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Hi, I'm familiar with the importance of orientation between power transformers and chokes.
But what about...
- interstage transformers near chokes?
- chokes near chokes?
- power transformers near filament transformers?
Do I need to be careful relative to the place of these.
Thank You.
Follow Ups:
For some of the examples you're asking about, it depends on how the components are being used. The first choke in a choke input filter will generate a large AC field that can couple to any nearby choke or transformer attached to the signal path. A chokes with smaller AC voltage across it, such as a choke in a CLC filter, is relatively benign. I wouldn't be concerned about coupling from a power transformer to a filament transformer.
to one of the power transformers only, if a filament and plate transformer are side by side, while measuring output Voltage. If you see it go up slightly then that is the phase at which the two transformers are not loading each other.
The stereo amplifier I built a couple years ago called for the PT and OPTs to be positioned relatively close to each other. Before cutting the chassis, I tested the coupling this by powering the PT and moving the OPTs around on the bench. It turned out that the best null was produced when the OPTs were not only at 90°, but also positioned slightly in front of the PT. There's no way to know things like this unless you physically test them. Reversing phase is like that too, you just have to try it.
Even though L2 has a relatively small signal across it, it can still 'pick up' radiated fields and limit its effectiveness. I once had a situation in a LCLCLC filter for a phono stage where the ripple at L2 and L3 was the same rendering the last filter stage ineffective. As it turns out L3 was picking up radiated fields from L1 and a 90° rotate of L3 made the earth flat again.
dave
"As it turns out L3 was picking up radiated fields from L1 and a 90° rotate of L3 made the earth flat again."
That's exactly what I said - "The first choke in a choke input filter will generate a large AC field that can couple to any nearby choke..."
This is why I don't use choke input filters in audio equipment. I once miswired a CLC filter in a high-gain guitar amplifier, accidentally leaving out the first small cap. The buzz from the choke was everywhere. I'll bet that if you had added even a small capacitor at the input to L1, maybe only 0.47uF or 1uF, the problem would have disappeared. Voltage spikes at the input terminal of a rectifier-driven choke can be huge.
I've found ~ .22uF works pretty good for small C before L-critical to stop buzzing with my 5H hammond chokes.
With my 10H giant hammond chokes not needed.
For L2 coupling my answer has just been keep the main power supply in its own chassis and put L2 in my audio chassis.
I'm addicted to using L-critical at this point - so I live with the trade offs. :)
I was referring to this follow up:
A chokes with smaller AC voltage across it, such as a choke in a CLC filter, is relatively benign.
to which I make the point that it may not generate any appreciable field, but its positioning is just as critical to avoid picking up fields so it is far from benign.
dave
You're quoting my comment referring to capacitor input, but the noisy example you described was choke input. Obviously very different. Anyway, I don't believe the choke in a CLC filter generates an AC field of significance. Normal rules apply, don't position it next to a stage with a gain of 100 for example, but otherwise not a problem.
Do the same "rules" apply (90deg) if a choke/IT/OPT is in close proximity to a power transformer but separated by an aluminum plate of a chassis?
An aluminum plate can provide attenuation from the power transformer, but there are many variables. Minimal coupling really requires the components to be at 90°. I wouldn't place an IT next to a power transformer under any conditions, other than maybe the use of a thick mu metal shield.
I'll keep all the recommendations in mind. Thanks for mentioning mu metal.
I do have sheets that I never put to use, I believe 3M... it's on the thin side so I'll maintain the 90 rule.
I think you might be referring to 3M Ultraperm 80. I've used that myself, but it's only moderately helpful. Like you say, it's thin (.004"), and that just isn't enough to be very effective at 60Hz. The last time I had a coupling problem, I wrapped the output choke (under a cathode follower) with Ultraperm. It attenuated 60Hz at the output about 6dB. That wasn't enough, so I removed it and installed an aluminum plate instead. That reduced the hum about 10dB to 12dB. I didn't try using both, but probably should have.
I suspect mu metal needs to be at least .020" to .030" thick to provide good shielding at these frequencies. I once worked on a RF project where we made the entire chassis and all the sub-compartments from mu metal. The goal was to push unwanted EMI (including 60Hz and 120Hz) into the noise level, which was about -170dBC. IIRC, the panels were about .080" thick. The boxes weighed a ton, but the shielding was excellent.
