Shielding with Mumetal and Copper

[Deke609]

Going down a bit of a rabbit hole here. I'm interested in playing around, just for fun, with mumetal and copper shielding on my SII.

Regarding mumetal - I'm particularly interested in "zero gaussian chambers" and open-ended tube variants of same, made of 3 concentric, separated layers of mumetal.  From what I read, the shielding effects of each layer are additive. In a 2012 thread about modifying a 300B amp, the poster, PJ and Doc discussed shielding input transformers with mumetal, and PJ suggested just such a chamber.

My first question: is it sufficient to separate mumetal layers with copper conductive adhesive foil (mu-m, cu, mu-m, cu, mu-m)? Or is it a requirement that the mu-m be separated by either an air gap or a non-conductive material? All the examples I saw online had an air-gap using spacers made of an undisclosed material.

Second question: Can anyone suggest some targets for experimenting with shielding on the SII? Topside, I was thinking of the PT-8 and maybe between the plate chokes and output transformers.  On the bottom side, I thinking about the insulated signal wire between RCA inputs and the fine attenuator, both twisted pairs of green wires from the PT-8, and the signal wires to/from the plate chokes -- my thinking being that with a lot of time and patience I can probably get shielding around these parts without desoldering (but maybe not - desoldering and resoldering isn't off the table).

Again this is just in the spirit of "let's see what happens if" - I'm not to trying to fix a hum problem. This kind of experimentation is a lot cheaper than playing around with different values of expensive Mundorf caps!

Any suggestions or insights would be much appreciated,

Derek

[Paul Birkeland]

When performing experiments on noise reduction, it's really important to start with a noise figure, apply your treatments, then remeasure. 

I mention this because the noise floor in the S2 will be dominated (assuming you have the DC filament upgrade installed) by the tubes themselves.  While you can certainly apply magnetic shielding here, it will be difficult to learn anything from it when the amp is otherwise the same as when you started.

For treatment of something like the S2, the first course of action is proper layout (this is already done for you), then perhaps the next thing would be a flux band around the power transformer. 

If you want to mess around with copper and mu metal shielding of audio transformers, I would suggest doing so with moving coil step-up transformers.  The results will be insanely audible and very easy to measure. 

[Deke609]

Many thanks PB.

I have all three SII upgrades, including the DC Filament.

... it's really important to start with a noise figure, apply your treatments, then remeasure.

I have the gear/software you recommended for testing distortion, frequency response, etc.: a usb audio interface essentially identical to your M-audio, ARTA, dummy loads and various connectors/cables/leads.  I assume I can measure the noise floor using these by passing a tone through the amp -- or perhaps even just measuring the signal output of the amp with no signal input (assuming that the amp produces noise simply by being powered on).

a flux band around the power transformer. 

Fascinating stuff. Based on my quick research, copper does not block EMI, whereas mumetal does.  However, If I'm understanding this right, a closed loop of copper film around the transformer can capture any current that might otherwise be induced in circuitry farther afield.  So instead of blocking EMI with a mumetal enclosure, I trap its inductance with copper. This looks doable.  Do I connect the flux band to ground? 

Any point in adding similar flux bands to the OT's or PCs?  Or is the potential for induced voltage from the OT negligible?

If you want to mess around with copper and mu metal shielding of audio transformers, I would suggest doing so with moving coil step-up transformers.  The results will be insanely audible and very easy to measure.

I will need to do some learning about this.  My quick internet search turned up lots of references to using moving coil step-up transformers with phono cartridges - but from your post I take it that they can be put to other uses.  They look pricey and a little more involved than my simple plan of cladding things in sandwiched layers of copper and mumetal, but the promise of something "insanely audible" is irresistible.  A mod that is INSANELY AUDIBLE !!!?. I will will have to pursue this!

cheers and thanks,

Derek

[Paul Birkeland]

Yes, the M-audio and ARTA will let you see the noise floor pretty well, but the assumption that magnetically induced noise is measurable with those tools may be erroneous.

The flux band should be connected to safety ground (for safety).  Another advantage of the flux band is that it's maybe two orders of magnitude less expensive than buying mu metal shielding.

