A few days after meeting in person the Rune Audio staff (at their stand in Udine, for the Open Source Day) I decided to give it a try and build - or better to hack together - an autonomous box with the fewest possible connections to the outside world.
It is just an experiment, made recycling what I already had at home. It uses a raspberry Pi version B, an HiFiBerry DAC, a class D Sure amplifier based on the ST508 chip, a 27V 13A switching power supply for the amp and a 5V 1A (actually a phone battery charger) for the Raspberry and the DAC. Network connection is working via WiFi (a TP-Link dongle), mass storage is a single 16GB USB stick.
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The output and input capacitors of the amplifier have been changed since the original SMT ones were of poor quality. As soon as I can get a couple of good 2.2uF caps for the input I'll remove the two grey polyester ones I put in a hurry.
The look may be is a little creepy (my friends told me I built a box for "the hand" of the Addams Family ), but the wood too has been recycled from scrap I had from other projects. Anyway it sounds nice, and everybody gets very curios and amazed to see music coming out from that strange box
Besides improving the input capacitors I'll have to try and add a large electrolytic to the 5V power supply, at times if the song is almost silent it is possible to hear some (very small) noise that looks like it is correlated with the activity of the CPU. May be the phone charger is not stable enough and the current peaks propagate up to the DAC output. We'll see.
One more thing I believe could be useful is an analog pot to manually regulate volume... the RA interface works great (on my Galaxy Note table I found the best browser to be Opera, it almost never hangs) but sometimes you just want to quickly change the volume and waking up the phone or the tablet could be a little slow. Useful but it would break the minimalist look... what do you think?
This is a closeup of the Raspberry and DAC... sorry but the forum only allows me to upload 3 pictures with each post.
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And yes, the on/off switch is awful and the loudspeakers connector is junk. Just used what I had at home
[EDIT] I also added a 220ohm resistor in series with the fan power supply to slow it down, the amp is rated 100+100W RMS over 4ohm speakers but at home it would be very unlikely if I ever come to a tenth of that, and the fan running at full speed only adds useless noise.
Last edited by davidemania on 15 Dec 2014, 07:54, edited 1 time in total.
Ehm... this is embarassing at the moment I have nothing worth talking about, next step will be to build a decent pair of loudspeakers...
Sure Electronics offers a very good value for the price, there are lots of projects using their amps on the web. Almost everybody agrees that they need a few improvements (like the capacitors upgrade) but then they are fine. I never tried them but someone says that hifimediy amps are better built (and a little more expensive).
I improved a little my system with a few modifications, as follows:
1) input capacitors upgraded to 10uF 400VAC motor run capacitors (cheap and they are performing very well)
2) 4700uF electrolytic capacitor added to the 5V power supply to reduce the noise I was having during CPU activity peaks
3) amp gain reduced to about 1/3rd adding a 47K resistor in series with the 22K one in the input stage. Now the volume knob usually stays in the 1-50 range, that's much better than the 1-10 I had before. Besides, this reduces the resolution loss you have with software amplitude control (1 bit lost every time you halve the signal) since I don't have HW volume in my DAC.
4) old speakers are now in the bin, I got a pair of Magnat Monitor supreme 200 and I am very happy with them. You can get them (while it lasts) for 60 euros shipping included, and they perform like (or better than) many speaker I could listen to in the past that cost much more.
5) amplifier power supply voltage increased from 20V to 24V. I am not sure if this is actually making any difference but 24V feels nicer to me
Here is a picture of the modifications:
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