closed circuit. For one example please see: https://www.diyaudio.com/community/threads/fender-blues-jr.322222/post-5425331
Also, sometimes a piece of mild-steel sheet can help even if it isn't a full closed circuit. For one example please see: https://www.diyaudio.com/community/threads/fender-blues-jr.322222/post-5425331
Biscuits tins are effective against E-fields, but they do next to nothing against LF magnetic disturbances.
1 to 2mm galvanised steel is much more effective, but does not come close to mumetal and other similar materials.
As Mark just remarked, a strategically located magnetic piece can reorient the field lines in a more favorable direction .
These solutions aren't mutually exclusive, and can be mixed and matched for the best result.
That said, a distant transformer connected with an umbilical remains the best (and easiest) option
1 to 2mm galvanised steel is much more effective, but does not come close to mumetal and other similar materials.
As Mark just remarked, a strategically located magnetic piece can reorient the field lines in a more favorable direction .
These solutions aren't mutually exclusive, and can be mixed and matched for the best result.
That said, a distant transformer connected with an umbilical remains the best (and easiest) option
Good ideas going here guys. I had built the Pete Millett Phono pre which used inductors as well, and whatever I did wrong, I could NOT get the hum out until I broke down and made a separate box for the power supply, period. I spent time and money on you name it, but that was the only thing that worked. Yes, a battery supply would have also worked. I made boxes for shielding out of MU-metal and that is not cheap thinking I'll tell you that. Batteries will last longer than you think if it is SS.
