Groove Tubes VELO-8 User manual

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Groove Tubes
Velocity (Ribbon) Microphone
Reference Manual
VELO-8

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1 VELO-8 REFERENCE MANUAL

A little background on “Velocity” mics
The VELO-8 name stands for “velocity” microphone, with a traditional
“figure 8” polar pattern; hence, VELO-8. “Velocity microphone” is the
original name used by RCA, Electro-Voice and others for what we commonly
today call “ribbon” mics. They actually worked on reacting to sound wave
velocity (or audio wind), so the name fits if you understand the technology.
Most ribbon mics have a very thin aluminum element that looks like a silver
ribbon. It is moved (activated) by very small amounts of sound (or air velocity)
from instruments or voice. This ultra-thin strip of metal is a double-edged
sword.
First the positive: its ultra-thin element can capture details with a smoothness
not found in any other microphone technology. It can make a saxophone sound
sweet, not harsh. Ribbon mics can do the same for voice and many other
instruments that might record too bright or harsh using modern condenser mics.
Now the downside: while its thinness affords highly detailed sensitivity to
sound waves, it is also easily damaged or torn if the accompanying air waves
are too strong. Higher air pressure levels, like those generated by a closely
miked strong vocalist or a bass drum on axis, can easily rip the thin aluminum
ribbon element resulting in a costly and time consuming re-ribboning repair
adventure. Replacing the torn ribbon is a delicate maneuver, and only possible
if performed by a highly experienced mic tech or a factory repair department.
Unfortunately, both in the early days and today, a broken ribbon element is
quite common. If you find a good mic tech who can repair your ribbon
element, it can take weeks or months before you’re back in business. The old
RCA and Capital studios usually employed a full time tech whose only job was
to maintain their array of ribbon mics so studio time was not lost. But those
old ribbon tech guys are mostly gone today, and few younger techs have the
training and/or experience to do a good ribbon repair. So today, we usually
have to search for that rare “ribbon mic guru” and send it to him, or in the case
of a newly produced ribbon mic, it would have to go back to the factory... and
you wait!
This is one reason velocity mics fell out of favor with most the larger studios,
who embraced the more modern and highly detailed condenser mics which are
not as easily damaged from higher levels of wind or air velocity common to
rock recordings.
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Another factor of the ribbon mics’fall from favor with modern studios was that
they had very low signal output as compared to the electronically activated
higher output condenser mics. They therefore required more preamplification
to get similar output levels, which add noise floor to the signal from the
increased amplification factor. For an example, listen to the hiss on Elvis’s
“Love Me Tender” or Patty Cline’s “Crazy”, both recorded with early ribbon
mics.
In fact, ribbon mics can be up to 20dB weaker than condenser mics. This is
really a problem today, as most mixers and preamps do not have the low input
impedance to match the relatively low output impedance of the typical ribbon
mic (which uses a transformer to boost the impedance from 1 Ohm to maybe
100-200 Ohms). Also, the ribbon element’s output signal levels are too low as
well. Therefore, the “loading” is usually too high, and the preamp too weak to
effectively boost the level to the strength of the more common condenser mic.
Our ViPRE and SuPRE mic preamps were specifically designed to load the
ribbon with a matching balanced transformer winding of 300 Ohms, instead of
the industry standard of 2,000 Ohms or higher. These preamps also amplify
over 70dB, so this feature set was specifically intended to allow ViPRE and
SuPRE to address the special needs of preamplifying a ribbon mic, and
duplicate the loading and amplification designs of the old RCA consoles when
velocity mics populated the studios in the USA.
How the velocity mic works
Ribbon mics, both then and now, all work on the same basic principle; a thin
conductor (usually a strip of aluminum) is suspended and “tensioned” between
a magnetic field and is activated by sound waves, and this generates a small
signal voltage. This is similar to the way the more common dynamic mic
works, which uses its diaphragm suspended in a round magnetic field to
generate a signal voltage from its movement activated by sound waves, like a
small speaker in reverse.
