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Old 7th April 2007, 03:07 PM   #1
Peteruk is offline Peteruk  United Kingdom
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Default Class T Output Inductors : Litz or Solid Core?

Hey all,

For some time now, I've been at a loss over deciding exactly what sort of wire to wind my inductor cores with on my 41hz amp4 kit. Basically, the real dividing line is: solid core or multi-strand. There is a thread on Audiocircle I've started , but I thought I'd make one on here too. My thoughts go something like this:

Solid Core

The Metglas Inductor calculator gave me a recomendation of 16AWG wire, 10 turns. However, this calculator doesn't factor in the actual application of the inductor (that it is in a switching amplifier), merely that it is an 'output inductor'. I was happy to go ahead with this design until it was suggested to me that there would be a serious issue with skin effect on such thick wire.

- However, solid core is used by Red Wine Audio in the sig 30 (although a much smaller gauge [perhaps this is how they retain a sufficient skin], which is really my model for the amp4, and also on the charlizee amp (although, that uses air cores, something different all together).

- Micheal Mardis once stated this when I asked him:
'"Why use Litz Wire on the inductors?
Litz wire is normally use in RF to carry high frequncies better than conventional wire (it's very flexible, too).

But the reason the inductors are in the output circuit is to REDUCE/REMOVE the RF. So you want to send the signal thru a low pass filter to remove the RF, but use Litz wire to increase the high frequency transmission? Doesn't make sense to me. '

I've had mixed responce to this view.

- This option would result in considerably less resistance, in theory resulting in fewer losses, which can only be a good thing. I've heard people talking of inductors sounding 'better' with fewer windings.

- What's more, there's no difficulty in finding high quality (OCC, OFC) wire in any gauge when considering solid core.

- Insulation thickness can be a bit of an issue however. Quality wire is not easily available in AWG16 with an enammel coating. A slik braid for AWG18 would be the best I can do, but even that's 0.5mm thick. Though this might not be an issue:
Click the image to open in full size. (Duelund winds their top of line silver wound toroidal inductors with a cloth covered ( silk and cotton permeated with natural plant resins ) solid core wire.) Although admitedly, this is a very different scale.

Litz
- According to Jan on one of the amp description pages:
'Compared to single strand wire, it has lower loss at high frequencies (which is good for switching output amplifiers) and is easier to wind. It is especially used when thick wire is required, as the "skin effect" gradually becomes more important.'

Based on this idea, Litz is technically superior and is the way to go. It is in direct conflict however, with the other theories supporting solid core.

- Litz has the practical advantage of being easier to wind tightly and evenly.

- I'm not acutally aware of many sources for litz wire though...




So yeah, a really mixed up jumble of thoughts...but your input would be very welcome. I'd especially be interested in hearing about preffered sources for Litz wire, and from anyone who's done listening tests on Class D output inductors.

Thanks to all who can help!
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Old 8th April 2007, 12:12 AM   #2
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I would go with litz wire.

Tripath amps switching frequency is up to 1000khz: these are radio frequencies. It is good to carry RF, only you must avoid it's picked up by signal and speacker cables.

Wire thickness has small influence on inductance, while toroid permeability and number of turns are critical. As you have very high perm toroids you will get almost the same inductance value with 10-13 turns, whatever wire you use. Then, the bigger the better. With few turns, windings will be well spaced, so skin effect will be small.
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Old 8th April 2007, 02:03 AM   #3
v-bro is offline v-bro  Netherlands
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Hi, greetz men!

Litz wire certainly, especially for the 'easier to wind' effect. I also think it helps minimalizing air spaces between the core...

I should really try these high perm cores one day.....
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