Cheap stepped ladder attenuator. Coarse and fine.


2007-03-18 9:22 am
I need a volume control for my bi-amped system. I had the crazy idea last night of how to build a cheap stepped attenuator of reasonable quality. The typical volume control has a 24 position switch and attenuation steps are 2dB for most of the range with a few larger steps at greatest attenuation, Max attenuation is about 48 to 60dB

I thought of using two 8 position switches instead of a single 24 (or more!) position switch.
The first switch is wired in 8dB steps, the second in 1dB steps giving 64dB of volume control in 1dB steps, Think of it as coarse and fine adjustment.

Now you know what I'm thinking, here comes the question:

Are 1dB steps way too small? This could be significantly cheaper with 6 position switches (2dB and 12dB).
look up the aikido switched attenuator.

It uses coarse on a stereo switch (middle) and fine on a pair of mono switches, which also double up as balance control.

The kit gives all the info on substituting alternative resistor values to change the attenuator impedance and for smaller/bigger steps.

If I recall correctly the coarse is 6steps of 12db each and the fine is eleven steps of 1db each.

There is one slight mistake with the aikido. The two stages of attenuation should follow the normal passive attenuator rule of Rs<<<Zin. As standard the two switches interact with each other but it is very simply solved by lowering the first stage resistors by a factor of 5 to 10.


2007-03-18 9:22 am
that looks like a very well made board. Very cheap too. Those switches are tough, I used to use them for commercial public address speaker attenuators, they do feel very clunky but can be modified for a softer action. The two switches for left and right puts me off a bit. I won't need a balance control.

That does answer my question that 1dB steps are not too small.

With what I was thinking of I'll need two 6-8 way 4 pole switches.
I was looking at something like this done with two switches as coarse and fine: