Magnetic field of toroidal transformer with speaker

Thread Starter

Veracohr

Joined Jan 3, 2011
725
I'm designing a powered subwoofer for myself and I've been intending to use a toroidal power transformer I have (60Hz mains frequency). My plan is to have the PSU, amplifier, and speaker all in one box.

In theory a toroidal inductor shouldn't have a magnetic field outside of the toroid, but what about in practice? Suddenly I'm worried about the magnetic field of the power transformer affecting the speaker function, especially since it would be right in the middle of the frequency range of use.
 

sparky 1

Joined Nov 3, 2018
335
When things are apart the evaluating and testing might include setting the power supply next to the speaker.
Noise unfortunately is something that happens, artifacts can be elusive and complex in that stage of development.
Speakers with PS have been known to be difficult but not always. You never know until you try.
 

MisterBill2

Joined Jan 23, 2018
7,021
The systems that I have seen with the toroidal power transformer all put the transformer with the toroid axis at right angles to the speaker axis, usually on the bottom of the enclosure, and usually a couple inches below the speaker. Thesepackages had amplifiers in the 15 to 100 watt range, and never any magnetic hum is heard. But in each case the amplifier assembly is not next to the toroid. So that is what some of the good brands do for their bass boxes.
 

MaxHeadRoom

Joined Jul 18, 2013
21,426
In theory a toroidal inductor shouldn't have a magnetic field outside of the toroid, but what about in practice? Suddenly I'm worried about the magnetic field of the power transformer affecting the speaker function,
One thing you do not want to do, as I have seen done by some in the past, is to place a metalic shield cover over the toroid, this effectively creates a shorted turn on the transformer.! o_O
Max.
 

Thread Starter

Veracohr

Joined Jan 3, 2011
725
I suppose buying a PSU and generating the negative rail from that might be more simple, but probably cost more. I was trying to use this transformer I bought a while back for another purpose but ended up not finishing that project.

If the datasheet for a supply (Mean Well RSP-320-27) says the rated current is 11.9A, and that the current range is 0-11.9A, I assume that to mean the peak output current is 11.9A. Is that right? In simulation with a DC supply for the positive rail and a converter circuit for the negative rail, I'm seeing peaks of 18A (over the supply max) but rms is around 8.4A (within the supply max). Unfortunately I have to go up to a 500W supply (RSP-500-27) to get one that can supply that current at a 30V output.

I'm shooting for +/-30V and a max of 220W to the load from a TDA8920C class D amplifier IC in BTL configuration. The simulation is only showing about 167W in the load, so it could potentially mean even requiring more current than the 500W supply can do if I ran the amplifier at max output (which is unlikely).
 

MisterBill2

Joined Jan 23, 2018
7,021
Post #7 is the very first mention of voltages. What voltage does the toroid transformer provide, and what are the current ratings? What is the power rating of the transformer? Until we know that there is no clue what can be done. There are a number of ways to set up power supplies, and so if we know what we are starting with there are options. BUT guesses are useless and a wast of bandwidth.
 

Thread Starter

Veracohr

Joined Jan 3, 2011
725
I didn’t mention voltages before because my question wasn’t about general power solutions, it was specifically about magnetic fields. If I didn’t know what kind of solution to use, I would have presented a more general question with the requirements.

The transformer is 48V center tapped, 13A per side. I designed a SEPIC converter for the positive rail and a negative input buck-boost for the negative rail. I made rough inductance measurements of the transformer to plug into my simulation and the result was within the current rating.
 

Thread Starter

Veracohr

Joined Jan 3, 2011
725
By the way, I just opened up one of my nearfield monitors to see what they did. This is an ADAM A7 (100W total output max), a well-respected professional brand.

There's a toroid on the same axis as the speaker. About 4 inches below and 9 inches back from the front edge of the speaker.

The toroid is mounted to the back plate with a metal plate covering the whole transformer (can't see it much in the picture).

The amplifier board is very close to the toroid, separated only by the PSU board (the upper of the two boards which are perpendicular to the back plate is the amplifier board).


