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Re: DEM Pre-Amp



My .7db NF preamp arrived several days ago and just got put in the
system with the converted Drake (spec 3 db NF.) It seems to have a
problem or is causing a problem. It increases the S meter reading
from S1.5 to about S3, but the s/n ratio seems worse. It seems to
work only at about 7.5 V for a decent gain and NF.  S meter increases
to  >S4 at this supply voltage and the NF improvement is obvious
listening to the AO-40 beacon. My 12V supply isn't useable
for this preamp. The spec sheet says that the supply range is
7 to 16 V. The spec doesn't have a curve for NF or gain vs supply
voltage.
Does anyone know if this is typical of this preamp (model 13ULNA)?
I sure hate to send it back since it took so long for them to ship
and it's up on my tower now.  I'm tempted to just use a 7.5V supply
and keep the preamp. The schematic shows a variable resistor from the
IC that generates the internal negative voltage to ground and the wiper
to the gate of the FET.  It may be that this pot just didn't get
adjusted
at the factory to a proper voltage for the gate. I don't want to take
a chance on blowing the FET by trying to adjust  this for optimum
gain and s/n ratio. Any suggestions?


Ken   K5MBV

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