[sdiy] DC wallwart +/- on-board V Reg ?

[Sarah Thompson]

It is true. LDOs have a limited frequency response -- from DC to their design limit, they reduce noise usually by about 40db. Beyond that they do very little. This is why putting a passive pi filter first before the reg works -- a physically small pi filter only does anything useful above a particular frequency, typically a few kHz, but they make like a brick wall right up to the point where the capacitors and inductors are beyond their design limits, but that is typically in the GHz. Chain the two together and you block noise very effectively.

Sarah

Sent from my iPad

> On Mar 6, 2015, at 2:02 AM, Andre Majorel <aym-htnys at teaser.fr> wrote:
> 
>> On 2015-03-05 17:07 +0000, Tom Wiltshire wrote:
>>> On 5 Mar 2015, at 16:35, Justin Owen <juzowen at gmail.com> wrote:
>>> 
>>> I was hoping to get some advice on the potential
>>> advantages/disadvantages/benefits/costs of circuits that use
>>> a DC wall wart adapter (e.g. 9V boss-style adapters) to
>>> power a voltage regulator (e.g. LM317) which then powers the
>>> circuit - versus designs where the circuit is powered
>>> directly from the wall wart (e.g. guitar pedals)?
>> 
>> Noise is the biggest issue in my view.
>> 
>> With a 9V-from-external-wallwart, you have no idea what the
>> power looks like. If you've got a regulator in between, you
>> can be reasonably sure you're getting clean voltage of the
>> right level.
> 
> About 10 years ago, I read something to the effect that getting
> rid of the noise of a switch-mode power supply by following it
> with a linear voltage regulator does not work. Not as well as
> people think it should, anyway.
> 
> Sorry for making unsubstantiated claims but I cannot find the
> source now. I think it may have been some app note mentioned
> here. Anyone remember which ?
> 
> -- 
> André Majorel http://www.teaser.fr/~amajorel/
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