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Here is an instrument direct box that I've actually built and used myself. My schematic program forced me to use a LF351 as the first stage; my actual prototype used a TL071. The first stage needs to be a JFET input type. The second stage is a NE5534. I know certain people hate 5534's but hey, it's a direct box.

This circuit has 20dB of gain which makes it good for guitar. It overloads with a bass, unless you change BOTH R3 and R7 from 10k to 39k. That'll knock the gain down to 10dB.