Handheld electret project

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Chrisfromthepast

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I stumbled upon these donor bodies and they were way cheaper than any of the 603s clones that I’ve been playing with.
I just wanted to put it on more people’s radar before they disappear.

The electronics are completely useless, and the capsule rattles around a little bit, but its a fairly nice donor body.
I like that its easy to get apart, so prototyping is less painful.

the stock capsule isn’t terrible, and shoves in the end of a 1/2” brass coupling, so I got some use out of it afterall.
I replaced the capsule with a 165a, and the friction fit was perfect.
The capsule mount is milled out of aluminum, but if a smart person were handy with casting or 3d printing, this piece could be made of a less resonant material.
Stock, this mic has serious rf and handling noise issues. With the 165a and electronics replacement, I found handling noise to be manageable.

the first one, I built with a km84 type circuit (omitting capsule polarization stuff) with nte 10:3.
It still had some rf issues until I added 100pf from each xlr signal leg to ground, and was still suffering from some of the brightness of the capsule and head basket, but overall is a delightful cheap project.
I also tried this circuit, which is a little closer to the circuit inside of the kms 105 this donor body is emulating:

this circuit is all about the op amp, which seems like a solid performer with low current demands.
Seems like a cop out compared to transistor circuits, but dang was it easy to troubleshoot!

Here is the github with gerbers and kicad files:
The pcb will also fit into mxl603s and similar, and 3/4” brass tubing
 
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Chrisfromthepast

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Mar 5, 2018
Messages
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kingkorg

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I have two of those with a Neumann logo. Take care of the resonant chamber behind the capsule, it is creating acoustic mess. I damped it with some foam.

Mine came with tiny bm800 board soldered to xlr connector. It can be quite usable if modded.
 

Chrisfromthepast

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Mar 5, 2018
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image.jpg
Thats nice that yours came with a more thorough logo! I wonder how many iterations there are. Price has already gone up since I bought a pair.
This is what is inside of the knock off.
its output is super hot, and whenever I tested it in a live performance environment with industry standard equipment, the RF interference rendered the mic useless.
Im not going to pull apart our real deal 105, and its probably best for the forum if we don’t dissect a current production model.
I was mostly interested in the mechanical differences in capsule mount, and similarities of headbasket.
Also pictured is the original capsule, shoved in a new mount awaiting a new block of project time.
The 165a capsule is definitely smoother though.
 
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Chrisfromthepast

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Mar 5, 2018
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I went ahead and tried out the silver version so that I could tell my mic pairs apart more easily.
The finish on these is not nearly as nice as the black ones I got earlier. I will sand them eventually.
Other than the finish differences, the rest of the mic is about the same.
I still think its a pretty good value for a donor body in a form factor we don’t see as often. #Zramo104silver.
Any old circuit board that would fit in the 603s type body will work in these donor bodies, and I’ve been able to turn various parts into working SDC mics faster than ever with this platform. As a vocal mic though, you really need more protection from plosives. The real Neumann version has a plastic mesh that helps with this.
 

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kingkorg

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I actually made a hole through thin mesh sitting under the thick outer one, and squeezed in some foam in the cavity between the two. It helped a lot.
 

Chrisfromthepast

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Joined
Mar 5, 2018
Messages
45
I actually made a hole through thin mesh sitting under the thick outer one, and squeezed in some foam in the cavity between the two. It helped a lot.
Great tip!
I haven’t done a scientific test, but it seems to have fixed the plosive problem and it looks more professional to boot.
 

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