Yesterday we covered one of the advantages of the higher power supply voltages afforded by tube circuits: linearity. Today we’ll jump into the second advantage of increased voltage, headroom in power amplifiers.
Headroom is a funny thing. If you take a SET amplifier of perhaps 18 watts and try and get 20 watts out of it, you’ll be well into clipping of the amp – that is if you are trying to play continuous sine waves. But if you are playing music and hit a peak that needs 20 watts, perhaps even more, headroom with high voltage allows you to exceed the amplifier’s wattage rating – if only just for a moment – and for music that’s really all we need.
A small power SET amplifier has plenty of voltage in its power supply to deliver far more watts than it’s rated at – yet it’s limited not by the voltage but by the output devices’ maximum rating and the wattage of its power supply. Clearly, if one wanted to, you could add more and more output tubes with a bigger wattage power supply and get a bigger SET amplifier – but it may not be important if you have a very efficient pair of speakers and keep your listening levels in check.
The key here is headroom: the ability of the amp to produce short peaks of music when needed, peaks that exceed the rated power of the amp by up to 3dB (twice the power) in some cases. Much depends on the capacitors in the power supply as these are the energy storage devices used by the amp to supply peak power.
High voltage also exists in solid state power amplifiers. A typical 200 watt per channel amp will have a little more than half the power supply voltage of a tube – 160 volts or so – and bigger power amps have up to 200 volts approaching that of a tube.
One of the advantages of building a high power solid state amp – an amp with far more power than you might need – is the higher voltages needed to make that higher power.
As an example our new power amplifier is a brute – 350 watts into 8 Ohms, 700 watts into 4 Ohms and 1200 watts into 2 Ohms. This is huge and far more power than almost any system really needs to play at live levels. However, even on an efficient loudspeaker, this amplifier will sonically outperform smaller solid state amps because of the increased linear region and the utter lack of compression at any level.
Tube amps of any wattage only have less than 1/3 higher voltage than does a high powered solid state amp – but they too are saddled with something else that a solid state amp doesn’t have to deal with – an output transformer.
Let’s take a look at that tomorrow.