Getting it just right

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We covered what a stepped attenuator is and how it works in yesterdays post. The mechanical versions of this complex volume control are not used much anymore because of their many limitations: restricted number of steps, inability to use a remote control, expensive to build, big jumps in level between steps.

To make an effective volume control you need to control a great deal of range. Most designers want to cover at least 70dB from loudest to softest, but 50dB is the practical minimum.

If you’re going to divide the control into steps you don’t want the steps too big. Imagine trying to get the level just right when each step is a giant change in volume. To be usable we recommend that steps be no greater than 1/2 dB—the protocol we adhere to when building our own stepped attenuators (like on the BHK).

I had previously mentioned the physical limitations of building a mechanical switch means that few have greater than 30 steps. If each usable step is 1/2dB; and you need to cover 50dB; and you only have 30 steps…

You see the problem.

Which is why most mechanical attenuators have larger steps. 1dB is common, but we’re still 20 dB short of the minimum required.

Enter the audio taper.

It turns out our ears are less sensitive to volume changes at lower listening levels. This allows the designer to have large jumps in volume at the softest end of the control, and finer increments at the upper end. Pots, attenuators, and all manner of level controls use a tapered response when adjusting volume. Bigger jumps at lower levels, smaller at higher.

The BHK Signature preamplifier uses a stepped attenuator too. Unlike the restricted mechanical ones we’ve discussed so far, the BHK has a whopping 100 steps and 1/2dB increments!

How the hell did we do that when mechanical switches are limited to 30?