Wild Blue Pixel
TIP OF THE DAY
Friday, February 18, 2011
Last time I left you with a shopping list for a no solder pre-amp. Let's not mess around then, let's get building!
First job is to cut the store-bought audio leads to give us a plug on a long bit of wire. Try to make sure you leave enough wire to get to your instrument and amp. If you don't have any spare wire lying about then you'll also need to cut about 10cm/4 inches from the center wire of one of the store-bought audio leads for use later. Then strip about 5mm of insulation from the ends of all of the wires and twist the wires between finger and thumb. Twisting them keeps them together and makes it easier to get them in the holes of the block. See figure 1.
Next connect the four leads from the M040 to four connections on the block. See figure 2. Make sure that all of the connections are secure by giving them a little tug. If the block isn't gripping them well, try stripping off some more insulation and folding the exposed wire in half. This should give the block some thing to grip. Now connect the battery clasp, making sure that the leads go red to red and black to black. See figure 3.
We need to connect the signal wires now and this can be quite tricky. You'll see in the figure that the module has in and out written on it. This means in from the instrument and out to the amp. Whatever lead you've chosen to go from your instrument, connect the center wire to the in and whatever is going out to the amp connect the center wire to out, as can be seen in figure 4.
We've now got two shield wires hanging loose. These need to go in the negative or black side of the battery input as shown in figure 5. This is the tricky part I mentioned, I have great big fingers that are more at home working on oily machinery so getting three wires in the tiny hole and doing the screw up took me a few attempts. I have kept the wires short for neatness but you might find it easier to leave the wires longer. The last bit of wiring is to connect the 10cm/4 inches of wire to the other side of the negative or black wire from the battery and strip 12mm/ 1/2 inch from the other end just like in figure 6.
As I said last time I'll be putting my amp in an old tobacco tin so I need to get the wires out. Nothing fancy here - I just cut the tin with some scissors and bent the tang over. See figure 7. I expect the sharp edges will damage the wire sheath in time but I'll worry about that later.
Before we power it up and shut the lid on it there's one more job. That wire hanging loose needs to be connected to the tin so that the whole thing is shielded. I used some sandpaper to get back to bare metal and the lid will keep it in contact. See figure 8.
Testing the Pre-Amp
As there is no switch yet, the only way of turning it on and off is to connect and disconnect the battery. Plug the battery in and connect your kalimba to the in lead.
Plugging in to an amp. Make sure the amp is turned off and the volume control is set to zero. The amp is always the last thing you plug in and switch on. Making sure again that the volume is set to zero turn on the amp and give it a few seconds to warm up. Slowly increase the volume on the amp. You may hear a slight hiss or hum, this is normal. If you get a loud buzz that goes away when you touch the shield wire attached to the box then the shielding connections aren't very good. Give all the connections a wiggle and tighten if necessary. If nothing comes out of the amp then something is very wrong. Go back to the top of the page and read through again.
Plugging into a computer. Go to your sound settings and turn the input volume of the line in socket all the way down. Make sure you have it plugged in to the line-in socket on the computer and slowly increase the input volume. If all goes well, there should be some sort of indicator to show you there is sound getting in. Play a few notes and set the volume so that the indicator bar never quite goes to the full extent. If nothing is happening, then back to checking connections.
You'll probably find that you won't need to put much volume on either the amp or the computer. This is because the pre amp is running flat out all the time. Next time I'll run through adding a volume control to the pre amp.
If you've any problems with this project or you just can't get it to work then drop me a line and I'll be happy to help.