Heirloom

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.

High Voltage

Most tube power amps have an advantage over most solid state amps: high voltage.  If we look at a typical tube vs. solid state design, that tube design might have up to ten times more voltage than the solid state design – at least in a preamplifier.  The tubes need the high voltage to operate properly while the solid state devices do not.

This has several advantages: increased linear region and headroom.

Every amplifying device has a region that it is perfectly linear – meaning what comes out is an exact copy of what went in, only larger.  This linear region varies from device to device but generally we could suggest that up to 25% of the total operating range of a typical amplifying device could be considered linear.

If the device has a power supply of 30 volts, then we could say that 7.5 volts (25%) would be the linear region and if we manage to keep the music signal inside that range, then we don’t have to use feedback and other tricks of the trade to keep it linear.  But 7.5 volts isn’t a large area for even a preamplifier, so we’re going to be relying on crutches to keep things linear.

But if the device has a power supply of 300 volts, like many tubes do, then our linear region just went from 7.5 volts to 75 volts and that’s a lot of region in anyone’s book.

This is one of the reasons why all PS Audio products are so power supply intensive.  Not only do we subscribe to bigger is better, but higher as well.  Our typical operating voltage for our DACS and preamps is not the standard 24 to 30 volts, but 60 instead.  This gives PS source products twice the linear region of most solid state devices – yet far lower than a tube would have.

From our perspective, if you can keep the signal you want well within the linear boundaries of the amplifying device, you’re fine – so 15 volts of linear region is far more than the maximum output signal is going to get to – hence we stick with that.

The additional linear region of a tube is wasted once you pass over a certain level.  Wasted, that is, on linear performance – but valuable on another front – headroom in an amp.

Tomorrow let’s cover headroom.

Paul McGowan – PS Audio, Intl.