R-core transformers poor choice for amp PSUs?

[Bosk]

I've recently built a 15V regulated PSU to power a Tripath 2020 amp I'm running, using LM317 for regulation, fed by an R-core transformer.
The amp is running great, and has never sounded better.

However, I know that toroids generally seem to be the optimum choice for amplifier PSU's. Why, I'm not sure as my electronics theory knowledge is rather modest.


My question therefore is should I consider replacing the R-core transformer with a toroid, or wouldn't I be likely to hear the difference?

[pjp]

There is no real reason. People just assume that toroids must be better because they're different and they cost more.

There are three differences between transformer core types:

1. the radiated magnetic field differs. EI transformers have a significant radiated field, which audiophile types will tell you is a problem since they can induce currents in the amplifier electronics.
In my opinion if you take a little care in transformer placement this is never going to be a problem.

2. The size/weight of a toroid for a given power is less. This might matter to commercial manufacturers since extra weight means extra costs. And bigger amps are just unwieldy.

3. The capacitance between the primary and secondary winding is higher for toroids. The capacitance lets mains noise through into your amp, and in this respect an EI or an R-core transformer actually is better than a toroid. Again, in a properly designed transformer this is unlikely to cause an audible problem.


As far as R-Core vs Toroid is concerned, I'm certain you're not going to hear any difference at all.

[richie00boy]

I'd be more concered about powering an amp from an LM317.

[LinuksGuru]

Originally Posted by Bosk

My question therefore is should I consider replacing the R-core transformer with a toroid, or wouldn't I be likely to hear the difference?

No reasons whatsoever to change transformer.

[h_a]

I'm in the lucky position to have some hands-on experience with R-cores and toroids in exactly the same amp, in the same position.

From all the spectrum measurements I did, it appears to me that R-cores are incredibly close to toroids with respect to stray magnetic fields*. Vastly better than EI-types.

So, if you don't need the last bit of performance in terms of signal-to-noise ratio (because, for example, your enclosure is large enough that the humm is inaudible anyway) you can go for the R-core if you feel like it.

Also so far mechanical humm was much less an issue for the R-core as many toroids mechnically humm slightly even if a DC-filter is used. But I would recommend a proper DC-filter anyway.

Sonically maybe others would like to comment.

Have fun, Hannes

*closer to toroids in certain directions than in others though.

EDIT: on why generally toroids are used: toroids have (or can have) very good technical specs (weight, parasitic capacitance, stray field) and are easy to get. R-cores can have the same or close to the same tech specs, but there exist only few manufacturers - so are hard to get. Industry always prefers products where long term availability and low cost! is guaranteed.

[Bosk]

Thanks very much for your help guys, after reading your advice I'll stick with my R-core and spend my money elsewhere where it will improve the system.

[SY]

One more thing- toroids are generally a worse choice for performance reasons. They have superb coupling and bandwidth, so they'll cheerfully and efficiently couple every bit of mains noise and dirt right into your amp. R-cores are much superior in this regard.

[h_a]

Hi Sy,

I also have heard that a couple of times, but so far never seen a measurement showing that.

All I can say is that the R-cores I'm having here have also an electrostatic screen, which should be unnecessary if this was the case. Anyway, going for an electrostatic screen in toroids is also certainly a good idea.

Have fun, Hannes

[VivaVee]

Toriods can be built on a low profile core. This enables the amp manufacturer to put the whole electronics in a slim-line case. I see this as the one true advantage of a toroid - in other words, this is a cosmetic design issue

But this can be easily abused because the most efficient core cross-section (for a tape wound toriod) is square. It is easy to 'squash' the efficient square cross-section into an inefficient rectangular cross-section when you are trying to achieve a low profile.

Like all components, they can be designed and manufactured very poorly without much trouble.

Common design 'features':

1. Running the core at a high flux density so you get a higher nominal VA rating for a given physical size. The result is higher external flux fields that effectively negate the theoretical advantage of the toriod and the transformer vibrates bady.

2. Specifying the VA rating at a large temperature rise. This gives the illusion of a high power transformer but everything else in your case is exposed to the higher temperatures. A special award goes to the manufacturer that rated their encapsulated toroid for a temperature rise that caused the nylon case, that it was potted in, to melt.

[star882]

The power supplies for 1U rackmount servers and small PCs pack a few hundred watts into a box 1.75" tall. One I have seen the inside of just used an EI transformer, but it is a special one that is mounted sideways. There are some toroids, but they're inductors.

Some of the new laptop power supplies are just a little larger than a deck of cards and can supply over 100w. I have not seen the inside of one, however.