Mod Out Your SM57

If you're involved in music, you (more than likely) know what an SM57 is. And if you don't, you need to Google it immediately. Being such a popular microphone for both studio and live applications, they're easy to come by and also very cheap. People use them all the time on guitars, snare drums and other percussion, and in a pinch, they can work on many sources.

To add to the diversity of this microphone, there are multiple modifications available that can alter its sound and change its flavor. This post is meant as a resource of available DIY projects and links to already modded 57s.

Impedance Modification

I found this mod in Recording Magazine a few years back. Read the article that is linked below for all the details, but to make a long story short, most preamps are not loaded correctly for many ribbon (especially old ones) and dynamic microphones. They generally have an impedance around 1500-2500 ohms, suitable for most condensor mics, but if you check out the spec sheet for the SM57, it states that its "rated impedance is 150Ω (310Ω actual) for connection to microphone inputs rated low impedance." Simply, this incorrect loading does not adequately damp the diaphragm of the microphone, allowing it to move more freely than it should, thus causing high frequency ringing and intermodulation (read the article for more details).

One remedy to this problem is having a preamp with switchable impedances. However, the vast majority of them don't. The other solution is to build a small contraption that will lower the impedance of the preamp when used inline with the microphone. This is pretty simple to do if you're at all handy with a soldering iron, and it only costs around $10. All you need is two XLR connectors (a male and a female) and a short length of balanced cable OR an XLR barrel connector, and one resistor with a 1% tolerance per contraption. The resistor value coupled with the impedance of the preamp will dictate the resulting overall impedance. The article recommends a 698 ohm resistor, which will give you a total load of 500 ohms, ±10%, with most preamps, whose impedances range from 1500-2400 ohms. If you want to customize the total load based on the impedance of a specific preamp, see the box below (this is copied directly from the sidebar section of the linked article).

Click here to see how to custom size your own resistor

If you want to calculate a load resistor that’s tailored to your preamp/board’s actual resistance, it’s easy to do. The formula is:

1/Rg = 1/Zd – 1/Za

Where Rg is the resistor to be used in the Gizmo, Zd is the desired total load impedance, and Za is the actual load impedance of the input.

So, for example, if you want to create a load impedance of 500 ohms, as we did in the experiments, you would first look up the specified input impedance of your board. A Mackie board with XDR Pro preamps has an input impedance of 1300 ohms; plugging the numbers into the equation, we get:

1/Rg = 1/500 – 1/1300 = 0.002 – 0.000769 = 0.00123

Pushing the “1/x” button on the calculator, the answer is:

Rg = 812.5 ohms

The nearest value in the 1% tolerance series of resistors is 806 ohms, so that would be the one to use.

The build is very simple: you solder the connectors onto the cable as usual, then solder the resistor between pins 2 & 3 of the male end. Just make sure you do it between the correct pins or it will not work properly.

I did this mod back when I first read about it, and I can tell you firsthand that it definitely makes a difference and cleans up the high end quite a bit. I use 57s a lot on guitar amps, and this mod got rid of a lot of the high end trash that I always found especially irritating when recording loud guitars.

One thing to keep in mind is that you will lose a couple of dBs of gain from the 57, but unless you're recording something ultra-quiet (with a 57?), this shouldn't be a problem. Just worth noting.

Original article in Recording Magazine: http://www.recordingmag.com/resources/resourceDetail/330.html

Transformer(less) Mod

This mod is very popular, fairly easy to do, and actually changes the overall sound of the 57 quite a bit. I originally found it in a TapeOp article back in 2006, but it's been around since at least sometime in the late 70s. It's a simple process:

  1. Unscrew the top section of the mic that houses the capsule and cut the wires

  2. Unscrew the small flathead found toward the base of the body. This will allow the connector to come out, cut these wires as well

  3. The transformer is held in by lots of hot glue, so you need to heat up the body in order to loosen it. The original article recommended boiling the body in a pot of hot water, but it's far easier to heat it with a heat gun, hair dryer, or even by holding a soldering iron to the body

  4. Once it's hot enough, pull on the wires still attached to the transformer and it should come out pretty easily

  5. Wire up the capsule straight to the connector, the "+" to pin 2 and the "-" to pin 3

  6. DONE

This modification extends the low frequency response quite a bit, and flattens out the high-mids, so the overall response of the mic is much more even and less harsh. You do, however, lose 10-12 dB of gain, so it won't work as well on quiet instruments without lots of clean gain - for things like snares and guitars, though, it will be fine.

Personally, I like the transformerless on some things and notsomuch on others. Flattening the response definitely takes away a lot of what makes it "57ish", taming much of the aggressiveness that comes from the high-mid bump. I'm sometimes not as big of a fan of it on snares - it really depends on the particular drum and what kind of sound you're going for. It can work well on guitars amps, especially if there's a lot of mid gnarliness that needs taming. My advice would be to have some of each laying around. Also, if you click the link below, you can see pictures of the entire process from TapeOp Message Board user bayoucables. He rigged his removed transformer so it can be put back inline if he decides he likes that sound better on a particular source. Pretty smart.

If you want to take the next step with this modification, you can upgrade the transformer. Mercenary Audio sells just such a transformer that you can find here. The description says that it maintains a 57 type character while extending the high frequency response and bringing the overall sound closer to that of an SM-7. I have no experience with this modification, so I can't comment.

Bayoucables transformer wired to XLRs: http://www.bayoucables.com/altec/projects/sm57/sm57.htm

The 57 Family

There are several other mics that are related closely to the SM57. Again, I haven't used any of these personally, but I wanted to include a little info for anyone interested.

Granelli Audio Labs G5790

This is basically a regular old SM57 modified so that the capsule is at a 90 degree angle to the body. It seems that it's specifically made for snare micing applications where it would be difficult to squeeze a regular 57 in there without interfering with the hihat. I don't generally have too much trouble with this, but on occasions where snare micing does get tight, I really wish I had one of these laying around.

Shure SM77

The SM77 was produced by Shure for a while in the 80s. It is a transformerless design, and, although I've never used one, I've read many accounts where people describe it's sound as being similar to that of a transformerless 57. They look similar to the 57, but are smaller and have a rubberized sort of coating. Would be worth checking out if you ever happen to run across one.

Unidyne IIIs & IVs

There are various microphones in the Unidyne family that Shure has manufactured since the 70s. They still manufacture the 545SD, which has a slightly different frequency response pattern than that of a 57. You can check out Shure's website for more info.
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