I understand that in order not to potentially overload an amplifier one should connect a speaker with an impedance no less than that specified, and I have just recently come to understand that for maximum power transfer the amplifier and speaker impedances should match.

So here, I know the AMP and speaker impedance should match for the best possible power transfer, and that a speaker impedance should not exceed the amplifiers specification:

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However, I don't fully understand the implications for a constant voltage speaker system, like this:

enter image description here

The constant voltage speaker system makes perfect sense to me, I understand that like national power transmission, the high voltage low current means less loss... but... my question is...

Does the transformer coil impedance now have to match the amplifiers impedance/specification, OR, because of the dark magic of reactance, or other forces beyond my comprehension, does the presence of the two transformers become irrelevant so that one must still match the amplifier and speaker impedances?

I'm afraid that despite reading numerous articles I must admit I have yet to fully grasp impedance, reactance, inductance, capacitance, admittance, conductance, susceptance. I obviously now have SOME understanding, but really it's mostly far to shaky for much practical use yet.

  • \$\begingroup\$ As Andy said, you don't want to impedance match amplifier and speaker. That is a misconception. The coil impedance of the transformer is not a concern in this application. If you step up, then step down by the same ratio, then it is like the amp is directly connected to the speaker. But if the step up and step down ratios of the two transformers are different, they will transform the impedance of the speaker to a new value. \$\endgroup\$
    – user57037
    Dec 29, 2015 at 18:19
  • \$\begingroup\$ @mkeith Thanks, that's useful, I got the impression that somehow the transformers were irrelevant, but no one says it, and I didn't have the understanding to feel confident. I have read that the step up step down transformers for this use case should be matched, and it certainly seems sensible to match them to avoid complication. I wish I hadn't said the bit about the max power transfer, it's not really relevant with hindsight, max power doesn't mean good efficiency, and it might mean distortion... I was just trying to get across my understanding of such matters as far as it went. \$\endgroup\$
    – Michael
    Dec 29, 2015 at 18:32

1 Answer 1


I have just recently come to understand that for maximum power transfer the amplifier and speaker impedances should match.

Not this is not the case with audio - an audio amp can have an output impedance that is substantially lower than 1 ohm yet nobody makes (as far as I know) 1 ohm speakers. If the amp had an 8 ohm output it could only deliver half the voltage to an 8 ohm speaker and the rest of the power would be wasted in its output impedance.

It's only RF circuits that you need to be concerned about matching impedances but this is more to stop reflections down PCB tracks and cables.

The rest of your question is based on a false premise about audio impedances so it's not worth attempting to answer. However I will try and give some insight about the transformers used.

Like any power transformer without a secondary load, ideally you want to be able to apply a voltage to the primary and have zero current entering the transformer - that would be perfect and, when you connected a secondary load that consumes power, the power needed to be input to the primary would be identical to that consumed by the load. Reality isn't that bad actually.

Primary magnetization inductance is basically what the primary impedance is when the secondary load is disconnected - it can't be infinite but it can be relatively small but, not as small as a speaker impedance because then a lot of the power amp's energy is wasted driving a reactive current that serves no purpose.

If it were a 50 Hz power transformer connected to 230V ac, a 10 henry mag inductance would take a "standby" current of 73 mA. If such a transformer were designed for audio and, you weren't too bothered about below 100 Hz (deep bass) then a 10 henry inductance would take 35 mA at 100 Hz BUT, it's possibly a 20V RMS drive and not 230VRMS so a 100 mH mag inductance would do and it has an impedance of 63 ohms at 100 Hz. This, of course will only get higher (better) as the audio frequency rises into the mids and the treble.

63 ohms is fine for an amplifier that can drive an 8 ohm speaker so that hopefully takes care of that side of things. Next - there are turns (windings) on the primary that do not couple power to turns on the secondary and these can be a right royal pain for audio transformers because they are in-series with the power transfer and at high frequencies these "leakage" inductors are going to somewhat attenuate high frequencies. The bottom line is that audio transformer designers try to make sure that approximately 99.5% of the magnetic flux in the primary is coupled to the secondary so, if the primary is nominally 100 mH open circuit then less than 500 uH is seen as useless to the transformer and a detriment to high audio frequencies.

Even so, 100uH as a blocking impedance is nearly 13 ohms at 20 kHz.

Bottom line is that audio transformers are really good at providing low loss power transfer across a wide range of frequencies. No impedance matching is necessary.

  • \$\begingroup\$ I probably should have said less pre question. I was trying to set the scene and broadly sketch the level of my understanding as it stood to do my best to elicit an answer that was appropriate for me... As I understand it though, any driver and load would achieve max power transfer when impedance matched, but where the driver has lower impedance vs load this will actually be more efficient, and in the case of amplifiers the ratio doesn't matter as long as the amp max volume used with speakers of specified impedance doesn't overload the amp! Such is my current understanding anyway... \$\endgroup\$
    – Michael
    Dec 29, 2015 at 17:56
  • \$\begingroup\$ However, what I do understand, rightly or wrongly, doesn't help me to deal with the so called constant voltage speaker system... It seems daft that something as elementary as a transformer should exhibit electrical properties I don't really fully get all the implications of, but there you go - it's one thing to use a thing or even build a thing, and another to understand it. \$\endgroup\$
    – Michael
    Dec 29, 2015 at 18:01
  • \$\begingroup\$ @Michael hee hee I've taken pity on and given some more words in the answer because you were so polite. \$\endgroup\$
    – Andy aka
    Dec 29, 2015 at 18:33
  • \$\begingroup\$ Hahaha! Never let it be said that being polite was a waste of effort eh! I definitely consider my current level of knowledge and understanding to be pretty poor, and I'm sure being humble in the presence of my electronic betters is no bad thing :) Thanks for your additional words, I have read them, and a surprising proportion of them actually feel like they might eventually almost make sense, when I've read them a few more times and worked through them slowly :) \$\endgroup\$
    – Michael
    Dec 29, 2015 at 18:48
  • \$\begingroup\$ Have a ponder for an hour or a day and leave a comment about what you might not uderstand. \$\endgroup\$
    – Andy aka
    Dec 29, 2015 at 19:03

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