History of Naiant microphones
The MSH-1
The development of the MSH-1 began as a result of Joel Cameron's Tape Op mic (note: PSW doesn't seem to allow deep linking anymore, or maybe they discontinued the Tape Op archives :( ). 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.
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The MSH-4 tube microphone
The
MSH-4 was, to my knowledge, the smallest tube microphone in the world
(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 values for R5 and
R6 may need to change to accomodate different capsules.
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, merely a way to use that tube
within the constraints given.
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Schematics
My schematics have been published not only in the Homerecording.com thread, but also on forums such as the DIY forum on prodigy-pro.com, and the Micbuilders forum on Yahoo!
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.
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The "Linkwitz" mod
I sometimes get questions about whether or not I modify the capsules I use for my microphones. Usually I don't. 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 already suffer.
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 mic input impedance 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 mic has low output impedance).
So if you use Linkwitz's mod, use his entire circuit! Or design your own circuit that accomplishes the same goal.
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The future of microphones
Microphones and preamplifiers are drastically inefficient devices in the modern age. But before we explore that thesis, let's review the development of microphones.
We can very crudely divide the world of microphones into dynamic and condenser microphones. Dynamic microphones are characterized by very low output (voltage), but good current drive capability (low output impedance). Condenser microphone capsules are the opposite: good output level, but extremely high output output impedance, such that condenser microphones require an onboard amplifier circuit to act as a current buffer.
The first solution was derived before the invention of the transistor: the tube condenser microphone. By its nature, an inefficient design. But when the first FET condenser microphones were invented, the power demand was small enough that a dedicated power supply was no longer required. Phantom power came into common use.
Early FET designs, such as the Neumann U87, used phantom power mainly as a source of the capsule bias voltage. The internal FET needed only a small amount of current, and an output transformer was required to yield the necessary low output impedance. The Naiant X-T and X-M/T microphones used a similar approach.
So for either FET condensers or dynamic microphones, a significant amount of voltage gain was required for any purpose, whether live sound reinforcement or magnetic tape recording.
Fast forward to 2011: many things have changed. Most condenser microphones are transformerless, electret condenser microphones of excellent quality do not require a high supply voltage, and digital recording is the most popular choice. In spite of these factors, there has been a trend towards increasing demand for phantom power supply current.
At the same time, portable and battery-powered devices are growing rapidly in popularity. Full-specification phantom power is extremely expensive from a portable perspective: a 9V battery can supply about 5Wh, and full-spec P48 at 10mA per microphone costs 1W for a pair of microphones--far too expensive for a portable power budget.
What's more troubling is that the vast majority of that power is wasted. When you draw 10mA from a standard P48 supply, that means 0.34W gets dissipated in the phantom supply resistors, and only 0.14W actually does any work inside the microphone.
Let's think about how much power is truly necessary. Many digital converter ICs run off of a +5V supply rail. Let's assume the load of that IC is 10K, that means the IC only needs a tiny 3mW to run full scale. And yet, our microphone could be using 160 times that much power!
But wait, we haven't accounted for the preamplifier yet. Most professional preamplifiers are designed to output a signal at +4dBu with over 20dB of headroom. The +4dBu is fine; that's pretty close to the maximum level our converter wants to see. But wait, aren't we supposed to leave digital headroom? Yes we are! So we don't really need more than +4dBu, do we? That extra unnecessary headroom creates wasted power in the preamplifier, and that too-hot signal is probably just padded down in the converter before it hits the ADC IC.
It's time for a new way forward, but one that still respects the need for backwards compatibility. It already exists! The P12 standard is far more appropriate for the modern portable digital world than the P48 standard. Yes, +12V is too low for externally biased condenser microphones, but high-efficiency DC converter technology is very cheap now, and electret capsules of high quality now exist in a range of sizes and polar patterns. Without the large loss of power to the supply resistors and microphone circuits, efficiency is massively improved.
Next, preamplifiers should be regarded as a technology of the past. Condenser microphones already possess increasingly sophisticated amplifiers; they should be designed to output a high sensitivity that always ensures that their maximum dynamic range can be captured without requiring external amplification. Converters should be designed with variable input attenuators to cope with loud sources, and should supply the required P12 phantom power. Dynamic microphones must also incorporate P12-powered amplifiers.
Naiant is ready for the future today--every Naiant U and X series microphone supports the P12 phantom power standard. Of course, Naiant microphones are also fully compatible with the P48 standard.
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