Audio Matching Transformer Miniature Output Center Tap Inter Stage

This is one item of new, sub-miniature audio transformer commonly used in small, low voltage supply transistor radio audio output stages. Three different types are available; An inter-stage phase splitter, an output phase-combiner and impedance matcher, and an unusual inter-stage device with three separate windings. Power rating is estimated at 150mW. Overall dimensions are 14.3L x 12.5W x 11.3H (mm). Basic test results at 1kHz are shown in the secondary pictures.

These are largely obsolete for the original intended purpose, but they do have their "cunning plan" uses, like obtaining beyond-rail drive from a voltage amplifier stage into a small class B output stage running on a 3V battery supply, without having to resort to boost supplies.

Output Yellow

The primary side is the tapped side. This has a D.C. resistance of approximately 10.6 Ohm, and that of the secondary is about 1.8 Ohm. The secondary is intended to drive a small 8 Ohm radio speaker. Ignoring the centre tap, the impedance ratio is about 6.25:1, making the primary impedance 50 Ohms.

Inter-Stage 1 Green

When used conventionally, the untapped side is the primary side. The primary side has a D.C. resistance of approximately 232 Ohms and the secondary side shows 208 Ohms, 104 Ohms between the centre tap and each outer connection. Ignoring the centre-tap, the impedance ratio is about 2.4:1

Inter-Stage 2 Blue

This type is slightly unusual, as it has two separate secondary windings. The voltage ratio from primary to one secondary is approximately 2:1 giving and impedance ratio of 4:1. The primary has a D.C. resistance of 155 Ohms and each secondary measures around 68 Ohms.