[AMRadio] RE: Resistive load on interstage audio transformers

D. Chester k4kyv at charter.net
Thu Mar 13 12:13:49 EST 2008

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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