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Old 22nd July 2008, 01:02 PM   #11
AndrewT is offline AndrewT  Scotland
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r5=680 is too big.
If the supply runs at 24Vdc (this will vary with mains voltage and amplifier load) then you have just 13mA available.
At max mains voltage and min amplifier draw the Zener should be near maximum dissipation. That makes more current available at other operational conditions.
If Zener dissipation is a problem then use a zener+medium power transistor to make more current available for the low level stage when mains is at minimum and amp load is maximum.
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Old 22nd July 2008, 01:31 PM   #12
Ted205 is offline Ted205  United Kingdom
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thanks for your help andrew.

can i ask how you calculated 13mA
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Old 22nd July 2008, 01:56 PM   #13
AndrewT is offline AndrewT  Scotland
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17*sqrt(2)=24, [24-15]/680=0.013
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Old 22nd July 2008, 02:06 PM   #14
Ted205 is offline Ted205  United Kingdom
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Thanks for you help
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Old 22nd July 2008, 03:11 PM   #15
Ted205 is offline Ted205  United Kingdom
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Quote:
Originally posted by woody
It might be best to tell us what you realy need. Like how much
current do you need, how tight do you need this voltage regulated, how much noise can you tolerate, do you need a
single supply voltage, is it located in the amp ect.

I'm making the additional power supply for a discrete buffer (Andrew Rothwell's design from Decibel dungen)
which i need to investigate as to what current is needed.
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Old 22nd July 2008, 06:01 PM   #16
AndrewT is offline AndrewT  Scotland
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the negative and positive rails draw similar current of ~6mA while no signal is present.
But the positive rail current is modulated with a signal passing.
Allow at least +6mA and -6mA.
but probably better to go to +12mA and -6mA, to ensure that current limiting does not occur on asymmetrical peaks. i.e. 0mA to 18mApk in +ve rail and 6mA continuous in -ve rail.

This analysis depends on R1=120r & R2=27k with +-15Vdc to +-20Vdc supply.

I suggest you add an RF filter to the buffer input. 300uS should be sufficient. 470pF & 680r are about right, but there is much leeway in these values.
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