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.
#radiorepair #radio #electronicsrepair #restoration

[@petersharpe1516]

Brill! Very informative and patiently delivered

[@bca10101]

After much mental struggle to grasp this subject, it was this video that finally gave me that eureka moment. Thanks--that made my day!

[@FluxCondenser]

Glad to hear that, Bruce. Your comment made my day! When I make these videos I think about what first confused me most about the subject and try to create the video I wished existed to help me. The process of making the videos has actually helped me understand things better and funnily enough, I sometimes watch my own videos to re-teach myself when I inevitably forget something technical!

[@grahamamorrisonsr1135]

Excellent description. I wanted to know why old radio speakers (tube type) use these transformers and you explained it perfectly. Thanks!!

[@FluxCondenser]

My pleasure. Glad you found it helpful!

[@illajayscrewedup5886]

Great video. very informative.

[@iRepairElectronics]

you do a beautiful job in translating this stuff. nice visuals as always.

[@FluxCondenser]

Thanks so much for taking the time to leave a kind word. Much appreciated.

[@deerounet]

Great job! Speaking about audio transformers frequency range, what mechanical aspect of the transformer determines how much range you will get? I'm not quite sure to grasp the difference between power transformers and audio transformers. Thank you.

[@FluxCondenser]

Thank you! It’s the core of the transformer that affects its magnetic flux and frequency range. Air core transformers are appropriate for very high frequencies. A ferrite core for high frequencies. And an iron core for lower frequencies such as power or audio applications. A power transformer designed for 60 hertz actually crosses into some of the audio band which is between 20-20,000 hertz. A power transformer and audio transformer are similar, but the core will be a bit different, I imagine, to tune them to the appropriate frequencies. That’s about the limit of my knowledge on the subject, but you’ve now got me curious to learn more about how this frequency tuning is done.

[@deerounet]

@@FluxCondenser Thanks it's clearer now! I'm trying to gather audio transformers from random audio equipment but I'm not very lucky so far...on the other hand I have loads of power transformers, what range covers a 60hz power transformer according to you ?

[@FluxCondenser]

You can experiment, but I don’t think you’ll have much luck using a power transformer for audio. For very low fidelity applications it might work, but a transformer that’s designed for 60 hertz just isn’t likely to cover the entire audio band. Keep in mind, too that your primary concern (no pun intended!) is that your audio transformer provide the correct impedance. To replace an audio transformer, a power transformer not only has to be able to handle most of the audio frequencies, it has to provide the correct step-down for the circuit. Note that I can’t simply give you a transformer impedance to use as this will vary greatly depending on the circuit. A tube amp will have different requirements than a solid-state amp, for example. Look on the audio transformer or schematic to find the value. You can also do a calculation. Calculating impedance is complex, but you can roughly calculate it by measuring the transformer’s DC resistance and multiplying that by 1.25. If you have a power transformer which provides a suitable impedance for your audio circuit give it a try. I’d be very curious to hear about your results.

[@deerounet]

@@FluxCondenser Thanks again for such a detailed explanation! I might be wrong but, the audio transformer I'm looking for doesn't need to match impedance because I'm just using it to multiply 2 different signals (ring modulator attempt). the ideal ratio should be 1:1. In the meantime I found a hum destroyer with 2 full range audio transformers, such a shame I damaged one while desoldering it. I guess it was the price to pay to learn how to correctly desolder a pin component. (create 2 parallel rows of tin solder and heat them alternately as quick as possible while holding the circuit upside down to let gravity do its job ;) I tested the good one, I have around 500ohms on both sides. In my audio transformers searching quest I found a very old cassette recorder (telefunken MC310) I inspected the schematic and found a transformer. It is cylindrical, and it has a flat screw in the middle that you can move around 360° (never seen that feature on any transformers before), I tested it : 30 ohms : 4ohms But it doesn't seem to work as good as the one I got from the hum destroyer. I will try other power transformers.

[@CHANDANSINGH-ou6ll]

Hindi bole nahin aata hai

[@FluxCondenser]

Sorry to hear that. I’d try a 100 ohm resistor wired in series. Also, double-check the ground connection.