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.