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So why do ribbon mics sound so different? These obvious sonic differences
result from variations in the few simple mic components; the ribbon, the
magnetics, the transformer, and the interfacing design of these elements. The
craft and design to combine these three components is most important, as with
any transducer… little things can make BIG differences! Let’s look at each of
these four areas in detail, and discuss how we created VELO-8 to be the best
ribbon mic possible.
1. The Ribbon element: RCAmics had ribbons ranging in size from 2” long
by 1/4” wide, down to about a quarter of that size. The thickness of the
aluminum ribbon could vary from as thin as 2 microns to as thick as 8 microns.
As most of the old ones have been rebuilt (usually with thinnest material
available), few can be found with their original RCA-produced elements intact,
and have usually been rebuilt many times using whatever thin aluminum they
could find. Also, the thin aluminum was hard to come by when RCA got out
of the ribbon mic business in the late ‘50s. One common trick of the old studio
techs was to use the aluminum peeled off a gum wrapper, which is about 3
microns. Hand tensioning of the ribbon, or stretching it across the magnetic
field, was critical and rarely measured in any meaningful way… the old guys
just had a feel for it (or not!). Of course, the way the ribbon is crimped (or not
crimped) also contributes to the end audio result as well. This is why so few
vintage RCA mics today, even of the same models and era, sound exactly alike.
But some common truths are found; the larger the ribbon element the more
output level it generated (i.e., the RCA 44), and the thinness contributed to
more (or less) detail as well as more (or less) reliability. For example, a thicker
ribbon meant more durablity but gave less detail.
We like the old classic velocity mics VERY much, but didn’t want to just
“reissue” an Elvis impersonator mic, as many have done. However, we did take
a page from RCA velocity mics of the ‘50s, and a few tricks from Bang &
Olufsen (B&O) ribbon mics of the ‘60s, and made ours a unique offering
which stresses the advantages of the ribbon mic, but also addresses the
shortcomings. For example, our ribbon element is big and thin; VELO-8’s
aluminum ribbon is over 2 inches long, 1/10” wide, and is just 2.5 micron thick
(human hair is about 10 microns, so it’s a quarter of the thickness of a strand
of hair!). The ribbon element itself is also uniquely shaped; it is very precisely
“dual formatted” to have two shapes, and both contribute to detail and
reliability. VELO-8’s ribbon has both a curved area (in center) and a crimped
suspension area at either end of the ribbon to help it hold its precise tensioning.
This allows for maximum signal output and good long term stability. It is also
precisely tensioned by a special proprietary GT computer controlled system;
we call this “ribbon tuning”, so they all sound exactly the same (a must for
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stereo recordings!).
Lastly, our VELO-8’s ribbon is field interchangeable by the user (more on that
groundbreaking feature later).
2. The Magnetics: The classic RCA 44 ribbon mics were relatively high signal
output devices (as compared with other ribbon mics) which was the result of a
larger element and massive ceramic or Alnico alloy magnets placed around the
ribbon element; more magnetic field gives more output, just like guitar
pickups or speakers. These RCA 44 mics are favorites of engineers both
today and then... and captured early Elvis and Sinatra tracks quite nicely. These
early Capitol tracks were smoother than the later tracks by these same artists,
as they were mostly recorded with more highly-detailed and brighter condenser
mics like the Neumann U47. You will also notice more noise floor on the earli-
er recordings, but who cares? The “feel” of these performances could not have
been captured any other way, and the truth is that both were great recordings,
but for different reasons!
