[R-390] R390 whistle filter data

[Tim Shoppa]

Perry Sandeen <sandeenpa at yahoo.com> wrote:
> In Chuck Felton's r390 article in Electric Radio he showed the addition of a 5Kc whistle filter
> uing a 1 Henry choke and IIRC 690pf cap in series.

My calculations do NOT put this filter at 5kc, but at 6.061kc. Of course
winding or stray capacitance might bring the resonant frequency down
to 5kc.

> Three questions.  Was that a mis-print for the inductor value?  If not does anyone know what he
> used for the inductor?  Has anyone tried the mod?

Audio filters using L's and C's tend to use inductors in the 100mH to 1H
range. The "classic" surplus example is the 88mH telco line-loading
toroid popular in every single RTTY decoder for many decades. I think
there were Brit and Euro equivalents at slightly different values.

Some very small filament transformers might exhibit a winding inductance
in the 1H range if you want to play around (although most will be larger).
Hammond sells iron-core chokes in this range (well, generally larger,
but I think they go down to 1H) new today.

For AM reception on crowded SW bands I find the 8kc filter width does
a pretty good (but not perfect) job of getting rid of a carrier 5kc away.
Tune a little away from the QRM and it does even better (even if
introducing some distortion, it still is slightly better than 
switching to the 4kc IF bandwidth.) I can see how 6kc filters
became a "standard" on crowded bands!

Tim.