Mu metal is strain sensitive. If it becomes mechanically strained, it loses permeability. It still shields, but you don't get "max perm". Steel also works well and isn't as expensive. If I remember the curves correctly (don't have them anymore) a "dropped" sheet of mu metal has lower perm than a dropped sheet of steel.
At work, I had them replace some mu-metal shields that were (randomly) not working to spec with steel shields. Problem went away. The "bad" mu-metal shields all looked like they were a "little bent" to me, but looked fine to every else.
Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.
. . .Oh!. . .Remember: Modifying things voids their warranty.
"Mu metal is strain sensitive."
Ultraperm is sold as a product that can be bent and shaped. I haven't read anything on the 3M site or elsewhere to indicate that it loses effectiveness when used this way. Maybe it does though, or maybe it's an alloy that alleviates this condition? Would be interesting to know more about this.
The name Ultraperm is used in the past by the German Vacuumschmelze company.
However, they don't have Ultraperm 80.
Looking at the site of 3M I couldn't find ultraperm 80 either.
Vacuumschmelze used ultraperm with a "standard" nickel alloys, this means it is crystalline based. bending means you destroy the permeability.
For good magnetic shielding you need the old fashioned Mumetall and as thick as possible. Of course annealing if you have to shape it in the final form.
You're right it's no longer shown on the 3M Website. No idea why. That's odd, considering "3M Ultraperm 80" is sold all over the Internet. Even Walmart lists it. I guess we have to rely on the specifications published by resellers now. None of them advise against bending it, as far as I know. (Not that this means anything.)
Ultraperm is a brand name of Vacuumschmelze, probably that was a problem for 3M with their Ultraperm 80.Anyway, as far as I can see, Ultraperm 80 is not a very good shielding product to shield a transformer.
* Unknown datasheet, unrealistic.
Edits: 06/11/25 06/11/25
"Ultraperm is a brand name of Vacuumschmelze"
Just curious, do you know when they copyrighted or trademarked this name? 3M sold Ultraperm 80 for many years.
"Ultraperm is a brand name of Vacuumschmelze"
"Just curious, do you know when they copyrighted or trademarked this name? 3M sold Ultraperm 80 for many years."
I know that Vacuumschmelze has been using the name Ultraperm for over 50 years, but I don't know if they have rights to this name.
Thanks for clarifying. 3M doesn't appear to still offer the same product under any name, so perhaps it was just a marketing decision. I have several sheets of this that I bought at a good price about 15 years ago. It was much less expensive than the prices I see now online. I don't know why that is, or even who is making it (maybe it's all old stock). In any event, this material does work to attenuate line noise from transformers, but the effectiveness is limited. It's mostly useful as a "last mile" sort of thing when just a little more cleanup is needed.
I am not 100% sure this was ever a 3M product. I think the 3M branding is simply from the adhesive and is not indicative of the overall manufacturer. Put another way it could simply be a 3M adhesive attached to a Vacuumschmelze nickel by a third party.
There was a large amount of this specific material available from multiple surplus sources about 25 years ago at like $6 a sheet in packs of 100. People started buying them 100 at a time and flipping them on ebay for $20-30 a sheet about 20 years ago and as the surplus market dried up, the ebay price went up.... some kept the price the same and cut the sheets in half.... ahhh shrink-flation.
dave
Didn't think it was quite 25 years ago, but I did buy four or five sheets at about $4 each. I think it came from Electronic Goldmine, which also sells PC drill bits and such (my main need at the time). Anyway, I'm sure I remember looking this up on the 3M site and finding specifications that appeared to describe it as a product of theirs. They do still sell EMI absorption materials, but not this one. It's a mystery :)
Google led me to ad ad for Ultraperm, which said "The alloy contains 80.3% nickel, 14.3% iron and 5.4% molybdenum." I assume that Molyperm is the same thing. 20+ years ago we (Bottlhead) had a power supply transformer in a phono preamp that induced hum. The manufacturer resolved it with a copper flux band overlayed with a layer of molyperm tape.
I don't know which wrap was more effective, but together they worked well.
Thanks Paul, good info!
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