DIY SM7b microphone?

There are some things that are worth making for yourself, other things that are over-rated and it’s easy enough to buy a cheaper version and upgrade it, and there are a few that, after exploring the options, you decide it’s just best to bite the bullet and pay for the quality.

Perhaps surprisingly, at least for me the third category seems to be populated mostly with simple mechanical stuff that generally requires either elaborate machinery and/or major fabrication skill. Often it’s not that it’s impossible to do, it’s simply not worth it to re-invent the wheel. I’m reluctantly leaning that way with the Rode PSA1 “scissor” microphone boom- there seems to be little available between the $15-20 Chinese versions and the $100 or $120 versions from Rode or Heil, other than some ripoff dealers trying to sell the $15-20 Chinese things for much more. The Chinese ones are typically small, lightweight, flimsy (soft steel), cheap (I’ve got one that actually cost me about $7 shipped) and not easily upgradable in any very worthwhile sense. Fabricating one from scratch would require some machinery and a lot of work, and what I’ve seen of butchering Ikea desk lamps to make one hasn’t been impressive.

While my instinct tells me that the Rode should probably sell for maybe $50 tops, and that $100 (even $89 or so used) is just too much, it may turn out to be the most reasonable alternative. Sure looks like there’s an open market niche in the $30-$70 range that somebody should be exploiting, but if nobody is then I’ll have to deal with what’s offered. I want a good boom, not a new hobby making them.

The other end of the question, the easily upgradable, seems to include these low-and-middle range condenser microphones. For instance, I bought a Chinese CAD GLX2200 on sale (might have been a price mistake) for $25 shipped (it normally goes for more than $55). It’s not terrible… but it’s also meant to look like a Neumann, and a little googling reveals that it’s considered extremely upgradable, capsule and electronics, with parts made for people trying to make less-expensive copies of Neumann microphones (which cost in the thousands), so… lots of upgrade stuff available, capsules and circuit board kits. Seems silly putting hundreds of bucks into a $25 microphone, but you can replace everything inside with much, much better, you only need to be able to solder and some simple mechanical skill. I may do just that over time, as I have time and a few bucks and get bored.

I’m on the fence, though, with regard to where on the potential DIY spectrum the Shure SM7b microphone is.

Maybe this dynamic microphone classic is worth the $400 price tag, “Made in Mexico” and all. Maybe not. It has certainly proven itself many times over. Thing is, I’ve got no business owning one even at a quarter of that price. I’ve got negligible microphone experience in recent decades (when I last sang on stage the Electro-Voice EV664 was the mic of choice- ancient history). There’s no way I can remotely justify spending $400 (not to mention $150 for a Cloudlifter CL1, which seems to be the “trick” setup) on this thing.

Of course, I still want one.

So, how feasible is it to try to duplicate one?

After some investigation, it might not be too hard.

There’s a lot on the web about the cartridge being virtually identical to/based on that of the Shure SM57 or SM58 (both usually $100 microphones, got a couple of the latter). Umm… maybe, in some technical ways that aren’t evident at a glance. They sure don’t look much alike.

A key question is whether the diaphragm of the SM7b really is different than that of the SM57/58. Maybe. The SM7 certainly does bias more toward the bass, but a lot of that might be the increased size of the airspace (plenum) behind the cartridge. It’s pretty tiny, just the unoccupied space in a “stage” microphone handle in the 57/58, and by contrast pretty huge in the SM7, that whole big can. That can’t be an accident. Dynamic microphones share some characteristics with speakers, basically an enclosed diaphragm and a voice coil, and if you think if it in those terms it’s most like an “acoustic suspension” speaker in that the enclosed air space is not ported. Does the amount of space matter in an acoustic suspension speaker? Yeah, it’s a huge factor.

I really wish I could find some dimensions somewhere on the web. Even if Shure doesn’t want to publish too many specs, somebody has to have measured that can. On the other hand, I don’t really want to fabricate a shock mount/holder for the whole thing from scratch, and I have a couple of 45mm shock mounts of the style usually used with condenser microphones not being used for anything. 45mm is a lot like 1 3/4 inches, so I suspect that’s the diameter I’ll end up using for the main body part if I do this. Length is an open question. I toyed momentarily with the thought of fabricating some of it from PVC pipe (and painting it black), which is probably acoustically closer to “dead” than a metal can, but I suspect that EM shielding is a big factor in this model, and shielding non-conductive materials is a pain- I’d probably end up using aluminum tubing, or possibly butchering an aluminum bottle of some sort, maybe a Sigg.

I’d also like to have the dimensions of the other end, the grill. It would be nice to just be able to use the same windscreens that the SM7 does.

If the diaphragm on the SM7 is actually different than that on the SM57/58, or if it’s worthwhile to include the boost/cut circuits, it turns out that the cartrdige for the SM7 line of microphones is available as a replacement part. I typically see it for special order at $160, but at least one listing I’ve seen seems to have it in-stock at $140.

It took a little digging to get an idea of how much of the electronics are within the capsule and how much are outside of it, that’s not shown in the schematic. From looking at photos it appears that there’s really not all that much going on outside of the cartridge. Shure states that the diaphragm of the SM7 series was altered to enhance the low-end response. What I can say is that that the SM7 cartridge is larger, in the sense of longer, and apparently has four coils inside, at least two of which are not in the 57/58, they’re for the mid-range boost and the low-end cut switch. The electronics are basically a couple of switches (DPST, it looks like, but bridged to work as SPST, perhaps for reliability, perhaps for reduced contact resistance dealing with the tiny voltages), a .22mF capacitor (I’ve got some nice polymer film ones from my electric guitar experimentation, maybe even a paper-in-oil or two) and a male XLR connector that could be mounted to the back panel or could be a pig-tail coming out of the can. There is a LOT of shielding, and with analog signals this weak it’s all going to be critical- that’s clearly a main function of the perforated tube/grill, the can enclosure, possibly even the often-discarded switch cover/blanking plate.

All in all, some aluminum tubing, some bits and pieces, some soldering, maybe some tricks like using insulating foam and/or spray-can pickup bed lining for acoustic damping, willingness to go through some trial and error, it seems very feasible to build up some sort of Franken-SM7 from the real SM7 capsule. Whether or not I’ll actually do it someday is anyone’s guess. I like to work out details of a mental model first. Sometimes that takes months, sometimes it never happens.

I’m sort of surprised though that nobody seems to have done it. I’ve run across forum conversations where the possibility has been discussed, but I’ve never seen it said that anyone actually fabricated one.

– Robert the Wombat