Something like this will be a good transformer to use for picking up hum:
https://www.ebay.com/itm/UTC-O-25-150-600-2K-Audio-MIC-LINE-Transformers-UTC-0-25/223173687434?hash=item33f630388a:g:-0sAAOSwJjNbtMBf
I tried to use a pair of similar transformers as LOMC step-ups and they hummed almost everywhere I tried them. 

[Doc B.]

EMI (electromagnetic interference, and in high frequency ranges also called radio frequency interference) is generally considered as a radiated phenomenon and it is indeed blocked by copper. Small signal transformers often use copper to shield from external EMI. Power transformers often use a grounded copper electrostatic shield between primary and secondary coils to divert primary noise to ground.

Mu Metal (80% nickel) is used to block a magnetic field by attracting and redirecting the field. It is very high perm, easily saturated and generally used in small signal transformers. We have also used a material called molyperm (80% nickel with a few percent of molybdenum) over a layer of copper to reduce the radiated field of a power transformer designed for a phono preamp.

Low signal level transformers sometimes also have a steel cover over that to deal with stronger external magnetic fields that would overwhelm the mu metal.

A copper flux loop in a transformer develops a current that produces a magnetic flux that, to some degree, reduces the magnetic field of the transformer. This is not the same as an electrostatic shield.

[Deke609]

Many thanks to you both.

@PB: So the ARTA/USB audio interface may not be sensitive/accurate enough to look at changes to magnetically induced noise in the SII.  Assuming, for the sake of speculation, that playing around with shielding may be a sonically worthwhile exercise (but based on yours and Doc's responses, I doubt this), is there something else that you would recommend for taking noise measurements?

@Doc B: thanks for the clarification.  I mistakenly thought EMI and RFI were two completely different things.  I think you're saying that RFI is just a high frequency species of EMI.  But I take it, just based on my recent internet research, that copper shielding (not in the form of a flux band) does little to block low-frequency EMI, and is more effective against high frequency interference?

So the copper flux band creates a competing/partially-counteracting magnetic field, similar in purpose to using tightly twisted pairs of wire?

So my take-away, other than I still have a ton of reading to do about all of this, is that beyond maybe adding a flux band to the PT, there really isn't much if anything to be gained from encasing innards of the SII in copper and/or mumetal. I've already clad my SII base in copper sheet (purely for look and feel), so it's probably pretty well shielded from externally generated RFI, if there is any. 

cheers and thanks,

Derek

[Paul Birkeland]

@PB: So the ARTA/USB audio interface may not be sensitive/accurate enough to look at changes to magnetically induced noise in the SII.

No, this is not what I'm saying.  You are trying to reduce a specific source of noise in the Stereomour that may be -10 to -20dB below the noise floor of the 2A3 itself.  If it isn't visible/audible to begin with, there's not way to measure your success at reducing it (if it is there at all for that matter).

[Jamier]

Derek, I've been reading an article about hum and noise in valve amps. It is pretty comprehensive and touches on most of the sources of  noise. The web address is: https://www.dalmura.com.au/static/Hum%20article.pdf

     It's not too technical, which is good for beginners like me.

Jamie

[Deke609]

Jamie - this is great!  I only gave it quick skim b/c I really need to get some paying work done today, but it certainly does look comprehensive.  I'm looking forward to reading it before bed tonight.

Thanks so much for passing this along.  If you come across any other good, not-too-technical reference material (absence of ancient Greek math symbols is a plus) , I'd really appreciate you posting links to that as well.

cheers,

Derek

[Deke609]

No, this is not what I'm saying.  You are trying to reduce a specific source of noise in the Stereomour that may be -10 to -20dB below the noise floor of the 2A3 itself.  If it isn't visible/audible to begin with, there's not way to measure your success at reducing it (if it is there at all for that matter).

[Preface: I drafted a response yesterday, but must have failed to hit "post" b/c it has disappeared into the ether. I just noticed this.]

Thanks PB. I misunderstood (again).  So you're saying that the effect, if it exists, of any EMI coming from components within the amp on the signal may be so minute that they're buried in the noise floor of the amp, and therefore may not be measurable.  And so any changes brought about by shielding experiments may also be non-measurable. That makes sense.

