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Soundelux ELUX 251, October 2001

Most people would rank the multipatterned Telefunken ELA M251E/250E as one of the three most sought-after tube

Most people would rank the multipatterned Telefunken ELA M251E/250E
as one of the three most sought-after tube condenser microphones, alongside the Neumann U47
and the AKG C-12. Priced at $5,000, the Soundelux ELUX 251 is the
closest thing to an exact copy you’d ever want of the vintage

Not to denigrate the original 251, but, after all, it is a
50-plus-year-old mic using 60-year-old materials and manufacturing
technologies. For this reason, I did not base this review A/B’ing the
new ELUX 251 and any particular Tele M251. With antique mics, what I
call the “vintage factor” becomes a significant issue.
Vintage factor—the physical condition, upkeep, modification
history, how and where they were used and how gracefully they have
aged—makes finding two M251Es that perfectly match impossible.
Without an established “baseline” performance for the
average Tele 251, an A/B seems pointless and of little value.

The original Telefunken ELA M251E (always called the ELAM 251 because
the letters all run together on the mic body) was designed and
manufactured by AKG for Telefunken and used a CK 12 capsule and 6072A
vacuum tube just like AKG’s C-12. AKG also made a non-import version
using a Telefunken AC701 subminiature tube called the M251 (no
“E” suffix). Sister mics are the M250, an omni/cardioid model, and the M252, the exceedingly rare
twin capsule stereo version. The Tele used the much smaller T14/1
audio-matching transformer rather than the larger Type V1248 used in
the first few hundred C-12s. With a larger transformer, a mic should
have a more linear response at higher SPLs, especially in the low
frequencies. The ELUX 251 uses a special wide-bandwidth transformer
with the same primary/secondary turns ratio as the Tele and provides a
more uniform impedance vs. frequency than the original.

The same plated/loaded amplifier circuit as the original is used but with
“tweaked” part values to obtain a quieter noise floor. Like
the original, only half of the 6072A tube is used, running at a low
120-plate voltage and only six volts on the filament. Lower voltages
mean longer tube life. The other half of the tube is not even lit up,
so some enterprising person could invent a socket adapter to
“rotate” the tube around and use the newer other
half—if and when the other side wears out in about 10 years.
Sovtek and new-old-stock GE tubes are carefully tested and selected by
Soundelux for lowest noise, maximum SPL and minimum distortion.

Modern manufacturing techniques address many of the serious problems
that arise in 50-year-old mics, such as oxidation that can occur even
in reconditioned vintage mics. AKG used polystyrene plastic for the
terminal board in the M251E to avoid excessive moisture
absorption—a problem that many old microphones suffer from,
compromising sonic performance and noise floor. The ELUX 251 uses a
Teflon terminal board with a very high dielectric constant (the ability
to insulate high voltages) to take care of moisture. Mechanical
components such as the tube socket and wire-wound resistors are all

resonance-damped to disallow any mechanical noise. The point-to-point,
internal handwiring on the ELUX is silver-plated copper with Teflon
insulation for maximum conductivity and, over time, minimal oxidation
and negligible temperature influence within the heated microphone

Although some early Soundelux U95 mics were Chinese-made, all of the
company’s mics—including the ELUX 251—are now manufactured
in the USA. Designer David Bock selected a German-made capsule with a
6mm Dupont Mylar diaphragm and the same close tolerances and
asymmetrical design as the famed CK 12 capsule used. Like every part of
the ELUX 251, the capsule is hand-built, tuned and strictly tested. The
head grille’s internal chamber resonance closely matches the Tele M251E
in equivalent volume and “mesh count,” which affects HF

The ELUX 251 comes in a large aluminum briefcase with the 110/220VAC
P251 power supply, the mic itself, all cables, instructions and
shockmount. The black-colored shockmount is a stout affair with upper
and lower knurled thumb screws that tighten two constricting metal
bands around the mic’s body. The bands are covered in felt to avoid
scratching the “Institution Green” painted microphone
body—an exact color match to the M251E. You have to take care
that the upper band does not compress over the pattern switch…bad
news! Unlike a Neumann U47, there are no concerns about placing the
ELUX 251 capsule up or down, because the mic generates very little
radiating heat.

The P251 power supply is a significant improvement over the old
M251E’s unregulated power supply. The original supply emitted an
acoustical noise and, as it was unregulated, “shocked” the
capsule with an excessive momentary spike of polarizing voltage when
the microphone was first turned on. The ELUX 251 uses a power
transformer twice the size needed and a shunt regulator (zener diode)
for the high voltage. This linear shunt design is preferred sonically
over series regulators and/or switching power supplies common in
computerized gear. A constant-current source circuit provides regulated
DC filament voltage. Soundelux could do a good business selling power
supplies for old M251Es—instantly improving the sound of those

The ELUX 251 connects to the P251 power supply with a
double-shielded cable using 6-pin Tuchel connectors. The Tuchel
threaded and locking connector was chosen over a multi-pin XLR connector for two reasons: less likelihood of
disconnect and zero mechanical noise. An XLR can wiggle around in its
socket and cause crackles or microphonic noise. Mechanical inertness of
all components within a sensitive microphone is important so that loud
sounds vibrating them don’t contribute to the mic’s sound.

My first trial for the mic was in a session with a loud, male rock
vocalist. In the past with loud singers, the ELA M251E would not have
been my first choice, because the Teles tended toward a brighter and
edgier sound near or at clip a lot of the time. I also find more need
to de-ess every time I use a typical-sounding Tele 251E. Comparing the
frequency response curves of the ELUX and the original, the ELUX is
smoother with a less boost in the upper midrange and high

For my rocker, the ELUX was really good—better than any Tele I
have ever tried for loud guys. I got a bigger sound with good low end,
even though it was placed 10 inches back from his mouth. At that
distance, older condensers do not typically have an overload problem,
and there was certainly none here. I did use an industrial-strength pop
filter and slightly tilted the mic out of the direct wind path. With a
modicum of EQ (a bit of cut at 2.5 kHz) and compression (4:1 ratio, RMS
compression of about 3 to 6 dB), my singer loved “working” the
mic, as its cardioid response is even all around the front.

One problem I often have with old Teles is noise. After setting a
good, hot mic gain level, using a little EQ and compression, there was
usually a constant background noise. These days, with 24-bit digital,
everything else is so quiet that mic noise is unacceptable. The ELUX
251 is quiet for a vintage design at about 27 dB “A”

Female rock vocals fared equally well, but I had a chance to use the
mic more dynamically on the song as the verses were quieter. My singer
liked the fat sound of this mic, and I tried recording in omni pattern,
with smooth results. In a friendly sounding room, the omni pattern adds
a more transparent “openness.” Soundelux points out that
the main focus of the mic is the cardioid pattern, but besides the omni
pattern, users will appreciate, I am sure, the mic’s figure-8
versatility for distant coincident M/S orchestra miking or just about
any other application. Two of them would “rule” for drum

Soundelux Microphones,