R & C values for LM833 active audio crossover circuit - Page 1
Forum section: electronics / beginners

author: Adhith
author_rank: Frequent Contributor
timestamp: May 29, 2017, 06:26:28 am
content: hello everyoneI'm trying to make an active crossover circuit for my DIY speaker project. I came to know that using a dual opam LM833 , it is possible to make a constant voltage active crossover. From the circuit below it is understood that R & C values could be changed according to the equation Fc = 1/(2x(pie)xRC). So what i want to know is that does the R & C values are just taken as random to match the equation for the required crossover frequency or is there any specific criteria to choose the resistor & capacitance value to get the required frequency??

author: danadak
author_rank: Super Contributor
timestamp: May 29, 2017, 12:06:21 pm
content: Some of the considerations in the design of integrators whichare the heart of your filter.https://en.wikipedia.org/wiki/Op_amp_integratorRegards, Dana.

author: David Hess
author_rank: Super Contributor
timestamp: May 29, 2017, 12:26:26 pm
content: The resistance have to be low enough to limit DC errors and low frequency noise while not being so low that the operational amplifiers have trouble driving them. The capacitors need to be large enough to overwhelm any parasitic capacitances and low enough to be realistic.

author: danadak
author_rank: Super Contributor
timestamp: May 29, 2017, 12:51:27 pm
content: "Normal" or "Representative" values would be R's in the 1 - 100's of K,caps 100 pF to .1 uF.Simple guidelines, there can be exceptions to this.Regards, Dana.

author: David Hess
author_rank: Super Contributor
timestamp: May 29, 2017, 02:42:55 pm
content: For noise purposes, the Thevenin equivalent resistance for each input should be about 6.5 kilohms. You get this by dividing the input noise voltage by the input noise current. Resistance above this will raise the output noise and resistance below this will lower it but not by a lot. The LM833 is specified using a 2 kilohm load so this value is not a problem.If you calculate the input resistances from the values shown, then you will find that they are between 5 and 12 kilohms so right around the 6.5 kilohm value calculated from the datasheet.

author: danadak
author_rank: Super Contributor
timestamp: May 29, 2017, 05:20:05 pm
content: These will help you to do noise calculations -http://dicks-website.eu/noisecalculator/index.htmlhttp://www.analog.com/media/en/training-seminars/tutorials/MT-049.pdfRegards, Dana.

author: Audioguru
author_rank: Super Contributor
timestamp: May 30, 2017, 03:15:05 am
content: An active crossover with 12dB/octave slopes or an even number of 6dB/octave is wrong:1) If the woofer and tweeter have the same phase then there is a -30dB notch at the crossover frequency.2) If the woofer and tweeter have opposite phase then there is a +3dB peak at the crossover frequency.A crossover with an odd number of 6dB/octave like 18db/octave slopes does not produce these problems. The outputs of the woofer and tweeter add perfectly across the crossover frequency.

author: ruairi
author_rank: Frequent Contributor
timestamp: May 30, 2017, 03:39:19 am
content: Quote from: Audioguru on May 30, 2017, 03:15:05 amI think it's fair to say that it's a little more complicated than that. 2nd order Linkwitz Riley crossoversdosum flat for example.To the O.P. - if you are interested in speaker building and crossover design buy a copy of Douglas Self's - The Design of Active Crossovers. It's a truly excellent book that answers your questions and many more.

author: Adhith
author_rank: Frequent Contributor
timestamp: May 30, 2017, 03:37:07 pm
content: thank you friends.. I'll do some research on the circuit & will let you guys know.

author: Adhith
author_rank: Frequent Contributor
timestamp: June 01, 2017, 07:25:29 am
content: could the circuit be expanded to make a 3 way crossover??

author: Adhith
author_rank: Frequent Contributor
timestamp: June 07, 2017, 05:23:58 pm
content: hello friendsi just found this circuit on the web & need some clarification regarding IC1a and IC1b. I came to know that there is no path for the IC1a and IC1b minus input bias currents (pins 2 & 6) which makes the op amp outputs to saturate. Could anyone help me out??

author: ruairi
author_rank: Frequent Contributor
timestamp: June 07, 2017, 05:49:31 pm
content: Adhith,Is this something you would like to build or are you just studying it for fun? At first glance it looks like a fairly crude circuit and it will be relatively noisy.If you are hoping to build something it's a great idea with something that is known to perform well and sound good. Again, I point you to Douglas Self who has a fantastic book called "Small Signal Audio Design". If you only have one book on audio circuit design this should be it. He details many EQ circuits starting with some very basic ones and gives the background on the component choices etc.Re your previous question on crossovers, take a look at the Rane 2/3 way design I've attached. Cost effective and well done like a lot of their older analog gear.Are you a member over at GroupDIY? If so take a look at the Studer 169 EQ project, simple and known to sound good.Cheers,Ruairi

author: Adhith
author_rank: Frequent Contributor
timestamp: June 08, 2017, 11:30:25 am
content: Quote from: ruairi on June 07, 2017, 05:49:31 pmThank you very much for your help. I'm doing it for my DIY speaker project. Unfortunately I don't have the Douglas book with me. What I'm looking for is a smaller circuit(if possible). I thought of using LM833 for making a 2 way crossover since it schematics is provided in the datasheet. But I'm not sure about its efficiency due to noise interaction. Just saw the post in groupDIY and it seems really good.If you have some time please have a glance through my previous speaker projecthttps://www.instructables.com/id/LIGHT-BOX-a-Portable-Speaker-With-Vu-Meter/This time I'm planning to have little bit smaller speaker unit which makes the PCB size much more limited. I don't know whether its possible to do within the size limitation but I'm just trying that's all.