Want to build an active direct injection box (D.I) - diyAudio
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Old 10th July 2005, 01:27 PM   #1
cosmox is offline cosmox  Sweden
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Default Want to build an active direct injection box (D.I)

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
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Old 10th July 2005, 07:00 PM   #2
Brion55 is offline Brion55  United States
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Here's one I've built several times:
Attached Images
File Type: gif direct box.gif (8.0 KB, 1825 views)
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Old 10th July 2005, 09:18 PM   #3
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I've got a schematic for an all-opamp (no xfrmr) DI box published in an old guitar mag. Email me if interested in a copy.
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Old 11th July 2005, 12:32 AM   #4
Brion55 is offline Brion55  United States
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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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Old 11th July 2005, 01:58 AM   #5
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Quote:
Originally posted by Brion55
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.
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?
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Old 11th July 2005, 05:25 AM   #6
Brion55 is offline Brion55  United States
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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.
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Old 11th July 2005, 03:50 PM   #7
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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.
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Old 11th July 2005, 09:27 PM   #8
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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
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Old 29th October 2008, 01:40 AM   #9
nwsound is offline nwsound  United States
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Quote:
Originally posted by leadbelly
I've got a schematic for an all-opamp (no xfrmr) DI box published in an old guitar mag. Email me if interested in a copy.

I would love a copy of that schematic if you have it still.
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Old 16th January 2009, 11:27 AM   #10
jman 31 is offline jman 31  United States
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Quote:
Originally posted by nwsound



I would love a copy of that schematic if you have it still.
Hey leadbelly or nwsound, If you are still around, I was wondering if you still had a copy of that schematic?

Jman 31
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