The diyAudio First Watt M2x

[cubicincher]

answer to MEPER

Hello MEPER,


very interesting test of the M2! Was fun to read.


I am not sure if you can get the metalshield for the transformer in a perfect fit.
I've ordered a pair metalshields which are too big. And I did not install them yet. Have to make them smaller.
Greets Dirk

 

[cubicincher]

M2X experience

Shielding case for EDCOR.

Greets Dirk

 

[MEPER]

So you build your own little metal box shield.

It seems Edcor shields some of their transformers using a thin copper shield like this:
EDCOR - MXLcs Series

It looks like tape. I wonder if it is "enough" just to apply the tape like on the picture and not cover the whole transformer. It would be easy to do. Mu-metal tape is a bit expensive but does not need much…..just a small sheet.

If the shield is for not picking up noise it is probably enough just to cover the windings.

 

[WKCox]

I just read the 6moons review of M2. He had the top off the amplifier:
6moons audio reviews: FirstWatt M2


Was the shields on the Edcor's "handmade" or something the factory makes? ….is it mu-metal or "just" steel?

Pass uses mu-metal. I believe Nelson posted earlier in this thread. I tried steel but that didn’t work well, so I purchased some mu-metal foil. Expensive but effective.

 

[qwertyl]

I tried steel but that didn’t work well, so I purchased some mu-metal foil. Expensive but effective.

I had similar experience, steel boxes didn't make any difference, mu-metal foil works great.

 

[MEPER]

I will get a small sheet of mu-metal tape. It would be most easy just to cover the windings with tape. One piece of 17mm all the way round like they do at the Edcor factory (where they use copper...if must be for RF shielding not magnetic).
I know that the steel used for magnetic shielding has a special grain structure. They write a bit about it at Don-audio as they sell shielding for e.g. toroid transformers.

 

[MEPER]

The Lundahl's like this one is by default in a mu-metal case:
https://www.lundahltransformers.com/wp-content/uploads/datasheets/1674.pdf

 

[cubicincher]

EDCOR shielding

MU-metal, selfadhesive shield sounds very good to me (yes, expensive).
I did not want to cut those cases into 2 halves, make them smaller and soldering/welding back together. A lot of work.
And then the metal case was very close to the pins / contacts of the EDCOR.
Because of this I tried the rubber-welting.

Greets Dirk

 

[MEPER]

I ordered a couple of these sheets:
Ultraperm 80 Metal Shield; MuMetal Mu Metal Alloy Shielding Sheet Audio HiFi USA | eBay

Will see how it is to work with. Mu-metal should saturate easy so it is not for shielding strong magnetic fields…..but that should not be the case inside the M2X chassis. Some light stray fields from transformer and PSU wires.

 

[maty tinman]

Talking about Mu-metal, a book.

Audio Transformers by Bill Whitlock

[PDF] http://www.jensen-transformers.com/wp-content/uploads/2014/09/Audio-Transformers-Chapter.pdf

Page 9

The effectiveness of magnetic shielding is generally rated in dB. The transformer is placed in an external magnetic field of known strength, generally at 60 Hz. Its output without and with the shield is then compared. For example, a housing of 1/8" thick cast-iron reduces pickup by about 12 dB and a Mumetal can by about 30 dB. Where low-level transformers operate near strong magnetic fields, several progressively smaller shield cans can be nested around the transformer. Two or three Mumetal cans can provide 60 dB and 90 dB of shielding respectively. In very strong fields, because high-permeability materials might saturate, an iron or steel outer can is sometimes used.

Toroidal power transformers can have a weaker radiated magnetic field than other types. Using them can be an advantage if audio transformers must be located near them. However, a toroidal transformer must be otherwise well designed to produce a low external field. For example, every winding must completely cover the full periphery of the core. The attachment points of the transformer lead wires are frequently a problem in this regard. To gain size and cost advantages, most commercial power transformers of any kind are designed to operate on the verge of magnetic saturation of the core. When saturation occurs in any transformer, magnetic field essentially squirts out of the core. Power transformers designed to operate at low flux density will prevent this. Often a standard commercial transformer, when operated at reduced primary voltage, will have a very low external
field.

 

[Mazeppa]

Okay, I'm going to go for max shielding of my Edcors, and Cinemags (if they ever respond to my inquiries) prior to mounting on the boards instead of trying to get by without and finding out I should have after the amp is up and running requiring a retrofit of shielding.
I believe I'll go with covering the windings rather than a cage for the entire transformer; applying the Mu under the RF shield to facilitate attachment of RF shield to chassis ground.

1. What thickness of Mu Metal self adhesive strip is recommended for the M2x application?
1.a.Thinner means easier application, okay to use multiple layers to achieve desired overall thickness?

2. Any recommendations for RF shielding?
2.a.Material, thickness, brand?

 

[WKCox]

Okay, I'm going to go for max shielding of my Edcors, and Cinemags (if they ever respond to my inquiries) prior to mounting on the boards instead of trying to get by without and finding out I should have after the amp is up and running requiring a retrofit of shielding.
I believe I'll go with covering the windings rather than a cage for the entire transformer; applying the Mu under the RF shield to facilitate attachment of RF shield to chassis ground.

1. What thickness of Mu Metal self adhesive strip is recommended for the M2x application?
1.a.Thinner means easier application, okay to use multiple layers to achieve desired overall thickness?

2. Any recommendations for RF shielding?
2.a.Material, thickness, brand?

I bought a 2' x 4" x 0.01" piece of non-adhesive Mu-Metal foil from McMaster Carr.

Before the Mu-Metal shielding, I was seeing noise levels of around the 500 - 600 uV, which is close to the First Watt specification for the production version. After shielding, I measure around 250 - 300 uV (0.25 - 0.3 mV), which is quite an improvement and the limit of my test equipment.

Multiple layers of 0.01" Mu-Metal showed no additional improvement. Note that I used a 5-sided cover with no shielding between the transformer and the PC board.

 

[MEPER]

It looks good. What about the pcb side of the Edcor is it also covered with mu-metal or is it a "top case" only?

 

[WKCox]

Top case only.

 

[Mazeppa]

Household AL foil good for RC shielding?

 

[WKCox]

Household AL foil good for RC shielding?

Maybe for RF shielding but it will have no effect on blocking the magnetic interference from the power transformer which is the primary noise generator.

 

[Mazeppa]

Top case only.

Good question, MEPER.

Thank you for the information, WKCox.

EDIT:
1. Can anyone speak to the effectiveness of the shielding built into the Antek shielded trafo's?
2. Shielding on windings, or a cage more effective?

 

[tsmith1315]

Some earlier discussion on shielding, including copper tape and mu metal:

The diyAudio First Watt M2x

 

[maty tinman]

Bad idea.

In the case of magnetic fields of 1 kHz, steel is better than copper, and this one better than aluminum, but at 100 kHz, although steel it is still better, the difference is very small.

Between 100 kHz and 1 MHz copper is clearly better than steel.

At 1 kHz the mumetal is more effective than steel, but at 10 kHz steel is more effective than mumetal.

At 100 kHz both the steel and the copper and aluminum are better than the mumetal.

Throughout the spectrum of said frequencies, copper is better than aluminum. And much more expensive too!

 

[maty tinman]

To finish, RF shielding, I have a bad notice: a lot of DIY or not aluminum cases have 1-2mm thick. You need at least 4 mm.

I am talking about audio band.