Help Design an unbalanced consumer Line level to Balanced interface

[Whoops]

Hi fellow mates,
for my Live Sound work and duties I have many times Keyboards, Samplers, Drum Machines and Roland SPDS instruments.
All of these instruments have unbalanced Line Level outputs which fall into the -10dBV consumer Line Level.
The signal in already Low Impedance so there's no need for an Hi-Z input like Instrument Level.

I need to build very simple boxes to interface these instruments to the FOH console Balanced Mic Inputs.
Normally I use DI Boxes from the local PA company, but as those DI boxes are different every gig they range from alright to pretty bad or non-working/malfunctioning. So for next year I want to start using only my devices, so I can have more consistency from gig to gig.
Also I don't really need all the commodities of a DI box for these type of instruments, no need for an Hi-Z input, no need for Galvanic isolation (so no need for transformers), and might be fine without any attenuation or step down.

What I use sometimes and works well, and that would be preferable for me if it didn't have it's short-comes would be to use a Jack TS to XLR cable (Pin 3 connected to sleeve). By the way for those not familiar with Live sound, all inputs on stage to the console use XLR connectors and connect directly to the console's Mic Preamp.
This cable works very well, no circuit is connected in the middle of the signal so it's quite transparent, but there's 2 downfalls:
- The instrument output is not protected from Phantom Power if by accident it is turned on in that particular channel
- Also the signal is unbalanced all the way, and in a sub-box on a stage the signal can have 30 meters of cable or more before it reaches the Mic Preamp's input, all those meters of cable are traveling side by side with lighting equipment, mains electricity and RF so they are prone to external interference/noise.
Example below:




So here is my goal, and I really need you guys to help on this and I'm only a Sound Engineer and not as educated in electronics design as you guys.
I would like to do a circuit that:
- it's really small (as small as possible as I will be traveling), ans possible of having a small box with 4 or 6 of those circuits inside.
- Converts -10dBv unbalanced Line Level to Balanced
- completely transformer-less
- it's the most simple and basic circuit possible, with the least amount of components
- and now the hardest part, that works solely on Phantom Power

 

[Whoops]

I'm researching a bit and I have 3 possible ideas for the circuit.
Using a That Corp 1646 IC (Balancing Opamp), using a TL072 or having a complete discreet circuit using transistors.

Here are the 3 possibilities:



Phantom Power can deliver 10mA, so that will be fine to Power the TL072 and also a discreet circuit.
I'm not sure if it's able to power the THAT 1646, it says in the datasheet it's "Quiescent Supply Current" is a Maximum of 5,75 mA, but I don't know it that consumption goes higher when working, I don't know the meaning of "Quiescent Supply Current" that well.

I also don't know circuit wise how can I take the Phantom Power the unit receives and supply the Opamps with it.
I don't know how could I separate it from the audio and also if I should supply the Opamps with a single rail or if it's possible to derive 2 rails from the +48V, a positive rail and a negative rail, don't even know if it's needed.

If you guys could help an hobbyist designing a circuit I would be quite grateful.
Any help, circuit examples of Opamp and Transistors employed to balance a signal, or suggestions are very welcome.

Thank you so much

 

[MicUlli]

Connecting an unbalanced signal source to a balanced signal destination is one of the easier things:
You have to know what output impedance the source provides (in most cases found in the device manual) let's say R=200 Ohm. Connect + input to source output. Connect - input via R=200 Ohm to GND of the source output directly at the source. Connect source GND to cable shield. That's it.

 

[Bo Deadly]

- and now the hardest part, that works solely on Phantom Power

Not sure that’s possible. Phantom just doesn’t provide the power to drive long lines without a step down transformer. You can try but without the transformer you would have to keep the signal level really low to the point where the noise of the active amp would be unnecessarily high.

A battery would solve a couple of problems at once.

One thought that I’ve had from time to time about this scenario would be to use a lipo jump pack for a car. It’s pretty small but gives you many amps of clean floating 12. They are vey common. You can buy one at many big retail stores and charge it from USB in your car on the way to the gig.

 

[scott2000]

Came across one of NYD's things. Maybe it can spark more relevant ideas? Or not....