I will try taking measurements just to see what I can see.  And, subject to what I read in the article that Jamie linked to, I think I will play around with shielding the signal wire in a few spots. First target: the run from the rca inputs to the first attenuator. When I assembled the SII I noticed the care that had been taken to keep this run shielded and pushed as close to the outer edge and as far from the OT as possible. I also noted that in the pic of the Neothoriator innards, some copper tubing encases some long signal wire runs that pass close to what look to me like OTs - so maybe there's something to be had by adding some additional shielding.  Don't know - and if I can't measure it, I'll be left with my hearing -- but that can't really be trusted because I will know I made a change and my mind may fabricate a perceived difference in sound.

@Doc B: I've read elsewhere on the forum that you've experimented with shielding quite a bit, so I'm hoping you may know off the top of your head the answer to a question I posed over the weekend regarding the construction of "zero gauss" chambers - typically 3 concentric, separated layers of mumetal or similar: to get the added shileding benefits of multiple concentric layers, is it sufficient to separate the mumetal layers with copper conductive adhesive foil (mu-m, cu, mu-m, cu, mu-m)? Or is it a requirement that the mu-m be separated by either an air gap or a non-conductive material? 

The shielding for the signal wire I have in mind will be open-ended, so not true "zero gauss" chambers. All I'm after is some of the additive shielding benefits of separated layers of mumetal - if feasible. I don't think I could make a cylinder of air-gapped layers of mumetal small enough to put around the signal wire.  But layers separated by copper tape or a non-conductive thin plastic is probably doable.

Later on I may look at different ways of shielding the tubes themselves.  I read yesterday in Vol 1 1999 of Valve that "Buddha" shielded radio tubes by soldering copper sheet tightly wrapped around the lower 2/3s or so of tubes! And then grounding the shield. I will not go so far (I fear damaging the tube).  Instead, I may make so some fairly wide and open copper cans, half the height of the tubes, that leave a good inch or more of air gap between the shield and the tube -- just to shield against anything passing to/from the tubes and the amp's innards below.  For shielding the tubes from any stuff that may be coming from above, I think it would simpler to affix a sheet of copper to the the underside of the shelf directly above the amp.

[Aside: I realize that this may be crazy, but after I clad my SII in copper I swear the sound changed.  And at that time I hadn't even thought of the "shielding" issue and wasn't expecting any sonic change - I just wanted a real copper encased amp because I like the looks of copper and I think it is representative of the incredible sound that the SII produces. But after letting the newly cladded amp warm up, I instantly thought I'd broken something b/c it sounded "off". But as I listened more, I concluded that it wasn't off, it was just different: the soundstage was smaller (not a plus for me), but there was better detail in the higher frequencies - I heard a few new details in music I am familiar with.  That said, the amp is still breaking in, so it may just be coincidence - or I am imagining it. Dunno.  It also occurred to me that the copper cladding, while it might be shielding the amp's innards from some EMI from the outside, might also be inwardly reflecting back EMI produced by the amp - I have no idea as I don't yet understand the nature of magnetic flux and other possible sources of radiated noise, or how copper shielding interacts with them. All of which is to say: I am aware that "shielding" could possibly worsen the sound of the amp.  But nothing I'm doing is irreversible.  After another month of burn-in, I will experiment with taking the cladding off - it can be removed without damaging it]

cheers and thanks,

Derek

 

[Doc B.]

Exactly what are you trying to shield, and from what kind of noise? Different kinds of interference require different kinds of shielding. The spacers in a zero gauss chamber need to be non magnetic and are typically made of strips of stainless steel. Alternating copper and mu metal should be OK, but make sure all the copper is tied together and grounded for it to function as an EMI shield. The mu metal containers will work best with no gaps or holes. This is overkill for anything short of a small signal step up transformer.

[Paul Birkeland]

If the copper cladding on the base of your Stereomour touches the chassis (which I presume it does), then you have effectively extended the ground plane of the SII, which is certainly not a bad thing!

[Jamier]

How can it touch the chassis and still allow the chassis plate to drop into the base?

Jamie

[Paul Birkeland]

It's very, very thin material.

[Jamier]

"...then perhaps the next thing would be a flux band around the power transformer." Wouldn't placing a flux band on the PT-8 be a bit tricky. Those solder terminals could be easily shorted if a DIY band migrated. Also,you probably couldn't use the bell end without modification, not to mention the hole in the chassis would have to be resized or the trans would need to be elevated to keep the band from touching the chassis. Sounds like a lot of work for very little improvement. I'm in! This wouldn't be the first time I signed up for something like this.

Jamie