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Old 7th May 2013, 02:58 PM   #38991
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J.N.

We ain't arguing. But how does magnetostriction correlate with saturation?

There was the acoustical consultant who specified the amplifiers be flown with the loudspeakers in a theatre. As he was US based and specified US made products and the theatre was in London, the amplifiers made an annoying buzz (pun also intended.)

The acoustician's cure was to specify a sound isolation enclosure. I would have used a buck transformer on the power supply.

ES
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Old 7th May 2013, 03:11 PM   #38992
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Originally Posted by simon7000 View Post
J.N.

We ain't arguing. But how does magnetostriction correlate with saturation?
Both can cause noise.

Magnetostriction will be at 2f.
Loose laminations will pull in at 2f, and can buzz as well.
External metal will move as 2f.
An external magnet will move as f.
Loose windings as 2f.

Saturation will cause a change in the input waveform because the reluctance will drop at the magnetization extremes. That additional current at the peaks will be more audible than the primary f.

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There was the acoustical consultant who specified the amplifiers be flown with the loudspeakers in a theatre. As he was US based and specified US made products and the theatre was in London, the amplifiers made an annoying buzz (pun also intended.)
Sounds like a spec concern. Seems to me that perhaps there was a difference in powerline frequency, no?

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The acoustician's cure was to specify a sound isolation enclosure. I would have used a buck transformer on the power supply.

ES
I probably would have spec'd the correct line frequency.

Course, maybe the cover plate to the amps resonated at 50 hz, so were quiet on this side of the pond..

jn
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Old 7th May 2013, 05:59 PM   #38993
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J.N.

When the core is saturated more flux leaks. This can vibrate other parts, but what the buzz is about is the linear to nonlinear shift in the movement of the actual transformer parts (mostly the core laminations.) Now most laminations when moved at 50, 60, 100, or even 120 hertz don't make as much noise as say 480 or 960. The there is the Fletcher Munson effect coming into play. So that is why you get a buzz when you saturate.

The other cause is of course "DC" on the AC line preventing symetrical movement on the B-H loop.

Now of course you can't have theoretical DC on an AC line due to the infinite time requirement. So lets just leave it at a voltage offset you can measure with a DC current meter in series with a resistor.

Now as to amplifier noise, there is no spec for the acoustic noise produced by an amplifier. So one rated for 50 or 60 cycle use is in spec when it is slightly noisy. Keep in mind a theatre should be NC25 or lower, thus very sensitive to noise levels. I have a theatre in Alfred N.Y. that has a similar problem. One of the power amplifiers makes more noise when the lights are dimmed. That leads me to believe the dimmer system uses pairs of SCRs and is not perfectly symmetric. Of course the solution in that case would be to add a wall to the amplifier room, as it is open to the stage.
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Old 7th May 2013, 06:14 PM   #38994
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J.N.

When the core is saturated more flux leaks.
Agreed.
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That leads me to believe the dimmer system uses pairs of SCRs and is not perfectly symmetric.
Wow..I haven't seen a Saturable Core Reactor in decades..Now that's a really old theater..

jn
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Old 7th May 2013, 06:38 PM   #38995
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Agreed.


Wow..I haven't seen a Saturable Core Reactor in decades..Now that's a really old theater..

jn


No, Not Saturable Core Reactors, Silicon Controlled Rectifiers (for the hard of Humor.) SCRs pair instead of triacs for higher power handling. A slight difference in trigger level or on state voltage would cause the problem.

I have at my bench a linear slide transformer removed from a very old theatre dimmer!
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Old 7th May 2013, 06:41 PM   #38996
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I'm sure I told the story somewhere of the Chicago orchestra pit I played in one night on a tour in 1972 that had d.c. supplied to the stand lights, tubular incandescents which, when supplied with a.c., generated, collectively, an unacceptable level of acoustic noise.

The hapless guitar player, who plugged in to one of the convenient outlets just before the performance, wasn't warned by the staff. The line fuse in his amplifier was not equipped for interrupting d.c. and the unmistakable and acrid smell of burning power transformer wafted out of the pit. Fortunately no one shouted FIRE, but needless to say the guitar parts were absent that night.
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Old 7th May 2013, 06:56 PM   #38997
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Originally Posted by simon7000 View Post
No, Not Saturable Core Reactors, Silicon Controlled Rectifiers (for the hard of Humor.) SCRs pair instead of triacs for higher power handling. A slight difference in trigger level or on state voltage would cause the problem.

I have at my bench a linear slide transformer removed from a very old theatre dimmer!
Not nearly as sexy as a saturable core reactor...

jn
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Old 7th May 2013, 07:09 PM   #38998
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One can easily catch (= measure in time domain) the transient inrush current - that's the best way to understand (I mean real understanding, not guesses) the phenomenon.
.... but that means real work.

Example of switching on at the wrong moment.

jan
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Old 7th May 2013, 07:23 PM   #38999
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.... but that means real work.

Example of switching on at the wrong moment.

jan
Your example switches on at peak.

Which has been agreed produces higher inrush.

It does not however, indicate that it was a result of core magnetization.

jn
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Old 7th May 2013, 08:42 PM   #39000
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The interesting bit is the front versus back slope of the current. When the capacitors are not fully charged the back slope contributes energy. Once charged it drops the contribution. This indicates a transformer that is far oversized for the load. A typical audio application.

ES
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