Quick Start Guide - sort of...
I know you guys are happy we've started shipping, but also wondering where the documentation is. Some of you are smarter than the average bear (a Yogi Bear Cartoon reference for non-US folks) and have gotten things going despite our terrible lack of help. For now we are posting a set of 3D renders that shows the main connectors and switches. Also some simple notes about using. So here goes.
1. The 503HTA is designed to connect to any Pi with a 40-pin GPIO. Simple line up the holes and the connector and gently press in.
2. When you fire up your Pi and run your chosen player (that we can not help with as there are many to choose from) simply select the Hifiberry DAC (not DAC+) as your audio device/card.
3. For tube rollers there are the two pots and a small 3-pin header labeled R-G-L, for Right-Ground-Left. Be VERY careful not to short either L or R to G! It will destroy the bias transistor and will require a repair trip back to us. We suggest using some female to male jumper wires such as these from Adafruit:
Of course, you must also be aware of the 6V/12V jumper and make sure it in the right location.
4. At the factory we adjust the voltage at R and L to 13.4V. This places the output center with enough margin for up to ~14V P-P or 5Vrms. Biasing lower (12V for example) will give more margin, but, at least with the 6922/6DJ8, the distortion is the best when the bias is higher. For low ohm headphones that are reasonably efficient (90db+) you could go as high as 16V. As always, your experience may vary and is highly dependent upon the headphones used.
5. The 3-position slide switch selects between 0, 47 and 100 ohm output resistance. IF you are using low ohm headphones (32-50) you should select the 100 (right most) position. This reduces the load on the amp and provides the lowest noise operation on the Pi.
6. The toggle switch next to the volume pot selects 0db (down) or 10db (up) attenuation. This applies to both the Pi DAC output and the Line In signal.
7. When you insert a 3.5mm jack into the Line In connector, the amp automatically switches to that signal. This input is rated for up to 2Vrms (same as the DAC).
8. If you want to drive a downstream amplifier, you should select the 10db attenuation. With the DAC at full output and the volume pot all the way to the right you will get ~2Vrms out. Be careful as you can easily over-drive (and possible damage) downstream equipment if you drive to high.
One more note, as mentioned in the campaign and pledge descriptions, we do NOT supply a power cord. We had a previous campaign and it turned out to be a real problem getting the right cords (UK especially) here in the US. If we could have used a smaller supply we might have gone with a wall wart type with interchangeable plates. But the 24V 2A supply is bit higher power (55W total) to be put into a wall wart. We're sorry if this was not noticed by you and made you scramble around to find a cord. Please accept our apologies.