Nady Multi-transformer Mic Mod

[dgatwood]

I just finished a mod to one of my Nady RSM-2 mics that some of you might be interested in. The original plan was to replace the transformers with Lundahl LL2911 transformers. When I got in there, though, I decided to experiment a little more.


Step 1: Wiring the Lundahl transformers to the board

In this step, I needed to create a board that could comfortably mount in a relatively small space with the transformer hanging off of it. I started with a chunk of breadboard, drilled out the holes, then wired it on the back using copper tape. I used some cut-off ethernet cable strands (either 22 or 24 AWG solid conductor) to bridge across in an X shape because half the primary contacts were on the opposite side of the transformer.


Step 2: Staring at it

The old transformer was fastened on one side of a metal brace. The brace roughly divided the compartment into two halves, but they weren't quite equal in size. The old transformer was held on by a pair of screws through a printed circuit board that didn't really do anything other than provide four pairs of solder contacts. The transformer had wire leads that soldered to one contact on the board, and the mic wires and cable soldered to the opposite contact, and the board bridged between them. I quickly realized that this was wasting lots of space for no reason.

After removing the original transformer and board, a light went off in my head. If I left out that basically useless printed circuit board, I could use the space to hold my board and the Lundahl transformer on the smaller side of the frame member and fasten the original transformer on the other side. Thus, I could make both transformers available....


Step 3: Mounting stuff

First, I wired in the new board and got that working and tested. To conserve space, I fastened it on by using thick double stick foam tape between the rows of pins and sticking the other side to the frame member inside where the old transformer had been mounted. (I couldn't easily put screws in the original location and still make the board fit into the smaller of the two halves of the compartment.)


Step 4: Drilling and filing

The hardest step was filing a square hole in the pot metal bottom to hold the slide switch. I bought some tiny files at Home Depot and slide switches at the Rat Shack, and used a titanium-coated drill bit for the big hole. I drilled the small holes for screws with a brand new bit that was also some specifically coated bit, though I don't recall with what. Drilling in that stuff was really, really slow, but eventually it cut through. (I drilled from the inside out just in case it couldn't cut through the metal and just skimmed across the paint.)


Step 5: Wiring the switch

I used a DPST slide switch. I realized that I'd be screwing up the impedance of the ribbon if I just switched the secondary, and I'd be screwing up the output impedance if I just switched the primary, so I switched one end of each.


Step 6: Testing

Everything worked the first time.


Step 7: Reassembly and Customizing

The last step was to put all the pieces together, whereupon I realized I needed to file on the square hole a little more. No big deal. Finally, I had everything together and tested again, and added a label. On one end, it says "CH", the other end "DE". (I thought about doing TLDs and going with CN, but I figured "Why bother?")

I don't have photos of this one, but I plan to take some pictures of the second one as I mod it over the next few weeks (short attention span ).

Anyway, maybe next weekend I'll do some recording samples with the switch in both positions....

[kojdogg]

that's awesome man-- like shinybox L "on" and "off"

[demto]

Was there any improvement to the mic's. If so can you give us a detailed description
of what the improvements did for the mic's.

[dgatwood]

Okay. I've just finished the second one. Check out the photos.

[dgatwood]

Originally Posted by demto

Was there any improvement to the mic's. If so can you give us a detailed description
of what the improvements did for the mic's.

I can hear the difference even while singing into it through the headphones. It sounds a lot less... uh... "honky" I guess would be the word. Lower harmonic distortion in the transformer, I think.

In the samples on Shinybox's site, it sounded like the high end was a little more open, too, but I couldn't really tell that on my voice while singing. That part of the effect was too subtle to hear under those conditions.

I was honestly expecting little difference, but IMHO, the difference is huge. It's like night and day between the two switch positions (and yes, I'm sure that neither of them is wired backwards). All I can say is "wow".

I'll try to do some recording on Monday if I get around to it, and I'll post samples as soon as I have some samples to post.

[dgatwood]

The wiring diagram:



Note that the back of the breadboard has pieces of copper tape crudely stuck to it, and the pairs in each corner are solder bridged together and soldered to the nearest piece of copper.

[kojdogg]

again, that's real cool. the picture link didn't seem to work. will you post sound clips too?

[dgatwood]

Originally Posted by kojdogg

again, that's real cool. the picture link didn't seem to work. will you post sound clips too?

Sorry about that. I moved it just a minute ago. Should work now.

[skipwave]

Great work, and nice documentation!!

[dgatwood]

Originally Posted by skipwave

Great work, and nice documentation!!

Thanks. Aptly, I write (computer programming) documentation for a living....

BTW, just to correct something I said earlier... I almost didn't do the switch on the second one, but decided to do it anyway. I'm glad I did. The first mic, the mod made very little difference. The second one was the one where the difference in sound was dramatic. It's amazing how mismatched the two stock transformers were. Cheap Chinese crap, I suppose.