However, today we have a new magnetic material, Neodymium, that is 100
times more powerful than the best of the old Alnico magnetic alloys that RCA
and others used. Also, unlike natural occurring magnets, the new materials can
be cast into special shapes. All previous ribbon mic magnetics, both yesterday
and today, have been made from off shelf square bars placed in a metal
conductor. But instead of using commonly available off shelf magnets, we
designed and custom cast a totally new “roof top” shaped magnet that
concentrates the magnetic field on the edge of the ribbon and increases signal
output. We also used the highest quality aerospace Neodymium material that
costs 20 times more than the common Neodymium magnets you’d find on your
money clips and refrigerator decorations. Furthermore, we choose to use a
“high output roof top” magnet on BOTH sides of the ribbon. Most ribbons
today (and then) have just a single sided magnet that uses a steel chassis to
pass the magnetics to the opposite side of the ribbon element. This was the
most costly approach... but of course, as we are suckers for quality and
performance, we believe it was well worth it, as this approach boosts both
signal output (while lowering the floor noise) and improves detail (with more
depth and frequency response). In short, it simply makes for better recordings.
3. The Transformer: The transformer’s job is VERY important as it interfaces
the ribbon output to the preamplifier’s input. The ribbon element is typically
very low impedance, just one or two Ohms, and also needs a significant level
boost as well. Therefore, the transformer must boost both the output level and
raise the impedance of the ribbon element, with as little degradation as possible
5 VELO-8 REFERENCE MANUAL

to the signal. It can contribute much to the signal character, or it can degrade it
as well. Cheaply made transformers degrade the signal a bunch, but good
transformers are costly. This is the main reason studio product designers have
moved away from using transformers in general and moved toward the MUCH
cheaper “active” differential input stages made with FET or, worse, op amps
for both output sections of microphones and input stages of preamps or mixers.
However, in the golden years of recording, there were excellent quality
transformers placed in both the output stages of all ribbon mics, as well as the
input stages of the mixers... life was good, and nicely balanced too!
Today, however, due the the high cost of quality transformers and the very low
cost of solid state devices to make active input and output circuitry (and with
product catalogs spinning this as some kind of positive verbage like
“transformer-less inputs!”), high quality transformers at either end that can do
the math BUT not degrade the signal are a rare luxury. But luxurious sounding
they are! Adding a high-quality transformer to your mic or SSL console can
cost you hundreds of dollars each, yet still many engineers swear by
transformers over cheaper and common active input stages. But they also
have the added advantage of total electrical isolation, which is not possible
with active circuitry. Certainly a cheap transformer can add unpleasant
distortion and frequency loss and/or shift, so we don’t recommend that. But
it’s interesting that many customers of lower cost Chinese ribbon mics are now
retrofitting them a higher quality Western-made output transformer made in
Europe or here in the States that easily cost twice what they spent on the mic!
There is a noticeable improvement of course, as less degrading of the signal
path occurs. While it’s safe to say that the lower cost Chinese components in
general just do not measure up to the Western-made product, it’s also a fact
today that any highly labor intensive hand assembled product MUST be made
offshore if the target is to keep the price within reach of the common man’s
studio. That’s why we make mics in the same GT captive mic factory where we
have made our condenser mics for nearly 15 years, and use the similar formula
of importing the higher quality Western-made critical signal path elements like
capsule elements, tubes and transformers.
So the “GT Rule” with transformers is to go big and stay home! We went big,
and we looked no further than the 118 freeway to find our favorite
microphone transformer builder, Cinemag. This company has been a GT
partner/vendor for many years. Cinemag designed and produces our custom
multi-stage variable input transformer at the heart of the ViPRE mic preamp,
which sounds incredible! The Cinemag company is co-owned and operated by
Tom Reichenbach in Chatsworth, California. Making transformers is a lost art
today, but it’s a family tradition for the Reichenbachs; Tom learned from
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perhaps the very best “wire winder” ever, his dad Ed Reichenbach. Ed owned
and operated Reichenbach Transformer company during the golden age of Los
Angeles studios. He made transformers for all applications, including for the
studios. One of his outside sales guys was Dean Jensen. Dean specialized in
studio customers and audio manufactures and helped design transformers to fit
their demanding needs. Dean was a sales engineer, Ed was the designer and
manufacturer. Later, Jensen formed his own sales company and named these
“Jensen” transformers gaining (at that time) a high reputation for making
perhaps the best sounding transformers of the day. But Dean’s offices were
always located inside Ed’s transformer factory and Ed actually made these
classic Jensen transformers! Unfortunately, when Dean died, his business went
into a bankruptcy proceeding which resulted in the Jensen brand name being
sold to a former employee, and Ed simply retired and sold the Richenbach
factory. Soon after that, Tom started his own transformer business under the
direction of his dad. Frankly spoken, we’ve sampled all the best transformers
at every price point made today, and Tom’s transformers out perform them all
every time... so we stayed with Tom on the VELO-8 project and it really paid
off. Here’s what Tom and GT came up with in this latest collaboration.
