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Old 12th April 2011, 08:41 PM   #1
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Default Noisy IC Universal Regulator

Hi everyone,

Recently I constructed a fet based pre amp for my guitar. The sound performance seems good and overall I am happy with it. However, I was somewhat disappointed initially with the background noise it produced - almost a white noise/hissing sound.

On a hunch I swapped the power supply (its one of those multi-voltage ones where you can select a number of different voltages from 1.5-12V) for a 9V battery and the noise disappeared. Good times.

However, I would like to continue to use my DC power supply and perhaps others which could be just as noisy. I would like to keep the circuit as simple as possible, therefore my question is this: How can I clean up a power source using non-regulated techniques initially then if this doesn't work what regulated circuits have people used?

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Old 12th April 2011, 08:49 PM   #2
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Location: Canandaigua, NY USA
Does your circuit have any bypass capacitors for the supply? If the noise is hiss, bypasses and maybe a bit of series resistance should take care of it, depending on exactly where it comes from.
I may be barking up the wrong tree, but at least I'm barking!
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Old 12th April 2011, 10:12 PM   #3
kubeek is offline kubeek  Czech Republic
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It probably comes from the switching regulator. Does the adaptor look something like this?
Then it is a SMPS and some LC filter should help get the noise down.
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Old 13th April 2011, 10:56 AM   #4
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Thanks for the messages!

Conrad, I have a capacitor but no series resistance. Am I right I thinking that this creates a low-pass filter? If this is the case then I believe my cut-off is too high (the capacitor is only 10uF) hence why I can still hear the noise.

If I were to use a resistance of 100ohms and a capactor of 100uF then this would set the cut off at about 15Hz?

Kubeek, yes its very similar. Say I use a 100uF cap, would I be correct in saying that using an LC filter could produce the cut off required without needing the such a high series resistance? In my comment above I said I could use 100Ohms which seems a little high?
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Old 14th April 2011, 02:26 PM   #5
kubeek is offline kubeek  Czech Republic
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LC means inductance with capacitance, which will be far better. If you donīt have access to a suitable inductor, you can use RC filter.
You donīt say how much current does your amp need, but the 100ohm resistor would limit the current to 120mA. I would use something smaller, like 10ohms, which with the 100uF cap will make a 160Hz low pass filter, which will attenuate -42dB at 20kHz and that should be enough.
If it is not, you can split the resistor into two 5 ohms and make a two-stage filter with two caps, connecting it R-C-R-C, which will yield -70db attenuation. If you got different caps, use the larger as the first one, preferably with a 100nF ceramic one in parallel.
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