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Re: PP Iron for SE - repost of a 1996 rec.audio.tubes by "Paul"....

Posted by dave slagle (M) on February 20, 2004 at 09:03:34

In Reply to: Re: PP Iron for SE - repost of a 1996 rec.audio.tubes by "Paul".... posted by Steve Eddy on February 19, 2004 at 23:30:59:

(ds)the use of the -Bdc is to combat remenance which is exactly the reason we never return to the (0,0) point.

(se)Is it or is that just a guess on your part?

Not a guess… I have only seen a few references to this, nothing that goes deeply into it since even though it works, there are other "better" (read cheaper) ways to do it.

(se)I don't see how the DC bias can combat remenance as the restorative force of the field produced by the DC doesn't get returned to the system in order to restore anything.

Ok simple test… bias a core with a Bdc, now hit the core with a negative unidirectional signal to take it to the –Br point. Now apply a very small signal, where will the minor loop center on? The Bdc point or the –Br?

(se)Well you haven't given any reference so far. :)

nor have you :-), the only place we seem to disagree is on what happens with the remenance when there is a Bdc present. And that is a subject I have never seen mentioned specifically anywhere.

Reuben lee touches on the remenance in pulse cores, but that’s about it.

(ds)but what happens when the signal is removed?

(se)Same thing that happens when the signal is removed from a non-biased core except where it ends up will be offset depending on how much of a bias there is.

OK lets try this… it might get confusing but try to follow.

When we have a minor loop centering upon Bdc, and if suddenly the signal is removed say at the Bmax of the minor loop, we know the core will return to the vertical axis of Hdc. Now is it your contention that due the the remenance, the core will actually remain magnetized to the new point equivalent to Brdc which will be the point in the loop where the top of the minor loop crosses the Hdc axis and not at the original point Bdc.

(se)Tell me, where does the restorative force come from which brings the core back to the Bdc point once the signal is removed?

If we are still talking about remenace here, remember that remenace only exists because of a DC offset to begin with. It’s a catch 22, most if not all things we look at wrt tubes uses symmetrical cyclical AC which has no DC component, but what about music? We say its AC but dosn’t it also have a DC component?

(se)But my system doesn't include speakers that are made up of nothing but a tweeter.

This is what always gets us into trouble, you can’t talk in general concepts then refute the concept with a specific case. The points you are making only hold true when you attach numbers to the concept. What I am suggesting is that we first agree on the concept, then we will know how to properly attach the numbers and draw the correct conclusions. This all came about by placing DC through a nongapped transformer. Look at the graph of the incramental permeability of a core vs. a dc magnetizing force. Now if we are talking linearity, I will have to say that the perm is most linear with a higher value of H. Now if we are talking values its obvious that the H=0 gives you the biggest numbers but is decidedly less linear.

What confuses things is when someone puts in their own numbers and draws what I see as an incorrect conclusion. The higher but less linear perm often makes a complete circuit more linear, but that’s a number thing and not a conceptual one.

(ds)and again it depends on the design. lets take a 1:1 bifilar tranformer and i'll say it will be much more linear without the core than with it.

(se)Ok. But of what practial use would it be?

That isn’t the point now is it?? Its nice that we have made it this far into the discussion without introducing specifics (numbers) now all of the sudden you want to say if there isn’t enough inductance, the bass will roll off? And if the bass rolls off the circuit is non-linear?

Fine… I agree but it has nothing to do with anything I have been trying to talk about. I could easily state that just wind your bifilar air cored transformer with lots of inductance but then we would just rehash the obvious and get into the choice of compromise :-)

in any event... lets get back on track...

looking at the above graph, can we agree that increasing the value of H increases the linearity of the perm.

since perm relates directly to inductance, if we assume that the final result gives us adequate inductance (judged by ear) could the increased linearity possibly explain why some people have found good results with PP iron SE??? (please note we have no idea what value of H they are using so we have to assume there is enough inductance to give the desired bass response since no mention is made of the roloff.)

dave

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