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Re: Which is the BEST step attenuator

192.169.41.33

Posted by DG on August 9, 1999 at 09:36:21

In Reply to: Re: Which is the BEST step attenuator posted by Greg on August 9, 1999 at 08:00:44:

If you used the formula that I posted, you will note that while what Thorsten said is technically true, it is not the whole truth (don't read this as an attack on him, I've read many of his posts and have the utmost respect for him).

While what Thorsten said is true about the output impedance of a shunt design at little to no attenuation, it is unlikely that you will ever use your attenuator near max setting.

What he said about input impedance can be overcome by selecting a higher series resistor of say 5k to 10k ohms depending on your equipment. You will note that input impedance of a shunt design rises with reducing attenuation (ie. louder settings) which makes it easier for the source to drive.

It is true that a shunt design has input and output impedances changing with settings, but with proper selection of series resistor and shunt resistor values you wouldn't have any problems. A ladder attenuator is usually designed so that input impedance is constant but its output impedance changes with setting as well (in the same way as a regular pot). Whether fixed input/variable output impedance or variable input/output impedance is better depends very much on how you have chosen your resistor values in either design to match your components.

A shunt design can be made using 3 methods of varying the shunt resistance.
1) A volume pot (worst)
2) A series step attenuator
3) A selector which selects only one shunt resistor each time (best)

You will note that in case 3, at each vol setting there is only 2 resistors involved and one contact point. In a ladder attenuator, you also only have 2 resistors but you now have 2 contacts points. In the ladder design you also end up having to buy twice as many resistors as in Case 3.

So for a given price (or at any price if properly designed) the case 3 is a superior design. You could spend more money and improve on the design by using switching relays instead of a mechanical switch but the ultimate (in sound quality - you should try at least once) is to calculate you favourite volume setting and just hard solder one series resistor and one shunt resistor in place. This is my control case to which I compare all other designs to see how they degrade the sound.

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