[R-390] What have I done?

[Charles Steinmetz]

John wrote:

> I tried the diode load AF amplifier hi-fi method tonight and was stunned
> over how much better the R390A sounds that way.  If you haven't tried it
> with yours, do it!!!!  AC couple the diode load output through a
> electrolytic capacitor to a good AF amplifier and a hi-fi speaker.  Wow!  I
> won't say it's FM quality, but it's really, really good.

An electrolytic capacitor at this location is not a good choice, because 
it gets no DC to keep the electrolyte layer biased.  Also, a shunt 
capacitor is a good idea to filter out the residual 455kHz (and 
harmonics) that appear at the diode load terminal, so your audio 
amplifier doesn't have to deal with them.

I posted a recommended circuit for a diode load tap some while back:

<http://www.ko4bb.com/getsimple/index.php?id=download&file=05_Misc_Ham_Equipment/Collins/R390_R390A_diode_load_line_level_audio_tap_schematic_STEINMETZ.pdf>

Best regards,

Charles