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John
Transistors. For solid-state loudspeaker amplifiers, we need (at least) two per channel to drive a pair of loudspeakers: one (X) for the upper half of the music signal’s waveform, another (Y) for the lower half. X hands off to Y as the waveform crosses the ‘zero’ point and then Y hands the signal back to X as the waveform once again crosses the ‘zero’ point.
In a Class A circuit, X and Y are always on. When X is working, Y sits idle. When Y is working, X sits idle. And when a transistor sits idle, it generates heat. This is why Class A amplifiers tend to run hot. When no music is being played, both transistors sit idle. This is why Class amplifiers tend to run hot but even hotter when they’re switched on but not playing music. An ‘always on’ design means Class A amplifiers aren’t kind to our electricity bills.
Why don’t we turn the transistors off when not in use instead of letting them run idle? Why not let transistor X amplify the upper half of the input signal, have itself shut off whilst transistor Y amplifies the lower portion of the waveform and then turn on again once Y has finished (which would itself move momentarily do an idle state? The amplifier would have two states: X on, Y off OR X off, Y on. This is called a Class B circuit.
We can mentally picture the differences as a 400m relay race. In a Class A race, two runners would run at the same speed around the track, handing off the baton to each other every 200m. Both runners would always be in motion. But relay races don’t work like this in real life. The first runner would run the baton to the 200m marker; where the second runner would be waiting to take the baton from him and run the second 200m. It’s in this handover that problems arise.
Just like relay runners, transistors don’t go from zero to full operational power instantaneously. They ramp up into action causing a momentary lag. As the music signal crosses over from X to Y, Y takes a moment to get going. And vice versa. This momentary hand-over delay produces something called ‘crossover distortion’ — it can make amplifiers sound bright and harsh.
One way to sidestep Class B’s crossover distortion is to have each transistor always on for the first few watts of its possible output. These first few watts are often cited by amplifier designers as being the most crucial to music’s low-level detail. Each transistor’s remaining, less crucial watts could be engaged only when called for by large dynamic swings. This hybrid approach is called Class A/B. Every Class A/B amplifier has a different amount of Class A bias (usually set internally by a trimpot). The more Class A bias, the warmer the amplifier runs.
Schiit’s Vidar (US$699, intro-d here) is Class A/B. According to the manufacturer-supplied specifications sheet, we get 100wpc into 8 Ohms, 200wpc into 4 Ohms. Run a pair as monoblocks and the power output rating doubles again: 400wpc into 8 Ohms. The Schiit website doesn’t specify the amount of Class A bias other than to say “It’s got a decent amount of bias on it. No worries—we have internal thermal sensors that the microprocessor monitors. They’ll turn off the amp if it gets too hot.”
Those microprocessor managed thermal sensors turn up again in Schiit’s latest power amplifier, the Aegir (US$799, intro-d here). From the outside, standby button aside, it looks similar to the Vidar. However, the internals are quite different. The Aegir is closer in spirit to a Class A amplifier offering (according to Schiit) 20wpc into 8 Ohms, 40 wpc into 4 Ohms and 80wpc into 8 Ohms when a pair are run as monoblocks. Aegir runs hotter than Vidar.
Beyond 10 Watts of Class A standing bias, Schiit’s explanation outpaces my ability to boil it down into simpler terms:
“Technically, Continuity is a way to eliminate transconductance droop outside of the Class A bias region, and extend the benefits of Class A biasing. It also solves the NPN and PNP device mismatch problem, since it uses both NPN and PNP devices on both rails. It’s still a very hot-running amp, though, with over 10W of Class A standing bias.”
Instead, we pick up this Schiit amplifier story in the real world with one very specific pair of loudspeakers: Klipsch’s RP-600M (US$599/pair, reviewed here).
I give you Klipschiit:
Camera: Olaf von Voss | Editor: John Darko | Motion GFX: John Darko
