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omnis and pressure sphere attachements
Old 28th November 2015 | Show parent
  #91
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John Willett's Avatar
 
🎧 10 years
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Quote:
Originally Posted by studer58 ➡️
Steering it all somewhat back on-topic, here's a nice paper from Neumann with some great photographs: http://www.neumann.com/download.php?...d=lect0043.PDF

It's a download link to the PDF paper
Interesting - thanks for this.
Old 10th November 2021 | Show parent
  #92
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🎧 10 years
A query of those who have made their own APE/pressure spheres (30-50mm diam) for omni mics via DIY drilling....which sphere material have you found easiest to drill through and work with, in terms of stability, appropriate hardness, freedom from melting (!) etc ?

A few possibilities: hardwood, delrin, acetal, acrylic, polycarbonate, nylon, phenolic resin etc
Old 10th November 2021 | Show parent
  #93
Gear Maniac
 
🎧 10 years
Quote:
Originally Posted by studer58 ➡️
A query of those who have made their own APE/pressure spheres (30-50mm diam) for omni mics via DIY drilling....which sphere material have you found easiest to drill through and work with, in terms of stability, appropriate hardness, freedom from melting (!) etc ?

A few possibilities: hardwood, delrin, acetal, acrylic, polycarbonate, nylon, phenolic resin etc
Acrylic is VERY difficult to deal with.
I had success with 38mm billiard balls, i think phenolic resin is what they are made of.
Hardwood should be great, too.
Old 10th November 2021 | Show parent
  #94
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Quote:
Originally Posted by bremusound ➡️
Acrylic is VERY difficult to deal with.
I had success with 38mm billiard balls, i think phenolic resin is what they are made of.
Hardwood should be great, too.
Yes, you'd likely have a trade-off between drill speed and heat generation > possible melting and/or splitting, as acrylic is also quite brittle ?

I can imagine billiard balls might give the drilling bit some challenge, while hardwood is probably a good compromise material ?
Old 10th November 2021 | Show parent
  #95
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🎧 5 years
Tap Plastics sells clear acrylic spheres (just like in M50); I've had them drill the proper holes in them for me at reasonable price.

I've also made them myself from hardwood drawer/cabinet pulls from the craft store, using good Forstner bits. Some stores sell full wood spheres, but the drawer/cabinet pulls are easier for DIY because they already have a flatted area with a hole in the center, to guide the Forstner.

The only rub is that this being the US, these things are not made in Metric dimensions, so usually only available in 1.5" or 2". I've found the 1.5" (38mm) quite suitable; the shelf begins at only a bit higher frequency than the M50's 40mm - the 2" is very close to 50mm, which the Decca book claims sounds closer to the M50 (because it approximates the effects of the M50's head grille [?]), but I find that it's lower shelf produces a pushy upper mid-range.

I see no reason for them to be of anything more exotic than acrylic or hardwood. Josephson used to make hardwood ones, but no longer.
Old 10th November 2021 | Show parent
  #96
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🎧 10 years
Quote:
Originally Posted by studer58 ➡️
Yes, you'd likely have a trade-off between drill speed and heat generation > possible melting and/or splitting, as acrylic is also quite brittle ?

I can imagine billiard balls might give the drilling bit some challenge, while hardwood is probably a good compromise material ?
Never tried wood as i bought pre-drilled wooden spheres from an ebay dealer. I was surprised how easy it was to work on the billiard balls.
Old 10th November 2021 | Show parent
  #97
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🎧 5 years
Billiard balls (at 57mm) would produce a shelf at a much lower frequency than M50 (40mm); too close to the upper midrange for comfort.

Also, as you can see in the DPA graphics, use of a 40mm sphere requires a capsule with a fair amount of top lift (as the one in the M50 did), because the pressure boost falls back down to zero well below 20k. Not so with the 50mm - the boost extends all the way to the top, so one would want a flat capsule with that.
Old 10th November 2021 | Show parent
  #98
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🎧 10 years
Quote:
Originally Posted by M50k ➡️
Billiard balls (at 57mm) would produce a shelf at a much lower frequency than M50 (40mm); too close to the upper midrange for comfort.

Also, as you can see in the DPA graphics, use of a 40mm sphere requires a capsule with a fair amount of top lift (as the one in the M50 did), because the pressure boost falls back down to zero well below 20k. Not so with the 50mm - the boost extends all the way to the top, so one would want a flat capsule with that.
There are 38mm billiard balls available, that‘s what i used.
Had also success with (wooden) 50mm spheres on the 4006 (silver grid).
Old 10th November 2021 | Show parent
  #99
Gear Maniac
 
Quote:
Originally Posted by M50k ➡️
Tap Plastics sells clear acrylic spheres (just like in M50); I've had them drill the proper holes in them for me at reasonable price.

I've also made them myself from hardwood drawer/cabinet pulls from the craft store, using good Forstner bits. Some stores sell full wood spheres, but the drawer/cabinet pulls are easier for DIY because they already have a flatted area with a hole in the center, to guide the Forstner.

The only rub is that this being the US, these things are not made in Metric dimensions, so usually only available in 1.5" or 2". I've found the 1.5" (38mm) quite suitable; the shelf begins at only a bit higher frequency than the M50's 40mm - the 2" is very close to 50mm, which the Decca book claims sounds closer to the M50 (because it approximates the effects of the M50's head grille [?]), but I find that it's lower shelf produces a pushy upper mid-range.

I see no reason for them to be of anything more exotic than acrylic or hardwood. Josephson used to make hardwood ones, but no longer.
I have had some made out of hard wood, but haven't used them yet.

