Steppin' out

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Audio is much like fashion. Today we're in love with something, tomorrow it's passé. Stepped attenuators used to be the rage. Not so much anymore. There are two types of attenuators: fixed and variable. We've already covered variable attenuators (pots), and now we're on to fixed (stepped). A stepped attenuator volume control is built upon the same idea as our input selector. A rotary switch selects different resistor sets. The number of steps depends on the switch type used, but few of them have more than thirty. So, how does this work and why is it considered better than a pot? All attenuator types divide voltage. If I take two equal value resistors and tie them in series (one after the other, like a train) I can reduce whatever is fed into this string in half. It's not much of a trick. One end of the chain connects to the signal, the other to ground. My half-volume output is taken from the junction between the two resistors. Change the ratio of the two resistor values, and a different division of signal occurs. This is the same thing happening with the variable attenuator (a pot). The difference is simple. In a pot, the middle junction moves when you turn it. With two fixed resistors, that same junction does not move. It should be obvious that for every different volume level in a stepped attenuator, we need a different set of two resistors. If we wish 30 steps, that's 60 resistors—per channel. That's a lot of resistors! Here's a picture of one such device. stepped2 Why is this complicated device supposedly better? The quality of the resistors is better. By a lot. But, there are drawbacks, of which we will discuss tomorrow.
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Paul McGowan

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