[TenTec] Heil ProSet adapter with Orion -- hum

Thu Jun 24 21:26:46 EDT 2004

Actually no.  The input stage toady is typically a differential device
having a non-inverting and an inverting input.  The output of that stage is
coupled back to the inverting input via a R network including a bias R to
reduce or establish the input stage gain.   This then causes the two inputs
to be of different Z and thus unbalanced yet above ground.  The audio ground
is used the carry the difference in current  which is a result of the
different loads presented to the + and - because of difference in Z of the
two inputs.  In a true balanced configuration, no audio ground is needed as
there is no difference in I and R load between the two audio lines.  Another
way of looking at it is the 230 volt power distribution with our homes.  The
neutral is only there to handle the difference in current between L1 and L2.
Considering non inductive loads, if the current on L1 is 10 amps and the
current on L2 is 8 amps the neutral current should be 2 amps.

In our classic differential device, if a voltage of equal magnitude is being
induced from the outside world into audio + and audio - lines and these
connected to the non-inverting input and the inverting input the resultant
output is zero while a differential voltage from the mike produces an output
equal to the input voltage plus the gain of the stage.   In one companies
mike cable they use a single conductor shielded cable.  If the audio signal
line (center conductor) is connected to the non-inverting input and the
shield to the inverting input then any noise on the shield is amplified and
thus appears at the output of the device.

Traditionally the XLR connector has been used for both balanced and
unbalanced service in professional and broadcast audio for years.  It has 3
pins which have been designated as audio + audio - and ground.  But what
about shield?  In a balanced audio system, no ground is needed.  However,
shield is provided and is connected according to IEC standards to pin #1
with pin #2 being audio high or + and pin #3 being audio low or -.   Where I
find most problems arise is that shield is connected to pin 1 on both ends,
thus when connected between two pieces of equipment and may present a
condition that allows current to flow on the shield conductor.  Now we all
know the laws of physics with regard to current flow and magnetism, the
right rule of thumb and such.  The point is that under these conditions the
shield can not effective perform as intended purpose with no current flow
and its intended purpose is thus largely negated.

Enter 3 conductor shielded configuration.  In this configuration, we provide
conductors for audio +, audio -, ground and shield.  In using XLR connectors
there is a 4th connection which picks up the shell of the connector. Its
intended purpose is to provide continuity of shield without providing a
current carrying path.  I was startled to see a line of plastic/non
conductive connectors introduced some years ago.  At one installation that I
serviced they created havoc.

Now enter the unbalanced mike.  With the above holding true, the unbalance
should occur or be created at one end of the cable only, thus typically the
input.  The accepted rule is to treat it as balanced all the way to the
load, in this case, the active input device.

I stated earlier here on the reflector that all of my audio lines are 3
conductor shielded.  Oh yes I may have 2 of the wires and a shield connected
together on the RCA plug that goes into the radio with one wire to the
center pin.  That is the point that it gets unbalanced.  If the source/mike,
is unbalanced then so be it.  The wiring is standard and works in all
configurations, balanced, unbalanced, balanced to ground or unbalanced to
ground.  of course in a balanced or unbalanced to ground condition an RCA
connector is not suitable for the terminus.

Perhaps this helps answer some of the concerns of those surrounding balanced
vs. unbalance conditions.  I wish we only had a way to present drawing here
as it would make the picture very clear without the extensive verbiage.

73
Bob, K4TAX

----- Original Message ----- 
From: "George Skoubis" <george.skoubis at verizon.net>
To: <tentec at contesting.com>
Sent: Wednesday, June 23, 2004 11:33 PM
Subject: RE: [TenTec] Heil ProSet adapter with Orion -- hum

> Bob,
>
>   Is the audio ground only present when using a balanced mike? With an
> XLR connector?
>
>   On a Heil headset I can only envision +, -, and shield.
>
>   I'm not trying to be a smart alec with this question, just wondering
> where the audio ground is found on a headset.
>
> Thanks,
>
> George / KF9YR
>
> -----Original Message-----
> From: tentec-bounces at contesting.com
> [mailto:tentec-bounces at contesting.com] On Behalf Of Robert & Linda
> McGraw K4TAX
> Sent: Wednesday, June 23, 2004 10:55 PM
> To: tentec at contesting.com
> Subject: Re: [TenTec] Heil ProSet adapter with Orion -- hum
>
> I shouldn't bring this up again for someone will blame it on the "Pin 1
> problem".  The fact is there's typically 4 connections for microphone
> audio.
> There is the Audio Hi or +,  Audio Lo or -, Audio Ground, and Audio
> Shield.
> That's 4!  The big problem lies in the misunderstanding that Audio Lo,
> Audio
> Ground, and Audio Shield can be the same or connected together.  They
> can't
> and shouldn't.  Shield is shield and ground is ground.  They aren't the
> same
> as ground may be a current carrying conductor and shield should not be a
> current carrying conductor.
>
> In all of my mike lines there are 3 conductors which are surrounded by
> one
> shield.  None of these are used for DC or PTT. They are all for proper
> audio
> connections.
>
> 73
> Bob, K4TAX
>
>
>
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