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Old 19th September 2004, 02:42 AM   #1
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Default Active lowpass filter + high rails

Hi all,
I've built a simple 2nd order active lowpass filter using some small signal opamps, and now I'm trying to figure out a way to incorporate this into my little solid state amplifier. I've got +/- 38V or so rails, but the only opamps I have that I can use for my filter will only take a maximum voltage differential of 40V. How can I go about regulating my rails down to something that the filter opamp can use? Would using a 7815/7915 pair work, or is there a different solution you guys would recommend?

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Old 19th September 2004, 12:27 PM   #2
Eva is offline Eva  Spain
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This topic has been previously discussed many times. Try searching harder

The alternatives are essantially : An extra transformer, zener pasive regulators, transistor-buffered zener regulators, IC regulators, IC regulators with pre-regulation/pre-droppng and complex discrete regulators
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Old 19th September 2004, 12:29 PM   #3
Optical is offline Optical  New Zealand
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not sure about the 7815/7915's, check the data sheet for the maximum input voltage, i have a feeling its around 30V so 38 would cook them..

otherwise you can use zener diodes
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Old 19th September 2004, 07:23 PM   #4
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Default I would use pre-regulators

From the positive supply, connect a resistor, about 5.1k, to a 22V zener to ground. Bypass to ground with 0.1F and 10F. Connect the base of an NPN to the zener voltage. The collector goes to V++. The emitter of this NPN goes to VIN of an LM7815 regulator. Bypass this Input and Output of the regulator to ground with 0.1F and 10F capacitors.

Repeat the above for negative side, substituting PNP xsistor and LM7915 regulator.

The NPN/PNP transistors will need to be sized appropriately to match the load. A TO220 device will likely provide more than adequate thermal capabilities. Do however measure your current then do the math to make sure.
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Old 19th September 2004, 07:47 PM   #5
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Thanks for the idea about zeners - now that I know where to look, I've seen a bunch more posts about this - I appreciate the help.
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