The diyAudio First Watt M2x

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#1,421
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
 
#1,422
M2X experience

Shielding case for EDCOR.

Greets Dirk
 

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#1,423
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.
 
#1,426
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.
 
#1,428
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
 
#1,430
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.
Click to expand...
 
#1,431
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?
 
#1,432
Mazeppa said:
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?
Click to expand...

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.

709106d1539472003-official-m2-schematic-img_2444a_small-jpg
 
#1,439
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!
 
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