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Old 18th August 2014, 03:31 PM   #1
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Default Critique my first drawing / possible build [Active DI]

Hello all,

I'm designing a 6-channel active DI for my band, to use with an unbalanced drum trigger module out. It will be used only for live shows, so the emphasis is mainly on weight (airplane weight restrictions) and ruggedness (non-stationary use). This is decidedly NOT an audiophile build, but a lightweight workhorse that has to deal with a lot of transients (kicks, snare, toms) and no steady music.

I've done some searching and want to limit the build to a transformerless op-amp based DI. After searching the internet I found Bo Hansén's workhorse (not transformerless, so not what I'm looking for), ESP's project 35 active DI and This DI from Fivefish.net. Now, because the FiveFish DI looks simpler in design and uses external power rather than battery or phantom power, this seems to be my thing. So my plan is to build 6 individual DI's and stick them into a 1U rack case, along with some extra patchbay stuff. Essentially, the thing I want will replace two Behringer 4-channel DIs and a 1U patchbay, clearing up our live rack and cutting down on weight and size.

Now, since I'm new to building stuff like this, I've downloaded TinyCad and re-drawn the FiveFish schedule. Here's my questions.
  1. Does this look feasible?
  2. Would this work?
  3. If you look at the original FiveFish schematic, the unbalanced + signal seems to be shorted to the unbalanced GND. This can't be right, right? I've corrected this in my own schematic.
  4. What would you do differently, if anything, given my constraints mentioned above?
  5. I'm planning on powering the whole thing with a switch-mode laptop power supply (conveniently giving me 18V DC) and then converting it to + and -18V. Good plan? It's lightweight.
  6. Borrowing from ESP's project 35 amp, I've put two diodes on the 5532 outputs to protect them from any +48DC phantom power that may be present on the output XLRs. Would that work?

Now, here's the schematic modified from FiveFish for you all to point and laugh at (my first ever drawing). It will probably look awful, but I've heard that you need to make many mistakes to learn

Looking forward to any suggestions!!
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Old 26th August 2014, 10:37 PM   #2
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Subtle bump. Would really appreciate any feedback!
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Old 27th August 2014, 02:47 PM   #3
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by hvergelmir View Post
Hello all,

I'm designing a 6-channel active DI for my band, to use with an unbalanced drum trigger module out. It will be used only for live shows, so the emphasis is mainly on weight (airplane weight restrictions) and ruggedness (non-stationary use). This is decidedly NOT an audiophile build, but a lightweight workhorse that has to deal with a lot of transients (kicks, snare, toms) and no steady music.

I've done some searching and want to limit the build to a transformerless op-amp based DI. After searching the internet I found Bo Hansén's workhorse (not transformerless, so not what I'm looking for), ESP's project 35 active DI and This DI from Fivefish.net. Now, because the FiveFish DI looks simpler in design and uses external power rather than battery or phantom power, this seems to be my thing. So my plan is to build 6 individual DI's and stick them into a 1U rack case, along with some extra patchbay stuff. Essentially, the thing I want will replace two Behringer 4-channel DIs and a 1U patchbay, clearing up our live rack and cutting down on weight and size.

Now, since I'm new to building stuff like this, I've downloaded TinyCad and re-drawn the FiveFish schedule. Here's my questions.
  1. Does this look feasible?
  2. Would this work?
  3. If you look at the original FiveFish schematic, the unbalanced + signal seems to be shorted to the unbalanced GND. This can't be right, right? I've corrected this in my own schematic.
  4. What would you do differently, if anything, given my constraints mentioned above?
  5. I'm planning on powering the whole thing with a switch-mode laptop power supply (conveniently giving me 18V DC) and then converting it to + and -18V. Good plan? It's lightweight.
  6. Borrowing from ESP's project 35 amp, I've put two diodes on the 5532 outputs to protect them from any +48DC phantom power that may be present on the output XLRs. Would that work?

