DIY Microphone Design
History of Naiant microphones
The development of the MSH-1 began as a result of Joel Cameron’s Tape Op mic. In fact, it was after I had mentioned on a few occasions that it wouldn’t be too difficult to build a phantom powering circuit into the XLR connector instead of building an external power supply, which is expensive and less practical. But the Tape Op circuit is ill suited for phantom power; in fact, phantom power could potentially destroy the capsule! (although Mark Fouxman’s work on electret capsules has shown that they are actually very difficult to damage!)
I don’t know the origin of the circuit from which the MSH-1 was derived, but Tomi Engdahl’s Microphone Powering page on epanorama.net is an excellent resource; based partly on Christopher Hick’s PZM modification page. I have read speculation that similar circuits were probably found in an old Panasonic application guide.
It was the combination of those two ideas that yielded the MSH-1. In response to a DIY thread about the Tape Op mic on Homerecording.com, I finally decided to have a go at doing what I had said was possible. In April 2006, I built the first MSH-1 prototype, and on May 31, 2006, I sold the first commercial MSH-1 on eBay.
The MSH-4 tube microphone
The MSH-4 was, to my knowledge, the smallest tube microphone made (send me a link if you’ve got one smaller!) It was one of only three phantom-powered tube microphones on the market; along with the Audio-Technica AT3060 (one of my favorites), and the MT Gefell UM900 (which was first, and is stunningly beautiful, but I can’t afford it!). Among that rarefied company, the MSH-4 was the only mic that was transformerless and did not use a DC-DC converter circuit.
The MSH-4 used a NOS Raytheon JAN 6418 tube, which just fit inside. This is a subminiature tube that requires between +10V and +30V plate supply and 10mA filament current.
The MSH-4 was retired in November 2007 after a production run of 200 microphones. For those wishing to build the circuit, here is the final MSH-4 schematic. Nearly any electret capsule can be used in the circuit; the value of R6 may need to change to accommodate different capsules. Note that the capsule can see up to +13V, so if the capsule selected is not so rated, a regulator circuit should be implemented to supply the capsule. The trimpot VR1 adjusts for variation in tube and FET; and can also be set to change the operating point and thus distortion profile of the circuit. Lower settings will increase sensitivity and reduce distortion, but a slightly higher setting can be more tolerant of not-quite-compliant phantom power supplies. Set critically, the circuit will run on slightly less than 9mA, leaving at least +17V at the plate. Try 90Ω as a starting point and reduce from there. The 6418 tube should be selected for low microphonics, and well isolated from vibration in the case. Audio-Technica mounted the 6418 tube in a brass block in the AT3060!
Please note that the MSH-4 circuit was designed with these very specific requirements: phantom power supply, transformerless, no DC-DC converter. Changing the circuit in any of those aspects will strongly suggest changes be made elsewhere in the circuit. The MSH-4 is by no means a universal solution for using the 6418 tube, merely a simple way to use that tube within the constraints given.
In addition to the schematics on the epanorama.net page linked above, here is a basic circuit similar to the early MSH-1O model. It will offer high SPL handling across a wide range of phantom power supplies, and can tolerate loads to 600Ω. If you are building point-to-point circuits, this circuit can be built inside an XLR connector. The FET selection is not critical.
The Schoeps circuit offers better efficiency and lower output impedance at the cost of higher parts count, which requires a PCB or very clever use of perfboard! The Schoeps circuit, or any circuit that uses the capsule FET as a phase splitter and thus stays fully balanced from that point, will offer better noise immunity as well.
The “Linkwitz Mod”
I sometimes get questions about whether or not I modify the capsules I use for my microphones. Usually I don’t; it’s easier to simply purchase three-terminal capsules, but for the 6mm capsules I don’t bother. The Linkwitz mod is perhaps the most misunderstood aspect of DIY mic construction. I don’t mean Linkwitz misunderstands it! I think his work is quite good. But many DIYers misunderstand it.
Have a look at the Linkwitz circuit–the most important features are the use of a source resistor and a following buffer circuit. The majority of the reduction in capsule distortion comes from those two features. However, many people understand the Linkwitz mod as the mere cutting of the trace from source to ground. That’s actually not necessary to achieve high SPL handling. Instead, as my schematics above show, it’s also possible to float the capsule to isolate the source from ground, and still keep the capsule happily unloaded with the use of a source resistor.
The trace-cut will add a few extra dB to maximum SPL handling above what a floating design with source resistor will give you, at the cost of sensitivity. It may also have some effect on noise and induced noise immunity, but honestly I haven’t done enough testing to complete describe those phenomena. However, if you aren’t using a balanced topology from the capsule FET, your noise immunity will suffer, with or without the modification.
But the moral of the story is that you can’t simply cut the trace and add a source resistor, and then connect that capsule directly to a typical 1.5KΩ input impedance microphone amplifier without suffering some penalty in distortion or sensitivity, or both! You need that buffer circuit so the capsule FET doesn’t see a heavy load (and also so your microphone has low output impedance).
So when you use Linkwitz’s modification, use his entire circuit! Or design your own circuit that accomplishes the same goal.