double Bo Hansen active DI Box with modifications

[low.end.b]

Hello,

First of all, would like to thank @Bo Hansen and all people involved into making his DI box possible for everyone, from the design to the PCB and components availability.
Your knowledge and good willing make people like me able to build something special like it.

I would like to build this DI with some slight modifications:
It will be two DIs sharing the same signal input (an already buffered bass guitar), so I get two independent outputs, which is very convenient for my needs.

Also, I would like to be able to not depend on mixing console's phantom power (some gigs are not so fancy), and a small 48v power supply would be acceptable to be onboard for this. Better if could use +18v 250mA somehow, as I already have on.

After the two DIs, there will be a third output, balanced/unbalanced, amplified by probably an op amp (thinking of OPA2132 here), which will be line level output (+4dB) to drive personal monitor/power amp. I'm still thinking about this third section, don't have a defined schematic yet. The only requirements are be able to put +4dB level and have balanced/unbalanced outputs.

Hopefully my bass playing is better than my electronics knowledge, so I raised some questions, would like to know if someone could help me:
1. To use two units in parallel (considering an already buffered signal) I would solder the "Monitor Amp Out" of one straight into the input of the other one, keeping only one 10K resistor (not two) in between. Is there a better approach for this?

2. If I would use +48v from a PSU (Bo has even a phantom power design to deliver 10mA), where in the DI circuit should I solder the +48v and GND lines? Should I do something else to avoid the mixing console receiving 48v? I'm thinking of wiring a switch to choose between phantom power or PSU, if possible.

3. For the third output (balanced/unbalanced line level), should I follow the same idea and just solder "Monitor Amp Out" from the second DI to the third input circuit, keeping one 10K resistor in between?

Any suggestion on the "line level" approach, even not IC based (or other op amp instead OPA2132), is welcome. I'm not enlightened enough to design all this, but enough to read and solder.

Thanks

 

[My3gger]

DI output has microphone level output needing microphone preamp to amplify the signal.
You could add a simple mic preamp, THAT IC contains one but needs different voltages meaning you would have to build phantom and preamp psu at around +-15V. I would also add balanced output driver like THAT 1646, it allows playing with transformers if you ever want to.
If your 15 psu is symetric, powered by transformer it will be easy to get 48V with voltage trippler and +-15V for mic preamp.
If this was my project, i would build DIs as they are, use "monitor out" to get signal into second DI (dual mono). Adding mic preamp might be a little more complicated, veroboard is better for DI only, can't remember if anyone sells small THAT IC preamp or something similar.
Maybe build DIs first and leave enough space for modifications (like 10M input with fet)? That's easy with nice pcb, it is great DI.

 

[Whoops]

It will be two DIs sharing the same signal input (an already buffered bass guitar), so I get two independent outputs, which is very convenient for my needs.

Sharing the same input is easy,
you can have the signal to to the first DI circuit input (let's call it DI1) and then from DI1 you take it's buffered output out (unbalanced) and input that into the second DI input let's call it DI2.

Other option is just Multing the signal at the input, just feed the signal parallel to DI1 and to DI2

Also, I would like to be able to not depend on mixing console's phantom power (some gigs are not so fancy), and a small 48v power supply would be acceptable to be onboard for this.

The output of each DI box will be MIC Level, so you always need to connect it to a MIC Input, all mic inputs nowdays have Phantom Power (48V), even the cheapest small Behringer Mixer has 48V capability. I do Live sound professionally for many years, I did everything from small clubs to bars, to amateur kids garages and I never ever got a mixer, even the really bad ones, that didn't have 48V in it's Mic inputs.

So I really don't think you should overcomplicated this by not using Phantom Power.

What you could do is to buy a small and cheap 48V Supply and always bring it with you, it's a simple box that you put between the DI output and the Mic Pre. Something like this:

https://www.thomann.de/pt/millenium_pp2b.htm
It's very cheap and solves your issue completly

After the two DIs, there will be a third output, balanced/unbalanced, amplified by probably an op amp (thinking of OPA2132 here), which will be line level output (+4dB) to drive personal monitor/power amp. I'm still thinking about this third section, don't have a defined schematic yet. The only requirements are be able to put +4dB level and have balanced/unbalanced outputs.