Now, here's the schematic modified from FiveFish for you all to point and laugh at (my first ever drawing). It will probably look awful, but I've heard that you need to make many mistakes to learn

Looking forward to any suggestions!!
Should just say that I had no idea what a DI box was until I looked it up... however looking at your circuitry.

1/ The idea looks OK. I assume you just want a single input to balanced output.

2/ No it won't work. The opamps are all configured incorrectly I'm afraid.

3/ The fivefish circuit is incorrect as above.

4/ Make a working circuit

5/ That's OK. Small low power DC DC convertors are an option too.

6/ Diodes have to clamp to a rail to be effective. As they are they would introduce distortion. Lots and lots
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Old 27th August 2014, 02:59 PM   #4
Mooly is offline Mooly  United Kingdom
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Check out the links here,
bridging adaptor for single supply amp

For protection you need something like this per opamp. The diodes clamp to the rails.
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Old 27th August 2014, 03:11 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by hvergelmir View Post
Now, here's the schematic modified from FiveFish for you all to point and laugh at (my first ever drawing). It will probably look awful, but I've heard that you need to make many mistakes to learn
The top circuit needs a ground, like the bottom circuit has. The diodes should be removed.
The inputs on both op amps should be flipped. so that the feedback goes to the negative inputs.
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Old 27th August 2014, 03:19 PM   #6
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Quote:
Originally Posted by Mooly View Post
Should just say that I had no idea what a DI box was until I looked it up... however looking at your circuitry.

1/ The idea looks OK. I assume you just want a single input to balanced output.
Yes, of which I'll put 6 in a single box.

Quote:
Originally Posted by Mooly View Post
2/ No it won't work. The opamps are all configured incorrectly I'm afraid.
Serves me right for copying someone else's schematic. Thanks! Maybe I should stick with ESP's TL072-based designed after all.

Quote:
Originally Posted by Mooly View Post
3/ The fivefish circuit is incorrect as above.
Duly noted and abandoned.

Quote:
Originally Posted by Mooly View Post
4/ Make a working circuit
I was gonna!

Quote:
Originally Posted by Mooly View Post
5/ That's OK. Small low power DC DC convertors are an option too.
A step-down buck convertor is an option too, but I have a bunch of linear regulators lying around. But thanks for the suggestion, it does sound more efficient.

Quote:
Originally Posted by Mooly View Post
6/ Diodes have to clamp to a rail to be effective. As they are they would introduce distortion. Lots and lots
Thanks for this and your following post. Looking at ESP's schematic a bit better than I did before, the diodes are indeed clamped to the power rails.

Last edited by hvergelmir; 27th August 2014 at 03:29 PM.
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Old 27th August 2014, 03:21 PM   #7
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Quote:
Originally Posted by rayma View Post
The top circuit needs a ground, like the bottom circuit has. The diodes should be removed.
The inputs on both op amps should be flipped. so that the feedback goes to the negative inputs.
Thanks for that! Mooly has already suggested where to move the diodes to protect the opamps from +48 (like ESP's design has).
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Old 27th August 2014, 04:09 PM   #8
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In a balanced XLR interconnect circuit, the XLR pin #1 is not connected to the audio circuit common. It is a shield (not a ground) and is connected to the chassis at the connector.
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Old 27th August 2014, 05:36 PM   #9
rayma is offline rayma  United States
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Quote:
Originally Posted by hvergelmir View Post
Thanks for that! Mooly has already suggested where to move the diodes to protect the opamps from +48 (like ESP's design has).
Also, after flipping the op amp input terminals, the tip and ring should be flipped if you want pin 1 positive.
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Old 27th August 2014, 09:07 PM   #10
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Alright, after all your useful comments I've decided to ditch the FiveFish design. Too many unknowns and errors. So, I've opted to go for the ESP project 35 mentioned before.

It has some stuff I don't need (line/speaker switch, battery operation) which I've removed, and changed the 2x 9V battery by a single 18VDC power supply. So here's the altered schematic.

This should make much more sense, right?

In response to what Speedskater said, in this design XLR pin 1 seems to be connected to ground, with an 'earth isolation' resistor and capacitor...?
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