Odd or even?

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We started on Single Ended Triode amplifiers yesterday, better known as SET amplifiers. I mentioned that one of the three major benefits of a SET is its tendency to produce even order harmonics - rather than odd order - which tend to sound more musical or pleasing to the ear. But why does this type of amplifier produce a certain type of distortion that others don't?

The answer lies in the asymmetrical nature of the single output device on the amp. In a classic power amplifier we use two devices to provide the output power, one up and one down (as we say).

The top device handles the top part of the waveform and the bottom device the lower half of the waveform - this is known as a symmetrical arrangement because each half handles the signal in the same way. The only real issue with this type of setup is the point where one hands off control to the other, known as the crossover point, and here we can get crossover distortion if we don't engineer it properly.

A single ended device has no such crossover area because one device handles the entire signal - hence the name "single" ended. But here's the rub; the single ended device can only pump current for half of the waveform but cannot pull current for the other half - and this asymmetry is where we pickup the even order harmonics we talked about in the first part of this post. This limiting factor is why a dual output device is so nice and so popular, one half of the wave can pump power while the other half actively pulls power - because we use two different types of devices, one specifically for pulling and the other for pushing.

Here's a picture for you to try and understand how this single ended device works. Imagine a water hose that can go from zero water output to 100% water output. When no music is playing and the amp is at idle, 50% of the water is streaming out of the hose at all times (remember, it's a class A power amp so it gets good and hot just sitting there). When I put a signal into the amp, the water pressure increases (forming the top of the wave) to something close to 100% depending on the loudness of the signal. It then comes back down to the zero point (50% output) and then goes all the way close to off (to make the bottom half of the wave).

Viewed as a single sine wave, we go from 50% to 100%, back to 50% then down to zero, then back to 50% and we've completed one sine wave. But the problem is the top half of the waveform has all the pressure of the water, the bottom half has none. This creates distortion under load (connecting a loudspeaker) and we get harmonics - but because they are even order harmonics we find them euphonic and pleasant.

Tomorrow we cover higher voltage.

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Paul McGowan

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