Impedance transforming resistor

[K6JEK]

Do you know what happens when you put the output of your 300W AM transmitter into the input of your Titan amplifier?  Well after that you build a box called The Idiot Box.  In addition to preventing 3CX800A7 purchases it routes audio to a variety of transmitters and transcievers, line level and mic level, hi-Z and lo-Z.

But am I doing it the hard way with transformers?  I got a look at the schematic of the W2IHY box.  After a 1:1 (lo to lo)  isolation transformer and a variable attenuator circuit,  Julius provides  hi-Z output by simply putting a 47K resistor in series with the audio.  Voila!  hi-Z. 

Why not do that and stop scrounging around for lovely but expensive Jensen audio transformers?

Jon

[Bacon, WA3WDR]

That should work fine.  The most important thing is delivered audio level.  If you connect a speaker output to a mike input, the impedance isn't the issue, it's the mismatched dynamic range.  You can turn down the amplifier gain, but then residual hum and noise will dominate the throughput audio.  You want to pad things so that the source overload point is about the same as the input overload point.  Then residual amplifier hum and noise should not be an issue.  Maybe a speaker amp will be unhappy without a low low impedance load, but probably it will be OK.  You can stick a big ten ohm resistor on the speaker output, and then insert a high-value resistive divider to present about 47K source impedance and a low audio level to a high-Z mike input.  Of course this is an extreme example.

A line output into a mike input gives a similar but less severe problem of mismatched dynamic range.  In some cases with transformer input, you might get better response with the appropriate source impedance, so a properly designed voltage divider would be in order, and its source impedance would depend on the source the mike input expects.  Also consider the load the voltage divider and mike input present to the line source.  There are fewer issues with light loading than with heavy loading, but some transformer outputs may be sensitive to light output loads.

Keep in mind also that with high-Z audio, you can get serious high frequency rolloff with a long cable, because of cable capacitance.  You might want to put high impedance divider resistors in the mike-plug side of cables to avoid this problem.

The trick would be to provide a proper load to the source (usually a high impedance load is fine, occasionally not), and to divide the voltage so that the dynamic ranges approximately match, and present about the right source impedance to the audio input.  Transformer inputs are the most sensitive to the source impedance because of resonance effects, but for the very lowest noise, all inputs will be sensitive to the source Z.  You would choose the values of the voltage divider resistors to provide about the right level and the right impedance to the input.  And with high impedance audio, watch the cable capacitance.