From Tape Op: No. 25 • Sept/Oct 2001

A do-it-yourself tube mic & de-mystifying the Neumann U-47

Now let’s see about the Marshall MXL 2001

When the Marshall MXL 2001 first appeared on the market, it struck me as a likely candidate for modification. The microphone’s capsule is a decent copy of the capsule in a Neumann U-67 and, when mated to well-designed tube electronics, the performance is very presentable. It more than justifies the work and expense, particularly since printed circuit boards are available, which will remove most of the risk of wiring errors. Figure 3 is the schematic for the modified MXL 2001 microphone and its power supply. The circuit is adapted to a 5840 tube and it more closely follows American, rather than German, design practice. I used a Jensen DB/E transformer since it fits neatly into the space occupied by the MXL’s transformer and it is electrically suitable for the job. It is not quite a match for the U-47’s transformer, but it is better than the transformers in such tube mics as the Schoeps 221B and the Neumann KM54, since it won’t saturate with loud low frequency signals. The 5840 tube is electrically similar to the AC701K tube used in such microphones as the Neumann M-49.

Figure 3

The power supply is more-or-less conventional except for the use of a voltage tripler for producing the B+ voltage. I took this approach to allow the 95 volt B+ supply and 6-volt heater supply to be derived from a single 24 volt transformer. Voltage triplers work on the principle of charging capacitors individually from the outputs of rectifiers then adding the voltages in series. Voltage doublers and triplers have often been used where high DC voltages are needed and the cost or weight of a power transformer would be excessive. Figure 4 is a voltage tripler. The three capacitors are in series and are each connected to the transformer winding through a diode. The capacitors are charged on alternate half cycles and the DC output voltage will be about 3.4 times the AC input - the exact voltage depending on the capacitor values and upon the current being drawn. The 6 volt supply for the tube heater is tapped off the bottom capacitor of the tripler and the B+ supply is taken off the full output of the tripler.

Modifying the Marshall Microphone

The microphone’s case may be removed by unscrewing the collar at the bottom of the case - the sleeve then can be slid off to reveal the circuit boards with the microphone’s “head amplifier”. Removing the three screws on each side just above the base will allow access to the connector and the output transformer. Remove and discard the original output transformer, connector and printed circuit boards, cutting the wires as you go. HOWEVER, UNSOLDER, DO NOT CUT the two thin wires (you need the full length of these wires) that come down from the capsule to the terminals on the PC board.

The board (supplied with the kit, see next page), which replaces the two boards installed in the microphone, is pre-drilled and plainly marked, with the component numbers keyed to the parts list and the schematic diagram. Also, the board is marked to identify which output transformer lead goes to what solder pad. In any case, form the resistor and capacitor leads so that the parts fit neatly.

HINT: Install C2 (capacitor nearest connector end of mic) last, to assure that it will fit in neatly and to avoid blocking the solder pads below it. Also, use spaghetti tubing to cover the wires coming out of the tube and note that the grid lead (pin 1) of the tube is fed through a hole in the pc board, spaghetti tubing and all, and the right hand wire coming down from the capsule is soldered directly to this lead.

Figure 5a and 5b

The new printed circuit board and the new output transformer will fit neatly in the spaces that were occupied by the original parts. The 5 pin plug that is substituted for the original 3 pin plug will also neatly replace the original part. DOUBLE CHECK THAT THERE ARE NO WIRING ERRORS, PARTICULARLY THAT THE CONNECTIONS TO THE OUTPUT PLUG ARE NOT MIS-WIRED. I emphasize this since many failures with “do-it- yourself” projects are due to wiring errors. Figures 5a and 5b are photos of the Marshall mic with the new circuit board installed. The power supply is assembled in a standard ammo box. Assembling the power supply is straightforward - start by stuffing the printed circuit board, which is again supplied in the kit, being careful to observe the polarity markings on the silicon diodes and the electrolytic capacitors. (Reverse polarity with these parts will cause malfunctions and the destruction of the parts.) Upon completion of the circuit board, the final assembly of the power supply may begin. The AC cord, switch, fuse, and pilot lamp are mounted at one end of the box, the XLR connectors are mounted at the other end, with the power transformer and the printed wiring board mounted along one side of the box. The board should be mounted so that the large wire-wound resistors are along the top edge of the board and so the end of the board where the three power connections (ground, 6 volts and 100 volts) are made to the board is adjacent to the pair of XLR plugs. BE CAREFUL TO DOUBLE CHECK THE WIRING. Figure 6 is a photo of the power supply with the printed circuit board installed and the wiring completed.

Finally, the last chore is to assemble the special multi-conductor cable that connects the microphone to the power supply. This cable needs a female 5 pin XLR plug at one end and a male plug at the other end. Unfortunately, the Canare cable that I recommend has two blue wires and two white ones so you have to use an ohmmeter or an continuity tester to verify that each wire is connected to the same pin at both ends of the cable.

Figure 6. Finished mic & power supply (above) Finished power supply (below)

For example, the wire connected to pin one at one end of the cable must go to pin one at the other end of the cable, and so on - the cable shield being connected to pin 3 at both ends. When you are confident that the work has been done properly, connect the microphone to the power supply and connect the AC cord to an outlet. After the tube has been allowed to warm up, the DC voltages should measure as shown on the schematic diagram, Figure 3. NOTE THAT ALL THE VOLTAGE MEASUREMENTS ARE TAKEN BETWEEN POSITIVE POINTS AND GROUND, WITH THE MICROPHONE CONNECTED.

The voltages at the power supply will NOT be as specified if the microphone is NOT CONNECTED. If the microphone is properly modified, it will give excellent performance with any recording equipment that is set up to accommodate standard low impedance professional microphones.

Kit including parts and circuit boards

Printed circuit boards and a parts kit for this microphone is available from Mojave Audio through Royer Labs at 818-760-8472 or email David Royer at dave@royerlabs.com.

The kit is $160 w/o the Jensen Transformer, which is $66 from Jensen. The circuit boards only are $20.

-And you probably thought David Royer only knew about ribbon microphones. <www.royerlabs.com>