My particular application is the audio input to the HF-300 homebrew
transmitter. The setup is a follows: The nominal 600-ohm output of a UREI
BL-40 Modulimiter (solid state compressor/limiter designed for AM broadcast
service), feeds through a variable attenuator to a line amplifier, which
drives the transmitter audio section. The transmitter uses four 2A3's in
pushpull-parallel (cry your eyes out, audiophools!) to drive the class-B
modulator, presently a pair of 810's. The broadcast-quality input
transformer is a UTC LS-18, 500-ohm primary to 50,000 ohm split secondary.
I have a 6 dB fixed resistive H-pad inserted in the balanced line between
the line amp and the transmitter. A General Radio audio level indicator is
permanently connected to the output of the H-pad to monitor the audio level
going into the transmitter. I originally had a 25K carbon resistor shunted
across each section of the transformer secondary, to load it down to 50k.
Last night, I removed the 25K resistors and replaced them with a pair of
100k resistors, placing a 200k load across the secondary instead of 50k. I
immediately noticed about 6 dB more audio drive to the modulator tubes. I
reduced the signal level to the line amplifier until I once again had
approximately 100% negative peak modulation. I noticed the G-R level meter
reading exactly the same as it did before I changed resistors, so the
increase in output shows up as more signal going into the transmitter, as
the 500-ohm line feeding the transmitter is seeing a higher load impedance,
as reflected back through the input transformer. I listened on a receiver
to a recording played to modulate the transmitter while working into a dummy
load, and noticed little, if any change in quality, although I haven't tried
running a frequency response test.
My take on the situation is as follows. The nominal 500/600-ohm signal
source feeding the transmitter could be thought of as an a.c. generator
with zero internal resistance, wired in series with a 500 to 600 ohm
resistor. That effective series resistor loads down the transformer
secondary at 50k as reflected through from the primary, and the 50 kilohms
of resistance I had shunted across the transformer effectively loaded the
secondary of the transformer to 25k. instead of 50k. Although this might
have smoothed out variations and slightly extended the frequency response of
the transformer, the additional load soaked up 6 dB's of signal level, and
may have increased distortion, since both the limiter and transformer were
having to handle substantially more audio current to maintain the signal
level. The 200k's of resistance will still tend to damp out any ringing
effect of a totally unloaded secondary, but the effective load on the
transformer is now more nearly correct, supplied by the 500/600 ohm internal
resistance of the driving source. I could load the transformer to exact
manufacturer's specs by adding a 50-ohm resistor in series with each leg of
the balanced audio line feeding the transmitter, at a slight expense of
signal level.
Don k4kyv
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