Continuing with the DIY tradition started with my MP573 Preamp, I decided to try my hand at building some microphone kits.  Microphone-Parts.COM makes all sorts of mod kits and separate parts for upgrading or building microphnes. For a little bit of an up-sell price, you can get hand-matched pairs of capsules so I thought this would be a good way to get a nice matched pair of mics. Over the last several months, I kept my eye out for cheap MXL990s to use as donor mics and I eventually found a working one for $30 and a broken one for $10 on reverb.com. I ordered them while in the Caribbean this winter and they were waiting for me when we got home in April.

Mic-parts makes rebuild kits for many low cost chinese mics and the MXL990 makes a good donor mic. The 990 mod kit gives you a brand new circuit board and all components. The big improvement to make with any of these mics is to put a nice capsule on them. Despite the appearance of the 990, it is actually NOT a large diaphragm mic. The stock capsule is sized more like a small condenser but sits inside a ring which makes it look bigger. They sound awful but that's why they're only $70 new!

They will get new/improved circuitry (with a hand-picked FET/bias resisor combination) as well as a matched pair of RK12 Capsules. The capsules are hand picked to be within 0.5dB across the whole frequency range.  The RK12 is the "akg-ish" capsule (as in C-12).  They also have RK47 and RK87 (you can guess which capsules they emulate).  The mics will also get an improved (and much prettier) head basket. 

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Above, the MXLs torn apart ready to get new guts. The circuit boards are the existing MXL boards. 

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Most people paint these black. I decided to go for the vintage Army Green look.

 

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The parts as recieved from mic-parts.com.  

Let's Start Building!

The Parts Kits are complete with everything you need and no missing parts. There are some extra parts when you are done, but that's because they supply you with some optional parts to alter the voicing of the microphone.

The easy part is the circuit board. They are high quality boards and easy to work on. I finished both boards in a couple hours.

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I worked on both of them together

Decisions...

The capsules I went with - the RK12s - have a natural high frequency boost. This emulates the behavior of the real C12 capsule. The same is true of the RK87 (and a real Neuman 87). Those capsules are designed to be used in circuits which will compensate for that high frequency emphasis. Where the cheap chinse mics all get it wrong, is that they put a brightly voiced capsule into a linear circuit which results in a very shrill over-hyped top-end. Then, put a poorly biased FET on top of that, and yuk!  Mic parts makes the RK47 capsule as a drop-in replacement for these mics. The RK47 (like a real Neuman 47 capsule) does not have the high frequency emphasis and sounds much better in a linear circuit.  I opted for the RK12 anyway, just to be different.

So... one of the options in this kit build is to dial in some de-emphasis by choosing a capacitor. Below from left to right rea all the choices in order of brightness. The one on the left would be linear with no roll-off. To the right the mic gets darker and darker. I chose door number two which gives a very gentle roll off (-1.5db @ 10K) which is a good compensation for the Rk12 capsule.

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The Voicing Capacitor Choices

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Completed boards test fit inside the housing.

The Hard Part

The tricky part is definitely mounting and wiring the capsules.  First of all, you're handling $400 worth of very delicate stuff and one slip of a screwdriver or other accident could ruin the whole day. So you have to be very careful.  One interesting thing, is that they have you wire the capsule, the FET and a resistor directly to the Polar Pattern switch terminals rather than on the PC board. Usually when you see stuff like that, it's because of a revision to the design, but this is as-intended.  It does make the final assembly a bit more tricky though.  Speaking of the polar switch, yes it has one - but you need to open the mic to change it.  That's one downside of the MXL990 as a donor mic - it has no room for external switches. Still, it's easy to open the mic if you want to switch between Omni and Cardioid patterns.

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RK12 capsules on the bench ready to mount

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Strong glasses and precision screwdrivers are helpful

The MXL990 has at least two variations out in the world and I have one of each. The primary difference is that the circuit board mounts upside down in one of them.  This made wiring the capsule particularly tricky in one of the mics.

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Number One is done!

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And the twin

Now, Do They Actually Work?

There is a calibration step you must do before you can really call them done. There is a trim pot to adjust the capsule voltage. Ideally, you want 60VDC. To adjust them, you need to plug them into a preamp with phamtom power and stick a meter probe on one of the diodes on the board (It would be really nice if there was a test post for this). I dialed them both into 59.9VDC and called it close enough.  I was encouraged while it was powered up that it didn't smoke and I could see some signal on the preamp. But I still hadn't actually heard them.

Since I spent all this money to get the matched pair of capsules, I wanted to see just how similar the two mics sounded. I set them up to record a single source (acoustic guitar) simultaneously and gave it a whirl. I ran them through two channels of my Universal Audio 4710d "Twinfinity" preamp. I dialed in full transistor (no tube) mode and didn't overdrive it at all to avoid coloring the mic as much as possible.

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The Test Fixture

So far so good. As of now, they have not been really put through the paces yet but initial impression is that they sound wonderful - and are very well matched. They have a nice hot output. I could imagine that being a bit of a limitaiton (remember, no room for switches like a pad switch). They definitely do not have a harsh over-hyped top end and I'm glad I chose the gentle de-emphasis. I would like to shoot them out with my 1990s AKG414BULS to compare. That is still to come.

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They look good on me