 

[Bo Deadly]

Current" is a Maximum of 5,75 mA, but I don't know it that consumption goes higher when working, I don't know the meaning of "Quiescent Supply Current" that well.

Quiescent means when the circuit is at idle. Naturally if it had to drive a mic input load of say 1k to 1V that’s another 2 mA (1mA for each line). But that’s probably too hot for a mic in anyway. If each amp had phantom, that would be enough power. You could put a 20dB pad in front of each amp so that the source sees 10k and the impedance seen by the amp is low and the line level is low enough for a mic in.

Or use a battery and then you don’t have to be so cautious.

 

[Rob Flinn]

I realise that you want to DIY this so don't shoot me, but I use one of these 4 channel BSS DI's for the job you're talking about. I had considered making something but the price I got this box (£75 U.K) for it just wasn't worth spending my time making it. I probably couldn't even buy the parts for that money. I know it has a load of features that you don't need too

https://bssaudio.com/en/product_documents/ar416dspdf

 

[Whoops]

Connecting an unbalanced signal source to a balanced signal destination is one of the easier things:
You have to know what output impedance the source provides (in most cases found in the device manual) let's say R=200 Ohm. Connect + input to source output. Connect - input via R=200 Ohm to GND of the source output directly at the source. Connect source GND to cable shield. That's it.

That would make the signal travel unbalanced and as I explained that's not what I'm looking for.
I want to Electronically Balance the signal, so it travels though all the meters of cable in the stage balanced into the Stage Box inputs

 

[Whoops]

- and now the hardest part, that works solely on Phantom Power

Not sure that’s possible. Phantom just doesn’t provide the power to drive long lines without a step down transformer. You can try but without the transformer you would have to keep the signal level really low to the point where the noise of the active amp would be unnecessarily high.

I guess there's some misunderstanding...
Phantom Power is used to power active DI boxes (some of them completely transformerless) through many meters of cable, it's also used to power transformerless active microphones.

What I want to build is a similar to an Active DI box, but much simpler, without parallel inputs (or a buffered out), without the need for an Hi-Z input, probably without a PAD or step down and without a ground lift.

A battery would solve a couple of problems at once.

The premise is only to be supplied by Phantom power, that's the only goal

 

[Whoops]

Quiescent means when the circuit is at idle.

Thanks, that was my interpretation also, and made be think that if the consumption at idle is 5,75mA maybe when working the consumption is higher than that, that's what I'm not sure about.

Naturally if it had to drive a mic input load of say 1k to 1V that’s another 2 mA (1mA for each line).

The THAT Corp 1646 is a Balanced Line driver, it receives an unbalanced Line in signal and provides a Line out Balanced output (2 signal outputs).
It is note built to drive Mic input loads.

Or use a battery and then you don’t have to be so cautious.

As I said in the first post,
the premise is only to be supplied by Phantom power

Thank you so much

 

[Whoops]

Came across one of NYD's things. Maybe it can spark more relevant ideas? Or not....

Hi Scott,
I checked the circuit but can't really see an application of it for this purpose.

Thanks

 

[Whoops]

I realise that you want to DIY this so don't shoot me, but I use one of these 4 channel BSS DI's for the job you're talking about. I had considered making something but the price I got this box (£75 U.K)

Hi Rob,
I'm aware of all the offerings in the market regarding DI boxes, I also use or have used all of them. The BSS DI boxes are the most used DI's in Live Sound, they're pretty well known.

As I explained in the first 2 posts, I'm looking to Build, to DIY, a very simple unit (and small) to balance -10dBV unbalanced line level and that works on Phantom Power

If you have any suggestions about similar circuits I could study or any ideas on how I could implement any of the 3 possibilities I described that would be great

Thank you so much

 

[Whoops]

Searching for discrete and simple ways of having a differential output I found this circuit fom Jazk Orman of Amzfx.

It used just one Mosfet and it provides 2 outputs, Out 2 is non-inverted and Out 1 has the polarity reversed:



What you you guys think? Could this be a good candidate for the "Discrete" possibility of doing this?

If this could be applied in this scenario I think I would just have to derive +9V for the circuit from the 48V phantom power and provide some circuit protection for the 48V the outputs receive

Let me know what you guys think

 

[kags]

Has anyone seen these units? I know a lot of studios in Los Angeles have used these for years for interfacing keyboard rigs. All active - maybe a schematic is floating around or the manufacturer can be contacted.
http://www.deskdoctor.com/products/di_active/di_active.pdf
I see you wanted one that works on phantom, which probably rules this box and its components out. I'm sure it's an opamp-based driver.

