Building Another Tube Mic: The Royer Mod Again

I lost all my studio microphones in a house fire in 2011. I miss the large diaphragm tube mic I built in 2004, a modified MXL-2001 microphone. I have collected some parts and I am ready to build another one.

December 13, 2017. I found a nice heavy metal box in the garage. It's an old external hard drive case. I got a pack of little blank circuit boards from China and I have a collection of bigger boards (thanks, brother) from which I cut out a board for the power supply. To the left are some instructions, circuit diagrams and parts list from the original Tape Op article from issue #25. See my old web page for possible links.

I used clamps as a vise to drill mounting holes:

Test fit the board in the box:

I drilled holes as appropriate with an old hand drill:

I took some bits off the old hard drive circuit board and hot-glued them to the new board: a fuse holder, an AC connector and a ground lug. The last two will connect to the AC jack and ground strap on the case:

I drew the circuit on the bottom of the board:

I soldered insulated solid copper wire to make the connections:

Power supply all wired up:

January 3 update: I fixed the photo above to show the correct wiring of the power transformer!

The top of the board:

I got most of these parts from Mouser, got the bigger caps from China on eBay, got the diodes from Texas.

December 14. Disassembly of the MXL-2001. Take screws off the base, pull off body. Some "before" pictures just in case. One side:

Other side, showing transformer in its can:

Apart:

I had to take a peek at the gold diaphagm:

I'll use my own pictures from the first time I build this microphone to help lay out the parts on mic circuit board. Here I test fit the little board:

December 18. My $4 bag of resistors finally arrived from China. Be careful what you wish for:

Just kidding, sort of. Many of the strips of resistors were poorly marked or unmarked. I used my ohmmeter to verify values or I read the resistor color codes, then marked each strip more clearly.

I was also waiting for the audio transformer from Cinemag, which I received on December 27 or so.

December 28. I mounted and soldered the components on the mic circuit board. On the right you see I pulled the teflon standoffs from the original board and mounted them on the new board:

To help verify the wiring, I made a diagram from a picture of the board and clearly identified the tiny components:

I checked the connections visually while making check marks on the circuit diagram. Then I soldered the tube wires on the other side. I first painted the wires near the base of the tube with "liquid tape" and I added insulation where necessary to further protect the tube from short circuits:

I looked up the pin indentifications on line, found this diagram for the 5840 tube:

December 28. I drilled holes in the metal box for wires and the XLR jack and 1/4" holes on the metal partion inside the case. Later I hot-glued rubber tubing into the 1/4" holes to protect the wires:

The XLR jack fit after a little work with a round file. The 1/4" hole almost exactly accommodates the microphone cable:

I checked all the power supply connections, then mounted the circuit board in its box. Nice fit:

December 31. Here is the the mic with the transformer wires connected, transformer stuffed in its case:

I turned it on, checked voltages, found voltages too low, a little more than half what they should have been. Rats. I went over the power transformer data sheet, found I had wired it for 12 volts (parallel) instead of 24 volts (series). I fixed that problem and tried again. I got voltages I expected and got the tube to glow!

I closed up the mic case, took the power supply and mic to the studio and tried it out. Nothing. Not a whisper from the mic. What the heck?

December 31. After going over the circuits and documents from Jensen and Cinemag I discovered that the Cinemag transformer wire colors are reversed. Red and brown wires are the input wires on the Cinemag while red and brown are the output wires on the Jensen on which the Royer Mod circuit is based. So I made a change on the diagram...

...and reversed the transformer wires:

On testing, I found voltages too high! C'mon! I checked and rechecked my wiring. Found no errors.

January 1, 2018 I continued to recheck wiring and pondered my problem. I started checking power supply components since high voltages meant the power supply was misbehaving. I thought I might have overheated a diode and though a multimeter check did not suggest it was bad, I replaced it anyway, and I got my voltages back:

I closed up the mic and power supply box, plugged the mic into my studio and ... it works! Best mic I ever had, and now I have it back. Happy New Year!

Here is an approximate cost breakdown of the parts I used to modify the MXL-2001:

Item Cost
MXL-2001 $89
Film capacitors $2
electrolytic capacitors $5
diodes $2
resistors $2
power transformer $12
Cinemag audio transformer $50
5840 tube $5
audio cable $8.0
total $175

To compare, in 2004, I spent about $160 to build this mic. Is it worth $175? Absolutely.

Next? Ribbon mic? Yes! Here is a page about a ribbon mic build.

January 2018