[1000mp] Sound Card - FT1000MP

[Tom McDermott]

Hi Joe,


> Using an attenuator between the sound card and the patch 
> input is absolutely the wrong thing to do:  
>
>   1) the patch input is at mic level.  Use the DVS input 
>      for voice ... it bypasses the mic preamps and requires 
>      a more appropriate level. 
>   2) Do not attenuate the line out of a typical sound card, 
>      turn down the sound volume.  The line out amplifiers 
>      in most sound cards do not have sufficient headroom 
>      to handle full output without distortion (IMD).  Run 
>      the "Playback" and "Wave/MP3" level sliders at no more 
>      than 50%.    


	It's good advice not to run the soundcard at too high an output
level,
as you indicate. My particular soundcard achieves about 13 ENOB (effective
number
of bits) around 85 dB dynamic range at the best setting. Reducing it's
output
level with slider controls by 60 dB to prevent overdriving the radio throws
an 
additional 10 bits away resulting in about 3 effective bits. Well, that
sounds pretty
bad on my radio.

	However, using an external attenuator prevents having to throw
away so many bits, and the results in my case are rather dramatic in terms
of
improvement. You will find other web pages that report the same result.
So it's possible to have too much attenuation, but also too little
attenuation. I've had good results with the RS attenuating cable, but also
just using two resistors.

	Using the higher-level inputs is a really good idea as you suggest.
I
would think most hams would probably not use the DVS input, and thus the
attenuator
suggestion.


> An (high quality) audio isolation transformer is a good idea 
> to help eliminate hum (ground loops).  However, make sure it 
> is of high quality and does not saturate the core in the 
> presence of RF or you will end up with distortion problems.  


	I totally agree. I had some left over from a modem project, and they
perform very well and were inexpensive. They also privide 1 kV of isolation,

I hope never to need that. I have not tried the RS transformers, some
(George, W5YR) report problems with certain ones. I trust George's
measurements.


> Further, using an attenuator ahead of the patch input is 
> a good prescription for RF feedback problems.  The patch 
> input has two high gain stages ahead of the gain control. 
> Any RF feedback on the low levee audio will be much harder 
> to resolve on the patch input.  


	This is not my experience. One version I use includes an isolation
transformer and attenuator in a small box right next to the patch input.
This
box and the radio are about 3 inches away from a 1.5KW amplifier, and
absolutely
no RFI is present. The only case I can guess where RFI might occur is if the
atteuation
occurs a large distance away from the radio, thus increasing the length of
cable
carrying low level signals. This seems to be the situation when using no
attenuator and
letting the soundcard control the level.

	I've also used a MIC preamp for an old Telex headset that has
exceptionally low
output (and most uncomfortable earpads). The amplifier contains about 20 dB
of gain, 
but is provided with RC filters on the input and output leads. It was very
susceptible to
RFI before the inclusion of the RC filters, caused by RF rectification in
the op-amp.
If the soundcard output similarly rectifies RF, then the attenuator will
reduce that just 
like the desired signal. So I have come to the conclusion that the
attenuator only helps
the RFI situation.


> 73, 
>
>   ... Joe, K4IK 
>