The Shure SM81 was an amazing breakthrough when it was introduced in 1978. It was the first condenser microphone that was both rugged enough to survive in stage applications and also had very good sound quality.
The electret connection
To explain why this was so significant, let me go back a bit. Early condenser mics involved a light diaphragm of metal or metallized plastic, separated from a metal backplate. An electrical charge provided by an external power supply charged up the space between them as a capacitor, and the change in capacitance would be measured by a high-impedance input amplifier. This works very well, but it is extremely delicate and also extremely sensitive to leakage due to humidity, and it requires an external high-voltage supply to charge up the capsule.
To get around this, some folks attempted building capsules that used an electret as the diaphragm. Electret materials have a permanent electrical charge to them, but most of the better electrets are ceramic materials that do not make good diaphragms. This allows construction of a very rugged microphone, but not one that sounds very good. As a result, electret mics in the 1970s developed a very bad reputation as being usable for communications applications only, not for recording or high-grade sound reinforcement.
Now, some folks at Sony figured out that the backplate rather than the diaphragm could be replaced with the electret material, and the backplate would carry the charge. This allowed a very light diaphragm to be used, and some of the Sony microphones from the seventies using back-electrets sounded excellent at the time. Unfortunately they were rather sensitive. The diaphragm tensioning did not stay stable when the mics were knocked around, and the electret charge leaked away over the course of years. Today few of these microphones still work.
The crew at Shure spent a lot of effort in designing a back-electret capsule that was extremely rugged, could take extreme shock and wind, and had a stable electret element. Today, most of the original SM81 capsules out there still meet original specifications, and that is saying an awful lot!
Now, there is still some stigma attached to electret capsules. Despite some very high-grade electret types from the likes of DPA, most manufacturers don’t like to advertise the fact that their mics are electrets. But in fact, the electret capsule buys you an awful lot in terms of ruggedness and reliability if it’s well built, and it can sound excellent.
It’s hard to improve on the SM81, if you think about the original design constraints on the microphone. But a friend of mine with a European microphone manufacturer has done some study of the SM81 capsule and remarked to me that the SM81 top end might be a lot more accurate if the capsule windscreen plate were removed. This makes the mic more sensitive to air blasts, but I find that it makes a huge improvement in the top end response.
If you are willing to risk some loss in ruggedness and to do some careful surgery, I think this is a worthwhile thing to do. But it requires very careful work with some small parts, and if you aren’t used to working on small detailed items, I suggest passing. If you screw up your mic, I won’t take any responsibility.
Then again, I think this is no more delicate than, say, dis- and reassembling a model airplane engine. You need to be careful not to touch any of the parts and get fingerprint grease on them, but other than that it’s not too bad.
You will need a small spanner wrench (Edmund Scientific sells one, part # P36–413 from www.edmundoptics.com), a jeweler’s screwdriver, a small pair of needle-nose pliers or hemostat, and some dental picks. You will also need a very clean place to work, with good lighting.
The problem with detailing any changes to the SM81 electronics is that they have changed considerably over the quarter of a century that the microphone has been made. The Bevan paper cited below has a schematic of the original design, but if you open a current SM81 you will find the schematic is very different.
For example, a FET-IC is being used on the front end in place of the original (now unavailable) FET and resistor. Even making simple front-end changes becomes nontrivial because there are so many different front ends. I can’t give step-by step directions for any of this, so leave the electronics alone unless you’re willing to do the engineering work for your specific board variant. Most of the original electronics are pretty good anyway.
I have looked at replacing the ceramic bypass capacitor on the 10 dB pad with a higher-grade one, but everything I tried degraded the sound in some way. I basically gave up on the idea; I don’t think it is possible to build a bypass capacitor pad that doesn’t degrade the sound somewhat, and all I can suggest is to try and avoid using it. That’s not much help, though.
Step one: Remove the paper label on the back of the capsule and set it aside. You can do this with a pick or small screwdriver. This will expose the retaining ring that holds the capsule together. Mark the threads with a Sharpie so that you know how far in the retaining ring is screwed.
Step two: Remove the retaining ring with the spanner wrench. When you do this, the rear insulator and the spring contact will come right out.
Step three: Remove that rear insulator and spring contact. It will lift right out with a pair of needle-nose pliers. Just don’t touch any of the surfaces of the insulator. With it out, you should be able to look down into the capsule and see the teflon ring and gold contact that make up the backplate support.
Step four: Invert the capsule and drop the backplate support, the electret, and the spacer out on the bench. Use a dental pick to put them to the side and do not touch them. Notice the direction that they all went in. The diaphragm itself will remain in the capsule barrel after you do this. Tap the barrel hard against the bench until the diaphragm falls out.
The diaphragm consists of a very thin metallized mylar sheet wrapped around a metal ring. This is inserted into place with the ring facing toward the front of the mic, so the thin spacer is the only thing separating it from the electret.
Notice the disassembly diagram. Lay your parts out on the bench like this as an aid to help you reassemble the cartridge later.
Step five: With the diaphragm out, you should be able to look down the barrel and see the plastic windscreen. Use needle-nosed pliers to take it out. You will have to do some tugging and ripping, and it will almost certainly not come out in one piece. You may have to use dental picks to pry the outer plastic ring out, since it is glued into place. In the process, you will also be removing a thin layer of nylon mesh that is cemented to the windscreen. Make sure you get it all out, and that there are no stray pieces of plastic or nylon floating around in there. It is very easy to have a single nylon strand left in the barrel, and that is enough to prevent the mic from working at all.
Step six: Put the diaphragm back into the barrel. It will slide in nicely. Then put the spacer in and tap around the edges very lightly with a dental pick to make sure that it’s in place properly.
Step seven: Drop the electret into place and make sure it seats well. Put the backplate support in over top of it and make sure it seats.
Step eight: Seat the spring contact and rear insulator in together, and screw the retaining ring down slightly. Make sure the spring contact is making solid contact with the backplate support, by wiggling the contact pin around with a dental pick. Slowly screw the retaining ring down until it reveals the mark that you made in the first step.
Do not screw the retaining ring down any farther than it was originally! It will go in much farther, and you will damage the spring and possibly crack the electret. Too much tension is bad!
Put the paper label back on, and test out the microphone.
This is a true microphone modification. It doesn’t necessarily improve the mic, but it does changes its timbre, making it more suitable for studio applications where ruggedness isn’t as important and sound quality is more important. It requires careful hands but it’s not something that takes special equipment. Hopefully you’ll find your SM81’s new sound as appealing and as handy for certain studio applications as I have.
There is an excellent article which is not only a great description of the design goals and the design constraints going into the SM81, but a very good description of some of the design process of microphones in general. I strongly recommend reading it if you are even a little bit interested in mics:
Design of a Studio-Quality Condenser Microphone Using Electret Technology, Bevan, W.R., Schulein, R.B., Seeler, C.E. Journal of the Audio Engineering Society, December 1978. Vol 26, No 12.