Help Design an unbalanced consumer Line level to Balanced interface

[Newmarket]

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.

You might also want to include any capacitive impedance in the cold leg impedance to Signal 0V. As recently discussed on this forum. But even without it's fairly effective at attenuating "Ground Noise'. As used in many commercial implementations.

 

[Whoops]

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).

But if using an Opamp, any problem at all in using the classic TL072?

 

[Whoops]

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.

That's not possible at all.
I need one unit that will work with any Synth, Drum Machine, Sampler it's presented to me.
I don't work always with the same synths, bands or artists, even in the same band they can change a synth for a different one at any time.
I need a device that is flexible, like any active DI box is.

So let's forget that cable for impedance balance

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).

Thank you, will check that

 

[Whoops]

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.

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).

Thank you so much for this circuit suggestion,
this really does it.
I have no problems with the IC being SMD, I can design a PCB for it and have an hot air station if needed, but I can even solder those SMD IC's with a soldering iron with a thin tip.

It was great to see the circuit as it shows how the circuit takes the incoming Phantom Power and uses it to supply the OPAmp.

 

[radardoug]

Thank you so much for this circuit suggestion,
this really does it.
I have no problems with the IC being SMD, I can design a PCB for it and have an hot air station if needed, but I can even solder those SMD IC's with a soldering iron with a thin tip.

It was great to see the circuit as it shows how the circuit takes the incoming Phantom Power and uses it to supply the OPAmp.

View attachment 102928

That circuit needs an input cap or its not going to work.

 

[Bo Deadly]

Can you please provide examples of that?
any links?

Actually I looked into a step down SMPS for phantom right after posting that and found that the smallest SMPS like DETN01N-05 needs 11mA @24V so it's way above what phantom can supply. The series resistance of phantom just doesn't play well with a step-down SMPS.

Notice that for a 5V circuit (CMOS amp), you have about 12mA to work with. Using TLV6742 that's 1mA for each half leaving 10mA for driving the load. With a +-2.5 output swing, that's 4.2mA into 600 ohms so well below the 10 (assuming one amp is sourcing current while the other is sinking so load current it just 4.2mA and no 8.4). But a mic input is more like 1200 ohms and can't take that much signal. So you would need to put a 20dB pad, or maybe two 20dB pads, in front of the amp to get an output level that the mic input can handle.

If you use say a 12V circuit, that's 10.6mA minus 1.8mA for each half of a conventional op amp leaving 7mA (actually 4.4mA with an additional 2k2 in series) for the load. Still fine with or without pads but not as much margin.

 

[ccaudle]

That circuit needs an input cap or its not going to work.

Indeed, I didn't go into the details of how you would connect a line level device rather than a microphone capsule which can float to half the supply voltage. Doesn't need huge, I think 1.5uF range is fine, put a 100K on the outside to "true" ground to keep the cap from floating. Probably a polarized electrolytic, so put the + side on the interface side, the - side of the cap to the input connector.

 

[Whoops]

Thank you Radardoug and Ccaudle.
I redrawn the input, please check if it's like this

 

[Mr. Moscode]

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:

View attachment 102921

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

I would have a 1K resistor in series with the input to prevent parasitic oscillation and reduce RF. Put 2 12 volt zeners back to back from input to ground, before the cap. I would also want to see 33 to 100 ohms in series with both outputs. The BS170 is sensitive to static electricity which could short the gate to the body.
.1 uf output caps are way too small. Probably 10uf should do it. Off the top of my head.

Also unplugged TS phone jacks that fall on something that it shouldn't could send a charge or hum thru the gate and poof, silicon to charcoal converter. Don't ask how I know. . . .

 

[richiyobs]

Last summer I had the same needs... But no time to test or build anything
Attached the simplest solution I found... But no idea on how to power it with phantom pwr
So really Don t know if it can work

 

[MicUlli]

Yes it works. And it works only with phantom power. Performance is quite bad because of two issues:
1. Output impedance is not really balanced (1k versus 0)
2. Phantom power is asymmetrically loaded

Nevertheless a low cost solution that can be optimized very quickly. I will post it soon...

