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I found audio transformers online having the following specification: 600 : 600 ohms.

My question is what does it mean? Does it mean that it is simply an isolation transformer? Or is it in some way used for impedance matching?

How can it be used for impedance matching if both the primary and secondary have the same resistance? Lets say for example I have constructed an amplifier with output impedance of 600 ohms. The speaker has an impedance of 4 ohms. How can this transformer be used to match the impedance?

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  • \$\begingroup\$ Without a lot of context, this will not be answerable. Show a more comprehensive excerpt from where you're getting this. \$\endgroup\$ – Marcus Müller Jun 7 '16 at 12:40
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    \$\begingroup\$ It's a 1:1 transformer. The only way it'll do a good job matching to a 4 ohm speaker is if you dismantle it and rewind the secondary with fewer turns - sqrt(4/600) times the original turns, or about 1/13 the original number of turns, ideally in a wire with 13x the cross-sectional area. \$\endgroup\$ – Brian Drummond Jun 7 '16 at 14:33
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Impedance is just the ratio between voltage and current, like a resistor.

A transformer can change the ratio between in- and output voltage (and current as well) for AC signals.

So a 600 ohms to 4 ohms transformer lowers the voltage (and increases the current) so that 4 ohms at the output behaves as 600 ohms at the input.

That is useful when you want to connect a 4 ohms speaker to an amplifier which can only handle 600 ohm loads.

A 600 to 600 ohm transformer can indeed be an isolation transformer for an audio distribution system or a telephone line. The transformer is 1 : 1 meaning in- and output voltage stay the same (and the current as well).

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  • \$\begingroup\$ nothing wrong with this answer but could be a lot better \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 25 '17 at 0:35
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    \$\begingroup\$ @TonyStewart.EEsince'75 What a useful comment ;-) Any suggestions... ? \$\endgroup\$ – Bimpelrekkie Jun 25 '17 at 12:00
  • \$\begingroup\$ you can thank mr tweed for deletions ... 17hrs ago with comments on a critical answer from an expert in sound engineering, but poor format.. so... Dave Tweed tends to delete non answers in his mind, but does little to be constructive. A constructive answer might be... A 600 ohm amp would need to be high voltage ( tube) with a 13:1 transformer to drive 4 Ohms load with matched impedance actually at 3.6. With a 1:1 transformer, it would close to a short circuit. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 25 '17 at 19:15
  • \$\begingroup\$ 600:600=1:1 except usually high wire resistance , measure? thus 4ohm load on 600 ohm amp= attenuation ratio 4/(4+600) =0.00666... poor for this \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 27 '17 at 3:31
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It means this transformer is intended to isolate a audio signal between two different common mode voltages. It otherwise is intended to alter the audio signal as little as possible. This transformer is not intended to have a speaker connected to one side.

600 Ω is the official impedance of "line" audio, although line audio drivers are often lower impedance. The 600 Ω spec is giving you a clue that the secondary should be loaded with that resistance. That is the load resistance the frequency response and other specs are valid at. Anything else will probably yield a less flat frequency response over the audio range.

One use for such a transformer is at the receiving end of long cables. The driving and receiving equipment can easily have ground offsets between them, which would be added directly to the audio if it was a single-ended signal with ground as the common. At the end of the cable, both the signal and common are applied to the primary of the transformer. The common mode voltage then is largely cancelled out (only a little capacitive coupling between the transformer windings remains). The receiving equipment can then turn the result into a ground-referenced signal, if that's what it wants, just by grounding one side of the secondary.

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My question is what does it mean? Does it mean that it is simply an isolation transformer? Or is it in some way used for impedance matching?

It is an isolation transformer. It is for isolating signal lines with a characteristic impedance of 600 Ω. This may include audio and telephony circuits.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Using a 1:1 transformer to "unbalance" a balanced microphone for an unbalanced input while maintaining balanced operation over the length of the cable.

Signal level transformers have many uses such as isolation, ground loop prevention, differential signals, etc. Professional microphone and audio signal applications account are among the common applications where using various schemes are used to reduce any common-mode noise. As can be seen in Figure 1, any common signal on the two mic lines will be rejected by the transformer as it will only pass differential signals.

How can it be used for impedance matching if both the primary and secondary have the same resistance?

The impedance matching is required when both circuits are 600 Ω and isolation is required.

Lets say for example I have constructed an amplifier with output impedance of 600 ohms. The speaker has an impedance of 4 ohms. How can this transformer be used to match the impedance?

It can't. In any case, a 600:600 Ω transformer would be for mW signal levels and not 0.5 to 20 W levels, for example.

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A transformer is typically used for isolation and/or impedance matching.

Magnetic coupling between the primary (input) and secondary (output) provides the function of electrical ISOLATION. A transformer with the SAME impedance for both primary and secondary is clearly serving the function of isolation. You are correct, if the primary and secondary impedance is the SAME, is is not really used for the purpose of "impedance-matching".

IMPEDANCE-MATCHING typically implies that the transformer has DIFFERENT primary and secondary impedances. A 600 ohm transformer is not typically suitable to connect to a 4 ohm load. You would find a 600 ohm (primary) to 4 ohm (secondary) transformer used for impedance matching in that example.

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