Re: Playing Straight through the PA

> g/mail/postponed-mail/PLAIN; CHARSET=US-ASCII
> Regarding XLR connections:
> On Mon, 3 Jul 1995, Rkt wrote:
> > Mike Curtis wrote:
> > >Crystal mics and XLR connectors should never be paired without proper
> > >impedance matching
> > 
> > Mike is not referring to using an XLR connector as it is used on Hohner's
> > Blues Blaster(a JT-30 in a colorful case). The BB just employs an XLR
> > connector to make a hi-Z connection. On the other end of the cord is a 1/4"
> > phone plug.

The connector and cable itself is not inherently low impedance.  It's simply
standardized to be USED with low impedance equipment.  I'm not sure about
the distributed capacitance of balanced shielded mic cable, but would be
surprised if it is radically different from coaxial mic cable.  If this is
the case, the balanced cable itself would not necessarily be inherently low
impedance.  However, balanced line is never used with high impedance mic
equipment.  But this is mainly because balanced line is far more effective
with lower impedances.

Because the BB comes with its own cable, whatever they use on the mic end is
"their business", because they're making it "right" with a custom cable.  My
literature doesn't shw the Shaker coming with its own custom cable (they
didn't "fix the problem"), so in my opinion, it's not "properly" designed.

The XLR is not inherently low impedance - it's just a connector.  It's
industry standard to USE it to make low-Z connections.  Kinda like the
large square key is ignition and small round key is trunk for automobiles.

I'm taking a few liberties in the following, for the sake of simplicity.

Balanced line means we have a "plus" and a "minus" lead.  When the signal is
pushing on one, it's pulling on the other.  Because it's balanced, only
"different" signals reach the amplifier.  Hum and other undesireable grunge
are induced or coupled in "common mode" - that is, they "push" on both wires
at the same time.  The amplifier ignores these.

Think of a balancing scale, or a teeter-totter.  When you have less weight
on one side, it tips.  But if you put identical amounts on both sides, it
balances - whether it's 2 very light weights or 2 very heavy weights.  In
other words, it only registers differences.

Balanced line can be a simple twisted pair like we use for telephone (4
twists per foot is standard for phone cable), but for microphones, we
usually use shielded twisted pair, where the pair of wires are run inside a
braided wire "pipe" (or sleeve if you prefer.)

Unbalanced line has a "hot" lead and a ground.  The hot lead carries all of
the signal.  The ground is a "neutral" connection that carries no signal. 
Because it's neutral, it is used to shield the hot lead from interference. 
This kind of cable looks like a wire running down the center of a briaded
wire "pipe".

Because the signal is carried on the "hot" lead, the line is unbalanced.

While low impedance is _usually_ balanced line, it can be run unbalanced. 
This is less resistant to common-mode rejection and when run into a balanced
input, will reduce gain by 6 dB, but it does work reasonably well if you're
not real fussy and don't use a really long cable.  (20' max)

The JT-30 is crystal.  These are typically made from rochelle salts, which
is why they're water/moisture susceptible.

The R47 Green Bullet capsule is controlled magnetic.  The diaphragm has a
thick stiff wire attached to it.  This wire goes to an armature assembly,
basically a metal bar through the middle of a magnet.  Wire is wound around
the magnet, forming an inductor.  As the armature is moved through this, it
changes he magnetic field (ergo "controlled magnetic"), which indices
electric current into the inductor.  It doesn't have a transformer - it has
a tapped pickup inductor.  If you tap off a few turns, it's low-Z.  Tap 'em
all and it's hi-Z.  (It comes with 2 taps.)

Dynamic elements have a magnet with a cylindrical cutout in the middle.  A
lightweight cylinder of cardboard or light plastic is wound with VERY small
gauge wire, and a diaphragm is put over one end of this (think of an opned
tin can), which is placed in the cutout.  Sound moves this assembly and
winding, inducing current into the winding.

Controlled magnetics have more mass in the diaphragm assembly, producing a
more midrangey sound.  They're also much higher output than dynamics, good
for overdriving.  They used to be popular for 2 way voice radios.  Newer
radios use mostly electret condenser mic capsules.

 -- mike

This archive was generated by a fusion of Pipermail 0.09 (Mailman edition) and MHonArc 2.6.8.