12 Position Shorting Switch Attenuator



12pos_ss

Here's a picture of a new 12 position shorting switch and the old attenuator. I replaced the old one with a new characterized 60 db attenuator. This characterization creates a gentle volume increase for the first nine steps and the last two are gaint steps towards full volume.



Theory


The physics, math, and concepts of good design are the hardest for me to understand. Personally, I just like to build stuff and make stuff work, but I have discovered that to do it well it helps to understand the theory behind the design.

The typical volume control for audio is a resistive attenuator, a device that reduces or weakens the input signal. A typical volume control starts at a high level of attenuation and reduces the amount of attenuation as you turn the knob. To determine what resistive values to use we need to evaluate the volume control function. This leads us to our ears.

We perceive sound as energy transferred by sound pressure to our mind via the physiology of our ears. Our perception of loudness is not a simple mathematical function. When we first perceive a soft sound and we notice a change in loudness of the sound, it has to double in power for our ears to perceive an increase in loudness. To mathematically display and talk numbers like these these we developed the logarithmic scale. Using this scale the people at Bell Labs invented the ratio of reference level to the measured level and called it the bel, decibel, or db. Most human beings can detect a 3 db increase or decrease in sound pressure level. Thus using 3 db steps works well in audio attenuator circuits.

The math in calculating the 3 db steps is involved but fortunately Maarten (no itís not miss spelled) has made available his calculator (see next section). The other piece of information required for the calculation is the total resistance or impedance of the step attenuator. This is a functional requirement of the ampís input circuit. With this information and Maartenís calculator we can determine the resistive values required for each step of the attenuator based on the db value you enter. Sweet!

Functionally when building the step attenuator the volume control works from the greatest attenuation, for example negative 60 db and increases to 0 db. What this means is the input signal amplitude will be reduced by minus 60 db at the minimum position on the volume control knob and will increase as the volume knob is turned, e.g. -57db, -54db, -51db, Ö ,0 db. Where 0 db is the input signal at full amplitude (full volume) and will be amplified by the circuit down stream of the attenuator.



Attenuator Calculation


Thanks to Maarten of Platenspeler.com for making available his Attenuator Calculator. This link 59 db Calculation is the calculation I used for the above described characterized attenuator.





Construction Notes


installed

These are Sang's notes on the resistor values and a sketch of the circuit. If he build's it again he'll use use one 2 db step change for the first step, eight 3 db steps changes and then increase the remaining steps to achive a -60 db attenuation.



Installed Attenuator


installed

The advantage of using this type of attenuator is the low cost and simplicity. The draw back is the lack of resolution because it only has 11 steps.


installed

Another shot of the volume control and selector switch.