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Old 8th December 2015
@ ani ol
Just plug the line input into the mic input, and use the -20db switch on the channel so the level is not too hot. Start with the mic input level all the way down also. You just need a way to have the line level arrive at the board via an XLR connector.
HOWEVER, when you put a line level input into the mic pre, you are causing that signal to go through more electronics than it would normally pass, and adding extra noise. This can be seen in your schematic just by comparing the components from the mic and line inputs to where the two paths join.
You also have the danger of putting 48V phantom power back into your audio interface (or FX device), which could damage gear.
But don't let that stop you, if you want to try this. Just be careful.
As for installing transformers-this is something I do not suggest in the first place. As I mentioned above, I think you would be better off by making sure you have new capacitors in the output section, and putting bypass capacitors across them to keep the high end nice and clear. This can be done for a fraction of the cost of transformers on all channels at $50 per tranny, and the DDA mixers are easy to work on, since the channels remove easily.
I suggest you get your transformer "mojo" from your microphones and mic pres.
Why alter a well-designed circuit like the DDA D Series? Just make sure it is not pushing audio through 30 year old capacitors. Have the caps on your board ever been replaced? Electrolytic caps get old and the signal degrades with time.
And I will say again, if the same is true of the power supply, you should rebuild that first. At least measure, and make sure it is putting out correct voltages.
Sorry if this is not the answer you want, but it comes from experience. Get the board back into new condition, and you might like what you hear.