Want to build an active direct injection box (D.I)

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cosmox

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Joined 2004
#1
Hi all!

I have been looking for a schematic on a D.I box.
After some googling i found a couple but none of them had GND-Lift, phase (not hard to add, i know), Ratio etc.
The also lacked component quality.

Does any one know of a site that has more info on D.I box construction or is sitting on a service schematic (good inperation)?

Best regards, Hampus
 
#2
Here's one I've built several times:
 

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#4
The all op-amp DI boxes are cheaper to build, but you lose the ground lift ability, which may or may not be a problem, depending on what you are trying to do. Running a keyboard, guitar, or some other instrument direct, the non-transformer DI will work fine. Trying to run direct from an amp may require the ground lift to prevent ground loops.
 
#5
Brion55 said:
The all op-amp DI boxes are cheaper to build, but you lose the ground lift ability, which may or may not be a problem, depending on what you are trying to do. Running a keyboard, guitar, or some other instrument direct, the non-transformer DI will work fine. Trying to run direct from an amp may require the ground lift to prevent ground loops.
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Why? The ground lift is just a switch, just as it is in your schematic. Why is it any different for the op amp circuit?
 
#6
The transformer is what isolates the grounds. The op-amp's output must have a return path. Since the - supply is system ground, that is the return path. They do make special isolation amplifiers for data acquisition systems that have built-in isolation transformers, but I think they are quite expensive.

Ground "lifting" really becomes an issue when stage equipment is on a different mains supply than the mixing console. There can be an AC potential between the two "grounds" of several volts, causing terrible "ground loops" and lots of noise. If this kind of situation will never exist (as in a studio or small clubs where the sound board is on stage, etc.) ground lifting may never be needed. Also, if the instrument is only plugged into the direct box and it's monitor is being fed from the board (no stage amp) ground lift isn't needed.
 
#7
I believe you are confusing isolation and grounding. Yes the no-xfrmr circuit does not isolate one side from the other, but no the no-xfrmr circuit is capable of operating with the ground lifted thereby cutting the ground loop. If not, nobody would ever bother with balanced lines.
 
#8
Yes, the 'ground lift' switch doesn't remove the ground connection, it removes a SECOND ground connection, which prevents earth loops that can cause hum.

So the cable is still fully screened, and everything is still grounded, it just stops the hum.

As suggested, a transformer provides isolation, this breaks the ground connection totally - so there's no need for a 'ground lift' switch.

Instead of a 'ground lift' switch, you can do the same by using a lead with the screen disconnected at one end - but it's a GOOD idea to mark these clearly - or you could confuse yourself 😎
 
#11
Don't mean to be a jerk, but please stop requesting me to pass on a 20 year old schematic of no particular significance when 5 seconds with Google will get you more choices than you could ever build. The post was from 2005 when web resources were a little scarcer than now 🙂

If in doubt build this one, Rod Elliot's:

An externally hosted image should be here but it was not working when we last tested it.
 
#12
Whatever jackass! Nobody, that I saw, ever said it was a 20 year old schematic. An old magazine to me is a couple of months. Just ignore the thread if you don't want to answer. I did do several searches and I was trying to determine which I liked best.

I have had very good luck and very friendly help from this site and was looking for a little advice. I guess there's got to be one like you on every site. A simple "You probably don't want that one, it's very old" would have gotten your point across much better.
 
#14
leadbelly said:
Wow, you've got some edginess! 🙂
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I don't think its edgy bud, I think how you came across was uncalled for. It was a simple request that deserved a simple answer. Not everyone gets on a forum without doing their homework, I do a lot of reading and trying to understand something before I dive headlong into it. For you to imply that I don't or didn't was simply not necessary. And putting a smilie face after a rude comment doesn't make it not rude. 🙂
 
#17
leadbelly said:
An externally hosted image should be here but it was not working when we last tested it.
Click to expand...

Sorry to lift this old thread, but on this site ESP - Direct Injection Box for Recording and PA Systems
they write about this schematic:
Note that in some cases (especially with older equipment of US origin), pin 2 is 'cold' and pin 3 is 'hot'. This connection scheme is not recommended, and should not be used. The above is as close to an official standard as you will find, and should be used in all cases.
Click to expand...

What does this actually mean? Something to care about?
 
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