KX3 Audio output

[Jim Brown-10]

On 5/30/2011 8:50 AM, Rich - K1HTV wrote:
> Do a Google search for "Coby CA-745", an FM wireless transmitter that plugs
> into a cigarette lighter. You can find them at Amazon, Wal-Mart and Best Buy for
> between $16 and $19. The input is via a stereo miniplug at the end of an
> attached cable.

There is potential for significant difficulty using a device powered
from one point in the automotive DC power with another device powered
from a DIFFERENT point in the same automotive power system. The reason
is simple -- there can be a lot of DC resistance between different parts
of the system, and a lot of noise, both audio and RF, between those
points.  The audio interconnect is unbalanced, so any noise that is
present is likely be added to the audio feed.

Consider the use for which these modulators were designed -- a
BATTERY-powered personal music player, like an iPod or CD player.  With
these devices, there is only ONE connection to the automotive power
system, the lighter plug.  Ditto with a wired AUX plug-in for those
players.

BTW -- this issue requires VERY CAREFUL attention to the design of DC
busing and both audio and RF interfacing between the KX3 and the power
amp if those units are to be physically separated (for example, the
power amp in the trunk or under a seat).

73, Jim Brown K9YC


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[k.igor]

Knowing Elecraft design practices most likely the audio output is isolated,
so your concern is notwithstanding.
73
Igor, N1YX

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Jim Brown
Sent: Monday, May 30, 2011 1:54 PM
To: [hidden email]
Subject: Re: [Elecraft] KX3 Audio output

On 5/30/2011 8:50 AM, Rich - K1HTV wrote:
> Do a Google search for "Coby CA-745", an FM wireless transmitter that
plugs
> into a cigarette lighter. You can find them at Amazon, Wal-Mart and Best
Buy for
> between $16 and $19. The input is via a stereo miniplug at the end of an
> attached cable.

There is potential for significant difficulty using a device powered
from one point in the automotive DC power with another device powered
from a DIFFERENT point in the same automotive power system. The reason
is simple -- there can be a lot of DC resistance between different parts
of the system, and a lot of noise, both audio and RF, between those
points.  The audio interconnect is unbalanced, so any noise that is
present is likely be added to the audio feed.

Consider the use for which these modulators were designed -- a
BATTERY-powered personal music player, like an iPod or CD player.  With
these devices, there is only ONE connection to the automotive power
system, the lighter plug.  Ditto with a wired AUX plug-in for those
players.

BTW -- this issue requires VERY CAREFUL attention to the design of DC
busing and both audio and RF interfacing between the KX3 and the power
amp if those units are to be physically separated (for example, the
power amp in the trunk or under a seat).

73, Jim Brown K9YC


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[k1htv]

Jim,
   Looks like the addition of an inline audio transformer between the wireless
transmitter's miniplug and the KX3 (or any device powered by the vehicle) would
resolve the potential problem which you have addressed.

73,
Rich - K1HTV

= = =

-----Original Message-----
From: Jim Brown [mailto:[hidden email]]
Sent: Monday, May 30, 2011 13:54 PM
To: [hidden email]
Subject: Re: [Elecraft] KX3 Audio output

On 5/30/2011 8:50 AM, Rich - K1HTV wrote:
> Do a Google search for "Coby CA-745", an FM wireless transmitter that
> plugs into a cigarette lighter. You can find them at Amazon, Wal-Mart
> and Best Buy for between $16 and $19. The input is via a stereo
> miniplug at the end of an attached cable.

There is potential for significant difficulty using a device powered from one
point in the automotive DC power with another device powered from a DIFFERENT
point in the same automotive power system. The reason is simple -- there can be
a lot of DC resistance between different parts of the system, and a lot of
noise, both audio and RF, between those points.  The audio interconnect is
unbalanced, so any noise that is present is likely be added to the audio feed.

Consider the use for which these modulators were designed -- a BATTERY-powered
personal music player, like an iPod or CD player.  With these devices, there is
only ONE connection to the automotive power system, the lighter plug.  Ditto
with a wired AUX plug-in for those players.

BTW -- this issue requires VERY CAREFUL attention to the design of DC busing and
both audio and RF interfacing between the KX3 and the power amp if those units
are to be physically separated (for example, the power amp in the trunk or under
a seat).

73, Jim Brown K9YC




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[Jim Brown-10]

On 6/1/2011 5:31 AM, Rich - K1HTV wrote:
>   Looks like the addition of an inline audio transformer between the wireless
> transmitter's miniplug and the KX3 (or any device powered by the vehicle) would
> resolve the potential problem which you have addressed.

Yes -- IF the transformer did not pick up magnetic fields from the
vehicle's systems.  I saw that when doing testing of some pro mics for
RFI in a Volvo S80 -- the output transformer in the mic was not
shielded, and picked up trash.  That would be fixed with a transformer
that has a mu-metal shield, but shielded transformers are expensive.

73, Jim K9YC

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[k1htv]

Jim,
  I can see that magnetic fields in a vehicle could cause problems with a
microphone, as the output level is in the -50dBm range and is much more
susceptible to it . Using equipment like the KX3 at consumer line level (-20dBm)
there would much less of a problem with magnetic fields, so a less expensive,
unshielded in-line transformer would probably work just fine.

73,
Rich - K1HTV

= = =

-----Original Message-----
From: Jim Brown [mailto:[hidden email]]
Sent: Wednesday, June 01, 2011 13:04 PM
To: Reflector Elecraft
Subject: Re: [Elecraft] KX3 Audio output

On 6/1/2011 5:31 AM, Rich - K1HTV wrote:
>   Looks like the addition of an inline audio transformer between the
> wireless transmitter's miniplug and the KX3 (or any device powered by
> the vehicle) would resolve the potential problem which you have addressed.

Yes -- IF the transformer did not pick up magnetic fields from the vehicle's
systems.  I saw that when doing testing of some pro mics for RFI in a Volvo S80
-- the output transformer in the mic was not shielded, and picked up trash.
That would be fixed with a transformer that has a mu-metal shield, but shielded
transformers are expensive.

73, Jim K9YC



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