Impedance Matching with Line Matching Transformers.

I recently needed to match the 4K output of my BC-348 receiver to a standard 8 ohm headset. Looking through the parts pile yielded a Stancor A-8102 70.7V line matching transformer.

The catalog shows the A-8102 offers primary impedance values of 625, 1250, 2500, 5000 and 10000 ohms secondary impedance values of 4, 8, or 16 ohms. See the bottom of the following photo.

While the secondary taps are labeled in ohms, the primary taps are labeled in watts! So, how do you know which primary tap to use?

Let “E” be the voltage of the line matching transformer – in my case 70.7 V.

The impedance of the various primary taps is computed by

Z = E**2 / Pwr where E**2 = 5000 and Pwr is the labeling for the individual primary tap.

So my closest match would be the 1W tap (5000 = 5000 / 1).

73 Mark K3MSB

Copyright (c) Mark S. Bell 2019