LM358N SallenKey LPF not working? - Page 1
Forum section: electronics / beginners

author: lutkeveld
author_rank: Regular Contributor
timestamp: June 09, 2016, 04:34:51 pm
content: I made a 2nd order 1500Hz LPF with the LM358N.R1=1.2kR2=21kC1=68nFC2=6.8nFThe LM358N is powered from the 5V output from an arduino.The problem is that it doesn't filter anything. The output waveform is the same as the input.I'm 99% sure I hooked up everything correctly. I measured the values, multisim also says the circuit should work.What's happening?

author: Audioguru
author_rank: Super Contributor
timestamp: June 09, 2016, 05:18:36 pm
content: Did you use a calculator program to come up with your weird parts values? I use equal value resistors and two capacitors for the feedback capacitor.Don't use a lousy old LM358 for audio because it was designed for low power so it is noisy, has crossover distortion and has trouble with frequencies above 2khz at high levels. Use an audio opamp instead.

author: Andy Watson
author_rank: Super Contributor
timestamp: June 09, 2016, 05:22:03 pm
content: What defines the DC voltage level on Vin ? With a single supply it will need to be biased to one or two volts to allow enough working range for the LM358.

author: Audioguru
author_rank: Super Contributor
timestamp: June 09, 2016, 05:40:31 pm
content: Quote from: Andy Watson on June 09, 2016, 05:22:03 pmLike in most simulations, there is no power supply for the opamp so a dual polarity supply is assumed and the input DC is assumed to be at 0V.The OP said that the output is the same as the input so the circuit must be biased properly to work like that. I think the breadboard has a contact or two that make poor contact on the filtering parts.

author: Andy Watson
author_rank: Super Contributor
timestamp: June 09, 2016, 06:03:15 pm
content: Quote from: Audioguru on June 09, 2016, 05:40:31 pmThe OP did not mention dual rail supplies. I was assuming a single +5V supply. "Output is the same as the input" could mean 0V DC with the AC component being fed through the capacitor - i.e. the output of the op-amp is having little to zero effect because it is not operating in its specified voltage range. Whatever the reason, the input still needs some form of DC path to ensure that the op-amp is biased into its working range.

author: lutkeveld
author_rank: Regular Contributor
timestamp: June 09, 2016, 06:29:03 pm
content: I used this calculator for the values: analog.com/designtools/en/filterwizard/I know the 358 isn't suited for audio, but I had it laying around.I'll use something like the 5532 for the final circuit. It needs to be able to handle 21V DC max and have decent sound quality.It's powered from an arduino (5V), so indeed single rail.

author: Audioguru
author_rank: Super Contributor
timestamp: June 10, 2016, 01:36:24 am
content: The filter circuit must be fed from a very low impedance like the output of an opamp. This new opamp can be a follower with a gain of 1. Its input should be biased at half the supply voltage.How can this filter have decent sound quality when its midrange and high audio frequencies are removed? The sounds will be very muffled.

author: Zero999
author_rank: Super Contributor
timestamp: June 10, 2016, 09:55:41 am
content: Quote from: lutkeveld on June 09, 2016, 06:29:03 pmNo op-amp will be able to handle 21VDC at the input, when powered from a 5V power supply.

author: danadak
author_rank: Super Contributor
timestamp: June 10, 2016, 02:57:06 pm
content: One approach page 16 -https://courses.cit.cornell.edu/bionb440/datasheets/SingleSupply.pdfRegards, Dana.