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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