Bo Hansen DI layout

[Bo Hansen]

You must keep in mind that a "thru" Ground Lift switch that disconnects the XLR pin 1 does not work well in an active DI box.
(it works just fine for passive DI boxes)
This is because the pin 1 not only serves as ground connection to cable screen, it is also the negative/ground wire for phantom power.

If the XLR pin 1 is "ground lifted" must phantom negative/ground find a different and longer route, to get the phantom power to the DI box.

An example of a possible long route is via the mains plug earth connecting on the mixing console psu, and then through the house's electrical system to a guitar amp mains plug earth connection, and then the amplifier's input is connected to the DI-box "amp monitor out".

This can cause the DI box does not work, get lower supply voltage or becomes unstable, more sensitive to radio interference, buzzing from light installations, etc.

Do not overlook my "ground loop suppressor" is smart and works very well in an active DI box compared to a conventional ground lift switch.

--Bo

 

[dribro]

thanks! got to get it right then!

best
Adriano

 

[CurtZHP]

Just got done building my own.  First, a huge thank you to Bo for making this design available.  A fun, quick and easy build.

I used the OEP transformer in mine, simply because I wanted to use it for something.  I had just taken it out of my Gyraf G7 microphone, having upgraded that to a Lundahl.  So I had the OEP, and I was thinking, "What can I do with this now?"  I did a search on the forum, and found Bo's design.  Perfect!  I could use a direct box, and for about $10 in additional parts (I already had the metal box and the connectors) I set to work.  I etched my own board.

A couple questions....
I connected a wire from the "chassis ground" point on the PCB to the spare lug on the XLR, then ran a small jumper from that to Pin 1 of the XLR.  From what I've read here, this is correct, right?

I checked the voltages per Bo's instructions on his site.  On the "hot" side of the LED, I measure about 22VDC.  At the junction of the 3.9K resistor, the BC560, and the 10uF capacitor, I read about 10.9VDC.  Are these good?

When I connected it to my console (Tascam DM-24) and plugged my bass in, I noticed that I had to turn the preamp gain up very high to get a decent signal, and it clipped quite readily.  I didn't worry too much about it, because my experience with inexpensive console and interface preamps has been very similar, especially when using dynamic mics.  Connecting the DI to my Paul Stamler designed mic preamp yielded a much better result.  That said, short of getting rid of the red LED, is there anything else I could do to give this box a little more "oomph" when using it with less than ideal preamps?

Thanks again, Bo!

 

[Bo Hansen]

CurtZHP,

Thank you for enjoying my DI box design.

Your description of the ground connection seems to be quite accurate.

The voltage value you measured is quite ok, and remain within normal limits.
If the voltage over 100 uF capacitor is 22 or 26 volts, does not matter much, the main thing is that you have approx. the half of that voltage on the final transistor emitter. (at the junction of the 3.9K resistor, the BC560, and the 10uF capacitor)
In this case the electronics are functional as it should, clipping nicely and gives the most output swing.

I always go on about that you should use a red LED, as this gives a minimum voltage loss of all LED colors.
It makes no noticeable difference with or without this red LED, but if it starts to get a number of voltage drop in some weird colored LED, this may affect the DI-box output swing/head room, although it gets very little.

About low output.

About 14 dB loss from the high-Z input to balancsed output is suitable for a DI box that will feed into normal microphone input.
If the DI box has no loss, a guitar with high output microphones almost give 1 volt to the microphone input, in extreme cases, and this is too much.

Lundahl LL1538 and Haufe ST8456 provides approx. 14 dB loss in this application, but OEP A262A3E provides further 2 dB loss, and may appear to be somewhat weaker.
The OEP transformer has a very good quality for its price, even compared to the other two.

--Bo

 

[CurtZHP]

Great to know.  Thanks!

Like I said, I wasn't too surprised, considering the quality of preamps in my console.  They are OK, but not great.  And even with a condenser mic, I usually have to turn them way up.

 

[sisepuede]

Hi Bo,

Thanks a lot for this DI Project.

Is it possible to do this DI box using Jensen JT-DB-E wich is a Direct Box transformer and also coz I have several of them...  ;D ?.

I would like to use the Bo Hansen DI on Bass Guitar wich is at least 1 megohm Z.

Cheers!

Guillermo

 

[Bo Hansen]

Guillermo,

Sure you can use Jensen JT-DB-E transformer, but since it has a ratio of 12:1, it has unfortunately too much gain-loss for an active DI box, approximately 22 dB.

This transformer is made with high ratio to load a guitar/bass as little as possible when it is used in a passive DI box. (according to Jensen 140 Kohms when it is loaded with a 1 Kohms mic input)

What impedance guitar or bass to be loaded with is a matter of taste, passive and active DI boxes with impedances between 100 Kohms tuill 10 Mohm, color the sound at various brands and models of instruments.

I personally think that 1 Mohm is a good compromise to load conventional passive guitars and basses with, especially if you only have access to a DI box in your studio.

Of course, you should have several types of DI boxes, as this is a great way to bring out a special character of an instrument.
(the same way that you choose a microphone for a specific sound source)

--Bo

 

[sisepuede]

Hi Bo!

Thanks a lot for your nice and fast answer, seems like I should get a LL1538 and leave the jensens on the D.I as they are .

Cheers!

