Understanding impedance

[Richardcavell]

I once saw a stereo that had attachments for the speakers. Each attachment was marked “Impedance 50 Ohms“.

1. Does that “50 Ohms” refer to the vectoral sum of resistance and reactance?
2. Is it referring to the impedance inside the stereo, or the impedance of the speaker that should be attached?
3. May I assume that the reactance in question is inductive reactance?

[Zero999]

50 Ohms is a common impedance for RF. It probably refers to the aerial/antenna input, for the RF, normally FM/digtial audio broadcast, input. It has nothing to do with the speakers.

[CaptDon]

Impedance is the algebraic sum of resistance and reactance. The reactance can have a leading or lagging phase angle. Impedance is sometimes calculated as the square root of (R squared) + (X squared). In a right triangle illustration resistance is the bottom, Xc or Xl is the adjacent side and of course the hypt or slanted part is 'impedance'. As for 50 ohms, that is oddball for speakers but does agree with some headphone specs. What do you mean by the words "attachments for speakers"? I would understand that with component stereo the speakers are the attachments?

[Analog Kid]

2. Is it referring to the impedance inside the stereo, or the impedance of the speaker that should be attached?

Those two things (output impedance of the stereo amp and impedance of load--speaker) are supposed to match.
Usually 8 ohms output, 8 ohms speaker impedance.

50 ohms? Unless it's for headphones, never heard of that as an audio-amp impedance. More like for RF as someone else mentioned.

[Zero999]

2. Is it referring to the impedance inside the stereo, or the impedance of the speaker that should be attached?

Those two things (output impedance of the stereo amp and impedance of load--speaker) are supposed to match.
Usually 8 ohms output, 8 ohms speaker impedance.

Incorrect. The output impedance of a an audio amplifier should be as low as possible, in order to damp the resonance of the speaker. Look up damping factor.

[Analog Kid]

True dat: the actual impedance of the amp might even be in sub-ohm territory.
I guess I was referring to the nominal (rated) impedance.
In any case, the impedance of the load should never be less than that of the source.