Push or pull

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In yesterday's post I showed a classic SET tube amplifier. I displayed this because I wanted to show you how distortion products in amplifiers are made, then we can see how a balanced design help eliminate them. In yesterday's drawing we saw a single amplifier element. This is the simplest of designs: a single device running in class A mode. You can build one of these just as easily with a transistor as you can with a tube. I used a tube because that's what people are used to purchasing, a SET (Single Ended Triode). If you set this design on a test bench and measure its distortion you will be impressed. Impressed until you put a load on it; like a speaker. This is a classic problem with single device amplifiers: they are asymmetrical in their use of power. When this single stage device sits on a test bench reproducing a musical signal, and it has no loudspeaker on its output, it is a pure voltage amplifier; small voltage in, large voltage out. But now add a loudspeaker to its output and suddenly we have another issue to deal with, current (amps). When we combine both voltage and current we refer to the sum of these two as 'watts' (amps times volts equals watts). It is at this moment the SET amplifier type runs into a problem: asymmetrical power delivery. Sure, it is class A, which sounds like it should be perfect, but only for delivering voltage. When we want to also deliver current (watts) this is when we get into trouble. A single device only actively works in one direction. When it is producing music, it needs to push power in and out of the speaker evenly and this it cannot do. We'll use our trusted water analogy to explain. Imagine that we have a water tank (the loudspeaker), a faucet (the tube) and a drain hose (resistor). Our goal will be to push water into the tank, then drain an equal amount out. Let's look at a little schematic, similar to the one we looked at yesterday. SET The tube is labeled V2. The resistor is labeled R1. The speaker is labeled SPK1. So now let's follow along with our water analogy. The faucet (our tube) is an active pressurized device. When we want to fill the tank, the faucet turns on and pushes water into the tank; the pressure to fill the tank can go up or down depending on where we turn the faucet handle: the higher we turn it up, the greater the pressure and the quicker we fill the tank. When we want to reverse the process and drain the tank in an amount and speed equal to that of the faucet filling it we have two problems: the drain hose (resistor) is a fixed size and the pressure to drain is dependent on the water level inside the tank which varies. Bottom line, we cannot fill and drain the tank at the same pressure and speed. This is because the fill mechanism is active (the tube) and the drain mechanism is passive (the resistor). This is what causes the asymmetry in a SET amplifier. The net result is distortion, where the bottom half of the waveform is not the same as the top half. What's pleasing about this type of distortion is it is primarily 2nd order and the ear finds it to be a warm and rich coloration. There are ways to minimize this problem including making the drain resistor really small and adding feedback to compensate. Unfortunately, neither of these Band Aids really solves the problem. No, SET or single device amplifiers are not the best choice for power amplifiers when symmetrical low distortion music is the goal. What's the right way to fix the problem? Replace the resistor with another tube. That configuration is called push pull and gets us closer. It looks like this. Push Pull In this method one tube pushes the current while the other pulls it. In our water analogy we still have a faucet to fill our tank, but instead of a simple fixed hose to drain the water, we now add a suction pump. The faucet pushes the water into the tank (loudspeaker) while the suction pump pulls it back out at the same speed and pressure as it went in. This lowers the distortion without resorting to a Band Aid. Yet there is an even better means of doing this called complementary symmetry, but that's somewhat restricted to solid state devices which can pump or suck depending on which type of device you choose.
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Paul McGowan

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