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simple lowpass circuit help
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the_ufo



Joined: Nov 02, 2010
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Location: edinburgh
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PostPosted: Sun Nov 14, 2010 3:30 pm    Post subject: simple lowpass circuit help
Subject description: looking for info on this circuit
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How do I adjust the cutoff? is it just the ratio of that capacitor to the resistor? can it be done smoothley with a big variable resistor?

This is a digital filter, so I am assuming it will quantize the sound really nastily and give an output of a fairly low sample rate? or can I substitute some of these russian valves i've bought for that op-amp? (assuming I fire up the prewarmer circuit with the appropriate voltage?)


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cloudscapes



Joined: Feb 09, 2004
Posts: 100
Location: Montreal

PostPosted: Sun Nov 14, 2010 8:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

maybe I'm missing something
I'm looking at an analog filter.

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yusynth



Joined: Nov 24, 2005
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PostPosted: Mon Nov 15, 2010 6:14 am    Post subject: Reply with quote  Mark this post and the followings unread

As stated by cloudscape it is a basic 6dB/octave ANALOGUE lowpass filter. The triangular component with the + and - symbols is an operational amplifier (stuffed with BJT transistors). The cut off frequency is set by R1 and C. F = 1 / (2xPIxR1xC) ; However you must be aware that the ratio R1/R2 sets the gain as well. Therefore if you vary the value of R2 to change the cut-off you must change equally R to have a constant gain. The best way to do this is to use a dual gang pot for R1 and R2.


By the way forget about valves Wink

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Yves
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the_ufo



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PostPosted: Mon Nov 15, 2010 5:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

awesome, That is what I wanted to know.

I now realize it's an analogue component even though it's made of semiconductors.

If somone could reccomend such a dual-gang pot, i'd appreciate that greatly and defo use it. i've never ordered electronic components before.

w0000000000t, get the soldering iron out! Very Happy
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DrJustice



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PostPosted: Mon Nov 15, 2010 7:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

the_ufo wrote:
If somone could reccomend such a dual-gang pot, i'd appreciate that greatly and defo use it. i've never ordered electronic components before.


Since you're in the UK, maybe Maplin is a good place to shop. You can even pop down to your local branch. Hopefully they have these dual gang pots in stock.

Oh, and welcome to electro-music.com Very Happy

DJ
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yusynth



Joined: Nov 24, 2005
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PostPosted: Tue Nov 16, 2010 2:11 am    Post subject: Reply with quote  Mark this post and the followings unread

the_ufo wrote:
I now realize it's an analogue component even though it's made of semiconductors.

Semiconductors are components, analogue and digital are working modes, analogue circuits can be built with either semi-conductors or valves and digital circuits as well can be built with either semi-conductors or valves.

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the_ufo



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PostPosted: Tue Nov 16, 2010 5:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

brilliant!

thankyou everyone!!

I am finding electro-music.com to be most welcoming!!!
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enomys



Joined: Dec 19, 2010
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PostPosted: Sun Dec 19, 2010 12:06 pm    Post subject: 2nd order lowpass filter Reply with quote  Mark this post and the followings unread

Hi, just a question... For a 1rst order lowpass filter I can use a double pot to change the cutoff, but if I would like to change the cutoff in a 2nd order lowpass filter? thanks
Simon

yusynth wrote:
As stated by cloudscape it is a basic 6dB/octave ANALOGUE lowpass filter. The triangular component with the + and - symbols is an operational amplifier (stuffed with BJT transistors). The cut off frequency is set by R1 and C. F = 1 / (2xPIxR1xC) ; However you must be aware that the ratio R1/R2 sets the gain as well. Therefore if you vary the value of R2 to change the cut-off you must change equally R to have a constant gain. The best way to do this is to use a dual gang pot for R1 and R2.


By the way forget about valves Wink
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yusynth



Joined: Nov 24, 2005
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PostPosted: Sun Dec 19, 2010 12:18 pm    Post subject: Re: 2nd order lowpass filter Reply with quote  Mark this post and the followings unread

enomys wrote:
Hi, just a question... For a 1rst order lowpass filter I can use a double pot to change the cutoff, but if I would like to change the cutoff in a 2nd order lowpass filter? thanks
Simon


It depens very much on the type of 2nd order filter you intend to use. If you intend to cascade two of the above circuit th'at's a bad idea because you would need a quad pot for this. If you use a Sallen and Key or another topology such as those explained in detail here http://focus.ti.com/lit/ml/sloa088/sloa088.pdf

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enomys



Joined: Dec 19, 2010
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PostPosted: Mon Dec 20, 2010 4:32 am    Post subject: Re: 2nd order lowpass filter Reply with quote  Mark this post and the followings unread

thanks, i just checked the second order Sallen and Key, but my question still remians, I don't know to change the cutoff....
Simon

yusynth wrote:
enomys wrote:
Hi, just a question... For a 1rst order lowpass filter I can use a double pot to change the cutoff, but if I would like to change the cutoff in a 2nd order lowpass filter? thanks
Simon


It depens very much on the type of 2nd order filter you intend to use. If you intend to cascade two of the above circuit th'at's a bad idea because you would need a quad pot for this. If you use a Sallen and Key or another topology such as those explained in detail here http://focus.ti.com/lit/ml/sloa088/sloa088.pdf
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the_ufo



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PostPosted: Mon Dec 20, 2010 12:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yusynth explained how to do it for a first order filter... I dont know its applicable to a 2nd order. I haven't had chance yet to play around with this myself, so I can't really help.
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