John Curl's Blowtorch preamplifier part III

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Hi JN!

By two gaps I was referring to the air on either side of the coil (inside/outside) in series with the magnetic path.

So you are saying that the PM is strong enough in the driver you are using as a reference, even with the fairly large total air gap to saturate the front plate, center pole, or back plate in the driver you are referencing? I remember testing some Aura woofers some years ago with radial arrays of neo magnets which (big assumption here) should have had much higher flux density (2-5x) than a comparable size or weight ferrite structure. Those drivers did indeed have a generous vented one-piece yoke and front plate, but my thought here is: if the front or rear plates or center pole were saturating the additional PM B would be for naught. Without data here I am speculating, so many variables comparing two drivers....cross-sectional area and alloy of the plates/pole piece, grade of the magnets, etc...

Enjoying your brain dumps, magnet man!
Howie
To deliver 1.5 tesla to the gap takes some horsepower. Most manu's just slap flat and round pieces together, many use as little as possible, the normal trade offs. Of course the iron type matters as well. Without a true FEA it's almost impossible to tell if any edges at the gap are complaining. I would expect the faceplate gap back edge to be the most compromised as it is the shortest path.

That is also where I would expect the most eddies as well, especially as the coil edge broaches the front plate rear plane, as the ends of the vc is where the field diverges.

Jn

Edit: the shame of it is, that paper with the neo ring magnets was so totally excellent. I really loved the return field being used as an over excursion brake. It's a shame nobody has come up with the idea of using HTS and 77kelvin to make the correct magnetizer for a simple ring neo.
Or even easier, just flat rings with correct field maps, but just shape the profile of the inner bore to get the uniform field. They played with the magnet shape a bit, but not the place where profiling is most effective. The neos can be purchased to shape easily.
We have that exact magnet shape here albeit planar. It's called a three pole wiggler.
 
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And a happy new year...

Scott as usual, reciprocity acoustic measurements use one transducer as the source and the other as the microphone. Then the measurement is redone switching the source unit to the microphone and the microphone unit to the source.

Yes as usual that not what jn is doing. As in one of Demian's B&K links two separate sources are used so the IM only comes from the mic. NIST is shut down, I can't find any references to moving coil mics and reciprocity do you have any?
 
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Not surprising, the classic mic guys are pretty wedded to their simple one or two FET open-loop impedance converters. FET + op-amp circuits using feedback can be far better for THD and still meet the noise specs. Of course they will speak with a new "voice".

Just last week I found that the word 'Obsolete' is obsolete, so I guess that now 'Mike' (for microphone) is now also obsolete :) [maybe I am … :)]
 
Yes as usual that not what jn is doing. As in one of Demian's B&K links two separate sources are used so the IM only comes from the mic. NIST is shut down, I can't find any references to moving coil mics and reciprocity do you have any?

You are not limited to just two loudspeakers. Three will do and show what the inherent inter-modulation distortion limit is.
 
You are not limited to just two loudspeakers. Three will do and show what the inherent inter-modulation distortion limit is.

References to actually doing this with 6.5" drivers and no microphones? Yes I am quite familiar with reciprocity calibration using 3 condenser microphones and the attention needed to the coupling environment and other details. I don't easily find anything on doing this with dynamic microphones/speakers, not to say you can't. I already know phono cartridges are not in general as hinted at in the 1947 JASA series on reciprocity.

One may, however, anticipate the possible breakdown of these conditions when appreciable losses due to “relaxation” phenomena in the transducer media are present.

I don't see the point one speaker does 1kHz and the other 100Hkz there is no mechanism for IM you put the mic in the combined sound field and measure the IM what could be simpler?
 
Demian, something is wrong. The chart you put up is for the CAPSULES only and at 140 dB or more! The preamps are subject to cap loading, unfortunately, but with a little care should be virtually unmeasurable even with the capsule up to 100dB or so.
 

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Hi George,

In a PM speaker the magnet, pole pieces, front plate, airgaps, and VC are in a series circuit, and in most magnetic systems the part of the magnetic circuit with the lowest permeability determines the overall saturation fluxivity.

So here is my query: Since the magnetic circuit of a speaker has two air gaps, would there not be little chance of any saturation of the rest of the magnetic structureGreat discussion all!
Howie

Hi Howie
I haven’t designed a speaker motor yet, I am an armchair literature overviewer.:D
As I see, it is feasible to magnetically saturate the metal close to the gap (it is one gap by the way), paying the price of using the recourses of the PM inefficiently
In the series magnetic circuit PM is the source, the leakage flux at the gap is the wanted dissipative element.
Any other section which is magnetically saturated will constitute a parasitic leakage flux, which acts as an unwanted dissipative element.
Better set the priorities upfront and start from scratch than trying to patch ( “improve”) an existing magnetic circuit. Using a bigger or a stronger PM can easily fire back.
(I know you are an old style guy, you like sharpening your pencil :) )

http://www.extra.research.philips.c...chive/PTechReview/PTechReview-05-1940-029.pdf
http://www.extra.research.philips.c...ve/PTechReview/PTechReview-24-1962_63-150.pdf

The wild card here is the inclusion of the field radiated by the VC in series with the PM fluxivity...Curious minds want to know!

The VC field works as an external demagnetizing field (-Ha at the attachment).
This will reduce the working induction Bd appreciably if the load line crosses the intrinsic curve close to the knee

My wishes to all for a healthy and prosperous 2019 :wave:

George
 

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Yes as usual that not what jn is doing. As in one of Demian's B&K links two separate sources are used so the IM only comes from the mic. NIST is shut down, I can't find any references to moving coil mics and reciprocity do you have any?
My GR reciprocity calibrator can use a dynamic mike, a piezo transducer and a condenser mike all at once. It was listed as "absolute" so no higher calibration needed like a Cesium frequency standard.
 
Happy New Year Y'all from the great state of North Carolina! 5 hours to go in this god forsaken year, and before I head out for a bonfire and fireworks I wanted to say I appreciate just about everyone's technical contributions, y'all are great DIYers; from my perspective I am DIM...(Do It Myself...) :D

Cheers! :cheers:
:cd::eguitar::note::magnet::hug: :hphones:

Howie
 
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