Magnetic suseptibility of electronics inside chassis from output transformer on the underside

[pjd3]

Hello,

I realize this may be a nebulous question with a hard to answer response, but, I surely welcome input and ideas.

As I work on the layout of this stereo 20 watt plexi, I am up against variations on layout with a primary concern being how much of the output transformers magnetic fields are apt to cross the chassis and get into other things, particularly the zero-loss high voltage FET FX series send/recieve loop boards. Those are from Headfirst amplification and nearly identical to the Metro zero loss FX boards.

The optimal placement for those two FX loop boards would be right on the other side of the output transformers in the middle/rear of the chassis. Electronic fate has it that separating the PTs and OT, distance being one of the factors places the PT's in the far corners of the chassis rear with the OT's side by side in the rear/mid of the chassis, separated by about an inch.

I did the "headphone trick" as I do in all the amps I've built and as usual was intrigued by the relation of 60 cycle hum pick up verses position. It appears that the 9 or so inches away from the PT's was enough to bring the hum to a nearly inaudible level. But, what I can't determine yet is if the magnetic emissions from the OT's will permeate the chassis enough to be intercepted by the FX loop boards which I wish I could place on the other side of the chassis from the OT's - about 1/2" - 3/4" away from the bottom of the OT's. What I did notice during the headphone trick was that the bottom and the top of the OT's were the sides that were by far the most susceptible to the 60 cycle radiated field of the power transformers. I guess that makes me wonder if the tops and bottom of the OT"s would also be likely to emit the strongest fields when guitar signals are being brought through them.

I'm assuming that I won't know for sure until I actually build the amp up and try it, then its a matter of concluding how much, if any magnetic coupling is occurring between the output transformers and the FX loop board that are mounted on the other side of the chassis. The chassis one I found on Ebay, made in the USA, 18 gauge galvanized steel. I have really no idea how well the chassis is going to shield the magnetic field of the output transformers from the inside of the chassis. These are the "oversized Deluxe" output transformers from Musical Power Systems, 20 -30 watt.

Thanks for reading all of this, and I certainly would be interested in your take on what I might or might not be up against.

Thank you and Happy New year!
Best,
Phil (PJD3).

[R.G.]

Well, Maxwell's field equations say ...
No, I won't do that to you. It was bad enough when I had to do it. I'm going to blather some stuff that may help you think about it.

Magnetic materials are "low resistance" to magnetic fields in this setting. It takes a certain amount of magnetic "push" (as expressed in the number of magnetic "zoots") to force an M field through free space. Iron and steel need a lower amount of zoots to conduct the same amount of field. For most steels, it's several thousand times easier to force a field through them than through free space.

But that's only for a complete loop of iron/steel. Any time the M field in the iron has to jump back out into free space, it spreads out to wider and wider areas. In E-I lamination transformers, this happens where the E lam meets the I lam. There is a little air gap there no matter what you do. Even joggled as tight as you can get them, the air gap will have more "reluctance" (that is, magnetic field resistance) than the whole rest of the magnetic path through the iron. So M field leaks out of the E-I gaps, practically none out of the non-gapped sides. I think this is what your headphone test was telling you.

Sticking a plate of steel between the leaking gapped side of a transformer "shorts" the field trying to get through it, at least until the field gets to the ends of the plate, or until the amount of space on the other side of the shield gets big enough that even being a few thousands of times more "reluctant" there is enough sheer space in parallel to get down to near the iron's lower reluctance. That means that there is a "shadow" of lower M-field near the plate, declining as you get further from the plate's shadow and more of the field gets emitted again from the edges. Translation: plates shield the most in the center of the opposite side from the M-field, right up against the plate.

Distance is cheaper than magnetic materials because M-field intensity declines as 1/the square root of the distance. Double the distance, field goes down by a factor of four.

