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Old 4th May 2014, 08:50 AM   #1
PKN is offline PKN  Australia
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Join Date: May 2014
Default Active Low Pass

Hey guys i am a Auto Electrician and electronics is my pass time.
I am making a low pass filter based on the tL074 for my Amplifier i just installed. Here is a link to the Install. The amp has no setting for LP

forums.justcommodores.com.au/ve-holden-commodore-2006-2013/232418-amplifier-install-nxt-battery.html

Here is very rough drawing i did of the circuit it is very basic 1st order cascaded. Surprisingly it works pretty good.
The input is on the non inverting pin

I am trying to change the frequency via a variable resistor. Circled red on the diagram but does nothing is there a reason why? Changing the Cap has more effect on the frequency then changing resistor

Also circled in blue Are capacitors i put 100nf ceramic type caps is this correct.

Thank you
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Old 4th May 2014, 09:49 AM   #2
Zero D is offline Zero D  United Kingdom
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Hi, i'm presuming the reason it does nothing, is that you havn't connected the wiper ?

Yes the 100nf are fine.
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Old 4th May 2014, 10:08 AM   #3
PKN is offline PKN  Australia
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Quote:
Originally Posted by Zero D View Post
Hi, i'm presuming the reason it does nothing, is that you havn't connected the wiper ?

Yes the 100nf are fine.
Hey mate thanks for the reply.
I have connected the variable resistor so that the resistance fluctuates when you turn the wiper. leaving one terminal not in use here a pic.
Also i am not using a virtual ground i am using the earth from the rca if this helps.
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Old 4th May 2014, 12:03 PM   #4
Zero D is offline Zero D  United Kingdom
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Yeah that pot wiring would work too. If ALL the connections & soldered joints are 100% ok, i'm surprised you don't hear ANY change. You might like to try a different pot of say 47k as that should help.

Also you MUST put a resistor of @ least 1k in series with the pot, because the OpAmp won't like a capacitor between it's output & ground if the pot is at zero resistance !

*******

Ahh, i missed the fact that you're using the OpAmp off a single supply. Well you can't use that circuit like that then !

I'll post back this evening with a better circuit for you. I'm going out for sunday lunch etc very shortly
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Old 4th May 2014, 07:50 PM   #5
PKN is offline PKN  Australia
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Quote:
Originally Posted by Zero D View Post
Yeah that pot wiring would work too. If ALL the connections & soldered joints are 100% ok, i'm surprised you don't hear ANY change. You might like to try a different pot of say 47k as that should help.

Also you MUST put a resistor of @ least 1k in series with the pot, because the OpAmp won't like a capacitor between it's output & ground if the pot is at zero resistance !

*******

Ahh, i missed the fact that you're using the OpAmp off a single supply. Well you can't use that circuit like that then !

I'll post back this evening with a better circuit for you. I'm going out for sunday lunch etc very shortly
Hey mate thanks for your reply and help
Can you pls explain why I would not be able to use this circuit because it seems to work. I just want to know for future reference.
I tried sallen key topology but seems to have high distortion with the feedback cap.

Thanks.
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Old 5th May 2014, 05:38 AM   #6
Zero D is offline Zero D  United Kingdom
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As promised i'm back, later than advertised, but it turned into more than afternoon

I'm wondering why you're using a quad OpAmp. Is it because you're making a stereo filter, & only showed one channel, & will actually be duplicating the circuit ? If so then use a stereo ganged pot. If not use a TL072, but the pin numbers will be different of course !

Anyway, the fact that you're on a single 12V power supply, means that you have to power it differently, as you do need a Virtual earth. See my circuit below.

Your circuit might be working in some kind of fashion, but not correctly. Most OpAmps are designed to run off dual + & - power supplys, as are the TL074/72. If you want to use a single supply you have to fake it, by arranging for a half way point, = Virtual Earth/Ground. In this case 6 Volts. This is made with R2 & R3 & C3.

Plus you need electrolytic capacitors to block the DC getting out to the previous & later equipment. That's what C1 & C6 are for.

I've also given the circuit X 2 gain, = R4 & R5 which should help.

I've simmed the circuit & it appears to work fine & do what you require
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Old 5th May 2014, 06:09 AM   #7
PKN is offline PKN  Australia
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Join Date: May 2014
Quote:
Originally Posted by Zero D View Post
As promised i'm back, later than advertised, but it turned into more than afternoon

I'm wondering why you're using a quad OpAmp. Is it because you're making a stereo filter, & only showed one channel, & will actually be duplicating the circuit ? If so then use a stereo ganged pot. If not use a TL072, but the pin numbers will be different of course !

Anyway, the fact that you're on a single 12V power supply, means that you have to power it differently, as you do need a Virtual earth. See my circuit below.

Your circuit might be working in some kind of fashion, but not correctly. Most OpAmps are designed to run off dual + & - power supplys, as are the TL074/72. If you want to use a single supply you have to fake it, by arranging for a half way point, = Virtual Earth/Ground. In this case 6 Volts. This is made with R2 & R3 & C3.

Plus you need electrolytic capacitors to block the DC getting out to the previous & later equipment. That's what C1 & C6 are for.

I've also given the circuit X 2 gain, = R4 & R5 which should help.

I've simmed the circuit & it appears to work fine & do what you require
Hey mate thanks for your reply and help
You are correct in saying that i will be duplicating this circuit for left and right channel to the amplifier.

I will study this circuit and make it on my bread board and report back.

Thank you.
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Old 5th May 2014, 09:48 AM   #8
AndrewT is offline AndrewT  Scotland
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I think your cascaded RC filters result in an LR2 characteristic.
To maintain that, you must change both resistor values by the same amount.
To help with keeping the resistor change equal you can add a series resistor to each variable resistor. Try 4k7 as a starter.
It will allow you to change the roll off frequency of the LR2 from 50Hz (when VR is maximum) to 156Hz (when VR is minimum).

A 10k linear dual track is generally better matched than a log dual pot.
If you want a smaller range of adjustment then use a higher value than 4k7 for the series resistors.
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regards Andrew T.
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Old 5th May 2014, 07:47 PM   #9
PKN is offline PKN  Australia
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So your saying the 1st stage and the second stage should have the variable resistor so when you turn the wiper both the resistance change at the same time?

Thank you
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Old 6th May 2014, 04:40 AM   #10
Zero D is offline Zero D  United Kingdom
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@ PKN

If you want to use a variable resistor/POT in both positions, you can. But as you showed only one in your initial sketch, i thought that's what you wanted !

Either way will work though.
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