Newbie Help! Current vs Watts Question


2014-09-10 10:09 am
Hi! Hope I posted in the right section! I have a newbie question regarding amplification power.

A bit of a backgrounder. As I am finding an appropriate build for the amplification of a pair of Kef LS50s a friend of mine said that the Kefs aren't really "Watt Hungry" in terms of power but they are "Current Hungry".

I didn't get this at all and his explanation made my nose bleed.
He then tested the LS50s with a 100 watt emotiva and a 22 watt tube amplifier. My bet was on the Emotiva since it's of a higher wattage but was humbled by the modest output of his 22 watt build.

He said something about transformers and ohms though.

He said that the Kefs desired "current" instead of "watts" which I dont understand.

Is there such a thing as a high power, low current amp and vice versa?

Are tube based amps entirely a different animal in that they provide more current vs their solid state counterparts?

Apologies for such newbie questions but I'm racking my brain over current vs watts.

My friend's currently in HK for a business trip and I dont want to bother him for the time being.

Thanks for your answers in advance!


2006-10-27 1:34 pm
If you use a 8 ohm speaker, your amp have to "give" 1.66A to get 22W.
For a 4 ohm speaker you need 2.35A to get 22W.
So this way the speaker is more current hungry.
That speaker is 8ohm that is not considered current hungry. The 4 ohm speakers are, or some 2ohm speakers.
So the current instead of watt makes no sense.
Bit more sense current instead of voltage as the 8 ohm speaker needs 13.3V for 22W.
And for the 4 ohm speaker you need 9.4V for 22W.
All calculated from Ohms Law as mentioned before.

Normal speakers are usually from 4 to 8 ohm.
8ohm speakers are not considered voltage hungry, they are more or less the standard.
4 ohm speakers are considered current hungry.

Older speakers were 12-16 ohm. They are voltage hungry. 6-8 ohm is the basic these days.
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The speaker or dummy test load demands current.
That current comes THROUGH the amplifer from the Main smoothing capacitance with a bit of help thrown in from the Local supply rail decoupling.

Starting with an 8ohms resistive test load and your 100W amplifier.
Vpk = sqrt(Pmax * Rload *2) = 40Vpk, for a sinewave.
Ipk = sqrt(Pmax / Rload *2) = 5Apk
and as a check: Pmax should equal Ipk*Vpk/2
So you attach your dummy load and it demands 5Apk when the drive signal is a 28.284Vac sinewave.

If your amplifier is never asked to pass more than 5Apk, then all is well. It is rated as a 100W into 8r0 resistive load power amplifier.

But that is not the way speakers and particularly passive crossover speakers work.
They demand currents that are very different from that of a resistor load.
A single driver without a crossover can demand upto 150% (sometimes more) of the equivalent resistor load when presented with fast signals.
A two way passive crossover speaker can regularly demand 200% and some times 300% of the resistor current. On exceptional cases with real speakers and using real music waveforms some experimenters have measured quite different speakers all demanding >500% of the equivalent resistor current.

It's the current demand in the >100% regime that stresses the amplifier. Amplifiers not designed to meet these high current demands can current clip, (not the same as voltage clip).
Let's suppose your example 100W power amplifier can pass 7Apk into your load and the speaker demands 15Apk (300% of the 100W rating).
The amp current clips and you hear that as poor reproduction of the music signal.

If that amplifier had been designed to drive a 2r0 resistive dummy load at close to the 40Vpk that is part of it's specification into 8r0, then it could have met the 300% current peak with ease. Result: It sounds right.

This is transient current capability showing though.

This transient peak current capability is NOT short circuit current into a 0r1 test load.

Applying the same logic to your 22W transformer output amplifier, we get 18.8Vpk and 2.34Apk
Apply a 400% factor and if that transformer can pass a 9.3Apk into a reactive load, then it will perform (sound) better than the current clipping amp.

It about the ability to deliver current while maintaining an output voltage waveform that closely resembles the input signal voltage waveform.

Current capability rules when driving reactive speaker loads.
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