Transformer to give 12 volts

[Neil Purling]

For one of those cheap TDA2030 panels, 12 volts AC o/p.
Is there any noticeable benefit of a toroid transformer rather than a conventionally assembled type of transformer.

[SiriusHardware]

The thing I've always heard about toroidal transformers is that they have a much lower external magnetic field (typically 1/8th of that from a conventional transformer according to one source) so for audio equipment which includes sensitive / low level input circuitry a toroidal transformer would appear to be the better choice.

[joebog1]

The above is true!! Toroids also have 8 times the inrush current of EI style.
They are (usually) much lower profile as well, so a "smaller" case can be used.
They can also be obtained with interwinding screens, and external flux bands, which reduce the external field to almost nothing.
BUT, a quick question, why would you bother for a TDA2030? Its a very "ordinary" chip !
Some points I have noted.
The field is round like the toroid, hence the flux is smooth in the iron.
There are no "breaks" as in an EI transformer for the flux to leak out.
Regardless of how well made the iron is ( stamping quality) , there will always be small gaps where the E's join the I's, causing leaks.
There are no "square" corners for the flux to leak away.

You will have seen conventional toroids, but I have added the photos for your reference.
The screened and fluxbanded transformers are my design, specifically for valve phono stages. ( I DID warn you !!! I am an audiofool!!!! )
They are also wound on a much larger core than is "required".
BUT reducing the flux density, also reduces external fields, regardless of EI, or toroidal construction.

The toroid has another "drawback, if you will !!. The inside of the toroid can reach VERY high temperatures. By that I mean over 100C, when the measured outside "skin" temperature is perhaps 60C. I always drill "breathing" holes around the big steel washers that are generally supplied, and into the chassis also. The holes have to be aligned when you assemble the unit ( a no-brainer) but transformer temps are way lower!!!
DONT forget that the securing bolt used with toroids, MUST be insulated at some point, or a shorted turn will occur!!!!

Best regards
Joe

[Radio Wrangler]

The cores are made as wound-up strips, and the directionof magnetisation is always oriented along the length, so material with directional properties can be used to advantage. GOSS Grain-Oriented Silicon Steel. This allows the toroid and C-core transformer to be run to higher flux densities than with E and I lams. And the toroid has the lack of gaps as a benefit over C-cores. The high flux allows fewer turns per volt, and turns are costly on toroids. The tendency not to leak flux helps with the higher flux density. The lower Z and lower inductance gives the toroid a much larger starting current (Effectively the time constant applied to the DC component implicit in switching on not at a zero crossing of the AC waveform.

Cores can hum through magnetostriction, and the windings tend to be looser than bobbin wound assemblies, so the wires clatter and hum.

There are some very high quality toroids that didn't take the high flux route and are generally better behaved, but most manufacturers optimised them for profit.

But, as said, for a little audio amplifier chip, anything you've got will do.

The big toroids in my power amp in the lounge got vacuum impregnated twice and finally cast in a thermally-conductive silicone rubber in successive attempts to cut the fankenstein film set sound effects from distorted mains waveforms:

"MMMYOMMMMNNNMMMMNNNYOMNN"

David

[joebog1]

As a follow up to the photos I posted:re the flux density.
There is no "normal" flux density as such!! BUT by saying this, it is in regards to how much heat you can tolerate, and how long you wish the "service life" to be. I design "English old school", by that I mean build a tank when you need a kids push scooter!!! .
The screened toroids i posted are wound on 160 watt cores,BUT deliver only 62 watts, so 100 wats ( give or take) is "wasted"!!.
SO to the origional question: TDA2030 needs at most a couple of amps connected to a FAT cap, say 10,000 uF. ( DONT forget the .1uF and say 100 pF across the .1 farad!!!)
My 160 watt core would supply about 16 volts DC, which is OK for the TDA2030.
It would, under "standard" ratings ( about 1.6 to 1.8 Tesla's) 10 amps!!! .
With "my audio-phool" ratings it would give about 5 or so amps, with almost ZERO external field.
As sand amps ( read silly-con) need base current of hugely stupid proportions, it would be suitable. BUT my transformers are a specific design for very low noise!! They are valve rectifier with LCLC basic filtering!!!
I designed them for moving coil phono stages, and work OK !!!
so you need 12 volts AC at about 3 amps!
12 X 1.414 = 17 or thereabouts PEAK!! loaded down will be dependant on the impedance of the cap and transfomer impedances.
Current available will be 3 X .62 = 1.8 amps DC.
When you take into account the TDA2030 is pure class B ( as far as I remember) thats more than enough, as any peaks will be handled by the .1 farad cap.
FOR myself I would look for a filament transformer of perhaps 2 times 6.3 volts at 2 amps or so.
1/10th the price, and apart from the builder, nobody will ever know !!

cheers
Joe

[Neil Purling]

I have got a Gardner filament transformer, about six 6.3V secondaries. It's rather tall though.
The amplifier itself is 3.00UKP. The PSU will cost 10.00UKP for a 20VA transformer of conventional type. A toroid would add another fiver to the price.
I can see why you would use one if you had a low level input as with a magnetic phono cartridge, which is not the case here.

[Radio Wrangler]

Some very satisfactory low-level preamplifiers have been powered from conventionally wound transformers. There are two approaches to combating hum pickup, to reduce the field from whatever transformer or wiring is the transgressor, and to reduce its coupling into the victim circuit. Artful layout, minimising or balancing and offsetting loop areas for circuits carrying low level signals is important. It also reduces sensitivity to field from external equipment and wiring. Both approaches can be used at once, of course.

I used to design things that lived within triple, nested, mumetal shields. It was a surprise to find that the field from a 50Hz cooling fan was a full order of magnitude greater than that from the (conventionally constructed) mains transformer.

David