 

[MicUlli]

Here it is:
insertion loss <1 dB
frequency response 20..20kHz <0,5 dB deviation
THD < 0,01 % @ -10dBV
CMRR > 60 dB
EIN (input shorted) < -126 dBV(A)

circuit is protected against hot plugging and line shortages, C1+C3+C6 for EMI protection.

Hope it helps...

 

[Newmarket]

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).

If you are simply looking to balance the output resistance then it's relatively simple to load the output with a fixed resistance and compare the level to the open circuit level. A load equal to the output resistance would half the level "-6dB". But you can use any load resistor value that gives a significant drop in level (easier to measure than a small change) and calculate the internal resistor value to use in the cold leg using Ohm's Law.
Bit more involved if you want to include any associated output capacitor.

The cable solution wouldn't be my first choice though. Note that some kit can have relatively high output impedance eg 1K0

 

[Newmarket]

Last summer I had the same needs... But no time to test or build anything
Attached the simplest solution I found... But no idea on how to power it with phantom pwr
So really Don t know if it can work

Where is the supply votage(s) there ?

What's going on with the "Alternate C1" ?

 

[Newmarket]

Yes it works. And it works only with phantom power. Performance is quite bad because of two issues:
1. Output impedance is not really balanced (1k versus 0)
2. Phantom power is asymmetrically loaded

Nevertheless a low cost solution that can be optimized very quickly. I will post it soon...

Ah I see! It actually says Phantom Power on the diagram

 

[richiyobs]

gre

Here it is:
insertion loss <1 dB
frequency response 20..20kHz <0,5 dB deviation
THD < 0,01 % @ -10dBV
CMRR > 60 dB
EIN (input shorted) < -126 dBV(A)

circuit is protected against hot plugging and line shortages, C1+C3+C6 for EMI protection.

Hope it helps..

great thank you so much!
i ll try it
best

 

[Whoops]

Here it is:
insertion loss <1 dB
frequency response 20..20kHz <0,5 dB deviation
THD < 0,01 % @ -10dBV
CMRR > 60 dB
EIN (input shorted) < -126 dBV(A)

circuit is protected against hot plugging and line shortages, C1+C3+C6 for EMI protection.

Hope it helps...

Thank you so much MicUlli, that's a really simple circuit, very low parts count and cheap do build. Looks quite a great solution

What is "V1 48V" going to ground on the left side? is it an Electrolytic capacitor?

So the output is not fully balanced, there's no audio signal going out from Pin3, so this is "Impedance balanced", that's it?

Thank you so much for drawing this

 

[Whoops]

I'm more inclined at the moment to build the OAP Alice circuit recommend by Ccaudle and also the 1 transistor discreet circuit drawn by MicUlli and test both circuits out.
But for reference and study here are more 2 circuits of DI boxes that use 1 TL072 opamp, are completly transformerless and are supplied by Phantom Power:

This is Project 35 from ESP (Elliot Sound Products) by Rod Elliot:

https://sound-au.com/project35.htm



And here is another one from Sonelec Muzique, DI Active 002b:

https://sonelec-musique.com/electronique_realisations_di_002.html

 

[richiyobs]

Great! Hope you share any test/results
Best

 

[MicUlli]

View attachment 102983

Thank you so much MicUlli, that's a really simple circuit, very low parts count and cheap do build. Looks quite a great solution

What is "V1 48V" going to ground on the left side? is it an Electrolytic capacitor?

So the output is not fully balanced, there's no audio signal going out from Pin3, so this is "Impedance balanced", that's it?

Thank you so much for drawing this

Yes, the output is impedance balanced. V1 48V is a battery, together with R9 and R10 it forms the input of a phantom supply as commonly used for mic inputs. It's only scetched for clarity.
From my point of view there is no necessity to drive XLR Pin 3 with a complementary output voltage as seen on XLR Pin 2. Several microphone manufactures like DPA do it the same way.
C2 with + side to R3, C4 with + side to R8 and C5 with + side to R7 are electrolytic caps (voltage rating 50V).
By the way: the circuit is capable to process also +4dbu levels with only small degradation of THD...