Well, so you need to do a small MIC Preamp, take the output of one of the DI boxes and Mult it into a simple Mic preamp that will amplify the signal from MIC level into LINE level

Hopefully my bass playing is better than my electronics knowledge, so I raised some questions, would like to know if someone could help me:
1. To use two units in parallel (considering an already buffered signal) I would solder the "Monitor Amp Out" of one straight into the input of the other one, keeping only one 10K resistor (not two) in between. Is there a better approach for this?

That was my first reply.
Monitor Amp Out is a buffered unbalanced output, that will work well.
But as you said that your bass has an already buffered signal, I think paralleling the inputs could also work fine. If anyone disagrees please correct me

2. If I would use +48v from a PSU (Bo has even a phantom power design to deliver 10mA), where in the DI circuit should I solder the +48v and GND lines? Should I do something else to avoid the mixing console receiving 48v? I'm thinking of wiring a switch to choose between phantom power or PSU, if possible.

Check my reply with the 48V phantom power box.
2 channels of Phantom power, done

3. For the third output (balanced/unbalanced line level), should I follow the same idea and just solder "Monitor Amp Out" from the second DI to the third input circuit, keeping one 10K resistor in between?

Mult the Mic Level output from one of the DI's, one goes to the mixing desk, the other goes to your Mic Preamp

Any suggestion on the "line level" approach, even not IC based (or other op amp instead OPA2132), is welcome. I'm not enlightened enough to design all this, but enough to read and solder.

I don't remember the model now, but THAT Corp has an IC that is a Mic Preamp, very small and very simple to build.
Check THAT products/google it and try to find it

 

[low.end.b]

DI output has microphone level output needing microphone preamp to amplify the signal.
You could add a simple mic preamp, THAT IC contains one but needs different voltages meaning you would have to build phantom and preamp psu at around +-15V. I would also add balanced output driver like THAT 1646, it allows playing with transformers if you ever want to.

Thanks for the input, @My3gger. As a mic preamp, now I'm having a look at the JFP gain module, which is a JFET discrete and seems praized for bass. Thinking of two stages of this to get phase in place and all gain I may need. I read it was designed to operate with +20VDC 40mA, so my 18V should give enough headroom.

I didn't know about THAT company, and the 1646 seems to be just what I'm looking for. As @Whoops suggested, I'll try to keep it simple about PSU and will power the Bo Hansen DIs section only with phantom. I already have a 18V PSU onboard, but it's unipolar. Do you see some way I could power the 1646 with it, or would need something around +-15V? Thinking about biasing input signal for the lack of -18V., but don't know if it's a good approach.
The PSU I have on my board is an xformer isolated multiple outputs, so I have 9, 12 and 18V already on hand.

Sharing the same input is easy,
you can have the signal to to the first DI circuit input (let's call it DI1) and then from DI1 you take it's buffered output out (unbalanced) and input that into the second DI input let's call it DI2.

Other option is just Multing the signal at the input, just feed the signal parallel to DI1 and to DI2

That's the confirmation I was hoping to get. Thanks with your help with this and guiding me in the other thread.

The output of each DI box will be MIC Level, so you always need to connect it to a MIC Input, all mic inputs nowdays have Phantom Power (48V), even the cheapest small Behringer Mixer has 48V capability. I do Live sound professionally for many years, I did everything from small clubs to bars, to amateur kids garages and I never ever got a mixer, even the really bad ones, that didn't have 48V in it's Mic inputs.

So I really don't think you should overcomplicated this by not using Phantom Power.

What you could do is to buy a small and cheap 48V Supply and always bring it with you, it's a simple box that you put between the DI output and the Mic Pre. Something like this:

Were I live, all kinds of odd situations happen, sometimes for very bad equipament, sometimes for (very) bad people, so I try to be prepared for a battlefield. I'll see how this need for phantom power goes and keep a 48v supply in the bag if got this "odd" situations happening. As you suggested, is simpler and doesn't need to modify the PCB.

