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Old 5th April 2011, 11:41 PM   #1
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Default Just passing my PLXXO and trafo design by the experts :)

hi all

I normally keep to the class D sub forum - this is my first foray into this side of town!

Here is the plan: To produce a 4khz 12db/oct crossover for a sure 2*100 amplifier with a transformer to amplify MP3 players into the device.


I have used the equation from TLS.org | Passive Line-Level Crossover (and the attached spreadsheet) to design what looks like a working circuit to me! However, I am concerned about the low input impedance messing things up for me....

Input impedance: 22k

spreadsheet results:

Input Highpass
Crossover Frequency 4000
Amp Input Impedence 22

Second Order
R1 10
C1 5.7875
C2 2.2065
R2 100
R2' not possible
Use Ramp as R2 in HP 1.8086 <- Cant understand what relevance this value has :-(


(capacitors in nf, resistors in k)


If someone could possibly give me a hand it would be much appreciated. I seem to constantly be working out values, but no matter how much I look at it, am not sure if what I'm doing is even right!!

many thanks for your time, and hopefully you'll stop me pulling quite so much of my hair out!

ps. i plan to have this transformer OEP (OXFORD ELECTRICAL PRODUCTS)|A262A2E|TRANSFORMER, AUDIO, 1+1:2+2 | Farnell United Kingdom just before the PLLXO to help boost the signal

THANKS!

Last edited by zerokelvin99; 5th April 2011 at 11:51 PM. Reason: Confusion!
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Old 6th April 2011, 01:48 AM   #2
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Particularily in the HP section, the low impedance of your amp may be an issue with 2nd order. you may need to add a buffer on the end of the HP section. An OpAmp or souce follower. A reansformer might do it too.

But with an XO that high, you might want to do a 1st order PLLXO (an appropriate cap on the input) and then add a single cap in line with the tweeter. This will give you a 2nd order filter and give the tweeter some LF transient protection.

dave
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Old 6th April 2011, 10:36 PM   #3
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Trying to keep away from op amps to be honest - I am working off a 12v SLA supply (single sided) and I am not too hot with electronics.


Okay, so if I design the HP section as first order then add the cap across the compression driver then it'll act as a 12db/oct crossover? there are no worries about transient responses damaging it, as the amplifier simply isnt powerfull enough

Does this sound suitable to you?


Just to add to the fun, I am thinking of using a bass/treble/pan IC to control the crossover. (and add appropriate boost where required - v useful for me). I am looking at the NTE1576, is this a good starting point?

thanks!


EDIT: Apologies if this is too many questions
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Old 7th April 2011, 12:25 AM   #4
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Quote:
Originally Posted by zerokelvin99 View Post
Okay, so if I design the HP section as first order then add the cap across the compression driver then it'll act as a 12db/oct crossover?
Yes. Cap needs to be in series with the compression driver.

Quote:
there are no worries about transient responses damaging it, as the amplifier simply isnt powerfull enough
Don't count on it. Something fails and that SLA gets strapped across the compression driver, then the smoke comes out. The cap will prevent that

Does this sound suitable to you?

Quote:
EDIT: Apologies if this is too many questions
I just ignore the ones i canna answer

dave
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Old 8th April 2011, 11:00 PM   #5
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brilliant, many thanks

im working on another circuit now, combining an op-amp tone circuit to help get rid of this pesky input impedance problem

thanks again
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