boards etched and drilled.
is a closeup of one side of the microphone PCB board. As you can see
it is a little crowded.
copper pipe before work began.
The mic head’s windows are cut out.
Audio engineers the
world over appreciate the sound of vintage tube microphones like the
Neumann U 47 or the AKG C12, but the cost can easily run to several
thousand dollars, leaving them beyond the reach of all but the
biggest studios (or well-heeled dilettantes). What if I were to tell
you that, for under $600 — and with a bit of labor — you could
enjoy a taste of that vintage sound?
mic body’s main metal body materials are pictured.
mic PCB with the Lundahl transformer installed.
mic PCB With the cheaper OEP transformer attached with tie.
Denmark, is home to Gyraf Audio, a small boutique audio manufacturer
specializing in tube preamps and other processors. Their website,
features a DIY section showcasing several projects, some based on
products they produce. They don’t provide kits, but each project
includes schematics, parts lists and PCB layouts. One such project
is the G7 Tube Microphone. I recently took on this project, and here
I share my experience and the extremely gratifying results.
to Gyraf’s Jakob Erland, the mic’s designer, the G7 inspired by
the Neumann U 47 and U 67. It is a multipattern mic, capable of
omni-, cardioid and figure 8 patterns. It uses a Peluso Labs CEK-12
capsule, modeled after the AKG CK12 large-diaphragm capsule, the same
used in AKG’s C414 and the legendary C12. The tube is an EF86
pentode, and the audio output is transformer-balanced.
start, I had to etch three small circuit boards; one for the power
supply, one for the mic circuit and one for the tube socket, which
mounts to the mic PCB. I used MG Chemicals’ (www.mgchemicals.com)
positive presensitized process with a 4-by-6-inch board. I went
through a few very tiny bits drilling the holes. Fortunately, the
local hobby store sold them in packs of six.
the parts, most came from Mouser Electronics (www.mouser.com),
with a few from Newark Electronics (www.newark.com) and Allied
The capsule and its mounting saddle were purchased from Front End
and the Russian-made Electro-Harmonix EF86 tube and the socket were
found at The Tube Depot (www.tubedepot.com).
One important detail about populating the mic PCB — all components
except for the tube and transformer are mounted on the trace side of
the board. This was a bit confusing at first and made soldering a bit
of a challenge as the board became more crowded.
audio transformer is a Lundahl LL1538, available from K&K Audio
The mic PCB can accept this transformer directly. While its quality
is excellent, it comes with a high price tag (for me) — $90
lower-priced option that I also tried is the British-made OEP
A262A3E, available from Newark for under $20. It’s a surprisingly
good substitute, but it will not mount directly to the PCB. So I ran
wires from its pins to the board and then used a nylon tie-wrap to
hold it in place.
body was fabricated from a piece of two-inch copper pipe. Hardware
stores only seem to sell this in long sections, so I asked the
maintenance department of my station’s parent company if they had
any scrap and managed to score two feet of it.
head of the mic is formed by cutting two “windows” to the sides
of the pipe, about one centimeter from one end, with about one
centimeter of metal left on each side. This is where my Dremel tool
got a workout. Once I successfully cut the openings, I measured about
9-1/2 inches down from that end of the pipe and cut it off with a
pipe cutter. I did it in this order so if I botched the “window”
cutting, I could just trim off the mistake and start over.
I had to make the chassis that would fit inside the body and support
the PCB, tube and capsule. This was created using a pair of 2-inch
fender washers and a length of 1/4-inch square aluminum bar stock
from Home Depot. It was all held together using small 90-degree
brackets, screws and nuts. The bottom washer was drilled out to
accept the XLR-7 jack for the mic output and power input. The edge of
each washer had to be ground slightly to allow it to fit snuggly
inside the pipe. On the top washer, a small metal bar with two rubber
grommets provided a mount for the capsule saddle.
grille of the microphone was made from a piece of brass mesh. In my
case, my father happened to have a small roll of fine brass mesh
given to him by a guy who used to work for Mack Truck. They used it
to make fuel filters. A company called McMaster-Carr
sells brass mesh of varying sizes in small quantities.
Hammond 1411Q 7-by-5-by-3-inch box would house the power supply. It
needed to be drilled out for two XLR jacks, a couple switches and a
to Populate the Power Supply PCB
Guts of the Power Supply Inside the Box
finished power supply.
order to keep the cost down, and make parts easier to source, the
power supply was designed using two inexpensive toroidal transformers
rather than a bulkier, multiwinding transformer that is often found
in tube gear. A tube power supply has to provide a
low-current/high-voltage rail for the tube plate, and a
low-voltage/high-current rail for the filament. In this case, the low
voltage is obtained from the 9V secondary on one transformer. For the
high voltage, the second transformer is actually wired “backwards,”
connecting its 15V secondary to the secondary of the first
transformer, thus getting about 140VAC on its primary (which is now
Finally, the cable
from the mic to the power supply was made using a length of
10-conductor cable. Only seven wires are actually needed, so I just
cut off the unused wires.
it all was assembled, it was time for the smoke test. I tested the
power supply first, without the mic attached. The three polar
patterns are created by sending different voltages to the capsule —
0VDC for omni, 80VDC for cardioid and 160VDC for figure 8. With all
voltages looking healthy, and no shorts, it was time to test the mic
itself. It’s recommended to test the mic with the chassis inserted
into the body with the grille in place, since it provides electrical
shielding as well as physical protection.
can imagine the sinking feeling when I heard nothing. After checking
voltages at several key points and checking components, I discovered
the problem. A lead on one of the capacitors on the trace side of the
mic PCB was poking through and touching the shield can of the OEP
transformer. A slight “adjustment” and all was well.
worked like a charm and was very quiet (the good kind of quiet, not
the “I can’t hear anything!” kind). Using the Lundahl
transformer made it sound even better.
only other hurdle was building a suitable shockmount for it. Most are
too small to accommodate such a wide, heavy microphone. I ended up
making my own out of some 1/8-inch flat bar stock and nylon
tie-wraps. As a finishing touch, I used inkjet printable water-slide
decals to apply graphics to the power supply and a nice logo to the
mic body, followed by a few coats of clear lacquer to hold the decals
and protect the metal from tarnishing.
used this mic with excellent results on several projects. It’s
become one of my go-to mics for lead vocal tracks. I loaned it to a
friend who does VO work, and he wanted me to build him one (until I
told him how much my labor would cost!).
far as labor goes, it was a labor of love. It was very satisfying
watching it come together over several months, solving problems of
fabrication, hunting down parts and even chasing my tail a little in
troubleshooting. I gained an intimate knowledge of how it works and
what it can be made to do, as well as some new skills and some
improvement in old skills.
you feel brave enough to undertake this project yourself, the plans,
schematics, design notes and layouts are available at
I’ve posted my own parts list at www.radioworld.com/links under the Oct. 9 issue. For construction and troubleshooting
is an indispensable resource. Jakob Erland is a regular there. He and
others provided tons of useful hints.
Yengst, CSRE, wrote about the Plugin Alliance in the Sept. 11 issue.
He is assistant engineer at WAWZ(FM), Star 99.1, in Zarephath, N.J.
|The completed G7 microphone with the tube/transformer side of the mic board PCB.
completed G7 microphone with the back of the mic board PCB facing the
|The Finished Products All Assembled