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Old 5th February 2012, 08:24 PM   #1
gruni is offline gruni  Germany
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Default Three opamp 24db/oct crossover?

Hi everybody!

Is there any design that implements a full 24db/oct crossover in three opamps instead of four? Is it even possible?

The reason I'm searching for something like this is I want to squeeze the crossover plus an artificial ground into one quad opamp...

My first thought was to simply build a linkwitz sallen key low pass and sending the output with the signal input to a differential amplifier but of course that doesnt work (I think it is because of the phase shift caused by the low pass filter?)

hope you can help me!
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Old 5th February 2012, 08:48 PM   #2
Boscoe is offline Boscoe  United Kingdom
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It's done with two isn't it?

Click the image to open in full size.

You could fit a high and low pass crossover into one quad op amp but I wouldn't want too...
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Old 6th February 2012, 07:45 AM   #3
gruni is offline gruni  Germany
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why wouldnt you want to?

yes thats the well known circuit used for 24db/oct crossovers.

I'm not searching for good sound quality. it must be small and cheap...
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Old 6th February 2012, 11:06 AM   #4
Boscoe is offline Boscoe  United Kingdom
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Because I doubt you will able to find a great quad op amp but as your not going for the best sound quality then I guess your okay.
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Old 6th February 2012, 11:32 AM   #5
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by gruni View Post
I'm not searching for good sound quality. it must be small and cheap...
Does it have to be 4'th order then? I expect the capacitors will be more expensive than the opamps, and take up more space.

If you want to do everything with 4 opamps you could easily do a good 2'nd order crossover with one opamp each for: input buffer, low pass, high pass, and virtual earth.
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Old 6th February 2012, 11:59 AM   #6
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Quote:
Originally Posted by Boscoe View Post
Because I doubt you will able to find a great quad op amp but as your not going for the best sound quality then I guess your okay.
LME49740 is a great quad. It's bi-polar input though, maybe jfet would be better (opa4134?).
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Old 6th February 2012, 03:39 PM   #7
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You can do any order with just 2 opamps. Silly enough, you can do without opamps too, as long as your source output impedance and amplifier's input impedance are known and stay constant.

Make passive networks as you do with loudspeaker passive crossovers.

You can live without inductors - just use RC filter, and calculate the following stage as if it have input impedance of R+Rsource.

Gonna be messy.

3 opamps:
1. Input buffer
2. HP buffer
3. LP buffer

+ passive network in between.

You can go with 3rd order per single opamp in Sallen-Key way - 3rd order Sallen-Key Low-pass Filter Design Tool
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Old 6th February 2012, 03:50 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

If its driving diy amplifiers you can move the last low pass section to in
the treble amplifiers feedback loop. Can't do that with the bass unless you
slug the bass amplifier for unity gain stability. You can though build a low
frequency lowpass around the bass amplifier as boost / subsonic filter.

rgds, sreten.
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Old 6th February 2012, 06:21 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by s3tup View Post

You can live without inductors - just use RC filter, and calculate the following stage as if it have input impedance of R+Rsource.

Gonna be messy.
Could you explain how this is done?
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Old 6th February 2012, 07:58 PM   #10
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Looks troublesome for the mid section
Yet it got 12db slopes
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