Audio Transformers | George Cox

Double Primary Audio Transformers

After several months of using the toroidal audio transformer (#Building a Toroidal Audio Transformer:), I decided I needed more audio power output.

Since the amplifier is a stereo amp, I decided to try combining the outputs of both channels to get more power. Due to the transformer-less design of the amplifier, I could see that just parallel connecting would not work. I needed a transformer with two primaries and a step-up ratio to the secondary that will connect to the speaker.

I built two similar transformers based on this idea, one using EI-100 laminations, the other using EE 70/33/32 ferrite “E” cores. Because the ferrite Bmax as about 0.300 Tesla’s, as opposed to 1.000 Tesla’s for the laminations, I decided that anything under 300 Hz was unimportant for ham radio voice signals, I used this for the low end frequency in designing the ferrite version.

EI100 transformer:

Stacked to 1.5 inches

Primary 2ea @ 107 T 24ga

Secondary 150 T 25 ga

Undistorted 500 Hz output across 7.6 ohms: 4.5 Vrms = 2.66 watts

Undistorted 100 Hz output across 7.6 ohms: 4.36 Vrms = 2.50 watts

Undistorted 50 Hz output across 7.6 ohms: 4.35 Vrms = 2.49 watts

EE 70/33/32 transformer:

2 of these “E” ferrite cores taped together = 2-inch stack equivalent

Primary 2ea @ 72T 21Ga

Secondary 2ea @ 144T 23Ga (Paralleled)

Undistorted 500 Hz output across 7.6 ohms: 6.22 Vrms = 5.09 watts

Undistorted 100 Hz output across 7.6 ohms: 6.36 Vrms = 5.32 watts

Undistorted 50 Hz output across 7.6 ohms: 5.8 Vrms = 4.42 watts

Since the ferrite has far superior power out, I went with it.