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Old 16th July 2010, 01:28 PM   #1
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Default Maths help -Output cap selection

Can someone please help with the equation at the bottom of the diagram?

I have some headphones with a frequency response of 16Hz to 22kHz and and impedance of 24 ohm's, but no matter how I manipulate the equation, I can't seem to work out the output cap value (C2).

Thankyou.

Chris.
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Old 16th July 2010, 02:03 PM   #2
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You don't need to manipulate the equation, just stick in your values. They're assuming an LF cut-off frequency of 10Hz, I'd go a bit lower myself as a 10Hz -3dB point introduces phase distortion below 100Hz.

But assuming you want to follow their equation then C2 (in microfarads) is 1E6/(2*pi*10*24) which my calculator says is 663uF. I'd be inclined to use 2,200uF myself.

<edit> don't forget to heatsink the LM317
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Old 16th July 2010, 02:22 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

Its just a re-arrangement of R=1/(2piFC) for a 1st order filter.
I do not see why "must" comes into it, 2,200uF will be fine
for any headphones (the min is 8 ohms & any higher value).

/Sreten.
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Old 16th July 2010, 02:40 PM   #4
sreten is offline sreten  United Kingdom
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d.p. delete does not work
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Old 16th July 2010, 02:47 PM   #5
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Doh!!!!

Thank's, it was giving me a headache. I (stupidly???) assumed that the calculation gave the answer as farad's as opposed to uF's. That's why I couldn't see it.

I'm using 470uF at the moment. Will this be too low, and if so, how will it affect the circuit/sound?

Chris.
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Old 16th July 2010, 02:52 PM   #6
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470uF just means that you'll get more bass roll-off, and I guess that subjectively the bass will sound 'slower' than with 2,200uF. The bigger value I imagine will 'tighten' the bass.
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Old 16th July 2010, 03:06 PM   #7
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Will this value adversly affect the circuit i.e. extra heat?

Chris.
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Old 16th July 2010, 03:08 PM   #8
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No, can't think of a single adverse effect on the circuit operation.
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Old 16th July 2010, 03:25 PM   #9
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by abraxalito View Post
470uF just means that you'll get more bass roll-off, and I guess that subjectively the bass will sound 'slower' than with 2,200uF. The bigger value I imagine will 'tighten' the bass.
Hi,

Its actually quite the opposite with manipulating bass roll offs.

FWIW 470uF is a fine value for typical walkman type headphones.
They are around 32 ohm, yours are lower than typical but will
still be fine with 470uF. Only if you want 8ohm compatibility
do you need to increase it, to between 1,100uF and 2,200uF.

And FWIW the equation is wrong, it should read C(uF) = ....

/Sreten.

Last edited by sreten; 16th July 2010 at 03:27 PM.
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Old 16th July 2010, 03:28 PM   #10
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Quote:
Originally Posted by sreten View Post
Its actually quite the opposite with manipulating bass roll offs.
So you've listened and found that a smaller cap tightens the bass and a bigger one slows it down? Which 'phones did you hear that with?
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