Topband: Hi Gain Preamps, Noise Figure Comparison (KD9SV 994 x5, W1FB, W7IUV preamps)

[Bob K6UJ]

Don,

Thanks for sharing your research on the preamps.  Very informative.
Your study is very timely for me.
I have ordered DX Engineerings new phasing box, NCC-2.
Optional plug in preamps are available for the unit.  I am holding off
on ordering a preamp until I compare specs with other external preamps.
The KD9SV unit looks pretty good !
Now to find out how it compares.

Bob
K6UJ



On 11/8/16 1:54 PM, Don Kirk wrote:
> Gary (KD9SV) kindly loaned me one of his 994 x5 preamps (sometimes called
> KD9SV VLN (very low noise) preamp) so I could compare its Noise Figure
> against one of my W1FB preamps as well as 2 of my W7IUV preamps in series
> on 160 meters.
>
> Here is a link to a youtube video I created that demonstrates the
> differences in Noise Figure between the above mentioned preamps.
> http://www.youtube.com/watch?v=JpIiF1Hfovw
>
> The KD9SV 994 x5 preamp did in fact come out on top (best Noise Figure) and
> it's a commercially available preamp, whereas my W1FB and W7IUV preamps are
> home built.  Nevertheless in my current application (pennants that are
> 51.6% the size of full size pennants) the W1FB and W7IUV preamp Noise
> Figures are adequate and I didn't notice improvement in my signal to noise
> ratio on 160 meters when using the KD9SV preamp.  The lower Noise Figure of
> the KD9SV would allow me to use even smaller pennant receive antennas that
> would have more negative gain without harming my signal to noise ratio.
>
> I ran into a couple of interesting issues when working on this project.
> The biggest surprise was that the input impedance of my old HF rig (Kenwood
> TS-180s) was no where near 50 ohms on 160 meters (it actually measured 189
> ohms).  I added a transformer to the input of the Kenwood TS-180s to bring
> it closer to 50 ohms for my testing.
>
> Also as previously reported by Hardy (N7RT) a long time ago, the W1FB
> output transformer turns ratio is incorrect for a 50 ohm match, and I
> measured the output impedance of the W1FB preamp at 429 ohms.  I
> subsequently added a transformer to the output to bring it closer to 50
> ohms for my testing.
>
> *Noise Figure Comparison (average of two different measurement methods)*
> W1FB preamp: Noise Figure 3.3 dB higher than KD9SV 994 x5 preamp
> Two W7IUV preamps in series: Noise Figure 1.1 dB higher than KD9SV 994 x5
> preamp
>
> Note 1: my W1FB preamp is not stock, as I made modifications to protect the
> MC1350 IC from damage when transmitting.
> Note 2: The KD9SV 994 x5 preamp was set at its maximum gain (41 dB) for all
> of my tests.
>
> *Summary*
> While I suspect (through my measurements) that the KD9SV preamp absolute
> Noise Figure value is not the very low 0.7 dB value listed on a commercial
> website, it's indeed a very nice adjustable high gain preamp offering a
> maximum gain of 41 dB and it has the best (lowest) Noise Figure when
> compared with my other two types of preamps.  I highly recommend the KD9SV
> 994 x5 preamp if you're in need of a high gain preamp on 160 meters.
>
> 73,
> Don Kirk (wd8dsb)
> _________________
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>