Shielding with Mumetal and Copper

[Paul Birkeland]

Unwinding a foil inductor isn't a bad idea.

Copper slug and snail tape for your garden would work fine as well.  Soldering to it may not be all that fun, but it's possible (consider dipping one end in a solder pot if you have one.

 

[Deke609]

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.

PB: I don't understand. Just a guess: Are you saying that I could use a transformer like this to check for EMI?  If so, does passing the transformer through a magnetic field cause it to hum?  All by itself, without being wired to anything?  Or did you mean that I could power the thing and then play around with ways of minimizing its hum?

Many thanks,

Derek

 

[Jamier]

PB, I have a 14ga. Inductor I can unwind. Is 1 turn enough? 2? That stuff is easy to solder. Would an earth ground be helpful? I guess I might as well if I'm going this far. I don't think this would be happening if Caucasian Blackplate was still around.By the  way, How much is a new PT if I screw this up?

Jamie

 

[Paul Birkeland]

PB: I don't understand. Just a guess: Are you saying that I could use a transformer like this to check for EMI?  If so, does passing the transformer through a magnetic field cause it to hum?  All by itself, without being wired to anything?  Or did you mean that I could power the thing and then play around with ways of minimizing its hum?

A step-up transformer hooked up to a moving magnet phono preamp playing through your system will pick up magnetic hum.  If you have no wires connected to it, you will have no way to measure hum. 

 

[Paul Birkeland]

PB, I have a 14ga. Inductor I can unwind. Is 1 turn enough? 2? That stuff is easy to solder. Would an earth ground be helpful? By the  way, How much is a new PT if I screw this up?

I can't ever remember seeing a shorting ring that was more than one layer.  Yes, solder a wire to it on the bottom side of the transformer and run that to the safety ground of the amp.  A new Stereomour power transformer is quite expensive, but you aren't really taking any risks unless you skip the varnishing step.

 

[Doc B]

does passing the transformer through a magnetic field cause it to hum

No, but putting a small signal transformer in an alternating magnetic field can cause it to hum. Once again I want to emphasize that you have to deal with specifics regarding noise issues.

Is 1 turn enough?

By definition a shorting ring is one shorted turn of material.

 

[Jamier]

PB or Doc, Should the band run the full width of the Bobbin? I'm thinking yes.

Jamie

 

[Paul Birkeland]

I would keep it at least 1/4" away from each row of solder lugs.

 

[Deke609]

Thanks Doc. I was just guessing that the particular transformer linked to by PB might act as I imagine a really sensitive relay would if moved back and forth though a magnetic field. 

I am going to wait until I rewire my PTs for 8 ohms before playing with the shielding stuff on the outside chance that the added gain will make things more measurable. I currently have the PTs configured for 2 ohms with my LCD 4s. But I could use a bit of gain. I'm guessing that the switch to 8 ohms might produce some audible hum.  There is no hum at 2 ohms with the DC Filament.

In the meantime I will make some new power cords. I want to see if I can hear differences among a stock cord, the BH kit cord, and a BH-inspired cord with tinned copper braid shielding, with the shield wired to ground only at the mains power end.
cheers,

Derek

 

[Paul Birkeland]

Again, hum you will hear from an amp with directly heated tubes like the Stereomour will not be magnetic (unless there are major layout issues).  The noise floor of the tubes themselves will dominate what is audible and what is measurable.  Switching to 8 ohms will increase this noise. 

 

[Jamier]

Yeah, you need to put the flux band on, then varnish over it and the transformer to keep it in place

What kind of varnish? Regular old Man O' War spar varnish or something more specific for electrical applications. Do you need to dip the whole trans or just brush on a few coats?

Jamie

 

[Paul Birkeland]

Follow the directions on the can:
http://www.temcoindustrialpower.com/product_selection.html?p=insulating_varnish_overview

 

[Deke609]

So, I put a layer of copper (wired to ground) and a layer of mu metal ("MCF5" foil) around the signal wires running from the rca ins to first attenuator.  And while I was at it I also put a U-shaped copper/MCF5 shield between the newly shielded signal wires and the OT-2 - it wraps around the sides of the OT, well away from any contacts, and is separated from the signal wire shielding by a cut-open piece of shrink tubing.

[WARNING: What follows are my purely subjective impressions and could very well be entirely imaginary and the product of self-suggestion -- in which case I say Hooray for the power of self-suggestion, because I like what I hear!]

I immediately noticed that music sounds "faster" and more energetic.  There also seems to be some additional detail and clarity. No other changes noticed.

Pretty cool.

cheers,

Derek