The VELO-8 transformer uses pure nickel core laminates with multi-tapped
output windings that allow the level to be switched between 75 Ohms (most
detail) and 300 Ohms (most signal strength). We also added a High Pass filter
for both windings so there are four switched selections located on the lower
rear side of VELO-8’s body. The highest signal level is achieved in the 300
Ohm position. The 75 Ohm position will have less output but also will be
obviously “altered” and so will provide more natural detailed results, but it will
require more preamplification that may not be available on some mixers (but
perfect for ViPRE and SuPRE either way!). Also, the low frequency roll off can
be useful as ribbons really produce a lot of information in this the ultra low
bandwidth area which could make a voice, drum or guitar sound too boomy.
4. The Design: Actually the size and shape of the ribbon, placement of the
ribbon in the magnetic field, and quality of the coupling transformer which
boosts the impedance of the ribbon’s output are the most influential
components to the end result, but without a coherent design to combine them
all could be lost! Our VELO-8 design is an original (not another “Elvis
impersonator”, please!); it has higher than normal output levels as a result of
the increased magnetic field created by our custom formed dual “roof top”
magnets and special designed Cinemag transformer. It offers a dual contrasting
ribbon placement design that produces different, but desirable, sounds from
both the front and rear of the mic. Now, the engineer has two different
sounding sides to choose from; the back side is less bright than the front side.
7 VELO-8 REFERENCE MANUAL

The dual contrasting sides, combined with our four selectable impedance and
roll off options, give you eight different ways to use VELO-8! By the way, we
think you will use all eight for various applications... and that was the idea,
options are a good thing!
4a. VELO-8’s Field Replaceable Ribbon! This is perhaps the most
revolutionary part of our design: our exclusive field interchangeable ribbon
assembly. We anticipated that many of our customers will have never owned
or operated a ribbon mic before (mostly because more folks can actually afford
our VELO-8 as opposed to other ribbons). We also know that they will
probably NOT read our several cautionary statements about ribbon preservation
(i.e., STRONG AIR DIRECTLY ON THE RIBBON WILL CAUSE
DAMAGE! ALWAYS USE A POP FILTER WHEN CLOSE MIKING!).
So when and if the ribbon gets blown out with your girlfriend’s best attempt at
a Janis Joplin cover song, you can be back in business in about 10 minutes,
and not 10 weeks! How is that possible, you may ask? Because our VELO-8
ribbon is mounted and precisely pre-tensioned on a unique circuit board
material so it can be replaced in minutes; the process involves removing just
four screws on the chassis, and another four screws on the motor with a simple
procedure that takes about 10 minutes, start to finish!
“So How To?”: Notice when you unpack your VELO-8, there will be a small
pre-tensioned ribbon assembly mounted on a small PCB and sealed in an
anti-static package in your VELO-8 carrying case. By the way, the design of
the VELO-8 vertical storage case has an important purpose: keeping VELO-8
upright in storage will avoid “ribbon sag” which can occur over time and
degrade the performance if stored horizontally. The following chapter of this
manual will show you, step-by-step, how to quickly replace your VELO-8
ribbon assembly should it be necessary. So, if this should ever happen to you,
you can take comfort in two realities: first, you can whip out your spare ribbon
and replace it in 10 minutes (NOTE: the extra ribbon sub-assembly is included
in the VELO-8 package!), and secondly, you can send us back your broken rib-
bon sub-assembly in its special plastic package and we’ll return it to you for a
reasonable service charge!
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