Max/Actual Output Level vs. Distortion

[gootee]

At work, I was talking to a guy who puts together high-powered RF amplifier setups that are used to broadcast signals on outdoor test ranges. He told me that he ended up having to use power amplifiers with a maximum output power that was about eight times (8X) the desired output power, in order to be able to get low-enough distortion (and he might have mentioned intermodulation, there).

So I am just wondering, in general, if the same sort of thing is true for audio amplifiers. Or are RF amplifiers a completely different animal, in that sense?

So, for example, if I like to play music at about 30 Watts per channel, then should I be using an amplifier that is capable of around 250 Watts per channel? I imagine it would depend on the amplifier topology and implementation. But, if there is a reasonable "rule of thumb", what is it?

Regards,

Tom

[5th element]

Originally Posted by gootee

So I am just wondering, in general, if the same sort of thing is true for audio amplifiers.

It depends on what you call low distortion as this can vary significantly from person to person.

But in general a well designed and implemented solid state power amplifier should not show an appreciable increase in distortion as output level rises. Then of course things turn ugly when clipping is reached.

So in answer to your question - if when using a 30 watt amplifier you never hit clipping, then no, using a bigger amp shouldn't do anything.

In reality the bigger amp will give you greater headroom, so is less likely to clip. But it's all about system matching.

If you've got a sensitive pair of loudspeakers which are easy to drive, then you are not going to need a huge amount of power.

If you've got a low sensitivity pair of loudspeakers with an average 4 ohm, dipping perhaps down to 2 ohms and a large room. Now you're going to require a much more robust amp.

[h_a]

It's also largely a matter of definition.

Manufacturers often do not state how much THD is generated at maximum power. Sometimes it is 1% for solid state, for tube amps I've also seen even 5%.

Next, almost always THD depends on the load; the power rating is usually given for 8ohm, but THD will be higher for 4 ohms.

Next, THD is generally higher for high frequencies; specification is usually given for 1kHz and becomes much worse for 20kHz, as there's less negative feedback to correct it.

You don't state under which conditions he is using the amps, but I would not draw a general rule from his observation.

Have fun, Hannes

[Mooly]

RF amplifiers for transmission are totally different to linear audio power amps.
Transmitting RF is often done using Class C for efficiency. The load (aerial) is matched via tuned circuits etc, and that has the effect of reducing distortion.
RF transmission is done at one specific chosen carrier frequency normally... audio is over 10 octaves... a totally different ball game.