Variable radius coil

How would a coil with varying (let’s say increasing and then decreasing along a half a wavelength of a sine) radius of winding rounds act electromagnetically?
What would be a geometry for coils that’d yield a better efficiency than the standard straight and toroidal, sometimes rectangular ones?

probably just makes the calculation of the the value of the coil harder, especially if it were a ferrite cored coil :slight_smile:

I’m talking about coils in general, not necessarily wound around a core.
I trust that the geometry of the field is easily calculated and rendered with suitable software.

How would you vary the radius of a coil:

  • without using elastic conductors that have a resistance lower than copper, and that do not dramatically change resistance when being stretched
  • without changing the geometry
  • without changing the amount of windings

With core less inductors you have possibilities to change tighten the coil (increase windings), change geometry with complex, thus expensive, mechanical means to do it in a controlled manner. But how would you to that with inductors having a core?

And what purpose would it serve?

A multi layer coil per definition has varying winding diameters.

I frankly don’t know, this is why I’m eager to find out if any applications do exist.
Actually, I’m mainly interested in the field geometry in itself.

As to how to manufacture such a coil out of say, copper, why not cast it into the desired shape, then carefully weld its ends as part of a circuit?

Sure it will be more complex, way more complex.
Like the designs of class D amplifiers and variable speed drives are mathematically very complex.

If we must believe Bruno Putzeys, het spent quite some time in terms of years on the math.

Losses in inductors are determined by gaps, gaps between windings, gaps between windings and cores (if there are any) and gaps between the laminates of the core.

An example of welded coils are cage rotors in rotating machines, you can Google on squirrel cage rotor designs and I bet you get a lot of answers in to your questions.

Good luck with your endeavors!

Since we’re talking coils, what about a coil that’s coiled along its length (multiple times even), would this lead to a monstrosity of a field? So essentially a self-similar winding.
What are some truly useful coil geometries besides the toroid? Surely it’s not the absolute pinnacle of coils?

Any geometry that keeps the gaps small and the package as compact as possible.

A toroid happens to fulfill a lot of the above.

Open ends of coils always contribute to losses in the magnetic field, toroidal are closed loop designs and there are no open end losses.

The reason that toroidal are round and not square is that with round cores the windings can be wound most equal. Inconsistent winding distances act like open ends and just add losses.

Yes, I still don’t understand why rectangular wound transformers are used in tube amps. For a reason, assumably?

Do both of C and L play a role in a coil’s performance? I guess this translates to, is capacitance any notable issue with coils at all? I know that foil inductors notably harbor capacitance which is obvious given how structurally similar they are to… capacitors.
Then again, with a foil inductor are we talking about a coil anymore?
I’m interested in using foil inductors for crossovers but how should I counteract their capacitive nature? Can I null it with a passive component (or several to attain a suitable value) in series with it?
Whatever means are necessary for not letting capacitance bleed through, I want to ultimately understand it as a “counter-capacitive” measure, as in an inverse of sorts?

Because it is easier to custom wind C-I, U-I OR W-I cores than toroidal cores. Toroidal cores with multi layer windings are OK for power supplies that have rectifiers and precise DC Voltage regulators connected on the secondary. A rough secondary voltage with considerable tolerance is OK for that.

Also the capacity between primary and secondary is not well controlled with toroidal as the primary and secondary windings are payed over one and another.

The transformers in tube amps however must provide signals that are spot on. Controlled capacitance, a winding more or less on the primary and secondary, buck wiring to reduce inductance. All much better implemented on square shaped cores.

This is why I would buy tube power amps only from companies that perform in house winding. The transformers have a major impact on the sound.

Now you got me interested in tube amps again :sob:.

I just ordered a class-D amp (Eigentakt, NAD M33).

I had the privilege to be able to listen to a Octave V110 se from a German manufacturer that custom winds transformers for multiple generations and a genius who designs the amps, connected to Avantgarde UNO XD with T+A MP3100HV SACD/Streamer/DAC.

A absolute High End reference system. It is simply beyond the budget I can get approved, but sounded incredible.

