Stepped attenuators

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Controlling the volume level in your stereo system is an obvious and important function. The element used to turn the level up and down is important as well. If you look inside a preamplifier you'd discover it's one of the simpler devices in your chain. Most consist of only three blocks in the signal path: an input selector, a volume/balance control and a gain stage. The input selector is nothing more than a switch that connects the input you wish to listen to into the signal chain. This switch can be a mechanical one requiring you to turn the knob to choose or a series of relays that act like switches. Relays are simple switches that use a small electro magnet to open and close their switch contacts. This allows designers to send a tiny electrical signal to control the relay, rather than relying on mechanical force to move the switch from one position to another. There are even electronic versions of switches that use a transistor as the switch (usually a MOSFET). Once selected, the source you want to listen to is connected to the preamplifier's volume control; the subject of this post. Most preamplifiers use what is known as a pot to control the volume. "Pot" is short for potentiometer and is a simple device consisting of a resistive element and a contact point (known as a wiper). The resistive element sounds fancy but it too is simple and made from a material that resists electricity like carbon or certain types of metals that don't conduct too well; also known as a resistor (because it resists electricity). The contact point is a simple piece of good conducting metal that touches the resistive element (like silver). When you turn the pot up or down, you are moving the contact point (wiper) along the resistive element in a wiping motion. potentiometer1 The sound quality of this device is dependent entirely on the quality of the resistive element and the contact material of the wiper, once implemented properly. A good pot can be extremely expensive. If you look closely at the picture you'll note that the wiper is dividing the resistive element in two. In the example I show, A is a smaller area than B as divided by the wiper. This means that A has a smaller resistor value than B. If we were to actually measure these values we'd see that A is probably 25% of the whole and B the remaining 75%. If the pot is a 10k value (10,000 Ohms) then we could say it can be modeled with two resistors, the A resistor measuring 2,500 Ohms and the B resistor measuring 7,500 Ohms (the two resistors will always add up to the 10,000 value of the pot). I mentioned the sonic performance of this device depends on the quality of that resistive element and the contact material of the wiper. There's no doubt the best resistive elements can be found in individual resistors and the best contact materials can be found on expensive switches - none of which is true of potentiometers. Thus, if we decide to use discrete resistors and a quality switch to select those resistors, instead of a pot, we would have a better sounding volume control. This arrangement is known as a stepped attenuator. Stepped attenuator You can see in the picture such a device. The switch is nothing more than a series of contacts and, as you can see, there are four sets of resistors: two to make up the A and B combinations of our pot for one channel and another set for the other. The advantages of a stepped attenuator are the possibility of better sound because of the parts quality. The downside of this arrangement is the volume is controlled in a series of fixed steps and you may or may not like the step level. There are pots that can come close to the performance of a stepped attenuator, but rarely do they. Most high end preamps of today use an electronic version of the stepped attenuator, replacing the mechanical contacts of the pictured switch with either relays or electronic relays.
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Paul McGowan

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