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Old 15th May 2007, 03:27 AM   #1
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Default Active Crossovers, my flavor of the month

Hello DIY Audio Community!

I am currently building an active crossover for my Maggie SMGs. The Crossover can be seen here

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

The article says that an output buffer is highly recomended, however It says to use an expensive op amp. I was wondering if it were a better Idea to use something like a Darlington Pair.

If a Darlington pair is not practical, would someone please point me in the direction of a suitable op-amp for the output buffer, As I am completly new to Op-amps

Thank you all so much

-Moose
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Old 15th May 2007, 03:55 AM   #2
ssmith is offline ssmith  France
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hi alexmoose,

I'm in the process of building the exact same project. The board looks well designed -- and unless I got something wrong there is a space on the board for the output buffer.

For the opamps, I decided to go for the LM4562 for the whole board. These opamps are fairly recent, unity gain stable, and there is plenty of info (mostly very positive) on them in these forums. Digikey (order code LM4562NA-ND) has them for $5.50 each, which makes $38.50 for the 7 needed for the whole board.

other parts I will order from digikey for the P09 are:

3296W-200LF-ND - Bourns 20k trimpot
3013PH-ND - 100nf bypass caps
+ the various frequency caps

I'll get the resistors from Mouser -- Vishay/Dale RN60 1/4w

would be nice to hear what you decide on.
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Old 15th May 2007, 04:07 AM   #3
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Okay, I was going to use Dirt Cheap Op-amps for the crossover, but expensive ones for for buffer stages (As the website seggested). I'll look into the one you seggested

I will probably get the resistors, and crossover capcitors from Tubedepot. my crossover only needs to be 2-way, which means 1 quad op-amp per channel.

Keep me informed on how it goes!

-Moose
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Old 15th May 2007, 04:09 AM   #4
Tim__x is offline Tim__x  Canada
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It doesn't actually say expensive, it says high-quality. An inexpensive NE5532 is perfectly suitable, it's twenty some years old, but still a good preformer.
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Old 15th May 2007, 05:10 AM   #5
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Quote:
Originally posted by Tim__x
It doesn't actually say expensive, it says high-quality. An inexpensive NE5532 is perfectly suitable, it's twenty some years old, but still a good preformer.
Thank you for reminding me about one of the fundemental rules of DIY, and of life itself.

You say the NE5532 is perfectly suitable. Sounds great!

I do not mean to disbelieve you, but I am completly new to op-amps (i'm building a Solid State system after building a tube system). what Makes an Op-amp suitable for this? how does one determain what op-amps should be used for this property (perhaps something in the datasheet?) I do not know, and would forever be in your dept if you were to enlighten me.

Thank you
Moose
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Old 15th May 2007, 05:44 AM   #6
Eva is offline Eva  Spain
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For people with their feet on the ground there is stuff like LM833 for normal filter stages, TL072 for high impedance filters and NE5532 for output buffering.

Carefully matching active filter response to loudspeaker response is far more important.
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Old 15th May 2007, 06:23 AM   #7
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Quote:
Originally posted by Eva
For people with their feet on the ground there is stuff like LM833 for normal filter stages, TL072 for high impedance filters and NE5532 for output buffering.
Evita,what about the people with their feet 1 metre above the ground.....

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Old 15th May 2007, 06:35 AM   #8
ssmith is offline ssmith  France
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Quote:
Originally posted by alexmoose

I am completly new to op-amps

Me too.

Quote:
Originally posted by alexmoose

what Makes an Op-amp suitable for this? how does one determain what op-amps should be used for this property (perhaps something in the datasheet?)
from what I can gather, the ideal opamp for this design should be unity gain stable.

Rod says the "TL072 is the most economical option" -- though not necessarily the best.

There are also some threads on opamps in the
ESP forum , including recommendations in various places for the OPA2134, NE5532 and LM1458.
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Old 15th May 2007, 01:50 PM   #9
Nrik is online now Nrik  Denmark
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NYopu cannot use NE5532/34 for this purpose.

Input impedance is about 300kohm wich might sound like a lot, but in these filters this could affect the attack frequency and precision. You MUST use FET-input opamps for the filtering itself.

I will recommend OPA2134 as a cheap and good opamp for this purpose. Input impedance is almost infinite.

For output buffer you can use NE5532/34, darlington transistors or what you feel is right.
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Old 15th May 2007, 03:07 PM   #10
ssmith is offline ssmith  France
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Quote:
Originally posted by Nrik
NYopu cannot use NE5532/34 for this purpose.

Input impedance is about 300kohm wich might sound like a lot, but in these filters this could affect the attack frequency and precision. You MUST use FET-input opamps for the filtering itself.

I will recommend OPA2134 as a cheap and good opamp for this purpose. Input impedance is almost infinite.

For output buffer you can use NE5532/34, darlington transistors or what you feel is right.
Thx for the info on impedance -- checked the datasheet of the LM4562 I was planning to use and get two figures:
Differential Input Impedance: 30Kohms
Common Mode Input Impedance (10V<Vcm<10V): 1000Mohms

presumably the common mode figure is the one to go by?

I checked on the NE5532 datasheet for the figure you mentioned, and there is only one figure -- the 300Kohm you mention.
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