Passive DI

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Passive DI

Postby tall_phill » Fri Mar 17, 2017 11:28 pm

So I'm considering building some passive DIs, and I'm looking at Jensen application note AS007 http://www.jensen-transformers.com/wp-content/uploads/2014/08/as007.pdf.

That note lists an optional output pad for the DI, which in addition to the level drop, "will isolate the transformer output from the capacitive loading of the cable, preventing a rise in the higher frequencies." However the other DI application note, AS066, doesn't include any such output pad, despite being based around the same transformer, and the commercial radial JDI and Jensen Concert 1 DIs, which are also based on the same transformer, include input pads, but not output pads. So is this a legitimate concern? Are there other ways to mitigate a changing frequency response of the DI?

I should note that one of my objectives, if I am to do this is to simplify the DI as much as possible (yes, I know they are already tremendously simple) so it has little more than two jack sockets, a male XLR and a ground lift switch. I find the JT DB transformer appealing because it can already take a significant amount of level without distortion (such that a pad is basically redundant, unless you want to take the output of an amp).
However the DIs will be used largely on stages for live performance, so the behaviour should not be unduly influenced by cable length.

Can any of the more experience board members offer some guidance?

Thanks.

PI.
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Re: Passive DI

Postby chrisp » Sat Mar 18, 2017 10:16 pm

Hey Phil

The primary use of that U-pad is to add 15db attenuation to the output before sending it off to your preamp. Don't worry so much about that capacitance thing, think more about the level you want out of the DI.

In short, if you are DI'ing an electronic piano or keyboard that has basically a line level signal or near as dammit, the extra attenuation from the pad is useful to avoid overloading the preamp. Alternatively, you could just as easily use the preamp's input pad switch if it has one. If you are DI'ing a guitar, omit the pad as the 'basic' version of the DI is what you need.

You say you are looking to not complicate things - it would be dead easy to wire this pad with a DPDT switch to give you an 'output pad' option - but again its really only useful where you are dealing with keyboards, theremins and the such like rather than guitars AND where your preamp does NOT have an input pad switch.

Good luck.
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Re: Passive DI

Postby tall_phill » Sat Mar 18, 2017 11:01 pm

Hi Chris,

Thanks for the response.

Im might give it a shot without and see how it behaves driving a long cable.

Since pretty much all modern consoles for live sound can handle a wide range of levels at their mic inputs, from hot line level, to low output dynamic, without the need for a pad (at least that the end user operates), I see no need for a pad if the DI isn't distorting; and with the rated 21dBu before 1% THD @ 20Hz, I'm content that few applications on stage would distort the unit.

On the 'not over complicating things' note... It occurred to me recently that almost every DI I use has some features I almost somewhere between almost never and not at all. For instance, even on the simple JDI I never use the speaker switch, never use the polarity switch (the console has one), and almost never merge switches (maybe I used it once?), and Im pretty sure I have barely touched the Pad (not so on other DIs mind). So part of my thinking is that those switches are mostly just potential sources of mistakes.

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Re: Passive DI

Postby chrisp » Mon Mar 20, 2017 8:07 pm

It occurred to me recently that almost every DI I use has some features I almost somewhere between almost never and not at all.


Yep.

It will work just fine with the simple design I am sure.
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