Development of a "reference" class D starting point

[Bricolo]

If we have to use a cord for the coil, I see 2 esay solutions:

-we can all agree in a Farnel (or any other worldwide shop)
-using the small toroid cores we can find in computer power supplies (I have some of them)

[Bruno Putzeys]

Originally posted by classd4sure

How about counter wound air core coils in series, I've seen it used in a few amps. Worth doing?

That would provide some relief in the far field but the kind of dimensions we're talking about for a 100W amp (6x6cm?) is not "far field". The Q of the inductors would leave something to be desired because of their partial coupling, mutually and towards the ground plane.

I wouldn't settle for anything less than a closed magnetic circuit.

[classd4sure]

Hi,

-using the small toroid cores we can find in computer power supplies (I have some of them)

Yeah don't we all...I just checked out the two I have again...I don't think you'll find a matched set of them on it should you want 2 channels later. Same with the motherboards they powered, all different sizes.

Going from a large manufacturer of them that's been around a long time is a great idea, bulk buy or otherwise....I'll just cut one off the psu for now

[classd4sure]

Originally posted by Bruno Putzeys


That would provide some relief in the far field but the kind of dimensions we're talking about for a 100W amp (6x6cm?) is not "far field". The Q of the inductors would leave something to be desired because of their partial coupling, mutually and towards the ground plane.

I wouldn't settle for anything less than a closed magnetic circuit.

Sounds like it's not worth the effort at all. No further questions, yet. Take 5.

Thanks!
Chris

[Bruno Putzeys]

Originally posted by Bricolo
using the small toroid cores we can find in computer power supplies

These are either iron powder or ferrite. In the former case, they are invariably a "soft saturation" kind which produces tons of distortion even at low levels. In the latter case, an air gap needs to be ground in.

The cores Farnell has have no air gaps so some work is needed there too.
While trying to find info on how to machine ferrite I even heard large companies telling me it's impossible unless you have very specialised machines and tooling.
In the end I managed to order diamond coated burrs in a local shop (20 euros apiece or so. Manufacturer is called Pferd). They work fine for making air gaps in core halves. My drill stand, at 3000rpm is a bit slow though, which results in chipping.

For toroids I use grinding discs from Dremel (1mm thick or so) in a drill stand at slow speeds. It takes about 5 minutes to get through a toroid but it makes a clean cut.

[phase_accurate]

I once thought about using an "air-cored" toroid. The disadvantage would be the high amount of turns needed (=> skin-effect losses).

I once posted the formulae I used for determining the air gaps within my thesis. Maybe it is quicker to search them on my PC than to find the right thread on the forum. The accuracy of the derived inductance relied on the core type. It was better with RM cores than with EFD cores. It should work quite nicely with toroids. I guess this was due to stray fields etc. You will have to measure inductance anyway (not that dificult at all) and add or remove some turns. Simple as that.
Edit: forgot to mention that I introduced the air gaps by the insertion of insulating material between the two core-halves.

In the meantime I found the post:

http://www.diyaudio.com/forums/showt...536#post196536

The schematic diagram of my thesis' amp is found here. Under today's circumstances it is neither sexy nor refined but should serve well as an easy example for the understanding of PWM amps. But one has to add that it was developed when there was not much literature on class-d around and driver ICs weren't as good as today (neither were affordable OP-AMPs). The IR 2110 was already available but it had worse figures than today's versions, otherwise I'd have gone with that (I actually did experiment with it).

http://www.diyaudio.com/forums/showt...684#post194684

Regards

Charles

[johanps]

I fully agree about double-sided PCB - it's a neccessary evil .

Regarding filter coil(s), that's certainly an interesting area, where suitable recommendations will have to be developed. Possibly both pointing to ready-made parts, and instructions on how to DIY.
In my experience, air-wound cores can work with good results, at least up to the power levels I've been tinkering with (my current amp. uses +/- 55V for the power stage - unregulated of course).
That said, I am not advocating them, and I'm hoping to achieve equal or better results, with a much smaller size, using toroid cores instead.

Doesn't the UCD design counteract the non-linearity in the filter core to a large degree? Or is the core material still critical?

[phase_accurate]

Doesn't the UCD design counteract the non-linearity in the filter core to a large degree? Or is the core material still critical?

Although the statement might sound a little strange related to a self oscillating circuit:
The better an amp performs open loop, the better it will perform when the feedback loop is closed.

Regards

Charles

[Bricolo]

Originally posted by johanps
I fully agree about double-sided PCB - it's a neccessary evil .

Doesn't the UCD design counteract the non-linearity in the filter core to a large degree? Or is the core material still critical?

We could use the components side as a pure ground plane, so it wouldn't cause much trouble for diying double sided boards (having to align the 2 sides...)


As for UcD and non-linearity. See it as feedback on a linear amp.
Try to correct a heavilly non linear amp (or stage) with some feedback: it will work, but won't be as good as a much more linear stage corrected with the same amount of feedback

[Bricolo]

3 minutes too late