MODIFICATIONS
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NAKAMICHI CM-300
UPGRADES 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 big 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. Incidentally, the CM-300 and physically identical TEAC ME-120/Tascam PE-120 are said to have been manufactured by the Japanese company Primo. 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 mixed down with their MX-100. 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’ve 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. So I started doing some reading on how to use a shotgun mic.

    I was surprised to read that shotgun mics are not meant for sources beyond several feet, do not provide any "zoom in" ability to capture distant subjects and aren't good for music. Instead, they are designed to reject surrounding noise to capture dialogue, such as 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. But then I thought back to the bootlegs I have heard using shotguns and remembered how they were hollow and boomy. No offense to those who taped many live shows using them, but I have to wonder how things would have sounded using the CP-1 capsules instead.


    THE EXTINCT BATTERY
    As CM-300 owners we all know the original battery vanished long ago. I bought the modern replacement an Eveready 206 about 6 years ago but those are unavailable now too. The well-known solution is to stack smaller 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 ID...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 tube and trimmed the edges for neatness. Hard to see but I used a slightly smaller tube in the center...with a small spring passing through to pass the voltage. Not exactly elegant but they work and fit properly. Homemade Nak batteries!

    TIP: I found that the CM-300s are very sensitive to voltage levels. I had one CR 1/3N in a stack that was slightly low which made one stack 9.2V and one 8.7V. Using stereo mics the one with the 8.7v battery had slightly lower input level. Interesting.

    My taper friend told me to always remove the batteries when not in use. He claimed the off switch still allowed a small amount of current to flow and would eventually drain the batteries. I never thought to test this since batteries were so scarce but still remove them after use.


    CIRCUIT MODS

    Some people chop up the CM-300 or convert them to phantom power...but I wanted to retain the stock functionality. So I decided replacing the 25 year old parts on the circuit board was the best route. On opening it up I noticed something very interesting...several values were different from was was printed in the schematic...and even what was printed on the board(!) The 100pf cap on the schematic was actually .068, and the 47uf coupling cap was a 10uf. Not unexpected but surprising was that the 10uf coupling cap was a tantalum type. While tantalums have been used for decades as input caps in mics because of their small size, they are unreliable, obsolete and are widely considered to produce poor sonics. You really can't blame Nak as back in 1990 a tantalum or tiny electrolytic would have been the only caps that would fit. Photos of some modern Neumann mics still show tantalums in places. My opinion is in 2017 there is no valid reason to be using one in an audio circuit.

    My plan was to change the original 5% resistors to Dale CMF metal films because of their very low noise spec. The .068 mylar would became an Epcos MKT of Polyethylene Terephthalate (PET), a dialectric which I think sounds excellent. Because of limited space for the 10uf coupling cap there was only one choice: a modern audio-quality electrolytic. Not a bad choice, keep in mind that a tantalum IS an electrolytic.

    So I decided to use a 10uf/16V Elna Silmic II. To my ears Silmics provide an open, dynamically textured sound...very musical. Interestingly I find the Silmic can sound slightly dull at first, but after several days of use they suddenly open up; one of the few times I've heard an audible break-in of a board component. Lower-quality caps can have a brighter 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 shipped with my mic (FET resistor question mark added by me)
    Why Nak includes an incorrect schematic in their manuals is a mystery. Maybe they used the same board in the CM series
    and changed the actual values depending on the model.

    BELOW: To get to the interior of 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. Compared to the schematic the 1.2K resistor is 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...a 2SK118 transistor listed for "General Purpose and Impedance Converter and Condenser Microphone Applications".

    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 these critical parts touching the board can introduce noise into the circuit. Air is the perfect dielectric so I replaced the ancient 100M ohm resistor with a 1% MOX200F100 resistor (not shown). I situated the FET lead in mid air and soldered the new resistor to it.

    The FET may have been hand-selected originally and modern day replacements are scarce and questionable...so I left it in place. The brown ceramic cap for the -10db circuit was labeled '151' instead of 220pf...but since I'll never use the -10db circuit I left it alone too. NOTE: the foil trace on the bottom of the circular board is super thin and heats up instantly. I used a heat sink on the FET when soldering...take extra care here. I finalized things by using Chemtronics flux-off to clean any residue from the board.

    RESULTS
    Below is after I replaced the parts. I had to tilt the Silmic slightly to clear a rib in the mike 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 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 have them 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 resistors were poor sonic choices. Isolating the FET from the circuit board may also have lowered the overall noise levels. Take note.




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