I think you're seeing clean power with 'no load', but once you put around 200ma to 400ma load on it there begins to be significant ripple or hum.
Use a couple of 1000uf capacitors before the LM317, and perhaps a 1uf tantalum after the LM317.
Additionally, you can include a .1uf before or after for RF suppression (just in case of nearby transmitters).
For what it's worth, I used just the LM317 circuit from the Millet Max:
http://diyforums.org/MAX/schematic/MAXsch75.jpgI used 3 1000uf / 50V capacitors before the LM317 (instead of 4), from basically from CR1A to CR5.
(parts values are on the BOM on the same site,
http://diyforums.org/MAX/bom/MillettHybridMax.html)
Agreed viz. 'cheap adapters' - the little wall-wart cubes are little more than a step-down transformer and a diode, with maybe one capacitor on the DC side. Also, try a higher wattage converter - if the wall-wart is near 500ma output, that's too close to the maximum that will be required. Look for a 1000ma / 1A output, or at least 800ma.
I'm having swell luck with an old laptop supply - 18V, 1.5 amp.These are switching supplies - they will bring high-frequency switching artifacts to the output, but these are usually filtered easily enough through a good regulator circuit - 60hz (or 120hz as through a bridge rectifier) is somehow harder to filter.
High-frequency switching is usually done at 100khz - so it probably won't even show up in the audio output! I have a harder time with 100khz and other switching noise in RF applications (amateur radio).
Mike Yancey
Dallas, Texas
(edited: Hey! your English is great! ...better than lots of folks I know!)
AndrAKondrA wrote:
Connected the amp to the battery - here is the place the solution comes. 7-9-10 volt I hear the music without hum. OK it's supply voltage problem. Changing cheap AC/DC adapter to better quality adapter helps. But, still why until 7 volt zone i didn't hear the hum noise? Connected again the cheap adapter and oscilloscope to the output of the LM317 regulator. If the amp is disconnected no ripple seen in output voltage, when I connect the amp to the regulator I see ripple (after 7 volt zone) in the input voltage!
Bottom line:
1. I think its some sort of resonance between the amp and cheap power source.
2. Stay away from very cheap adapters they are unpredictable!