[amb]

Quote:

I have an ac adapter from a router with these specs: output voltage 15.6v, current 850mA. Plug seems too thick, 4mm diameter, but I can solve this... Is current value too much?

 
No, there is no such thing as "too much".  The amp will only draw as much current as it needs.  But chances are your router wallwart is a switching type, and may or may not be low in noise.  If you listen to the amp while the adapter is plugged in, you might hear noise.
 
Quote:

Also I would like to use a super bright orange led, what R7 value must I choose? According to its datasheet forward voltage is 2.1V and recommended operating current is 20mA. Then I go to the online calculator mentioned on the amb site and put these values:
voltage supply 9v --  led forward voltage 2.1v -- led current 20mA  and get this: resistor value 345 (ohms)

. I should get a much higher resistor value given that the default one for less bright leds is 10k. What am I doing wrong?
-is this knob ok? farnell  1282579

 
20mA is way too much current.  That's usually the maximum rating of the LED, and will be very bright, not to mention wasteful of your battery charge.  Most of the time 2-4mA is enough for good brightness.  I suggest just using the default resistor values, and make changes only if you find the brightness unacceptable.
 
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Since this is my first diy project, I've bought a 25 w solder with 1mm tip. Is the tip fine enough to solder the opamp and other smd chips? I've practised so far with through hole devices  but I guess that with smd's it won't be that easy...

 
1mm is good enough.  I would recommend a variable temperature solder-station, though.  Particularly if you will be doing many more DIY projects.  Don't forget the liquid flux, that's more important than anything else for SMD work.

 

[midoo1990]

i have a question regarding the ferrite beads.i soldered one of them mistakengily on level of the board and not 3.2mm above,is this a problem?

 

[amb]

Quote:

i have a question regarding the ferrite beads.i soldered one of them mistakengily on level of the board and not 3.2mm above,is this a problem?

I recommend that you fix it.  If you don't have any spare ferrite beads, you can instead solder clipped resistor leads into the PCB holes and then solder the existing bead to the resistor leads.  It's not as pretty but it elevates the bead off the board as it should be.