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Old 19th November 2004, 02:46 PM   #1
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Default To Match Impedance on DI Box, or Not to match?

Hello everybody! I'm trying to build a passive DI box, but I need to clear some things up first. So here's my plan: guitar line goes into a step-down transformer, into a mic pre. Simple enough? But what input impedance do I need on the transformer? I know for an op amp, or a transistor, I would want the highest impedance possible, as these don't need the voltage transfer, so much as current. But what about a Xformer? Wouldn't I want it to match the guitar so I could get the maximum voltage transfer? But I've seen a lot of people recommend a 200K input impedance, which is way up there! Wouldn't this give me a thin, trebly sound? But if I match them, I'll lose half my guitar signal! I already ordered the transformer when I thought I had to match them, it's 65K. But this can be adjusted, sort of, by running a pot in series or parallel with it to change what the resistance looks like to the guitar. But I saw a list of different pickup specs and their impedances at various frequencies change drastically! Like 8K for low frequencies, 80K for high frequencies! So if I do match them, or not, which of these frequencies do I consider the "guitar impedance? As you can see, I'm pretty confused, and the time has come to ask for some guidance. So could somebody please tell me what the input impedance should really be on a DI, and why?
Thanks!!
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Old 21st November 2004, 10:09 AM   #2
Giaime is offline Giaime  Italy
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You MUST have an input impedance of at least 1Meg, or you'll lose all the top end of the guitar! But, my friend, why to use a passive DI? Stick with a FET, a pair of resistors and you're done... You can also mount it into the guitar body! So you can use extra long cables, you can plug directly into whatever you find, etc... It's very useful.

I don't think I've ever seen a passive DI for the guitar... It can be useful for mics, where you need to raise the impedance... But here you want to lower it!
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