4 Ohms or 8 ohms

[TrevorN]

This may be a silly question. I understand the WiimAmp can output between 4 and 8 ohms, with 4 Ohms producing 120 watts and 8 Ohms 60 watts.

My floorstander speakers can receive the same impedance levels of 4 to 8 Ohms.

How do I know whether 4 or 8 Ohms is the impedance level when playing music through the WiimAmp? Is there a control to manage this, and does it actually make any difference anyway?

Thanks

 

[harkpabst]

Forget about that concept of ohms, things are a bit different.

Loudspeakers - like everything - put up a certain resistance to electric current. This resistance is a property of the loudspeaker, not the amplifier. Traditionally, nominal impedance (that's another, more general term for resistance) values for speakers are like 4, 8 or 16 ohms and none of these values equals the actual resistance. In reality, a speaker's resistance is not constant. It varies a lot with frequency. The reason why a nominal impedance is provided by manufacturers is that not all amplifiers can handle all impedance values.

Amplifiers are usually designed to act as ideal voltage sources. That means, they try to provide the same voltage (for a given volume setting) regardless of the speaker's impedance. The trouble is: The lower the impedance, the higher the current the amp must provide. Cheap amps often cannot handle 4 ohms speakers (where the real impedance may dip even lower). In this case the voltage will no longer be constant but drop to a lower value. If an amplifier's power output into 4 ohms is noticeably less than twice the power into 8 ohms, this is what's happening.

As long as your speakers' impedance is not lower than what's in the amplifier's specs, you are safe.

This doesn't mean that the amplifier somehow provides 4 ohms (or 8 ohms). It just means that the amp can safely work into, say a 4 ohms load. In the end that's all you need to care about. A speaker rating of "4 - 8 ohms" is pretty much useless, these speakers should rather be rated 4 ohms. Too low impedance values can damage amplifiers, too high impedance values will just result in less power output, thus lower sound pressure level. That's pretty much all you need to know, apart from the fact that the WiiM Amp can safely drive 4 ohms speakers.

These are just some basics. Much more could be said about the topic of speaker impedance and amplifiers, but not in this posting.

Fun with maths:
Power = Voltage * Current = Voltage * (Voltage/Resistance) = Voltage²/Resistance
=>
max. Voltage = sqrt(max. Power * Resistance)
max. Voltage = sqrt(120 watts * 4 ohms) = 21,9 Volts

Cross check:
max. Power into 8 ohms = (21,9 Volts)²/8 ohms = 59,95 watts

 

[onlyoneme]

And it starts to be more complex in real life scenarios when loudspeakers show reactive part of the impedance and some amplifiers designed to be voltage sources behave like current sources.

 

[cooky560]

Ohms was an attempt to match speakers to amps, however it's not really a useful measure because some "4 ohm" speakers drop as low as 2ohm at some points.

 

[harkpabst]

@onlyoneme, I didn't want to confuse @TrevorN even more than I probably did already ... but you're absolutely right, of course. A very famous example is the Quad ESL electrostatic loudspeaker (often referred to as Quad ESL 57), where the impedance in the upper frequency range is almost entirely reactive (and very low). Not a good match for any modern times Class D amp.

The price tag alone should keep anybody from combining those with a WiiM Amp.

@cooky560, in this case the speaker is simply rated inappropriately by the manufacturer. The idea of "matching" by some obscure ohm numbers certainly never was very helpful, indeed.

There are use cases for real "impedance matching" (e.g. output impedance of the source matching input impedance of the target to maximise power transfer). This is not applicable for general audio use because the the amplifier's low output impedance is required for proper damping of the woofer's fundamental resonance.

 

[TrevorN]

Many thanks @harkpabst
I have learnt three things
1. Give Ohms matching a wide berth
2. Post again with any queries on this board as there are clearly experts here
3. Enjoy my Wiim Amp

As this is such an inexact science I'm not surprised that manufacturers fail to provide any accompanying layman's terminology for those of us without the technical knowledge!