Author Topic: Designing approximately -12db/Octave passive HPF  (Read 198 times)


Designing approximately -12db/Octave passive HPF
« on: November 14, 2020, 06:25:56 AM »
Normal wisdom is that the input impedance of a stage should be at least 10 times larger than the output impedance of the previous stage. This reduces interaction and loading.

Hence while designing a -12dB/ Octave HPF by the very simple method of 2 cascaded RC filters, the second R should be 10 times more than first one, and the second C should be 10 times smaller than the first C (But the product of R.C should be same for each of the two stages)

This calls for a dual gang potentiometer where one part is 10K and other is 100K. That is hard to find. Some schematics suggest that the DIYer buys a Dual and 10K and and a dual 100K and does plastic surgery on them to get Dual 10/100K

I decided to see how badly would a second stage with a 10K resistor load a first stage with 10K resistor

On the left is a snippet of HPF from the VFE Standout and Focus pedals. There is a switch which disconnects the left part (R2 220ohms to ground) for 6db/octave and kicks it in for 12 db/Octave. It needs a dual 100k/10k pot

on the right is the same filter but with 10K pot feeding of a 10K pot ie dual 10k/10k pot, with a slight change of one cap value.

Red curve is original circuit of 10K RC filter feeding 100K RC filter

In blue are the 6 db/Oct filter by disconnecting one RC stage and also the 10K/10K filter that uses dual gang 10K ie not really ideal.

My personal conclusion : Of course there is loading due to 10K RC filter feeding 10K RC filter. But the final results are still suitable for the purpose of having a 6ish and a 12ish db/octave filter

(Oops I noticed an error in the left schematic. Lets see who spots it)
« Last Edit: November 14, 2020, 06:50:55 AM by Vivek »


Re: Designing approximately -12db/Octave passive HPF
« Reply #1 on: November 14, 2020, 04:40:53 PM »
you forgot to fill in R4

yeah, if you don't care about Q factor (fun fact: Q stands for quality), it doesn't really matter if you go up one order of magnitude for the 2nd filter or not, that's only for when you are designing for very precise filters where the q factor means a lot.

cheers, Iain
half man - half snail - 6 feet to scale - Snail man's - not frail - He's been - to jail - snail man is fuccing real