modding line amp to have DI input.

hey all.

i've recently bought some dual input line amps, and plan to mod them to have a mic input and an instrument input.

the models are WBS m490h.
a variation on this model (m490g) had mic input, so i know what to change to get mic instead of line.


my question is, is it possible to mod the second input to be an instrument in, for guitars or bass, or is this much more involved than the mic in mods?

the schematics are at
www.webforni.com/steen/M490H.pdf
www.webforni.com/steen/M490G.pdf

and my current progress is at
www.webforni.com/steen/miconly.jpg


many thanks in advance folks!
 
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Didn't look at the schematics... tried googling WBS m490h and got nothing of any use, only an obviously- biased wiki posting that was as useless as tits on a boar-hog.

From my experience with mixing boards and power amps, you will need a pre-amp for a mic-in to work. Build or buy a pre-amp, probably (notice I didn't type "prolly?")
 
thanks for the response man, but that's not what i'm asking.

The modules are preamps.

barring a couple of resistors and a cap, the m490g mic preamp is identical to the m490h line preamp, so getting mic input is not a problem.



it's the DI/instrument part that i'm needing help with.


cheers again!
 
You need a high impedance input (FET) with adequate headroom for your intended input. A bit of gain doesn't hurt but isn't absolutely necessary. Your basic source follower FET works, either self-biased if that works or biased as required for headroom. Doing that with gain takes a few more components.

I didn't see any FETs on those inputs, just BJTs, so you will need to tack something on the front of one of the inputs.
 
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