ruffrecords
Well-known member
Often there is a need to include a 10dB gain trim control in a signal chain. I needed one for a project recently. It is not hard to limit a linear pot to a 10dB range just by including a suitable value resistor in its ground lead but this leads to a cramped law for the pot with most of the gain changing in the second half of rotation (you get about 3dB in the first half and 7dB in the second half). What we really would like is 5dB at the centre point. You can achieve this with a slugging resistor connected from the wiper to the ground leg of the pot and adjusted to give 5dB loss in the centre. The math is not hard but painful and boring. It is made worse by the fact that pots have a tolerance. In my project I was using 10K pots but they all measured just over 11K (within the 20% tolerance allowed). This alters the value of both the resistors meaning you really need to calculate the value for each and every pot you use.
The attached schematic is annotated with hopefully easy formula to follow. R1 is the value of the pot. R2 is the resistor that sets the trim limit to 10dB. R3 is the slugging resistor that sets the mid point to 5dB. The formulae do give odd resistor values but I have found that using the nearest preferred value rarely makes more than 0.25dB difference.
For the really math challenged I have made a spreadsheet to work it all out for you. All you need do is feed in the measured pot value. I will post that separately.
Cheers
ian
The attached schematic is annotated with hopefully easy formula to follow. R1 is the value of the pot. R2 is the resistor that sets the trim limit to 10dB. R3 is the slugging resistor that sets the mid point to 5dB. The formulae do give odd resistor values but I have found that using the nearest preferred value rarely makes more than 0.25dB difference.
For the really math challenged I have made a spreadsheet to work it all out for you. All you need do is feed in the measured pot value. I will post that separately.
Cheers
ian