How much shielding you get from a galvanized steel chassis will depend on (1) the "permittivity" of the steel, literally how much it conducts fields better than free space; probably a few thousand times, different-ish for each steel; (2) how thick it is, thicker being better, of course; (3) how far the "driving" transformer is from the shield/chassis so the space between transformer and steel/shield can let some of the field weaken by inverse square law; (4) the area of the plate, to get the edges as far away from the circuit to be shielded as possible; (5) a sneaky one: the electrical conductivity of the shield. M-fields induce eddy currents in conductors, so slipping a thickish aluminum plate in between the driving transformer and the steel chassis will keep some of the field from getting through to the steel chassis. This is why super-shielded microphone transformers have multiple layers of copper and steel shells, alternating.

That's kind of the basis. Think about how this affects your thinking and ask more questions.

[TUBEDUDE]

As a practical matter the field generated by the O.T. should be much less of an issue than the power transformers continual 60Hz field. The field from the O.T. is the same as the guitar signal, so it will be effectively non-existent between notes, unlike the pervasive 60Hz.
Whether it adds or subtracts something to the signal i would think depend on the strength of the field where it is intercepted by a susceptible signal path element, and the phase relationship of both signal and field at that location. Sounds like experiment time, as i don't see magnetic circuit modeling as an aid.
Maybe it's not an issue at all.

[pjd3]

Thank you for all your pertinent information and thoughts on this. RG, you sure do give me a lot to think about, and Tubedude, as far as "time to experiment", it may be very feasible to look at the output of the FX loop board on a scope while physically moving the output transformer(s) close and away from its preferred position while looking for aberrations to a signal and at different frequencies especially related to the bandwidth of a typical electric guitar.

The idea if distancing the OT with an aluminum plate is interesting, not just because it could or would weaken the magnetic field from the OT, but because I discovered during the headphone trick that tipping one side of the OT upwards 1/4" made the difference between hearing a very low 60 cycle hum, and a complete disappearance of any 60 cycle hum, at least as far as the headphones to the full secondary would allow. It was a bit aggravating to wrestle the idea that a slightly angled plate of something would effectively eliminate any perceived 60 cycle hum from PT to OT.

Well, I could really kill two birds with one stone should there be any bothersome symptoms of either perceived PT hum or aberrations in the FX loop output as a result of close proximity to the OT across the steel chassis. I will surely be looking at those two things and it certainly would be a worthy educational exercise if nothing else.

Thanks again, your responses give me some good insight in how and what to look for, and things to try.
Best,

Phil D pjd3

[xtian]

Phil, please proceed with your experiments. But I will be very surprised if you can get the PT to induce any hum in your FX loop circuit. The only reason the OT picks up 60Hz hum from the PT is the big coil of copper in the OT.

[TUBEDUDE]

You cansweep the signal and look at any difference by using the XY function of the scope with the output of the loop in Y and the sweep generator output as the X input.
I've never had an issue with caps or resistors in a magnetic field, with an O.T. or inductor in the chassis. It was low power devices in those two instances though. ( a Hammond 124B in one case, and a 1.5H choke at 250mA in the other).

[pjd3]

Thanks xtian,

My concern was the FX loop board having magnetic fields from the OT couple into it. I'd like to be able to place the FX loop boards on the other side of the chassis right where the OT's are both mounted. The FX boards have their send and return jacks soldered to the boards themselves and then mounted to the back panel. That would make the board hover about 1/2" to 3/4" above the inner chassis with the OT's mounted on the other side of the chassis. I suspect there is a good chance there is nothing destructive that could happen magnetically in terms of coupling but, it erks me just enough to find out if it does.

Now that I'm thinking about it, I could even have one of my kids move the output transformers around while I'm playing, move it to the mounting position, then pull away a bunch of times to hear for any difference. That there may be all the experimenting that's really needed. Ha, if I don't hear a difference, then there isn't one! Right? Sort off.

Thanks folks,
Best,
pjd3

[danman]

I routinely place my OT at the opposite end of the chassis, directly over the preamp end of the board. I was concerned about creating noise in the preamp with the OT being right over it but so far, all of the builds have turned out very quiet.

[pjd3]

Thanks Danman, that is a promising thing to hear. I suppose having the OT right in proximity to the input section and not having any issues may be a good proof that it wont be an issue in my case. The FX boards are further up the signal path, just before the PI where I would suspect far less sensitivity that being near the input.

Thanks for that new!

Phil D.