Nakamichi CM-300 microphone History and Modifications

February 2015: CP-4s out of the box for the first time since about 1993.

When I worked at an audio salon in the 1990s that sold Nakamichi I remember people would come into the store simply because they were driving by and and saw "Nakamichi" on our sign, the name was that powerful. Legendary cassette decks, preamps, amps, Nak was considered consumer high-end back then. So when a friend who was a taper of Grateful Dead shows encouraged me to buy a set of Nak mics it made perfect sense.

The Nak CM-300 was sought-after mic of the period. Instead of a single pattern, the capsule on the tip was interchangable.You could use a CP-1 cardioid, CP-2 omni, CP-3 pinpoint omni, or CP-4 super-directional shotgun. The Nakamichi literature of the period recommended a three-mic setup: two on the source with a third farther back in the room to capture ambiance to be combined with their MX-100 mixer. Incidentally, the CM-300 and physically identical TEAC ME-120 and Tascam PE-120 are said to have all been manufactured by the Japanese company Primo. I decided to buy a pair of CM-300s which came stock with CP-1, CP-2 capsules and an optional pair of CP-4 shotgun attachments. My invoice lists the mics as CM-300S. Wonder what the "S" denoted?.

Since 1993 I used them a few times to record myself on acoustic guitar with the CP-1 capsule but that was about it. Recently I decided to dig everything out of storage including the CP-4s which had never been out of the box. The wind socks were distintegrating but the CP-4s were pristine.

I started doing some reading on how to use a shotgun mic. I was surprised to read that they are not meant for sources beyond several feet, do not provide any "zoom in" ability to capture distant subjects and aren't meant for music. Instead, they are designed to reject surrounding noise to capture dialogue in interviews or in a studio. Wow! I had always assumed that a shotgun mic was the correct choice for recording bands from the audience since so many tapers used them. I then thought back to the bootlegs I have heard using shotguns and remembered how they were hollow and boomy. No offense to those who spent so much time taping show using them, but I have to think they would have sounded more musical using a cardioid capsule.

THE EXTINCT BATTERY
As CM-300 owners we all know the original battery is no longer made. The modern replacement used to be an Eveready 206 but I haven't seeen those available since 2000. The modern solution is to stack small value cell batteries to achieve 9V but one caveat is that you should always use the same brand and voltage (or multiples of). The reason is that if you use different voltages (ex: 5v and 4v ) the mismatch can cause minor current flow and drain the batteries faster. Different brands can have different resistances which can also cause a drain. So for best results build a stack using identical voltages from the same brand.

For my CM-300s I used three "CR-1/3N" batteries. These 3V lithium-ion cell batteries are far superior to the old mercury/lead acid types and together produce about 9.3V which is what this mic is spec'd for. The batteries' OD is smaller than the mic's so I went to the hardware store and bought some clear flexible tubing the ID of the mic. I wrapped the batteries with a few turns of electrical tape to fit the inside of the tube. I used a spaver and spring between them to pass the voltage. So this method looks strange and isn't elegant but it worked for me.

CIRCUIT MODS
I wanted to retain the stock functionality and decided replacing the 30+ year old parts on the circuit board was the best plan. On opening it up I noticed something strange: the part values were different from what was printed in the owners manual and even what was printed on the PCB board inside(!) The 100pf cap was actually .068uf and the 47uf coupling cap was a 10uf. Not unexpected was that the 10uf coupling cap was a tantalum type. Tantalums are classed as electrolytics and have been used for decades as input caps in mics because of their small size. Photos of some modern Neumann mics still show tantalums in places. However by modern standards they are long obsolete and widely considered to produce poor sonics. Tantalum is also a conflict mineral which is unfortunate. You really can't blame Nak as back in 1990 a tantalum would have been the only cap that would fit. My 21st century opinion is there is no valid reason to be using a tantalum in an audio circuit as critical as a microphone.

I changed the original carbon 5% resistors to 1% Dale CMF metal films to take advantage of their very low noise. The .068 mylar became an Epcos MKT of Polyethylene Terephthalate (PET). Because of limited space for the 10uf coupling cap there was only one choice: a modern audio-quality electrolytic.

I decided to use a 10uf/16V Elna Silmic II. To my ears Silmics provide an open, dynamically textured sound which I find musical. Interestingly I find Silmics can sound slightly soft at first but after several hours of use they suddenly open up; one of the few times I've heard an audible break-in of a board component. Low quality caps can have a bright sound which is misleading. This brighter glaze may sound clean at first but produces flatter, less realistic dynamics to my ears.

ABOVE: Notice the small ground tab behind the mini-transformer. This grounds to the inside of the mic body, so I used contact enhancer on the tab as well as on the inside of the mic body. I also sprayed out the power switch with contact cleaner and reflowed the solder on the XLR connections.

Shown above is the schematic from the CM-300 manual that shipped with my mic (the FET resistor question mark was added by me)
Why Nak printed an incorrect schematic in their manuals is a mystery.

BELOW: To get inside the capsule body remove the C-clip from the bottom and press down on the brass pickup to push the plastic insert out the bottom. Once out the insert snaps apart. Just as before the values inside were wrong compared to the schematic: the 1.2K resistor was a 2.2K which I replaced with a Dale CMF. Next to the FET is a mysterious resistor not labeled in the schematic: Brown-Black-Violet calculates as 100M ohm which makes it a bias resistor to pass DC to the FET. The FET is a tiny 2SK118 transistor listed for "General Purpose and Impedance Converter and Condenser Microphone Applications". This FET may have been hand-selected originally and modern day replacements are scarce and questionable so I left it in place.

Hard to see but Nak connected the 100M resistor to one leg of the FET using a shared solder joint in the center of the board. I disagree with this as critical parts touching the board can introduce noise into the circuit. Air is the perfect dielectric. So when I replaced the 100M ohm resistor with a 1% MOX200F100 resistor (not shown) I bent up the FET lead in mid air and soldered the new resistor to it.

The ceramic disc cap is the -10db circuit and was labeled '151' instead of 220pf as in the schematic. I don't use the -10db circuit so I left it alone. NOTE: the foil trace on the bottom of the circular board is super thin and heats up instantly. So I used a heat sink on the FET when soldering to protect it. I finalized things by using Chemtronics flux-off to clean any residue from the board.

RESULTS
I had to tilt the Silmic slightly to clear a rib in the mic shell. Everything else fit perfectly. The new blue 15K Dale is a Mil-spec CMF/RN50, the 2.2K inside the capsule (above) is also a CMF/RN50. While I was in there I replaced the red/white/black wires that go to the XLR jack with new stranded Canare 22g.

Using the same preamp, headphones, batteries, and levels from before the mod I was flat out astonished when I powered the Naks back up. Before I had to get reasonably close to the microphones to pickup a clear signal, they had a dull, closed-in sound. To be honest I had never been too impressed. Now, the noise floor floor is vanishingly silent, with an articulate, spacious sound. I can hear creaks in the room, someone walking in another part of the house, my whispered breath, my computer fan, actual room ambiance. Switching the low cut filter on didn't diminish the magic.

I never expected such a difference from 3 Dale resistors and a Silmic. I can only conclude that the original tantalum was pretty horrific and that the original carbon film resistors were poor sonic choices. Isolating the FET from the circuit board may also have lowered the overall noise levels. Take note.