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[Recording Engineer]

A friend is building me a U47-type mic with a Neumann capsule. I'm currently gathering transformers from IOAudio, AMI, and Andreas Grosser as options and the IOAudio has come in first. It's the only transformer I've ever held in my hand that rattles... And apparently it's the way Neumann did it and should go away once mounted? Magnetostriction... Someone care to explain in laymen-terms and confirm this is indeed the way Neumann did it?

By the way, if anyone cares, I'm also gathering the currently-manufactured "tube substitutes/alternatives" by Telefunken, Phaedrus, IOAudio (should be available at some point this year), and my friend's wishful-thinking but completely expected to come in dead-last ("arrangement"?) with a widely-available and used cheap tube.

And we could careless that it won't sound like an original vintage U47; as we already know it won't! Just trying to make the best-sounding mic (to us) with a U47 circuit and parts-types as we can with the best readily-available parts... Andreas Grosser and some others are already do that, but this is the next-best alternative concerning price.

[klaus]

(...) It's the only transformer I've ever held in my hand that rattles... And apparently it's the way Neumann did it and should go away once mounted? Magnetostriction...

As I understand it, magnetostriction is only present in ferromagnets- those that remain magnetized at all times (i.e. permanent magnets).
Transformer magnets, as used in condenser mics, are not ferromagnetic, so that phenomenon would not be encountered there.
This notion has since been corrected by posters who know more about the subject than I. Please disregard the above.


Besides, I have never run into rattling magnets in microphones, new ones or old ones, aside those with badly constructed or sloppily assembled transformers. Which would indicate that, whatever rattles in yours will not go away with time, or until the core laminates are fixed.

I would suggest you inspect the transformer closely for manufacturing slop.

[AusTex64]

Sounds like the laminations are loose or something. I've never seen a mic transformer that rattles in the way you describe. But most of the transformers I've seen are AMI, and they apply some kind of varnish to the lams.

[Kai]

As I understand it, magnetostriction is only present in ferromagnets- those that remain magnetized at all times (i.e. permanent magnets).
Transformer magnets, as used in condenser mics, are not ferromagnetic...

Ferromagnetic means it can be magnetized, even temporarily. It's easy to check if a material is attracted by a magnet, but don't do that with an audio transformer, the remaining permanent magnetism will compromise its performance.

Of course the core of any transformer is (mainly non-permanent) ferromagnetic, as magnetism is one part of its working principle.
But, to make a transformer emit audible sound because of magnetostriction you would need lots of energy, causing a very strong magnetic field which is not the case inside a microphone.
Anyway it would not rattle, but act like a sound transducer.

These type of transducers exist and are used e.g. for the ping sound of submarine sonar (echolot), where they work very efficiently because they are coupled directly to the water, not to air.
In our case here the core parts are simply not mounted tight enough, causing movement between them.

[uwe ret]

Actually as defined in Wikipedia:
Magnetostriction (cf. electrostriction) is a property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetization.
As noted in a previous post magnetostriction requires considerable energy, not encountered in microphones or elsewhere in conventional audio electronic equipment.
This has no relation to the rattling of transformer lamination as reported in the original post. Possibly one or more layers of the core material may be missing and/or this rattle may disappear when the transformer is properly installed, thereby affixing the ferromagnetic components.

[uwe ret]

Transformer magnets, as used in condenser mics, are not ferromagnetic, so that phenomenon would not be encountered there.'

Practically all electrical transformers do rely on ferromagnetic core material and would be rather inefficient or non-functional without it.

[Recording Engineer]

The couple of people I've shown it to have years of audio electronics experience of all-type and I made no mention of the rattling; they independently commented on their own saying their only real concern would be the microphonics-potential.

I did contact IOAudio to see if this was normal for their product or defective. He says it's the way it was designed and will go away once mounted. I'm satisfied with the response should it really go away. However, I am quite surprised I've not seen anyone else express concern anywhere. I was even more surprised to hear him mention magnetostriction (the way Neumann did) and that I've never heard anyone ever mention that anywhere either! This is why I thought it would be best to ask about it here.

If this really is so or some-other-type of "changing-process" happens once mounted and used, would it be expected that original vintage Neumann transformers wouldn't ever rattle even after being unmounted?

[boz6906]

From my experience you want ZERO mechanicsl noise from any part, especially in a high-gain mic preamp.  High quality audio transformers like UTC are even 'potted' to prenent mechanical movement.

I would hesitate to say your transformer is defective, the mounting scheme may somehow clamp the transformer to prevent the rattle.

Do you have a website or product info about this transformer from the manufacturer you can share?

[klaus]

Though I now have egg on my face that does not wash off easily (my ignorant comments regarding "magnetostriction"), I am on firm ground with this part of your inquiry:

Nothing should ever rattle inside a microphone, regardless of type or brand, because such rattling is invariably transmitted to the mic's output as audible noise. 

If a transformer rattles in its installed position inside a mic, it is defective, and unsuitable to work as intended.

[Recording Engineer]

Well thank you to all the replies... I will report back if the rattle does indeed stop once properly-mounted; though it'll probably be another couple of months.

Boz, no product info or website... IOAudio is just a one-guy operation of Max Kircher as far as I know. His popular MK-47 is sold through 939 Studio and him with his contact info on GroupDIY.. I bought just the transformer directly. If this is indeed the design, I would have thought I'd seen a concern expressed somewhere by now... No thang at all if it goes away as was told though... We'll see!

[Brian Campbell]

I have two microphones with Max's transformers in them, they do not rattle.

[Recording Engineer]

Did you build them and did you notice them rattle before you mounted them? Or did you get them already mounted? Thanks!

[Brian Campbell]

No Max mounted them.