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Author Topic: How DI Boxes Work?  (Read 3079 times)

Geoff Doane

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Re: How DI Boxes Work?
« Reply #40 on: September 21, 2018, 10:39:25 am »

In the case of a passive DI you need to match a high impedance input such as a guitar to the low impedance input on your desk - say 20,000 ohm's to 200 ohm's so you need a transformer with a turns ratio of 100:1 hence the insertion  loss.


Peter probably already knows this, and just made a slip, but the impedance ratio is the square of the turns ratio.  Therefore if the impedance ratio really is 20KΩ:200Ω (100:1), the turns ratio is 10:1.  The voltage ratio is the same as the turns ratio, and that equates to a voltage change of -20 dB.

GTD
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John Roberts {JR}

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Re: How DI Boxes Work?
« Reply #41 on: September 21, 2018, 11:21:10 am »

As I said before, I searched before posting for this kind of DI discussion and couldn't find it in these forums, which may well be due to my poor search skills. I asked you ITT for a link to the previous discussions but got nothing. Which is not to say it isn't there, but I couldn't find it on my own.
Yes the search function here sucks... somewhat better using google ("prosoundweb + direct box"), but no i will not do your homework for you. Further the search engines seem happy to ASSume anything more than a couple years old is not relevant (probably a selling related algo bias.)  :o

ITT? I was able to search that
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With all due respect, it may be ad absurdum for you, but I for one am appreciating people's different viewpoints and explanations of how these things work with as much detail as they've cared to share, which has been considerable and enlightening.

Hopefully this thread can be found the next time somebody has a question about what exactly a DI box does and how and why it does it, since I couldn't find anything other than "what's the best DI?" and the like.
I am not as optimistic but resigned to same old questions over and over...

I particularly dislike the typical DI round and round since it gets confused by the number of different functional products all lumped together and called DIs despite significant differences. So takes pages to sort out all the variants... you guys missed a few but i won't feed this. Sorry I answered the question I did, but it's my nature.

JR

 PS: Maybe the forum's new policy to spam us with links to old written articles will help, while some of those are so-so.They could use AI to find questions in posts and provide articles, but isn't that what the search function should do? never mind.  :o
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Peter Morris

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Re: How DI Boxes Work?
« Reply #42 on: September 21, 2018, 11:21:47 am »

Peter probably already knows this, and just made a slip, but the impedance ratio is the square of the turns ratio.  Therefore if the impedance ratio really is 20KΩ:200Ω (100:1), the turns ratio is 10:1.  The voltage ratio is the same as the turns ratio, and that equates to a voltage change of -20 dB.

GTD

Oooops  :-[ ... Yes I did know that.  I actually picked an example I knew was 20dB ... well spotted - Thanks  :)


Itís probably worth adding a bit of an explanation of why this is so for others.

Its all about transferring power, the power dissipated across the load must to be equal to the power input.

In this case if we have 1 volt across 200 ohms - thatís    1^2/200 = .005 watts (watts = Volts squared over resistance)

If we have a 10:1 transformer we would have 10 volts on the input

For the power to be equal we have   .005 watts = 10^2/Z Ö Z =  20,000 ohms
« Last Edit: September 22, 2018, 04:05:41 am by Peter Morris »
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Dan Mortensen

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Re: How DI Boxes Work?
« Reply #43 on: September 21, 2018, 03:58:14 pm »

Yes the search function here sucks... somewhat better using google ("prosoundweb + direct box")

Thanks, JR. As I indicated or maybe hinted, my searching abilities leave something to be desired. Using your terms came up with a bunch of interesting and informative articles by Mike Sokol, Gary Parks, Mark Frink, Al Keltz, Gary Gand, and a couple threads with your comments in them. Those were just on the first page of results, but as interesting as all of them were, none of them answered the question in the original post of this thread, which had to do with the precise drop in amplitude from using a DI in a signal chain that didn't necessary need a DI.

All the articles discussed the attenuation switch(es) on DI's, but that switch is irrelevant to the question.

I am not as optimistic but resigned to same old questions over and over...

I particularly dislike the typical DI round and round since it gets confused by the number of different functional products all lumped together and called DIs despite significant differences. So takes pages to sort out all the variants... you guys missed a few but i won't feed this.

The original situation had some people using DI's in an unusual way, and I'm glad to learn about other devices which, with planning, could be used to solve a variety of problems that might come up. In that original situation they used devices on hand at the gig (none mine) to solve a perceived problem, so they did not have the luxury of planning. (Have to smile when typing that last phrase, in context of the gig, which was a unique one for which I was planning and testing my parts for two months.)

None of the articles on the first search page addressed any other variants, so further discussion is as welcome as are the several expansions of the original question but all of which fit into the title "How DI Boxes Work?".

You have immensely helped thousands of people here over the years, and we are all grateful for that help, knowledge, and experience. If you want to name at least some of the variants that would be appreciated, but you may choose as you wish. And that is my nature.
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Greg_Cameron

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Re: How DI Boxes Work?
« Reply #44 on: September 22, 2018, 08:02:20 pm »

In the original scenario you posted Dan, it seems they should have been using something I keep on hand for situations like this, Whirlwind ISO1 or ISO2 isolation transformers. Yes, I know they don't need isolation, but these transformers also serve as "line balancers". They've got 1/4" inputs as well as XLR and will take an unbalanced input and provide a balanced, "line level" output of either XLR or 1/4". The also have a pin 1 lift. They come in handy quite a bit. They have a pretty beefy transformer compared to the tiny XLR barrel style units so you can drive them with lower frequencies at hotter levels without saturation.


Greg



http://whirlwindusa.com/catalog/black-boxes-effects-and-dis/transformers-isolation-devices/iso-1


http://whirlwindusa.com/catalog/black-boxes-effects-and-dis/transformers-isolation-devices/iso-2-dual-line-level-isolator-and-balancer
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