Building a Tube Microphone: the Royer Mod
A cheap solid-state large diaphragm microphone can be refitted with a vacuum tube to make it sound much better. I got plans from an article in Tape Op magazine issue #25. It can be found here:
If this link doesn't work, you can scour the web to see if you can find a pdf of the tube mod article.
(2017 update: I recently found the original article here: http://recordinghacks.com/articles/one-tube-microphone-from-berlin/)
This page is in no way intended to be a step-by-step instruction manual on how to build a tube microphone. This microphone requires a dangerous high-voltage power supply. Do not try this at home.
Parts and Prices
I like to build useful things from scrap, junk, miscellaneous parts lying around the house. For this project, I had to order many parts, some relatively expensive. Maybe the next microphone will be more homemade :-)
Parts I bought:
The MXL-2001 large diaphragm microphone. This mic came in a two-pack with the MXL-603S, a crisp, quiet instrument mic. The pack cost $160 from Sam Ash Music so I will say that I spent $80 for the MXL-2001.
A high-quality audio transformer from Cinemag Audio Transformers. I got the CM-2480, a 10.5:1 mic output transformer, for $40.42 + $10 shipping. I exchanged many useful emails with Cinemag to determine which transformer to buy. How does this transformer compare to others on the market? How would I know? The mic sounds good.
A 5840 W Raytheon vacuum tube from Triode Electronics, $5.95. A minimum order amount was required so I also got some capacitors from Triode. I ended up using caps from Mouser, I think. Or Radio Shack. See below.
Various capacitors, resistors and power supply transformer from Mouser Electronics, about $14 plus $6 for shipping.
A capacitor or two from Radio Shack, about $3.
Parts I found in various junk boxes around the basement:
Total cash spent on the project: about $160.
The MXL-2001 was easy to take apart: unscrew the collar at the base, slide off the tubular brass body, unsolder a few wires, unscrew the transformer housing and circuit boards. I took pictures before I detached everything in case the modified microphone didn't work.
Original circuit boards, side one and side two:
The inside of the boards with their connections:
The fabled gold-sputtered mylar capsule. I had to look at it. A little wrinkle in this one. How does that affect the sound? Should I try to restretch it? No!
Construction of the Power Supply
Using the circuit diagram as a guide, I mounted the diodes (1N4004's), resistors and capacitors on a scrap piece of circuit board. Here is the underside of the power supply circuit board.
Nothing elegant. I admire beautiful circuit board work but it gets hidden away in box that no one ever sees. The power supply needs to function. This one does. Here is a picture looking into the power supply box. You can see the fuse holder and pilot light on the upper right.
Construction of Mic Circuit Board
I looked at the pictures of Royer's mic circuit board in his article and mounted the parts in a similar pattern.
The picture above shows the capacitor and resistor layout. Note the teflon standoffs where the capsule wires are attached. I took these off the orignal MXL circuit board:
I mounted the tube on the back, covering the tube wires with plastic insulation. Below is a picture of the mounted tube glowing. I secured the tip of the tube with a piece of copper wire.
This is the first electronics project I have built using a tube. Some web research led me to pages of tube circuits and schematics. Here I found a schematic for the 5840 tube:
Here is a pin-out diagram for the 5840 tube, to help clear up the circuit diagram:
I didn't know how to read the tube schematic! Initially when I tested the microphone, it didn't work. I found my error cruising the net looking for similar DIY projects. This page and its creator were very helpful:
Here is the working microphone in front of the power supply:
How does it sound? It is the best mic I have. The highs are crisp, the lows are rich but not booming. It is better than my Rode NT1 which is the only other large diaphragm mic I can compare it with.
I may try the tube circuit on the Rode NT1 capsule though I am hesitant to dig into that mic. I have some ideas about building a mic from scratch, winding my own transformer, etc., as time allows. I may do a different Royer mod on the instrument condensers as detailed in the second Tape Op article on Royer's website. I am amazed that a one-tube amplifier circuit performs so well. No wonder tubes are still used in audio equipment.