A7.jpg
 
I'm designing a powered subwoofer for myself and I've been intending to use a toroidal power transformer I have (60Hz mains frequency). My plan is to have the PSU, amplifier, and speaker all in one box.

In theory a toroidal inductor shouldn't have a magnetic field outside of the toroid, but what about in practice? Suddenly I'm worried about the magnetic field of the power transformer affecting the speaker function, especially since it would be right in the middle of the frequency range of use.
You can try some metallic shield cover.
 

MisterBill2

Joined Jan 23, 2018
7,021
With that transformer there is no need to go to switching power supplies for the two 30 volt supplies. With the center tap as common, a full wave bridge rectifier will give you plus an minus 24 prior to adding filter capacitors. With the caps and a load the voltages should be close to 30 volts. Much less complicated and much less chance of electrical noise from two switchers. The arrangement is a classic and fully mature power supply configuration.

The amplifier pictured shows that I was being rather conservative in locating the transformer, but in addition, heavy on the bottom makes an enclosure more stable.
One more thing is that in most commercially available equipment, minimum cost to produce it is a big design consideration. When I build something for myself or a client, best performance is the major consideration. That often means spending more effort on things.
 

Thread Starter

Veracohr

Joined Jan 3, 2011
725
Took me a while to respond because I wanted to test the transformer under load but I had to replace some parts.

The reason I decided on the switching supplies is that the maximum power output is directly proportional to the supply voltage for this amp, so I wanted a regulated supply:

Screen Shot 2020-09-23 at 8.30.23 PM.png

In my simulation it was dropping to ~+-27V under full load. Now that I've tested it with 220W load (albeit a DC load, not a switching load as the amp will present), I run into another problem. The voltage is higher and drops less than I expected, and even under a heavier load than I will likely ever give it, the total DC voltage is just over 65V, which is the limiting voltage of the amplifier. The instantaneous voltage is even higher. So with a more reasonable load, I need to at least limit the voltage, or regulate it preferably. I suppose I could throw an inductor on each rail to drop the voltage some, but it still wouldn't be regulated.
 

MisterBill2

Joined Jan 23, 2018
7,021
For at least 40 years many HIFI stereo amplifiers have used non-regulated power supplies with no complaints and no problems. But those were not class "D" amplifiers., more likely class "AB" or even class "A".
Switching regulators can be a real challenge, and at that power level noise might be an issue. That is the one negative consideration.
 

MisterBill2

Joined Jan 23, 2018
7,021
Did you mean *switching power supply* not switching regulator? If so you are perpetuating a myth - https://benchmarkmedia.com/blogs/application_notes/152143111-audio-myth-switching-power-supplies-are-noisy
OK, I did equate switching regulator with switching power supply, and I stand by my assertion, because the interconnections in the circuit do matter. Unintended coupling due to conductor resistance and inductance can be a big pain, and so the implementation of a circuit that works very well in simulation can have problems if the traces connecting the elements share the wrong currents.
So the reality is that layout really does matter a whole lot in practical switch-mode power supplies.
They are a lot different from the older analog kinds.
 

shortbus

Joined Sep 30, 2009
7,940
OK, I did equate switching regulator with switching power supply, and I stand by my assertion,
All I know is the last stereo I bought is powered by a switcher and has no noise from it. The powered sub-woofer is also a switcher power supply too. If you do a quick Google of switching power supply for audio amp they are used in most of them made today. Other those being sold to people that think *oxygen free copper* wire is needed for the purest sound.
 

ThePanMan

Joined Mar 13, 2020
41
A long time ago I put a car stereo inside a speaker box along with a 12 volt (adjustable) charger from an emergency light unit. In the other speaker box (was stereo) was the battery. The two speakers were hinged together and a strap was put across the top so the whole thing (heavy) could be carried. When the charger was plugged in the radio would play and use almost no power from the battery. When power was lost the radio would still play. When powered but the radio was off the battery would charge to 13.8 float volts. This was way back before toroidal transformers were widely used. Granted, my charger used old technologies for regulation, but it worked without any hum whatsoever.

On the subject of shielding the toroid: As has been said - don't do it. But you CAN shield the amplifier.
 
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