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.
Help Design an unbalanced consumer Line level to Balanced interface
- Thread starter Whoops
- Start date
Help Support GroupDIY Audio Forum:
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.
But if using an Opamp, any problem at all in using the classic TL072?
Last edited:
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
Thank you, will check that
Last edited:
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
Well-known member
That circuit needs an input cap or its not going to work.
Bo Deadly
Well-known member
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.
Last edited:
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.
Thank you Radardoug and Ccaudle.
I redrawn the input, please check if it's like this
I redrawn the input, please check if it's like this
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. . . .
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...
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...
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
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
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
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
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...
