What do you get when you combine the guts of a Porsche 911 with the body of a VW Beetle? You get a high-performance sports car that does not look like one. This is analogous to what is offered by Microphone Parts. They sell upgrade kits for inexpensive mics. I tried two of their modified MXL 990 condenser mics, and built a third one myself. One of the mics they sent was modified with an RK-12 capsule, which is their version of an AKG CK 12, the capsule in the venerable C 12 tube mic. The other was modified with an RK-47 capsule, which is their version of the Neumann K 47, the capsule in the revered U 47. Both mics utilized the Microphone Parts upgraded circuit, which completely replaces the circuit board in the MXL. Since the RK-47 is a dual-diaphragm capsule, the Microphone Parts circuit provides an omni/cardioid switch on the board - more on this later. Like the U 47, the Microphone Parts circuit does not support a figure-8 pattern.
Before building the kit, I used these two mics to record a female duo. I used the RK-47 in cardioid mode on the lead voice, and the RK-12 on the supporting voice. The RK-47 needed a pop filter, because the two inner screens were removed. This gives the mic an impressive look, as the dual-diaphragm capsule is readily visible. I always use pop filters anyway to keep singers from getting too close to the mics. The results surprised and amazed me. On playback of the first takes, I was stopped in my tracks, staring at the speakers. On listening to the lead vocal, my first unedited thoughts were, "I must have this," and "this sounds expensive." The vocal floated above the instruments without sticking out. The sound was a glossy, electric, slightly larger than life, "sounds like a record" tone that I associate with expensive pop productions. Not natural-sounding, but supernatural-sounding - and freaking beautiful. The RK-12 was a great performer as well, but to me, it was more realistic and less exciting than the RK-47.
For the initial takes, we recorded both voices at once, facing each other, and recorded electric guitar and bass through DI. There was considerable bleed, but it was good bleed and worked well in the final mix. Then we did acoustic guitar tracks with the RK-47, which also turned out great. We decided to do old-school backing vocals, with both singers simultaneously singing into the RK-47 in omni mode. Gorgeous. Since the polar pattern switch is on the circuit board, to change patterns, one must open the mic by first unscrewing the collar by the XLR connector, then unscrewing the mic body. The MXL bodies are not machined with great precision, so this was always a squeaky, awkward affair. I quickly learned that you must also remember the setting - or you have to do a soundcheck, or open the mic again, the next time you use it. I'm not sure what the best solution is - perhaps sticky notes on the mic before you put it away? Still, I applaud that this did not stop them from including the switch.
Already being sold on the mics, my next task was to see how easy they are to build. I was sent the 990B, a multilayer circuit board with solder pads on both sides. This sandwiched style of board improves the routing of the ground plane as compared to the 990A, but a downside of this is you can't see the traces. The build manual, which includes color photographs, is excellent and is probably the best DIY instruction set I've seen. Still, I don't recommend this as someone's first soldering project, mostly because you can't afford to make a single mistake, unless you have a de-soldering station. Warnings in bold warn the builder against soldering wick or a solder sucker, which is all I have. Microphone Parts told me that customers have assembled these boards with inexpensive soldering pencils that plug directly into an AC outlet, but personally, I can't imagine doing this without a temperature-controlled soldering station equipped with a precision tip. I went very slowly and carefully, sorting the parts first before beginning the build. The parts are high- quality, including WIMA capacitors and Vishay Dale resistors. The resistors do not use the standard striping system; numeric values are printed on the parts themselves, which I found challenging to read. I often had to use the magnifying glass on my "helping hands" clamp to identify them. The labeling on one of the diode sets was so obscured that I could only determine the value by process of elimination - it's a good thing I could read the other values. The manual says the circuit board takes about an hour to complete; it took me about two, though I think I'd be faster on a second attempt. Many of the components are tightly spaced, and I was being very careful not to connect solder pads. In one case, I thought I had mistakenly shorted two components, but it turned out that they were connected in the circuit; because of the hidden traces, there was no way to confirm this until I finished the mic. During one part of the assembly, a few components are left unsoldered on one end, to be attached to the polar-pattern switch later. I found myself wishing I had made the switch connections first, before soldering to the board, because some fine, jewelry-style wire-twisting with needle-nosed pliers was required before soldering to the switch, and breaking a leg of a transistor would have been tragic. Fortunately, Microphone Parts will replace user-broken components for a flat fee of $5, but since I don't have a de- soldering station, and the transistor was already soldered on two legs, I would have had to send the whole board back. The kit comes with extra capacitors to use for optional filtering, but these were not recommended for the RK-47. Once the board was done, it was time to remove the MXL board. I almost stripped one of the screws but eventually got it out.
