[Elecraft] Re: VOX sensitivity

[Baker]

On Mon, 5 Jan 2004 10:58:24 +0000, Julian, G4ILO wrote:
> I presume you're referring to his mod C: the addition of a 4.7K
> resistor to
> "prime" the compressor? No, I haven't tried that, but it looks
> simple
> enough to do so I might try it. I didn't realise that the VOX
> sensitivity
> was affected by the compression level. I use SSBC 3-1 already, and
> unlike
> G3RXQ I don't wish to use any higher setting. I can make VOX work
> with my
> desk mic by increasing the gain of its built-in preamp, but it
> doesn't help
> to have one mic with a much higher output than the others due to
> the lack
> of an effective mic gain control on the K2. In fact, I only want to
> use VOX
> with a head-set, which has no built-in preamp, and won't trigger
> the VOX at
> SSBC 3-1 even if I shout.

In the compressor IC there is a noise gate, and the mod of adding a 
resistor (preferably a pot) just lifts the un-amplified audio from 
most microphones over this threshold. It certainly does not add much 
compression.
>
> I think the lack of both gain and adequate control over the input
> audio
> level *before* compression is one of my few niggles about the K2.
> The
> number of people who have resorted to building preamps suggests that
> something ought to be done about it. I would have built a preamp
> myself if
> I was capable of doing it on a small board that could be fitted
> internally,
> as G3RXQ did. If someone came up with a small kit for this there
> might be a
> market for it. Even a schematic and a perf board layout would be a
> help for
> those of us who don't have the expertise to come up with our own
> circuits.
> Although there are plenty of preamp circuits around, designing one
> to fit
> into the K2 is a lot more tricky.
>
It was certainly a squeeze to get the preamp in position without 
shorting things out on either the front panel or control board.
I started with a perf board, but then made a "Kitchen Sink" one-off 
PCB. The biggest difficulty was stopping interference from the +5V 
line from getting on the audio. RF pickup also took some time to cure, 
so maybe with hindsight an external preamp would have been better.

For DX chasing/contesting  the effectiveness of an audio compressor 
comes behind an audio clipper and even further behind RF speech 
processing. I use a DATONG RF clipper for anything other than local 
contacts.

73

Stewart G3RXQ


> 73,
> --
> Julian, G4ILO. (RSGB, ARRL, G-QRP)
> G4ILO's Shack: http://www.qsl.net/g4ilo
>
>
> =3D=3D=3D=3D Original message =3D=3D=3D=3D
>
>> Julian,
>>
>> Have you considered the addition of a small compression to the 1:1
>> setting to increase VOX sensitivity, see
>> http://www.ac6rm.net/mailarchive/html/elecraft-list/2003-
>> 06/msg01036.htm
>> l and
>> http://www.ac6rm.net/mailarchive/html/elecraft-list/2003-
>> 08/msg00162.htm
>> l (G3RXQ)?
>>
>> --
>> 73,
>> Sverre
>>
>> ------------------
>> Sverre Holm, LA3ZA
>> www.qsl.net/la3za
>
>
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-- The Baker's Home 
-- baker@nildram.co.uk on 05/01/2004 
=A0