plug VS. XLR

tag@garfield.UUCP (Terry Greening) (10/06/84)

        I have a question on microphone cables I  would  like  to
have  answered  once and for all.  The question I have in mind is
the matter of the two popular types of plugs/jacks used  for  mi-
crophones.  I am refrerring to the basic 1/4" phone plug/jack and
the 3-prong plug/jack.  Are there any advantages or disadvantages
to  either  of them.  I have heard that one of them is better for
long cable runs but it may hinder audio quality.  I  have  gotten
several different responses from several different people.  Could
somebody that knows about these things please let  me  know  what
the differences in quality, length of cable run, etc. is????
-- 

Terry Greening
{akgua, allegra, dalcs!dreacad, ihnp4, utcsrgv}!garfield!tag
Memorial University of Newfoundland

vek@allegra.UUCP (Van E. Kelly) (10/08/84)

In article <1647@garfield.UUCP> tag@garfield.UUCP (Terry Greening) writes:
>
>        I have a question on microphone cables I  would  like  to
>have  answered  once and for all.  The question I have in mind is
>the matter of the two popular types of plugs/jacks used  for  mi-
>crophones.  I am refrerring to the basic 1/4" phone plug/jack and
>the 3-prong plug/jack.  Are there any advantages or disadvantages
>to  either  of them.  I have heard that one of them is better for
>long cable runs but it may hinder audio quality.  I  have  gotten
>several different responses from several different people.  Could
>somebody that knows about these things please let  me  know  what
>the differences in quality, length of cable run, etc. is????
>-- 

As a former designer of microphone preamplifier circuitry, let me take
a crack at this.

The physical configuration of the plug is not the determining factor
in audio quality by rather the following two factors:

1. Whether the microphone signal is being transmitted using a
   balanced (twisted-pair floating signal, ground shield, hence
   3-conductors) or unbalanced (ground-referenced signal, 2
   conductors) mode.  All other things being equal, a balanced
   interface will be some 20-40 db. less sensitive to interference
   from such things as 60 Hz hum and CB radio interference. A 1/4"
   phone plug usually has only 2 connectors, and can only be used
   in unbalanced mode.  An XLR connector has room for a three-
   conductor interface, and is usually wired in balanced mode.

2. What is the source impedance of the microphone element as seen
   from the cable's point of view.  A low impedance connection
   (150 ohms or so -- that's A.C., not as measured with your 
   ohm-meter) will be much less susceptible to high-frequency
   degradation due to the internal capacitance of long lengths
   of cable than will a high-impedance connection (50,000 ohms
   or so).  In the old days, XLR connectors went with
   microphones with low-impedance connections, while you could be
   pretty darn sure that a microphone with a 1/4" plug was
   high impedance.  With the new condenser mikes with built-in
   preamps, this is no longer a rigid rule, and microphone
   impedances can vary all over the map.

Still, for professional audio use, the universal standard remains a
low-impedance microphone with a balanced connection, terminated in
an XLR connector.  The major exceptions to this rule are miniature
microphones (tie-clasp and surveillance types) and electric guitar pickups.

The practical drawback in this scheme is that the low-impedance
microphone signals, unless pre-amplified at their source, will have
a much lower voltage coming into the mixer than a high-impedance
signal of the same total power.  This means that to achieve low-noise
operation you need an impedance matching transformer on every
mixer input.  An impedance matching transformer is a VERY expensive
piece of iron to build if you want combined wide frequency response,
high efficiency, good shielding, and negligible distortion.  Thus
many preamp manufacturers scrimp on the transformers, giving low-impedance
microphones a reputation for slightly "grittier" sound among some
audio purists.

In my experiences using microphones in a variety of settings (band tours
and the studio), I still wouldn't consider anything less than a 
low-impedance balanced connection feeding a high-quality matching 
transformer.   In a pinch, I'd take a two medium-impedance mikes (1-2 Kohms)
on very short (<10') cables directly into the mike jacks of a consumer
grade tape deck, but not unless I knew my locale was relatively free
from radio interference.  Only as a last resort would I try cheap
(read Shure) matching transformers outboard of the mixer or deck.

This much I feel may be of interest to the net.  Terry, if you
have more detailed questions, feel free to send me inquiries by
private mail.

Cheers,

Van Kelly
AT&T Bell Labs
Murray Hill N.J.

allegra!vek

The opinions expressed in this article are not necessarily those of
my employer.

lincoln@eosp1.UUCP (Dick Lincoln) (10/16/84)

> ....The practical drawback [to low-impedance microphones] is that the
> low-impedance signals, unless pre-amplified at their source, will have
> a much lower voltage coming into the mixer than a high-impedance
> signal of the same total power.  This means that to achieve low-noise
> operation you need an impedance matching transformer on every mixer
> input.  An impedance matching transformer is a VERY expensive piece of
> iron to build if you want combined wide frequency response, high
> efficiency, good shielding, and negligible distortion.  Thus many
> preamp manufacturers scrimp on the transformers, giving low-impedance
> microphones a reputation for slightly "grittier" sound among some
> audio purists......
  
> Van Kelly
> AT&T Bell Labs
> Murray Hill N.J.

Only one comment on this otherwise excellent response.  Expensive
transformers are not the only way to boost and convert the low signals
from low impedance, balanced sources.  Both my Carvin mixers have
operational amplifier integrated circuits on boards, four channels to a
board, that do this job very cheaply and very well.