[Lowfer] Preamp question

[Jay Rusgrove]

Mitch

The transformer should have 30 turns of bifilar wound on it. Connect one
wire from one end of the pair to the opposite wire at the other end of
the winding. If the winding is correct and the resistor values are
correct the feedback will establish the gain at 20 dB. Over the years
we've built about 2000 of these amplifiers and I've never seen the gain
stray more than a dB from the 20 dB mark.

The 100' loop here is untuned. In my case, I wanted a broadband general
purpose receive antenna good down fairly low in frequency (10 kHz) as
well as NDB and broadcast band stuff with no tuning. A rough calculation
of Q for this loop (if you tune it) puts it at a pretty low 40 or so.
The background noise output is high enough to overide the receiver noise
floor with it untuned anyway. Since the preamplifier doesn't overload (I
can only speak for my location) there really is no reason to tune it to
resonance anyway - right? The pattern of the loop should be identical to
any other loop - octo or box.

Let me know how you make out.

Jay

Mitch Powell wrote:

> Jay:
> Preamp working (with 3866) very nicely. Approx 30 dB gain
> and working with Vcc = 12V.
> Question about the toroid....
> I have wound 15-0-15   for the 30t bifilar.  Is that correct.
>
> Or should I read that as 30-0-30  ???
>
> Raining overnight and morning so all testing has been with the
> preamp inside house..... coming from the
> series tuned / toroid coupled 100 ft triangle.
> Obvious and definite peak around 185.3 where the loop is tuned ! !
> I will be making changes this weekend to make open-top and shielded
> loop - for totally untuned operation.
>
> Mitch
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