The M33 with top of the line Danish Audio Vector speakers sounded extremely good too, the best part of it also to my CFO and that is the best part of it as I see no point investing all that money for myself only.

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Can you tell by this image a lot about my amplifier’s topology?
I don’t know much about DC regulation and such, and frankly don’t know if such things are visible here, haha.
But yeah, this is pure class A, is specced to handle down to 0.5ohms, draws 500W, the toroids are 650VA and their field is strong enough to induce hum into a too nearby cartridge and through the ground lead of the turntable, though it’s nearly a meter away.
Made me realize that phono ground leads should be well shielded as well.
Or I need longer interconnects.

Is such strong induction characteristic of transformers like these or might I have problems with DC offset or poor grounding, or…? (The coils themselves don’t audibly hum)
I have read of amplifiers that use double-shielded toroids, so apparently the wide-range field of the transformers isn’t important but what happens at its core? How far from the coils should I place mu-metal to avoid any disturbance to their function?

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A 500 W pure class A? I very much doubt that.

Perhaps the input circuit is class A, the output stage at max class A/B with class A up to 5 or 6 Watt.

Otherwise you would have a very expensive heating element as pure class A amps draw their rated power even when no music is playing, they run cooler when they are playing music as the speakers get part of that energy when playing.

You can not hear hum from the coils of the transformers because the are perfectly fine.
They are also mounted in the right place and right direction.

The hum is not coming from your toroids. Ground loops in the external power or signal cables are typically the culprits for hum.

Try the turntable without ground wire or make sure the turntable is not grounded otherwise (via its power cable and motor power supply). In case the tonearm gets two paths to ground you typically get hum. Another possibility is that Phase and Neutral of either the turntable or the amplifier is the wrong way around.

Ground loops in cables to / from other sources may also cause hum like wise the Phase Neutral difference.

My recommendation is to check all your wiring to make sure you can also exclude those causes. If that does not help, ensure that the phono cables are not loose ore broken (assuming your amp has the correct MM or MC input (depending on your cartridge).

If that doesn’t help, the power supply of your turntable might be positioned close to your cartridge try moving the turntables power supply somewhere else.

Only if the above doesn’t help you should consider buying screened power and signal cables.

It does release a lot of heat. It states on the front plate that it’s class A so I trust it is.
The power draw rating on the backplate sure does say 500W.

Now this is an Electrocompaniet amp so it’s no wonder. The peak-current is 80A…
It’s a great warmish sounding SS amp.

The toroids are quite something in this one, admirable even.

Electrocompaniet are nice products. Their engineers know what they are doing, so you don’t need to worry about their design.
Have you been able to solve the hum issue?

Nope, it’s very clearly induced via the phono ground wire - bringing it closer to the amp makes the hum louder and louder.
I’ve tried switching everything around but it persists, maybe I really should invest in 2x longer interconnects to get more distance from the magnetic field.
It’s not a really notable issue as it’s very audible at listening position starting at 2 o’clock volume, but sure would like to get an actually black background.

Hi Arenith, you could try contacting Atlas cables in Scotland, they make dedicated phono cables in any price category I have their entry element series cables and am very happy with them. The manager, I think his name is Kevin, he is an expert in these things. Maybe you also have som power cables radiating into your phono connections .

Ist your turntable stacked on your amplifier when you have the hum?

Paul Mc Gowan has made a very good video once about the direction of radiation of toroid transformers. Toroids radiate through the center of their core and if your sensitive tonearm or cartridge connections sit right on top of the Center of a toroid transformer (meaning no shelf in between) there is a possibility of interference from the toroids.

Cambridge Audio has stacked their dual mono toroids in counter direction of their fields on top of each other to eliminate most of the radiation in their edge series stereo top fochten line amplifier.

That won’t be possible in your case so moving the turntable away, like just next to the amplifier might solve the hum issue. Not 2 m away as that might cause other interference on your interlinks. With phono cartridges the cable distance should be as short as possible. But try contact Atlas, they have always been helpful to me and they might have the cable solution for you.

I wish you good luck and am curious what the culprit was when you found it.

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