Innocuous question about voltage!

Is the current supply of an amp (volts ) more important than the nominal wattage.
Does the current supply vary over the frequency range of the amp?
How does a valve amps current delivery compare to solid state, ( lowish powered valve amps seem to control loudspeakers far better than they have any right to, is this just 'graceful' clipping or pleasant second hamonics ) thanks.

 

Power = Volts times Amps

That's a nice, simple relationship. Unfortunately, in the real world, the "power" number put on a piece of hifi tells you very little.

Let's start by looking at what the relationships between volts, amps, power and loads are. We have the above simple relationship which holds true in all circumstances (it's a fundamental law of physics).

Ohm's law also holds true. That is to say that Volts = Amps times Resistance, or by rearranging it that Amps = Volts divided by resistance.

Substitue ohm's law into the original statement, and by using the two arrangements we've made, you can get Power = (volts times volts) divided by resistance or Power = Amps times Amps times Resistance.

All well and good... However, you touched on frequency. In an ideal world, frequency would not affect things in terms of power. Unfortunately, in the real world, a speaker is not an ideal resistor - its effective resistance changes with frequency. A nominal "8 ohm" speaker can be as low as 3 ohms at a low frequency resonance, and as high as 30 ohms at a crossover point.

This is A Problem (tm) because as you saw above, power is related to resistance of the load. That means that should you put the same voltage to a speaker at all frequencies (which is what the amplifier tends to do - the volume control determines voltage gain, and we know amplifier frequency responses are basically flat within the audio band into a constant load) you could be supplying 10 times the power at one frequency than at another.

Confused yet? You're about to be...

An amplifier has its own resistance (known as its "output impedance") which is effectively in series with the speaker. This means that as currents increase (ie when the impedance of the speaker dips), more voltage is dropped across the output impedance of the amplifier - ie you get a smaller voltage output to the speaker as more current is drawn. This effect is written on amplifier spec sheets as "damping factor" - if you see a damping factor of 1000 into an 8 ohm load, that means that the output impedance of the amplifier is 8/1000ths of an ohm (usually at a given frequency). Just to make things more complicated, this output impedance also varies with frequency.

Spec writing on an amplifier is often an exercise in dishonesty - you will very often see numbers written down for a single channel being driven in isolation, when the power supply couldn't hope to keep all channels driven - for example, look at any number of sub £500 AVRs which claim "5x100W", while another line in the spec says "max power consumption 250W" - apparently a 200% efficient amplifier?

So, in summary I probably didn't answer your question. I probably did show that your question is a whole big can of worms for something as simple as power.

 

Isaac fascinating thank you, as you say fa more complicated subject than I imagined, so if I want to see the current delivery for a particular amplifier should i be looking at it's 'Q' or damping figure?
ie ,i had a pass labs 350 watt amp, but the bass on my speakers was still pretty woolly when i switchd to my ASR ( which I believe has a much higher current delivery ) the bass in particular was much tighter. Keith.

 

Not necessarily - you could have a low-powered amplifier that has an excellent damping factor, or a high-powered amplifier with a poor damping factor.

You touched on valve amplifiers in your first post. It comes on to the difference between Impedance Bridging (typical solid state amp, class D amp or OTL valve amp) and impedance matching (conventional valve amp).

Have a read: http://en.wikipedia.org/wiki/Impedance_bridging

http://en.wikipedia.org/wiki/Impedance_matching