Assembly
This note on assembly is germain mostly to the kit so maybe not something you need to read. There is a bit of info about pot selection towards the bottom to pay attention to.
Assembly is pretty simple. There is no tweaking to do afterward and nothing to match beforehand. Just make sure you get the right parts in the right holes. The holes for the resistors are pretty tight so a bent up resistor lead is going to be really frustrating. Keep them pretty straight and bent from the resistor at a 90 degree angle.
First flip the board over and solder in the ceramic surface mount capacitors. Don’t freak out. Its not that bad to solder any of this kit’s surface mount devices. My advice would be to put a drop of solder on one pad where the capacitor goes and then, with tweezers, grab the cap and bring it close to the blob of solder that you are keeping hot with the iron. As soon as the cap touches the solder it will get sucked in by the solder. Just make sure its positioned to where the other side of it is also sitting on a surface mount pad. Remove heat and it will be welded in place. Now hit the other side of it with the iron and a bit of solder. The solder must adhere to the pad so you must get the pad hot. It is absolutely going to solder to the cap but this pad might be tough so you need to be sure that a connection did happen between the pad and the cap via the solder. If you decide to throw in the towel on these caps its not a big deal as these caps are not absolutely necessary for proper operation.
Solder in all resistors and diodes next. Follow that up with the opamps and then terminal blocks.
Now the large surface mount caps get soldered to the topside. The four near the opamps should be easy. You want to be sure that the little metal tabs on their bottom side actually bond with the solder. A little flux doesn’t hurt to help this along. These tabs need to get hot and need to be forced down into the solder with finger pressure, then touch the solder right to the tab with the iron. If the cap is getting hot to your finger maybe give it a while to cool down. Test if the cap is soldered in place by trying to wiggle it back and forth. The two caps near the transistors you need to be careful to not fill the transistor’s through holes with solder.
Now solder in the transistors. The back of the transistors face the bold solid line of their silkscreen outline.
You need a way to control the volume. You can use an attenuator before the board with the output of the attenuator/potentiometer going to the terminal block. I’ll give you my opinion and then you can go do whatever you want. I like the sound of lower value potentiometers versus the higher value pots. A 10k pot, to me, is way better than a 100k pot. Any pot will do the trick though so probably you’ll want to find a stereo 10k potentiometer at EPO and wire it up to feed signal to these boards.
Why do I not include a potentiometer in this kit? You need to pick a pot that is right for your project. You should try to select a pot with a total resitance of 10 times or more than the output impedance of whatever signal source you are using. So if your CD play has a 250 ohms output impedance then you need at least a 2500 ohm stereo potentiometer. If its got 1500 ohms output impedance then you need at least a 15000 ohm stereo potentiometer. If you just want to be safe and cover all bases get a 20k ohm stereo pot and you will probably be fine. I prefer to go as low as I can and this way I get a super transparent and crisp sound. Going with that 100k pot gives you a very smooth but veiled sound. I just don’t appreciate that sound as much as the more crisp sound.
On the bottom of the board is a bit of silkscreen that looks like
Attachment:
atten.GIF
. Above this symbol are two solder pads. Solder a small piece of wire from one pad to the other. You might even get a solder blob to cover both of them and not need the wire. I had intended to put a potentiometer between the two opamps at this point but I think we should just stick with a potentiometer being off board.