Out of interest, how do you finish them (if at all) and how do you securely fix them to the mics? I wouldn't like them to fall on someone's head when the humidity changes in the hall...
Old 10th November 2021 | Show parent
  #100
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🎧 10 years
Non-standard pool/snooker/billiard balls seem like a good starting point eg:

https://www.amazon.com.au/Economy-Sp.../dp/B018XXTZC4
* https://www.amazon.com.au/Kids-Small.../dp/B01CGIM98E

* "We sell these balls in 38mm, 41mm, 44mm, 48mm sizes - (prices are different for other sizes, please buy from relevant listing if you require a different size)"

I could see them looking good with a final spray coat of Schoeps matte grey (or black or chrome/silver...to match the mic) ?

The intention regarding tighness is that they'd be an interference fit around the neck/collar of the mic (within drilling tolerances)...maybe some thin surgical tape layer-wrapped around the mic body could help with such a non-slip fit ?
Old 10th November 2021 | Show parent
  #101
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Quote:
Originally Posted by JGebauer ➡️
I have had some made out of hard wood, but haven't used them yet.

Out of interest, how do you finish them (if at all) and how do you securely fix them to the mics? I wouldn't like them to fall on someone's head when the humidity changes in the hall...
The spheres from Schoeps are attached via (that's Latin! :-) Plastazote foam.
Old 10th November 2021 | Show parent
  #102
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🎧 5 years
Quote:
Originally Posted by JGebauer ➡️
I have had some made out of hard wood, but haven't used them yet.

Out of interest, how do you finish them (if at all) and how do you securely fix them to the mics? I wouldn't like them to fall on someone's head when the humidity changes in the hall...
The bare wood balls, I gave a coat of sanding sealer, then painted them with artist's grey acrylic gesso (just because i had some on hand).
Old 13th November 2021 | Show parent
  #103
Before you take the effort to make your own acoustical pressure equalizers, take note of the following graph. Without proper testing methods you may well end up with something you do not want. Only a few millimeters misaligning the sphere can cause huge faults in the frequency domain.
Attached Thumbnails
omnis and pressure sphere attachements-mk221-40mm-sphere.jpg  
Old 13th November 2021
  #104
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I home-built my own heart-lung machine . . .

D.
Old 14th November 2021 | Show parent
  #105
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🎧 5 years
Quote:
Originally Posted by Orgeltonmeister ➡️
Before you take the effort to make your own acoustical pressure equalizers, take note of the following graph. Without proper testing methods you may well end up with something you do not want. Only a few millimeters misaligning the sphere can cause huge faults in the frequency domain.
One only need look at CU pix of M50 to see how the actual diaphragm lines up with the sphere.

The original capsule, the diaphragm is right up to the surface of the sphere; in the two later versions it's recessed a mm or so.

Looking at the scale, and frequencies involved, I wouldn't call what's shown in that graphic 'huge'.

Meant to post this from Josephson earlier.
Attached Thumbnails
omnis and pressure sphere attachements-m50-cap_mnt.jpg   omnis and pressure sphere attachements-josephson-making-apes.jpg   omnis and pressure sphere attachements-m150-capsule.jpg  
Old 31st May 2022 | Show parent
  #106
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🎧 10 years
One aspect I didn't realize was that some of the M50's HF rise was engineered into the tube circuit... it wasn't all generated via the sphere:

"The diaphragm was flush-mounted in a plastic sphere designed to allow the capsule to respond with some directional characteristics in the frequencies above 1 kHz while still maintaining an omni-directional characteristic at the lower frequencies. An additional +5 dB of high-frequency boost was added in the AC701K tube amplifier circuit, resulting in a favorable overall frequency response when the mic was used at a distance from its source"

...from Neumann M49/50 entry, in : https://www.coutant.org/12mics/index.html
Old 31st May 2022 | Show parent
  #107
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🎧 5 years
Quote:
Originally Posted by studer58 ➡️
One aspect I didn't realize was that some of the M50's HF rise was engineered into the tube circuit... it wasn't all generated via the sphere:

"The diaphragm was flush-mounted in a plastic sphere designed to allow the capsule to respond with some directional characteristics in the frequencies above 1 kHz while still maintaining an omni-directional characteristic at the lower frequencies. An additional +5 dB of high-frequency boost was added in the AC701K tube amplifier circuit, resulting in a favorable overall frequency response when the mic was used at a distance from its source"

...from Neumann M49/50 entry, in : https://www.coutant.org/12mics/index.html
I think that Coutant entry is just plain in error; I've never heard that claim made anywhere else. As you can probably deduce from my user name, I've been studying the M50 for a long time.

Everything I've ever read attributes the top end rise to the capsule itself; only the bump just below that is caused by the sphere. You can see in DPA's spec sheet for the 40mm APE that the bump doesn't go to the top (chart shows it used with their flat-response grill).

The capsule that was used for a long time in the M50 was the same as in the KM53a (KK53); same top rise. Other capsules were used over time - they all had a similar rise on top. Such a rise is inherent in small diaphragm omnis (wouldn't need to be added by the electronics), and as I understand it, Schoeps has to 'engineer it out' of their flat capsules.

Add the two together and you get the M50 response.
Attached Thumbnails
omnis and pressure sphere attachements-40mm.png   omnis and pressure sphere attachements-m50.png   omnis and pressure sphere attachements-response-capsule-m-50-top_km-53a-copy.png  
Old 31st May 2022 | Show parent
  #108
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🎧 10 years
There was related discussion in a much earlier thread on this mic:

Neumann M-50....vintage rising top end
Old 31st May 2022 | Show parent
  #109
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🎧 5 years
In the digital era, I've found the most useful mics for the 40mm APE are those with a top boost of only 2-3dB; closer to that of the APE: MK2H, Rode NT55, etc. - so you end up more of an even shelf on top (on-axis).
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