Circuit Help

[Cyparagon]

Wall warts aren't that noisy. I'd consider them good enough.

 

[Bionic-Badger]

Wall warts aren't that noisy. I'd consider them good enough.

Again with the harmful, blanket statements that don't consider the application of the circuit involved.

Wall warts can come in all different flavors, from unregulated types that will be completely out of whack with their stated voltage and fry your electronics, to switching power supplies, to linear regulators. You can't just assume that your wall wart will do the job. Also, if you're dealing with audio applications you want your power as clean as possible, and even high quality wall warts will have far more noise than their equivalent voltage regulated circuits.

I did a comparison of two different DC power supplies I had laying around. The first was a Meanwell MW07FA08TEL switching power supply (sorry, I have no idea where you can get these anymore) that can switch between various voltages (3.3V-12V). Another was an ELPAC MW1224 "medical grade" 24V power supply that I bought for $30.87 from Mouser back when they sold them. I compared them with both raw output and output after a LM317 using this extremely basic regulated power supply circuit that I modified with a pot so that I could change it to the voltage I need. Both circuits were loaded with a LM324 op amp, and measured using an oscilloscope.

Here are the waveforms of the power supply:

Meanwell (raw):

Meanwell (LM317):

ELPAC (raw):

ELPAC (after LM317)

As you can see there is significant noise in the signal in both the raw power from the PSUs at about 1khz. This is caused by the switching circuit in the PSUs. The ELPAC's switch circuit, while decent, still injects about 200mV of noise into the power system.

The output after the LM317 is a different story. There is still noise, however it is greatly diminished. The regulation circuit was not designed for noise rejection, and yet still does a decent job.

If you want to use a wall wart for audio applications, it should be a linear-regulated wall wart. Better yet, build the power supply circuit shown above and then you can accept any power supply you want without suffering from the noise. The circuit is cheap and easy to construct, and will reject most of the noise in your audio amplifier's power supply; the circuit diagram above specifically included the power supply section for that reason.

You can learn more about regulators and their use in such analog applications here.

 

[Cyparagon]

If that's 200mV, then my wall outlet is 340V, yet it powers 120VAC stuff without blowing it. :beer:

~70mV rms ripple on a 12V line isn't too bad, in my opinion. Good enough for the application? I don't know.

How does a simple filter cap (not electrolytic, that is) compare?

 

[lasersbee]

Am I missing something here...:thinking:

1) That is a an audio amplifier circuit

2) The transformer that is used in the schematic is a standard
120VAC input and 12.6V center tap output device easily available.
The simple split rail PS circuit will assure no DC hum due to the
60Hz filtering being used.

Here is one that will do the job...

http://www.digikey.ca/product-detail/en/241-5-12/595-1041-ND/952911

3) The cheap eBay Chinese Switching PS are useless to use in
Audio Amplifiers because of their switching noise unless you
go to extremes to filter that switching noise..
Been there.... done that.

I would go with the original PS circuit to be sure I won't have
added problems and headaches later..


Jerry

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