Old RCA w/ a modern twist.

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sr1200

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I have an RCA 74b that was re ribboned by AEA a few years ago. The output was horribly low before i had it done and only marginally better after. The magnets were barely magnets anymore in that there was very little “pull” from them. I tried a number of things like putting stronger magnets on the existing one(s), but nothing seemed to work. I then replaced the transformer thinking that I might be able to squeeze a little more headroom out of changing the ratio a little (got a cinemag 44 style tx) the signal got marginally better but was still (imo) unusable. So I decided to gut the mic (in true DIY fashion) and replace with something I could just drop in, so i wouldnt have to alter the chassis of the mic. I removed the old motor assembly and designed a motor that would utilize some dumb strong neodymium bar magnets (n52’s). I 3D printed the design, used some brass bars (screwed into/thru the motor assy) for the ribbon mounts and soldered a couple leads. The design of the motor keeps the magnets from clapping together. (No glue or anything like that). I used a wider ribbon than the original as i wanted a darker sound for this mic (I have 6 or 7 other ribbons in the locker already). I only have 2 micron silver, which is kind of hard to work with, but you still get decent high end even with a bigger/wider ribbon.

The result was fantastic! The mic is dead quiet and the output is actually greater than my sm7b! I may revisit it later on and go with a thinner ribbon design if I want more high end. All the original parts are stored away incase I want to sell it one day. (Likely my kids will have to deal w that after I’m gone, as I don't sell mics anymore)

The last pic is another design I did using the same idea of physically keeping the magnets separated. The entire insert is 3D printed with a mesh screen over the ribbon area to help stop wind damage as the windscreen on the head is only single or double ply of brass mesh and kind of wide spacing. This mic sounds like a slightly brighter version of my royer 121.

Just thought I’d share a bit of my insanity.
 

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Rusan

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Very nice work! And, a VERY cool looking old RCA. :cool:

I too recently drew up a monocoque ribbon motor design, with the magnets held in alignment by machined Delrin upper and lower frames only, and with a upper ribbon clamp that slides in a track for ease of tensioning. But, I haven't been able to build it yet.

I'd love to hear yours!
 

dmp

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Oct 28, 2009
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Nice DIY - and keeping the original guts is a good idea (because like it or not a non-working original RCA is probably worth more)
I may revisit it later on and go with a thinner ribbon design if I want more high end.
Isn't most of the frequency rolloff in the 74B due to the transformer? Are you running the cinemag in it now?
 

sr1200

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@shabtek AEA didnt mention anything about remagnetizing but they did mention it was a little weak at the time.

@dmp yes im running the cinemag trafo now. The original motor was slightly more open in the top end w the cinemag but just too low output. I wanted something darker for kicks and i kind of feel like the size and width of the ribbon makes more of a difference. My other design has a very clear top end and is less than half the width of the new RCA and shorter as well. I could try the original transformer and see if it gets darker or make a thinner ribbon version w the old trafo to balance it out.
 

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