Linkwitz-Riley filter input and output stage/gain question - Page 1
Forum section: electronics / beginners

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 08, 2020, 06:10:45 pm
content: Hi all,I breadboarded a Linkwitz-Riley 3 way crossover and thought about converting it to an isolator.An isolator is effectively a 3 way crossover (or fixed freq filter bank if you like) with adjustable gain in each frequency band.50kA pots are typically used in this scenario with their wipers in series with a summing resistor going to the final stage where max gain is set by calculating the Rf resistor in accordance to the wiper summing resistor.For example for 20db gain one would make the Rf resistor 10times larger than the summing resistor. (inverting amplifier)The problem is that the isolator needs to have zero gain at the midpoint of the pots and that's where my problem lies.I need the output stage to have about 9x gain or near the 18db mark so I used 22k summing resistors and 200k feedback resistors.For a 50kA pot the midpoint is around 10k.The problem I have is that if my input signal enters the circuit (with the pots centered) I get a gain of about 6.25 at the output since the wiper is at 10k+22k summing resistor = 32k -> 200/32 = 6.25x gain.If my input stage acts as an 1/6.25 attenuator I get unity gain at the middle but only 1.5 larger signal at the output.How should I go about this?I have an inverting input stage, an inverting output stage and 50kA potentiometers.I need to have aroung 9x to 10x gain in about 10k worth of my potentiometer's range (from mid to all the way to the right)I thought about going with something like Rf=15k Rsumming=1.5k but isn't it a bit low?The reason I'm asking is that quite a few isolators I've seen on the market use summing resistors in the range of 20k-47k..I'm clearly missing something but I feel stuck and though I'd ask!

author: Benta
author_rank: Super Contributor
timestamp: October 08, 2020, 06:27:09 pm
content: Linkwitz-Riley filters I know a lot about, but the rest of what you wrote is not understandable. A block diagram/schematic of what you're doing and an explanation of what you want to achieve would be helpful.

author: SilverSolder
author_rank: Super Contributor
timestamp: October 08, 2020, 06:47:15 pm
content: Sounds like an interesting project! A diagram of some kind would definitely be awesome, hard to keep it all in your head when you're not the one working on it!

author: dmills
author_rank: Super Contributor
timestamp: October 09, 2020, 11:46:11 pm
content: If the midpoint of a 50k 'logish' pot is 10k, then in that position the pot has a gain of -14dB (You can play with this a bit by adding a shunt resistor to one or the other end of the pot).22k is way too low when using a 50k pot (Which has a worst case (50% resistance) source impedance of 12.5k, you solve this with a buffer opamp after the pot, cap coupled from the pot wiper (or use a fet input opamp).You follow the buffer with your summing resistor and summing amp in the usual way, but you can now use much lower values so that you get a sane noise floor and whatever gain you need (I would suggest 14dB as a starting point!).You might also with to explore Bauxandall volume controls, lets you use a linear pot.