 

[ccaudle]

(MicUlli description of impedance balanced interface)

That would make the signal travel unbalanced and as I explained that's not what I'm looking for.

No, the "balance" in "balanced interface" refers to impedance balance. As long as the cold leg has the same impedance as the hot leg you can have asymmetric signal levels and still get interference rejection.

it's the most simple and basic circuit possible, with the least amount of components

I think the best shot would be to use the OPA-Alice circuit which is typically used as a microphone capsule buffer. There are single op-amp and dual op-amp versions. The dual op-amp version will give you 6dB higher output, but uses twice the current. The new low current op-amps that work best for this design are only available in SMD, so you need to be pretty comfortable with really small parts to build it yourself.

This is an example of a commercially available implementation:
OPA-Alice doc from JLI

GDIY thread about OPA-Alice

Relevant notes from GDIY user "rogs"
balanced asymmetric output drawing from rogs

You can buy that OPA1641 op-amp in the drawing for just under $2 in singles, and buying the resistors and caps will cost more in postage than the components cost. Since you are driving from a synth and not a mic capsule just put a 20k resistor in the location which shows a 1G resistor and save a little bit more.

 

[Bo Deadly]

A CMOS op amp would be good in this scenario because:

1) Very low quiecent current (< 1mA)
2) Very good drive
3) Very low THD
4) Higher noise at LF doesn't matter because no gain (and noise is not that high anyway)
5) 5V max supply voltage doesn't matter because you don't have the power from phantom to drive lines to a higher level anyway

Just searching mouser I found TLV6742 for example.

The fidelity of discrete circuits isn't not going to be anywhere near an op amp. Nobody complains about that in a mic because that's just how it's been done forever out of necessity. But now that there're op amps with 1mA quiescent current and triple-zero THD why not use them?

You could also use a step down SMPS and get a lot more power from phantom.

 

[Whoops]

(MicUlli description of impedance balanced interface)

No, the "balance" in "balanced interface" refers to impedance balance. As long as the cold leg has the same impedance as the hot leg you can have asymmetric signal levels and still get interference rejection.

Thank you so much for explaining, I will research and study a bit on "impedance balanced" as I'm not familiar with it.
I always think of a Balanced signal being 2 components the + polarity signal on Pin 2 and the same exact signal but Reverse polarity on Pin 3, but if "impedance balance" will provide the same CMMR then MicUlli suggestion might be the simplest possible then, but it's still missing one thing that is Phantom power protection to protect the keyboard's outputs, but maybe that can be added also by using a minimal ammount of components.

 

[Whoops]

A CMOS op amp would be good in this scenario because:

1) Very low quiecent current (< 1mA)
2) Very good drive
3) Very low THD
4) Higher noise at LF doesn't matter because no gain (and noise is not that high anyway)
5) 5V max supply voltage doesn't matter because you don't have the power from phantom to drive lines to a higher level anyway

Just searching mouser I found TLV6742 for example.

The fidelity of discrete circuits isn't not going to be anywhere near an op amp. Nobody complains about that in a mic because that's just how it's been done forever out of necessity. But now that there're op amps with 1mA quiescent current and triple-zero THD why not use them?

Why not really. I will check the TLV6742. Thanks

You could also use a step down SMPS and get a lot more power from phantom.

Can you please provide examples of that?
any links?

Thank you

 

[ccaudle]

MicUlli suggestion might be the simplest possible

The problem is you would have to disassemble each synth you wanted to use to determine the output components, or find or develop and accurate test setup to measure the output impedance of each synth, so that you could determine the correct components to use.
Just use one of the OPA-Alice designs and all that is done for you, plus can have lower output impedance than some synths (if a synth has e.g. 1K output impedance it is not going to do a good job drive a 30m cable).

 

[ccaudle]

Just searching mouser I found TLV6742 for example.

OPA1641 is only double Iq (1.8mA vs 0.9mA) and you don't have to deal with only using a 5V supply (OPA164x family is good up to around 36V).