Guillermo

 

[jasonallenh]

Hi Bo,

Speaking of Jensens... I'm trying to wire one up with a JT-13K6. I have to use it 'in reverse,' so I was hoping you could clarify how to connect it.

http://www.jensen-transformers.com/datashts/13k6c.pdf

What i'm looking at, I think I would just 'mirror' the setup, and have it so

Jensen "orange" is connected to the 10uF cap
Jensen "red" goes to XLR pin 3 and phantom resistor
Jensen "brown" goes to XLR pin 2 and phantom resistor

Then Jensen "white," "yellow," and "black" go together to the electronic ground? I'm mostly unsure about these and whether they all go to ground at the same point.

 

[spacecho]

Hello All,

I'm planning on building a few of these using the Haufe Transformers, and I have a Lundahl 1530 Transformer sitting here so I would like to build one using this part. I checked the Data Sheets - it is 1:7 instead of 1:5, so apart for a greater level loss, would it be suitable? or better used in another project.

Upon looking at these data sheets, I found that the winding resistances are similar, but the 1530 is a a bit different.
www.lundahl.se/pdfs/datash/1530.pdf‎
www.lundahl.se/pdfs/datash/1538_8xl.pdf‎

I found that the pinout is slightly different - so to use it in the circuit, would I do this with the Lundahl1530:
-pins 1-4, and 2-3 (stay the same as lundahl 1538)
-pin "e" on the 1530 goes to Gnd (which is pin 8 on the 1538)
-pin 8 on the 1530 (+) goes to + (where pin 6 goes on the 1530)
-and pin 6 on the 1530 goes to where pin 5 goes on the 1538)

The datasheet mentions that I need to add a 1k terminating resistor (for 1:7 wiring) for the LL1530 transformer, where the LL1538 does not need one. Where would I put this resistor?


Thanks for your thoughts,

 

[Bo Hansen]

Spacecho,

Yes, you can use the Lundahl 1530 transformer in 7:1 configuration, but it have a bit more ratio, so you get some dB's more level loss compare with LL-1538.

Connect the 1530 as the 1:7 configuration according to the data sheet.
Note that the connections 1 to 4 is output to the XLR, and 5 to 8 shall be feed from the electronics.

If the DI-box is connectet to a regular balanced microphone input, you not need any termination resistor.

Read also my grounding instructions, that you find here: www.hansenaudio.se/techpage.htm

--Bo

 

[tonycamp]

wanted to thank you Bo, and post up some pics of my stereo DI, I used a PSU case from my IOaudio Mk47 donor mic build! also wanted to mention i lent it to a friend, he told me(quote), "i had no idea direct bass could sound this good", now i'm worried about getting it back lol

 

[tonycamp]

heres another

 

[jasonallenh]

Hi everyone...

Still hoping for an answer to my question on this page about the reverse wiring of my transformer... Please help!  :-[

 

[Bo Hansen]

Jasonallenh,

Sorry, I missed your post.

Jensen JT-13K6-C  is a very good transformer for this purpose, perhaps somewhat impractical for our PC card, but with a little skill, it will fit.

Connect as follows:

Orange to 10 uF capacitor (amp out)
Brown to XLR pin-2
Red to XLR pin-3
Yellow, White, Black to the electronics 0V/ground (not chassis)

Note that the transformer case must be isolated from the DI-box chassis/housing.

--Bo

 

[drAk]

If I wanted to take the pad mod a little (okay, quite a lot) more to -40dB switchable, would I replace the 330K resistor with a 33.2K (105K for -30dB)? Would this drop the impedance too low?

Reason for this is to drop +4 line level down to mic level for FOH systems for live performance - multiple outs from audio interface feeding FOH via stage snake.

 

[Bo Hansen]

drAK,

When using the PAD circuit, the input have not any constant resistance/impedance, because it is added to a 2.2 Mohms resistor in series with the input.

But the lowest input resistance/impedance, even if you short the 330 kohms resistor to ground, (and it becomes infinite attenuation) will be 2.2 megohms as a minimum.

So you can change the 330 kohms resistor to a lower value for the desired loss, without having less than 2.2 megohms.

Btw,
The reason I'm not bothered to maintain the constant 1 Mohm input resistance/impedance when PAD is switched on, is because in this case is most often used a instruments with active outputs, where the impedance does not really matter anymore.

--Bo

 

[drAk]

Thanks for the clarification and education Bo. I'll proceed with my calculations to provide the 30 or 40 dB pad.

-drAk

 

[monkeyxx]

What would be the best way to wire up an extra input XLR for line input of, say, -20 dB, into the front end of the circuit?

I would want to get a balanced to unbalanced conversion with the lowest noise, is this possible?  All I know how to do is buy a second transformer for the job, or short one of the "hot" balanced signals to ground, but then you lose the balancing.  Is there a better way to do this?

 

[Bo Hansen]

monkeyxx,

I'm not sure I understand exactly what you're after, but if it's just a balanced line input with 20 dB pad which will then end up in an unbalanced output.

If this is the case, then there is no benefit to using an active Phantom powered solution.

Then I would rather recommend a completely passive solution.

Use a transformer with ratio 1:5 to 1:10, and connect it backwards.
For example Lundahl LL1538, Haufe ST8456, OEP A262A3E has about 15 dB loss, and the input impedance are 5 kohms or higher, depending on what impedanse the output will drive.

To obtain an unbalanced output, connect the output winding out-of-phase end to ground.

--Bo