Paul,
Would you mind giving more details about how the antivox output is connected in
your R-4B audio mod? I might do something similar to an R-4A.
Thanks,
Kihwal, K9SUL
________________________________
From: Paul Christensen <w...@arrl.net>
To: drakelist@zerobeat.net
Sent: Thursday, June 16, 2011 8:48 AM
Subject: [Drakelist] SM0VPO Audio Board in R-4C
The SM0VPO audio board worked so well in the R-4B, I decided to add one to my
R-4C -- which is a much easier exercise when the Sherwood PS-4 board is already
installed. The photos in the links below were taken last night after I
completed installation. Well, it was finished last week, but it took several
grounding iterations to get it right with no trace of buzz due to circulating
filament currents. Like my recent R-4B installation, there is absolutely no
hiss, hum, or buzz at any AF control setting. The output transistors run cold
to the touch and only get slightly warm when continuously running at high power
levels into the MS-4 speaker.
Getting near audiophile grade performance from a circuit that's designed to
work into *both* headphones and a speaker from a single-ended supply is no easy
accomplishment. I'm still amazed at the design. The photos below show the one
and only manner of installation (that I can figure out!) into the R-4C. The
board is mounted sideways on angled aluminum brackets such that the NB-4 can
still be plugged into the top without creating any mounting interference. The
location is good since it is away from the magnetic flux field of the power
transformer and AC HV line. Although not easily seen, a Zobel network was
installed, consisting of the usual 0.1 uF cap in series with a 10-ohm resistor
at the AF output. The large black cap is <gulp> 2,700 uF on the AF output
line. Ultra-low noise Toshiba BC550/BC560 transistors used in the high-gain
stage, with ample power supply de-coupling between the high gain stage and the
high-current TIP41 transistor
drivers.
http://72.52.250.47/images/R4C-1.jpg
http://72.52.250.47/images/R4C-2.jpg
http://72.52.250.47/images/R4C-3.jpg
Paul, W9AC
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