The capsule comes in a plastic container. It was difficult to remove. Tips on the website say to tap it against a table, but mine wasn't budging. I resorted to what the website said in bold not to do: I used a screwdriver. It was nerve- racking, but I eventually eased it out undamaged. When I did, a small piece of un-shrunk shrink tube came out. This was what was keeping it firmly in its container, as mentioned on the website. If it had been a little longer, I could have used needle-nosed pliers to pull the tube out first. Attaching the capsule to the base was also nerve- racking. One must hold the capsule without dropping it or touching the diaphragms, then thread some very small screws into it, along with a terminating wire with a small eyelet. I found myself wishing for some kind of plastic cover that could protect the capsule during installation. Also, I would have appreciated more hints on how to position the circuit board when attaching the capsule wires. You're instructed to make the capsule wires "as short as possible," but it was not clear how to do this. At this point, you can't actually attach the circuit board to the mic body, as that requires the screen. So it was awkward, but I eventually got the board to sit partway in the body while attaching the wires. Fine-tuning the polarization bias of the capsule backplate required many turns of a small potentiometer on the board, and then checking voltage at a particular point in the circuit, while the mic is phantom powered. If I had been instructed to leave the component lead at this connection intact, I'd have had something for an alligator clip to grab. Every time I touched a probe to this spot, it pegged the VU meter on the preamp that was supplying phantom. The manual explains, the more voltage to the capsule, the greater the sensitivity, until there is so much voltage that the diaphragms collapse. (The Microphone Parts circuits won't allow you to raise the voltage enough to make this mistake.) Less voltage means less sensitivity, but higher headroom. So one benefit of building these mics is you will know how to adjust the backplate voltage for different scenarios. This makes me wonder why we don't see more commercial mics with adjustable backplate voltages.
In the end, the mic worked - and worked great. From opening the box, to having a completed mic, I spent about four hours total. I opted not to remove the two inner screens. It was not clear on how to do this neatly, and they looked well glued. This gave me a chance to compare the fully-screened version with the open version. My build sounded slightly pinched by comparison, and the screens cause some resonance in the highs according to my measurements. Speaking of measurements, mine seemed to corroborate everything Microphone Parts reports about frequency response and noise; the self-noise of these mics is astonishingly low. I don't have a practical problem with the noise floor in the original MXL, but the Microphone Parts mods blow this away. For both the MXL and the RK-47, using the same mic preamp channel, I set levels with a test signal, then moved each mic to a silent room, covered it, and recorded. The results showed a remarkable lack of hiss from the RK-47. There was some popping down there, but so low that it would have been completely obscured by the hiss in the MXL.
Total outlay for the mic I built would be around $320 - including the purchase price of a new MXL 990 - plus about four hours of my time. Is there a commercial equivalent at this price? Not that I know of. Plus, even after you pull them, the MXL mic parts still work. The MXL capsule is covered with a screen (and looks suspiciously like the Monoprice interchangeable capsules [Tape Op #98] I recently reviewed), so you can just re-use it as is in another project. Heck, you could just cover the circuit board with epoxy and glue the capsule to a length of pipe - or whatever. There's a lot of information on the website that I did not cover. You'll want to make sure you have the required tools before ordering. You can check the tutorials. You can get matched capsules and circuits. Advanced shockmounts are even available for purchase. It's clear that the people at Microphone Parts really want you to get the most out of your DIY experience, and if you've mod'ed mics or assembled a preamp kit in the past, you would have a pretty easy time with the Microphone Parts kits. Despite some of the frustrations I describe, the whole experience was hugely educational and rewarding - and I'd do it again and again. There's at least one matched pair of RK-47s in my near future. (RK-47 & RK-12 capsules $109 each; 990B PCB Kit $129; www.microphone-parts.com)