Posted on Saturday, 10th August 2013 by Auw Jimmy
I have used several good brands of potentiometer. But once upon a time, I decided to stick with stepped attenuator. My first was DACT which using SMD resistor and configured in series. They were factory made, so just paid and enjoyed.
But later on, I decided to give a try to build myself. So I purchase several Grayhill Rotary Switch and off course several resistors to build it.
One of the most important thing when selecting the Rotary Switch is to make sure you have to select the ‘shorting’ configuration. This should be well described on the manual of the switch. Shorting means when you move the switch from one position to another position, it will short the connection to the next position before releasing the previous position. So it will not create any ‘pop’ or ‘clicking’ noise. All volume control should use this kind of ‘shorting’ switch.
The other type is ‘non-shorting’ one which will act differently. It will release the connection first, before connecting to the next position. This type of ‘non-shorting’ switch commonly used in selector switch (to select the input/output on the preamp), where the input/output should not be shorted.
Most of the manufacturer should come with explanation whether the switch is shorting or non-shorting one. Example below is the one from Grayhill. For volume control application, make sure you have to buy the ‘S’ type, while for selector switch, you have to take the ‘N’ one.
Off course, a bunch of resistors will be needed. The amount will depend on how many steps and what kind of configuration used. At current stage, I go with Takman REY Metal Film resistor. My future plan is to use custom Texas Components TX2575 resistor. But at 15x price tag compared to Takman REY, definitely I need to win some lottery first!
There are at least three types of stepped attenuator based on the resistor arrangement: serial, ladder, and shunt. You can search around for the detail explanation for each type (there are a lot out there, so I don’t think repeating them here will do any good, anyway here is the example from Goldpoint). But for the sake of simplicity: series is the simplest to design, ladder is the most ideal (shortest path, but will require double of the resistor counts), while for shunt, you better know what you are doing before using this one (this require good understanding about where you are planning to use this).
That’s why for combining the good from both series and ladder, some manufacturer will design series stepped attenuator with SMD resistor (which is very tiny to give shortest path). Each SMD probably only have 2 mm length, so even with 10-20 resistors on the loop in series configuration, the path is still very short (only 2-4 cm – this is nearly the size of 2 normal non-SMD resistor!). So we can have the advantage of shortest path (like ladder type) but with less resistor counts (with series configuration).
But off course, if we do ladder with SMD, the path would be the shortest than any kind of other configuration (at the cost of double resistor counts). If the resistor is $1 each, probably not an issue, but how about if the price is $15 each (I’m thinking of this)? Then the double number from 50 to 100 resistors definitely will rob our pocket quite fast!
Finally, we will also need to calculate the resistor value. I would suggest a small software from Jim Tonne, located here. This small software will simplify our life by calculating the resistor value (1% or 5% tolerance). All we have to do is filling the total resistance, number of positions, and the dB attenuation per step. Voila, you just go to the nearest online shop and start shopping for the needed resistor value.
Another thing that maybe interesting (although not needed for most casual people) is the deviation from the target attenuation. Most likely they are less than 0.4 dB which is quite tolerable. Off course you can go with higher precision 1% resistor value which will give even less deviation. It’s your call…