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Old 4th November 2002, 04:26 PM   #1
Ilianh is offline Ilianh  Canada
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Default Active crossovers

I'm making a second order active crossover for a sub of mine, i made some prototype on a breadboard using a pair of uA741, but there was some problems.
First of all, the noise levels introduced in the filter, its awful, at first i tought its the ground, now i think its either the psu or the opamps.
im taking the voltage off the main amp psu, using +/- 15v regulators, is that ok?

So, what are the best opamps for active filters?
i own the active filters cookbook, by don lancaster, this is where i got the designs and formulas for calculations.
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Old 4th November 2002, 05:13 PM   #2
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Good God man! 741s? You need to get updated!

Check out the web sites of Burr-Brown (now TI), Analog Devices, National Semiconductor, and etc. There are MANY, better op-amps than 741s around these days.

There are also plenty of DIY crossover designs on the web using more modern parts.

You can even download free spice simulators and op-amp models and try your hand at designing your own crossover...

MR
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Old 4th November 2002, 05:32 PM   #3
SY is offline SY  United States
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Very often, exces noise is symptomatic of high frequency oscillation. You might want to probe around a bit with a scope.

A 741 is pretty old technology- it should work fine for the low-pass section, but you'll probably want something a bit faster and with a stiffer output (not to mention lower offset voltage and bias current). Speaking of which, what sort of resistor sizes did you use in your prototype? If you go too big, the current noise of a 741 will kill you.
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Old 4th November 2002, 09:05 PM   #4
Ilianh is offline Ilianh  Canada
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thanks for replies
well yhea
i probed the in and out of the 741,
and the in is kindof clear, the out is awful, looks like 60 hz noise and some pink noise noise, i think the 60 hz noise is from the psu, guess i`ll make a better psu..

any idea of better suited opamps for these applications than the 741 and 318?

i used them becuz they are like very cheap... for the prototyping, also, could it be he breadboard? i got the dc regulators on the same breadboard as the filter itself
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Old 4th November 2002, 09:15 PM   #5
SY is offline SY  United States
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It depends on how fancy you want to get. My crossovers are designed with unity gain so that I can use buffers. The BUF03 is a classic for that application, but honestly, an LM310 will probably be just as transparent- as long as you don't tell your tweaky friends what you're using. Grab an Analog Devices databook and start browsing; they've got a terrific selection of fancy devices. And their analog guru, Scott Wurcer, has terrific taste in wine ;-)

Watch your grounding schemes!
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Old 5th November 2002, 01:49 AM   #6
Ilianh is offline Ilianh  Canada
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hahahaa heres the noise (dont worry about the dephasation, its what i wanted to acheive )

(its ugly.. isnt it?)
and theres a little bit of gain too
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File Type: jpg dephasation2.jpg (61.6 KB, 354 views)
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Old 5th November 2002, 03:13 AM   #7
SY is offline SY  United States
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Hard to tell for sure without my hands on the knobs, but I'll bet a kopeck to a doughnut that you've got a grounding problem.
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