Color organ Frequency filters - Page 1
Forum section: electronics / projects

author: Max12345
author_rank: Newbie
timestamp: October 09, 2013, 12:32:43 pm
content: Good dayI have attached a schematic of a color organ "Triple Deluxe Color Organ II" by Ledartist.I have a question please:The Red blocks contain, amongst other things, the frequency filters. Which components comprise the filters of the 3 bands?Thank youMax

author: Hideki
author_rank: Frequent Contributor
timestamp: October 09, 2013, 08:50:23 pm
content: Roughly like this, but since there's no buffering the filters will interact a bit.Low: R7/C3 lowpass, R9/C4 highpassMid: R12/C6 lowpass, R14/C7 highpassHigh: R19/C9 highpass

author: Max12345
author_rank: Newbie
timestamp: October 09, 2013, 10:10:23 pm
content: Hi HidekiThanks for replying.Just some background. I have already built this circuit and it works, but the separation between the bands is really poor.1. I want to replace the low filter (which seems to be a bandpass filter) with a more typical low pass filter.2. I have inserted another midrange channel, which meant I had to reconfigure the filters.3. I used a RC filter calculator (http://www.muzique.com/schem/filter.htm) to determine values for R and C, with I hope will be better separation.Questions1. Can I insert the new filters as shown in the new schematic?2. Does R9, R14 and R19 have anything to do with the transistors Q3, Q5 and Q7, as well as being part of the filter circuit?3. What is the purpose of R17 in the High circuit?Thank you so muchMax

author: Max12345
author_rank: Newbie
timestamp: October 09, 2013, 10:11:23 pm
content: Oh, and what do you mean "since there's no buffering, the filters will interact a bit"

author: Hideki
author_rank: Frequent Contributor
timestamp: October 10, 2013, 07:48:10 am
content: You will get poor separation no matter what you do because those are first order filters. The attenuation is only 6 dB per octave above/below the cutoff frequency. It looks like the original design tuned the filters far away from the actual given crossover frequencies -- probably in an attempt to make the bands seem narrower.Trying to squeeze four clearly defined bands out of that design may be asking too much.By "interact a bit", I mean that when you are connecting the two filters in series, the second will load down the capacitor of the first and change the response of the combined filter. Input and output impendances do matter. You could try modelling it in ltspice or tina-ti etc. and see what happens.R9 etc. also sets the transistor's operating point, so you can't mess too much with that value unless you want to recalulate it. You can change the value of the coupling capacitor (C4/7/9) to change the cutoff.

author: Max12345
author_rank: Newbie
timestamp: October 10, 2013, 07:58:02 am
content: Hideki, that's a GREAT reply! Thank you.Now, what can I do to improve the filters' separation and just to make them better?

author: Hideki
author_rank: Frequent Contributor
timestamp: October 10, 2013, 09:36:35 am
content: You can switch to second (or higher) order filters. Note that the complexity will go up really fast.If this was a high frequency design I would probably suggest using LC filters instead of RC, but for the audio range the inductors tend to be large, bulky and expensive (and have other negative issues) so this isn't done much.You can double up the RC filters but the Q-factor will be low, so this is usually done using active filters where the active element is an opamp (or transistor).

author: Max12345
author_rank: Newbie
timestamp: October 10, 2013, 10:03:30 am
content: Thanks Hideki, I'll look into the second or higher order filters...