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Old 9th September 2008, 01:24 AM   #1
exeric is offline exeric  United States
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Default Loosening up magnetic "stiction"

I read this AES paper http://www.mennovanderveen.nl/nl/dow...download_1.pdf
by Menno Vanderveen that was very interesting. It pointed out that output transformers have inherent low linearity at very low levels of audio input signal. Without going into all the details it shows that the worst effects occur between 20 to 1000 hz range and cause a loss of low level details. It further states that some of the holographic qualities of SE amps art probably due to a gapped OPT that has low output impedance that causes the tranformer to maintain its inductance at very low signal levels - in other words that magnetic stiction can't exert as much influence in that topology and has much more to do with the low level retrieval of sound and their holographic qualities.

Anyway, this all makes a lot of sense to me. He mentioned experiments where high frequency bias signals are sent along with the audio signals that seem to loosen up that stiction in OPTs. JLTi in Australia makes a device called a Lemm (I think) that does that, but its only sold in Australia and I haven't heard of anyone else doing it. It certainly makes sense to me as tape recorders have always used high frequency bias to make recordings if memory serves. Does anyone have personal experience with this, or thoughts on practical implementation of a high frequency bias device?
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Old 9th September 2008, 01:34 AM   #2
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I thought the word was "hysteresis". It's one of the reasons why you should size the output transformers appropriately for the job.

I hadn't heard of specific techniques to try to counteract it. The theory is intriguing.
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Old 9th September 2008, 01:47 AM   #3
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Yes, hysteresis is the correct term but I like stiction because it conjures up a more physical image that the average person can visualize. Hysteresis isn't commonly discussed as being a varying quantity with level so I'll stick with stiction.

Part of the problem with sizing the OPT correctly is that if you go for minimum distortion at low frequencies you inevitably need a lot of iron. And that invariably makes the problem worse at low signal levels. Compromises must be made.
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Old 9th September 2008, 01:54 AM   #4
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Quote:
Originally posted by exeric
Compromises must be made.
Yes, there are always compromises. Aren't toroids noted for being particularly good (with regards to low hysteresis)? But then you've got that whole DC saturation thing to deal with...
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Old 9th September 2008, 02:34 AM   #5
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There was a thread about this recently. You should be able to find it with a little searching. Bottom line: the effect can only be seen with really, really lousy core materials. With a pretty normal output transformer, I was unable to see it. And note the lack of that effect in MC stepup and line level input transformer measurements.
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Old 9th September 2008, 02:47 AM   #6
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Quote:
Originally posted by SY
Bottom line: the effect can only be seen with really, really lousy core materials. With a pretty normal output transformer, I was unable to see it. And note the lack of that effect in MC stepup and line level input transformer measurements.

I'm not so sure that's the case... take a look at some measurements from Cinemag of their 80% nickel vs. 50% nickel line output transformers. Rather stunning difference, unless I am misinterpreting here! Is there a noise difference? Or is that core-related?


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Old 9th September 2008, 03:03 AM   #7
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The discussion is buried in the Blowtorch saga, dating back to May sometime.

Measuring a standard EI output transformer (good quality but non-exotic) I saw absolutely no trace of this effect at the levels and frequencies that Menno was talking about. This one was a 1250:16/8/4 unit that I use in my bigger power amp.
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Old 9th September 2008, 04:26 AM   #8
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The distortion from core hysteresis is in the form of additional high harmonic magnetizing current. It gets converted to voltage distortion by the source impedance. So one obviously wants to use a low source Z for driving a steel core.

Looking at the Cinemag Fig. 1 and Fig. 2, I see they have Rs = Rl, or a damping factor of 1. The steel core drops to the nickel core dist. level at about 30 X the freq. This would indicate to me that one needs a source Z 30 times lower (or damping factor of 30) to get the steel core to equal the nickel core. So local neg. feecback to the driver stage could clear this dist. up.

Using both local fdbk and a nickel core would be even better yet.

One can inject a HF bias field into the core to remove this Hyst. dist. The best way is if it is orthogonal to the main field. But this is difficult to set up. (have done it with ferrite pot cores only, by winding thru the bolt hole)

Next best is to use two cores with the main winding wound over both of them, and the bias winding wound as two separate windings around them and connected anti-phase so as to null sum in the main winding. I've tried this once and it sorta works, but the Mu balance between the cores has to be perfect or HF effects show up in the audio. DC currents can wreak havoc to the balance too.

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Old 9th September 2008, 04:36 AM   #9
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Quote:
Originally posted by SY
The discussion is buried in the Blowtorch saga, dating back to May sometime.

Measuring a standard EI output transformer (good quality but non-exotic) I saw absolutely no trace of this effect at the levels and frequencies that Menno was talking about. This one was a 1250:16/8/4 unit that I use in my bigger power amp.

I wasn't able to locate the subject under "blowtorch".

If you read Menno's article carefully what he's really saying is that the human ear, at the threshold of audibility requires a change in sound pressure level 100000 times between 20hz and 4000hz. This isn't something he's making up! It's the ISO curve for the average human ear. He's not really taking about varying levels of hysteresis with level. What he's talking about is the extreme range of flux density required of the core in an output transformer.

A speaker is a power consuming device rather than primarily a voltage consuming device so an OPT feeding it most likely will require a greater range of flux density between loudest and softest passage than a MM xfmr. Isn't there some electrical law somewhere stating that?

Providing that is true it means one could probably create a MM xfmr with magnetic materials that is within that flux density range but far less likely you could get a permeable material for the larger range required for a OPT. Its not about the quality of magnetic material, but about the "range" of flux density the material can accomodate.
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Old 9th September 2008, 04:55 AM   #10
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One further clarification: What I gather from the article is that high frequency bias might be able to "extend" that flux density range on the low volume end.

If we can get back to my original question. Does anyone have a clue how one would implement this - i.e. where would you feed the bias signal, bias signal level, frequency etc? Whether one would have to filter it out before reaching the speaker? I sure don't know how one would implement it. Even if some disagree with Menno's audio philosophy in other ways it doesn't mean he isn't on to something here.
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