I don't remember the model now, but THAT Corp has an IC that is a Mic Preamp, very small and very simple to build.
Check THAT products/google it and try to find it

THAT1646 is probably what is going to drive the line level output, seems easy to implement. As preamp, I'm flerting with the JFP gain design, which I belive is known in this forum. Still making my mind around an IC or discrete preamp, have some days left to pull the trigger on the orderings.

 

[Whoops]

Were I live, all kinds of odd situations happen, sometimes for very bad equipament, sometimes for (very) bad people, so I try to be prepared for a battlefield. I'll see how this need for phantom power goes and keep a 48v supply in the bag if got this "odd" situations happening. As you suggested, is simpler and doesn't need to modify the PCB.

The only reason I'm writing in English is because this is an English speaking forum, otherwise I would write in Portuguese, I would much prefer it.

I worked in Brasil many times, I know the conditions can be quite hectic sometimes. Last time I was there we had a gig in Belo Horizonte, there was a small water streak next to the stage, when we got there the guys from the PA company had run the mains wires for all the event through the water streak. We were afraid we could get killed.
I think for the odd situations and bad conditions the 48V 2 channels PSU box will work great. Or even if you have a bad engineer that even doesn't know how to turn phantom power on, you are always sorted, it's very cheap and small

 

[Whoops]

After the two DIs, there will be a third output, balanced/unbalanced, amplified by probably an op amp (thinking of OPA2132 here), which will be line level output (+4dB) to drive personal monitor/power amp. I'm still thinking about this third section, don't have a defined schematic yet. The only requirements are be able to put +4dB level and have balanced/unbalanced outputs.

Mate I started to think about this.
I think to achieve what you want "to drive personal monitor/power amp" you don't need a mic preamp.
You can just use the buffered output from the second DI box, the signal will be already low impedance because it's buffered and it will be similar to Unbalanced Line Level , not +4dB, a bit lower, but still it will be strong enough to drive any personal monitor/power amp without any problems.

That way you are sorted and don't need an extra mic preamp circuit and going through the trouble of figuring out how to power it.

 

[low.end.b]

The only reason I'm writing in English is because this is an English speaking forum, otherwise I would write in Portuguese, I would much prefer it.

It's a little irony I came to an English forum and someone who speaks Portuguese is helping me, even more knowing you worked over here and know the odd things I'm talking about.

I worked in Brasil many times, I know condition can be quite hectic sometimes. Last time I was there we have a gig in Belo Horizonte, the was a small water streak next to the stage, when we got ther the guys from the PA company had run the mains wires for all the event through the water streak. We were afraid we could get killed.

This reminds me a venue I used to gig and in the rainy season there was a little waterfall running behind the stage. Some time after this, a singer got stuck in the metal structure because of some electrical leakage, same stage. Ended up in the hospital.
That's one of the reasons I want a DI with isolation transformer, at least some protection.

Mate I started to think about this.
I think to achieve what you want "to drive personal monitor/power amp" you don't need a mic preamp.
You can just use the buffered output from the second DI box, the signal will be already low impedance because it's buffered and it will be similar to Unbalanced Line Level , not +4dB, a bit lower, but still it will be strong enough to drive any personal monitor/power amp without any problems.

Thanks for your thoughts, really appreciate your help.
When I gig with my rig (no pun intended), the power amp is a Crown XLS, which is only +4dB level input and I can't drive it half power even using all the gain I have with pedals and a booster. I can clip hard any mic level, but this guy not even close.
When it's not my rig, usually it's a bass amp head powering, so I prefer to run my signal straight into its return loop to avoid the preamp stage, as I use my own. Sometimes It leads to the same situation than with the Crown.
For sake of simplicity, I'm thinking of giving up the balanced output in this stage, as the cable to my power amp (or bass head) won't be longer than 5 meters, and this avoids the power requirements of the THAT IC.
This way I can even put a stage of the Hamptone JFP to drive this (now) unbalanced output, get +20dB gain, don't mess with more PSUs and be able to drive any hungry input.