I was going to mod one of my Altec mixers for direct out, but rather than than mess one of them up, I made a combination direct box/mic pre. I used a Hosa passive direct box that I had. Originally I was going to use an Altec 15095 input transformer, but after testing it I found that the Hosa transformer sounded just as good. I moved the transformer to make way for the other components and removed the ground lift switch in the back and replaced it with the switch on the left. The mic input goes to a modified Altec 1588B preamp. I used a TRS jack for the output, which actually could have been a TS, since the output is unbalanced. The output is line level, somewhere a little above 600 ohms, close to 800 ohms at full volume. That wasn't a problem for me, since my M-Audio interface will accept up to 10K ohms input. The direct box circuit is intact and has a 150 ohm output. I can use a mic cable and run that output into the mic pre input and it becomes an active direct box! It has lots of gain and sounds great. For the power supply I am using a 14 volt dc power supply I built. I tried some generic wall wart supplies, but they were too noisy. Any DC voltage from 12 - 20 volts will power the 1588B.

This is the 1588B gain mod:

People are always saying that the Altec 1588B modules are noisy, thats BS. If you recap them there are no noise issues, its a very clean pre amp under normal gain settings, but gets wonderfully hairy when you drive it. I find it to be an incredible pre on rock drums, I can color each to taste. Anyway, the mod to give the 1588B module enough gain to stand alone and have a direct output pulled from all these altec mixers (1592a and B, 1599A and B, modular 1581A, etc) is fairly simple, plus while you have the shell off the module you can go ahead and swap out the electrolytic caps. Here are the pics describing the mod.

For the direct output mod the first picture shows you whats going on there. Note that the resistors are changed and  the 1 MEG output pot is changed to a 1 K pot. You can tap the output of the pot directly and add a buffer stage or an output transformer to taste.