The harp is the ultimate stringed instrument and I sympathise with all the harp players as it is the devil of an instrument to tune. They say mischievously that harp players spend half the time tuning up and the other half playing out of tune! (sorry harp players)
There are basically two ways to record a harp. If it's on its own as an overdub a simple high quality Condensor mike 30 - 60cm(1 -2 ft) from the instrument aimed at the striking point (hands) will cover it fully. You can also use two mikes as a stereo pair similar to the position A in pianos aimed at the top and bottom strings respectively and as close as the player finds comfortable.
I used to often have a harp within a big band/strings situation where it was so quiet relative to the rest of the brass etc. that a mike in this position was 80% spill so I had to find a better way. What I found is the other way of recording a harp using the sound board as the source. Like the holes in the frame of a grand piano the harp has a series of sound holes down the back. If you get a quality mike and wrap it in a cloth you can jam it into the centre hole. The cloth will hold it tight and stop any handling noise and you will get a good level signal that with a little top EQ will work very well and have a lot less spill and you'll be able to mix those beautiful glissandos over the brass. A combination of both mike positions will give a fuller richer sound if used together.
The banjo and mandolin are similar to the acoustic guitar and both have the same points - strike area, bridge and sound board - thus the same mike positions apply. I don't like to get too close to a mandolin or banjo because their sound doesn't fully develop until around a foot or two away.
The Dobro and lap steel both have a recording problem mainly the string noise as the slider moves up and down the strings. If you think of them both as guitars on their backs and place the mike in the acoustic guitar position B where the mike points back towards the striking position you can put the fingerboard off axis to the mike and thus reduce the slide noise. You can also mike the sound board and bridge as per the acoustic guitar.
The violin, cello viola etc. are all the same except different sizes.
They all have a strike point (bow area), a bridge and a sound hole and soundboard. For the violin and viola the typical miking is to place a quality mike 30 - 60cm(1ft - 2ft) above the instrument pointing down aimed at the strike area. This gives a balance between the bow, bridge and soundboard sounds. One technique I have tried is to mike the violin from underneath as well as overhead with the bottom mike in the opposite position to the overhead mike and phase reversed. By adding a little of the under mike you can add body and warmth to the sound because you are adding more of the soundboard sound. It can also be said that a violin player should be on a reflective floor as opposed to carpet because the sound emanating from the soundboard will reflect back off the floor and add to the fullness of the sound.
The cello is the same with the mike out in front of the instrument pointing at the bow area. Additional close mikes near the bridge and the sound board/hole can be used for effect if required. Again a reflective floor is recommended.
I have singled out the acoustic bass because it is one of the hardest instruments to record in my opinion. Being a classic stringed instrument it has all the sound areas - bow area, bridge, and soundboard and soundhole. It depends on the style of music as to how you mike it but the hardest is the straight plucked jazz bass. The traditional technique is to use a good mike (preferably with a large capsule like a U87 or U49 and put it about 5 - 10cm(2" - 4") from the bridge. This will emphasise the attack of the fingers with the added hardness that the bridge sound has. Another mike can also be added that is aimed at the sound hole which will emphasise the warmth and lower frequencies. A mix of these two should cover it nicely. Many bass players have an electric pickup on their bass and a combination of direct pickup and mike works well as the pickup adds presence.
Be very careful about the low end of the sound. It may sound silly but quite often to get a good bass sound you have to remove bass from the signal. A low end rolloff from around 80 - 100Hz can stop the bass from sounding muddy or a dip around the low mids at 200 - 300Hz will also work. There is a lot of energy in the low end of an acoustic bass and reasonable compression can help to contain it.
