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Old 23rd January 2008, 04:24 PM   #1
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Default active filter and preamp qs

Hi,
I am planning to build preamp and 4 way active filter for my two lm3886 and one lm1876 amplifiers.

here are the circuits for lowpass ,highpass and preamp.please let me know if the circuit is fine or it need some changes.
preamp will be added after active filter, the reason is, i want to adjust different gains for different drivers.

i have not planed the crossover frequencies, but have prepared the chart for different frequencies.

102.31Hz 11k 0.1uf
150.05Hz 7.5k 0.1uf
200.96Hz 5.6k 0.1uf
341.03Hz 3.3k 0.1uf
416.81Hz 2.7k 0.1uf
511.54Hz 2.2k 0.1uf
750.26Hz 1.5k 0.1uf
937.83Hz 1.2k 0.1uf
1125.4Hz 1k 0.1uf
3410.3Hz 3.3k 0.01uf
5115.4Hz 2.2k 0.01uf

Thanks
Aniruddha


Preamp Circuit
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Old 23rd January 2008, 04:34 PM   #2
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24 db/octave highpass filter
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Old 23rd January 2008, 04:38 PM   #3
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24 db/octave lowpass filter
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Old 23rd January 2008, 04:51 PM   #4
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please ignore the power supply part for NE5532 circuits in previous posts.
I will be using +/- 15V for NE5532

my plan is to have a 4 channel amplifier for one sound box,

2 channel 20Watt amplifier using lm1876 for dome tweeter and high mid speakers

2 separate lm3886 68Watts amplifiers with different power supplies for low mid and subwoofer.

crossover frequencies which i am thinking now are around 100, 800, 3000

please suggest me for this

Thanks,
Aniruddha
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Old 24th January 2008, 04:38 AM   #5
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Plzzz suggest and advise, should i proceed with this?
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Old 24th January 2008, 09:52 AM   #6
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can somebody please explain why the input for low pass(bass) 310hz is taken from the output of 3.1khz lowpass(midrange) and not from input buffer directly in the Block Diagram of 3-Way Crossover given in

http://sound.westhost.com/project09.htm

is there any advantage in doing so? or is there any disadvantage in taking input directly from buffer?

Please help!!!!
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Old 24th January 2008, 10:49 AM   #7
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I haven't made one of these yet, but have always intended to, so my comments are not based on direct experience.

I'm not sure about the volume control in your pre amp.
Would it not be more sensible to have the potentiometer before the input of the opamp, rather than in the feedback loop? I wonder if all opamps are always stable with differing values of feedback (unity gain opamps may not be for instance).
Having said that, I know some people do actually do that (I think I even spotted it in my Doug Self book).
I'm sure the capacitor values are fine, if you've done your sums right. Can't really comment on it. I think you should have some gain control for each filter output though.
You're right, it is a tad odd to feed the 310Hz filter from the 3.1kHz filter output, unless the buffer output can only drive a couple of opamps, but I find that doubtful, based on the input impedance of an opamp.
You'll see a dotted line on one of the diagrams suggesting maybe that you could do either.
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Old 24th January 2008, 11:01 AM   #8
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Hi philpoole

Thanks for the suggestions.

yes i will be using potentiometer before the opamp using the connector in the diagram.

>> I wonder if all opamps are always stable with differing values of feedback (unity gain opamps may not be for instance).

I couldn't get the exact meaning of the above sentence , may be i will have to read more about opamps, i dont think datasheet of NE5532 have something related to unity gain.
i will be more than happy if you can explain in layman terms if possible.

Thanks
Aniruddha
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Old 24th January 2008, 12:21 PM   #9
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For instance, some opamps may have some restraints on gain (might not work at unity gain or may not be able to work at unity gain), if you have a pot in the feedback loop, then you'll be changing the gain of the opamp. Only at one point would it be at unity gain, would it be stable at other gains?

If you have the pot in front of the opamp circuit, then it makes the design less dependent on the specific choice of opamp.
Hope that helps.
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Old 24th January 2008, 12:22 PM   #10
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Anyway, looking again at your preamp, perhaps your only using these two varaible resistances to tweak the feedback loop?
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