TenTec
[Top] [All Lists]

[TenTec] Re: Ten-Tec Omni-6 mic input from Write Log

To: <tentec@contesting.com>
Subject: [TenTec] Re: Ten-Tec Omni-6 mic input from Write Log
From: af5z@inetport.com (Bob Helms)
Date: Sat, 11 Sep 1999 17:54:03 -0500
Hi Barry,

I don't have an Omni-6 but the techniques of interfacing the audio to
other rigs should be very similar.  A few tips...

1.  Avoid end-fed antennas, balanced feedlines into the shack and
poor RF ground systems so you keep the RF in the shack to a minimal
level.

2.  Ground ALL your equipment together with short heavy gauge wire or
braid - - Including the PC, monitor, printer and any other
accessories in the shack.

3.  Use shields on everything you can - even DC power, speaker, keyer
and control wiring.  This helps avoid getting RF inside the cases of
various items in the shack.  Avoid cheap 'imported/Radio Shack' type
audio cables as the quality of the braid/shield on them is VERY poor
for RF frequencies.  You will have MUCH better results using RG-174/U
miniature (1/8" diameter) coaxial cable for use as shielded wiring in
the shack.  On critical low level audio cables (like microphone or
line level signals) put a decent size toroidal ferrite core on each
end of the cable near the connectors with several turns of the
shielded coax going through it.

4.  In severe cases you can use transformer coupling to break ground
loops and common-mode coupling.  To do this, put a small interstage
audio transformer in a metal box.  A 1K ohm to 1K ohm or a 1K ohm to
10K ohm transformer will work fine.  Bring in the sound card audio
output from the PC on a connector with a FLOATING ground.  This means
you use shielded cable from the sound card line output jack to a
two-conductor shielded plug on the  metal box.  Another choice of
connector here is a three-conductor (stereo) jack and plug at the box
end BUT MAKE NO CONNECTIONS TO THE BARREL/SHAFT OF THE PLUG.  INSTEAD
TIE THE CENTER CONDUCTOR OF THE CABLE TO THE TIP OF THE PLUG AND THE
GROUND/BRAID/SHIELD OF THE CABLE TO THE RING ON THE STEREO PLUG. 
Either way, the center conductor and braid/shield are wired through
the connectors to ONE winding of the audio transformer.  On the
second winding of the transformer, install a attenuator resistor
network to reduce the 1 volt RMS line level signal down to microphone
level.  This can be made from a 10K ohm trimpot wired so the two
fixed terminals connect to the second transformer winding wires. 
Connect a shielded output connector so that the center conductor is
wired to the wiper/moving contact of the trimpot and the
shield/ground/braid wire is connected to the junction of one of the
second transformer winding wires and either fixed terminal of the
trimpot.  The fixed terminal you choose will determine if the audio
level is increased when you turn the trimpot clockwise or
counterclockwise.  The ground/shield/braid side of the output jack
should be mounted to the metal box so the case is grounded.  DO NOT
tie the braid from the PC card output together with the braid of the
output cable going to the rig.  The output jack on the box mates with
a shielded connector and SHORT cable with the proper plug on it to
fit the OMNI-6 microphone jack.

To adjust the trimpot, set the microphone gain on the radio around
1/4th to 1/3 of the way up (near the same setting you normally use
with a microphone) and adjust the trimpot in the interface box to
provide the proper ALC action  when the sound card is producing
output.  Don't forget, if you change the MASTER volume or WAV file
volume on the sound card it will affect the drive to the radio.  Ohh,
it's a good idea to ground the interface box to the rig with a short
lead also.

PITFALLS:

The usual problems people have in trying to work with microphone
signals are:

A.  Hum due to inadequate or no shielding.  Transformers and/or
poorly filtered power supplies for microphone amplifiers are often
culprits.

B.  Bassy or tinny audio (poor audio frequency response) is usually
caused by either a severe impedance mismatch and/or excessive size or
number of bypass capacitors usually applied in an attempt to get 'RF
out of the audio'.

C.  Distortion/splatter is commonly caused by feeding FAR too much
signal level into the microphone input of the rig.  The DRIVE or MIC
GAIN control in most radios is in the circuit AFTER the first stage. 
This means if you overdrive that first stage, although you reduce the
level by turning the control almost off, the signal will still be
distorted.  A level equal to or slightly above the standard level
produced with the proper microphone is usually the best.

D.  Finally, particularly if the problem only occurs on some bands or
with some antennas, RF is likely 'getting into the audio'.  If RF is
combined with the audio signal fed to the amplifier in the radio, it
will cause your voice to sound like Donald Duck or like listening to
SSB with an AM receiver.  In severe cases, the radio will go into
oscillation and squeal or produce high output even when you are not
speaking into the mic (or when the  sound board is not producing
output).

Good luck.  With reasonable care you can produce a clean, high
intelligibility phone signal from the PC.  If precautions aren't
taken, you will have a distorted mess!

I took the liberty of forwarding your post to the Write Log EMAIL
reflector about using Write Log with a sound card to feed the mic
input of your Ten-Tec Omni-6.

It looks like I may get some of the replies as people simply clicked
on REPLY TO AUTHOR to respond.  So I'll forward these to you.

73,
Bob Helms, AF5Z
af5z@inetport.com

Message below from:     k4oj@tampabay.rr.com

...please share your experiences with the group (or at least me :-))

73,

Jim, K4OJ


Forwarded from:

n1eu@yahoo.com

Would like to interface my PC's Sound Blaster 16
output to the Omni 6 mic input.  I use WriteLog
logging software which has full capability to use the
PC sound card as a digital voice keyer, or to record
messages and trigger them with the function keys
during contests.

Would appreciate tips from anyone who's been there,
done that  ;-)

Thanks & 73,
Barry  N1EU


- --
Submissions:              writelog@contesting.com
Administrative requests:  writelog-REQUEST@contesting.com
Problems:                 owner-writelog@contesting.com

------------------------------

--
FAQ on WWW:               http://www.contesting.com/tentecfaq.htm
Submissions:              tentec@contesting.com
Administrative requests:  tentec-REQUEST@contesting.com
Problems:                 owner-tentec@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm


<Prev in Thread] Current Thread [Next in Thread>
  • [TenTec] Re: Ten-Tec Omni-6 mic input from Write Log, Bob Helms <=