Flaheu,
Yes, you can add a pad on the DI instrument input. as Chrispbass say, you can use same pad that you can find in my API 325 amp DI modification.
With a double pole toggle switch, 3 position SPDT on-off-on, and a additional 330 kohm resistor, you get a pad switch with 0dB, -10dB, -20dB choice.
My DI-box can handle +19dB (7,5 V) on the input before clipping, and this will be approx. +4dB (1,2 V) on the XLR output over 1kohm load, so a synth or keyboard with out pad is not any problem for the DI-box, but for the microphone input on the mixing console that not have any pad, it can be a headroom problem.
The DI-box can drive very long lines, (same as a good condenser microphone) and if the line are of good quality cable, it can be ok with 100 meters or more.
(but of course, it is also dependent on the console microphone input quality)
The DI-box have a build-in "ground loop suppressor" so in normal case you not need any "ground lift switch".
Disconnect the tx primary from the 10uF output cap, and connect it to 0V/ground. (maybee add a 100K resistor from the 10uF cap. to 0V/ground, this take care of switching pops)
Do not insert the effects loop before the tx primary, because the effect pedals can not drive the transformer, and the transformer can not drive the long cable line.
Place the pad on the instrument input as I say in the beginning here above, it can be ok to do a balanced U-pad un the XLR output, but for the best long line driving, I will go for the input alternative.
Miha,
The "ground loop suppressor" is a old trick, but with only one low ohm resistor to break up the ground connection between electronic ground and chassis ground.
I use a improvment version a "RCD-net", that also take care of high frequency disturbance as RF or light equipment buzz, and ground/earth protection if something strange happend with connected amps and similar.
A line input on modern equipment have in normal appr. 10 k input impedance, so this is not any problem, even older equipmqnt that have 600 ohms impedance are ok.
Chrispbass,
Yes, as I say here above, it is ok to use this pad on my DI-box, and a direct copy, it will be -10dB, because as in the API 325 amp, I use the 1 Mohm ground reference resistor, on the amp input, (or bias resistor in DI-box) as a part of the pad.
--Bo
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