Hot Rod 57

Modifying stock sm57 microphones for fun and sonics.

I've read that older 57s sound better because the quality of the transformers was better. Then my friend Dave said to rip out the transformer all together! So in the summer of 2007 I decided to try both.  Up first: the transformer replacement.

The good folks at Mercenary Audio sold me some T58 transformers ($75 each) designed specifically to replace the ones that come in stock 57s and 58s. First I unscrewed the back portion of the mic and de-soldered the contacts from the stock transformer to the microphone element. Then I released the XLR connector at the base and de-soldered it.

The next part was tricky... I had to get the old transformer out of the body, which was held in place by some sort of rubbery glue compound. The folks at Mercenary gave me a hot tip: put the body of the mic (not the element) in a toaster oven for 3-5 minutes. Despite his warnings I still managed to burn myself. And some of the glue spilled onto the metal tray (it's not going to come off, rendering it unsuitable for use cooking food). But it did break free the old transformer.

Next I needed to insert the new transformer and attach it inside so that it wouldn't bounce around. A dot of double stick foam seemed to work pretty well. Now I have to admit that this doesn't form the same acoustical cavity formed by that gooey glue. So the new transformer may not be the only significant change. But since I don't seem to have a large supply of industrial glue or the manufacturing setup to apply it without trashing other parts of the mic in the process, I decided the sticky foam dot would suffice.

I used a solder sucker to clean off the residue from the previous wiring. Then I tinned with some good Kester solder (yes, cheap solder sounds bad). I attached the XLR to the transformer.  Then I screwed the XLR connector into the base and attached the other end of the transformer to the mic element.

I screwed the body back onto the mic. Except for a little glue residue on the outside of the mic, this hot rod doesn't look any different from a stock 57. So I added a personal touch, a red enamel "T" for Transformer to help differentiate this mic.

Up next: the sawed-off 57 (no transformer). This turned out to be more difficult than I expected. Removing the back portion of the mic was easy enough, but then I went to attach the chassis of the mic to the drain wire and found out that the chassis wouldn't take solder. So I left the project and came back to it the next day. I decided to try another approach -- physically attach a terminal to the chassis and solder to the terminal. I took the chassis out into the garage to find out how difficult it would be to drill a hole in it. Piece of cake! The metal was soft enough to accept a standard wood bit in the drill. So far so good.

I quickly figured out it was going to be easier if I soldered the terminal to the wire before I attached it.  Then I had two choices for mounting: inside or outside.  In the second pic, see that black rubber shock mount on the right?  It fit much better inside when I screwed the terminal to the chassis with the bulk of the assembly outside the body.

I purposely cut the drain wire short so that it would bare any stress instead of the +/- connections. Having cleaned off the old solder and tinned the ternimals to the mic element, I finished the soldering by connecting the +/- wires.  Then I dropped the sawed-off into a SABRA-SOM SSM-1 shockmount.  I'd like something smaller, but that's a different project!

My drummer friend Austin brought his kit into the studio as a favor to me. The hot rod 57 sounded noticeably  better than the stock 57. And even in the somewhat bulky shockmount, the sawed-off 57 fit under the high hat better than the other versions. The sawed-off had 15dB less output than the other two, which was no problem on snare. And it sounded really great... significantly better than the stock 57 and the hot rod 57. A pleasant surprise. But I also noticed a lot more sound getting in the sawed-off from off axis.  Perhaps the normal frequency limitation of a stock 57 is also a key design feature for the cardioid pattern. It also seems very likely that removing the rear chassis of the mic exposes the back side of the element to sound that wouldn't otherwise otherwise get in.

About a month later, another friend of mine let me put up a bunch of mics while he sang and played acoustic guitar.  This is when the limitation of the sawed-off 57 became obvious. That 15dB loss without the transformer was an absolute deal breaker. I will want to use the sawed-off on loud sources like snare and electric guitar cabs, but the hot rod 57 will be needed for sources that have moderate volume.

Some other fun links for fans of the sm57

sm57 dropped 100ft inside cars for crash sound design over 40 times! Buzz by Chris Killoran. Full sound design article.

Bend your 57 90 degrees with the Granelli Audio Labs G5790.

Audio Wiki front page

Randy Coppinger - who I am and other stuff I'm doing.