How Does An Output Transformer Work? What’s Impedance Matching?

How does an output transformer work? What does the output transformer do? What’s an output transformer? What is impedance matching? How does impedance matching work? Is impedance the same as resistance? How do you calculate impedance? Antique radio restoration. The output transformer is mounted directly to the speaker assembly which is a common arrangement in antique radios. Today, output transformers are still used, but they’re usually mounted to the amplifier not the speaker.

Looking at a close-up of the schematic, the transformer is represented here, and the speaker here.

I told you earlier that the speaker requires higher wattage, not high voltage. This is because a speaker has low resistance, but requires high current. High voltage is not required to push through the low resistance coil, but high current is required to get the voice coil to move. I’ll explain more about how the voice coil and speaker work in the next section.

Unlike the speaker, tube circuits, have high resistance which require high voltage, but pass little current. The job of the output transformer is to transform the high voltage, low current signal into the low voltage, high current required by the speaker. This process is called impedance matching because the tube circuit is high impedance and the speaker is low impedance.

Like resistance, impedance is measured in ohms, but takes into account the added resistance that an AC signal, like an audio signal, faces due to induction or capacitance. This additional resistance to an AC signal is called reactance.

Calculating reactance and impedance requires formulas that are beyond the scope of this series. But for now, you can think of impedance as being roughly 1.25 times an equivalent DC resistance. So a speaker rated for 8 ohms impedance will measure about 6.4 ohms resistance.

The output transformer transforms voltage and current just as the other transformers in our radio. But the coils are arranged differently and there is an iron core running through them.

A coil’s core influences the magnetic signal transfer between coils. An air core is helpful to transfer high frequencies, while an iron one assists the transfer of lower frequencies like those in an audio signal. Other materials, such as ferrite can be used for medium frequencies.

While transformers can increase or decrease voltage, tt’s important to understand that they can’t increase or decrease power. A step-up transformer increases voltage from the primary to secondary, but the laws of physics dictate that power stays the same, so current correspondingly goes down.

The output transformer is a step-down transformer with a smaller secondary coil than the primary. A high voltage, low current signal goes in, and a low voltage, high current signal goes out.

As we’ve learned, transformer coils can be checked with an ohm meter, and the output transformer in our radio tested fine. No repairs were required other than a little rewiring which I’ll show you later.

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