TL072 - Microphone preamp (differential)

[Zero999]

You forgot the resistors.

[Ale]

How to chose the values? Why to use another resistors between C8 and C9 and the mic?

[Bassman59]

I meant condenser sorry :p

Standard phantom power requires the +48 V be applied to both pin 2 and pin 3, each through 6.81k resistors.

[Ale]

Yes, I found a schema online, it should be like this. I have a question, is it possible to put another switch on the xlr pin 1 to implement the ground lift? I can't find an active schematic with ground lift, only with passive transformers.

thanks

[Zero999]

Yes, I found a schema online, it should be like this. I have a question, is it possible to put another switch on the xlr pin 1 to implement the ground lift? I can't find an active schematic with ground lift, only with passive transformers.

thanks

No, because adding a switch, in series with pin 1 would break the circuit for the phantom power.

The only other way to float the mic, is to use an isolated 48V phantom power supply, such as a battery, on the mic side of the ground lift switch.
https://en.wikipedia.org/wiki/Phantom_power#Caveats

[Ale]

I found this schema on the net: http://www.poulpetersen.dk/xfiles/cir/gbhpdi.html, could it work? Here the ground lift is on the input side, in my case can I add it on the mic output? So I will still have the phantom power connected to gnd but disconnected from the next stage. I saw many other schematic with not only a switch, but with condenser and resistor, like this, but usually in passive DI and I don't really understand how it works. I mean... condenser and resistor didn't conduct enough current so it's like an open connection?

[Zero999]

I've edited my post to clarify the fact the isolated phantom supply, needs to be on the mic side.

Those DI circuits are not designed for condenser mics.

It a series resistor were connected between the 0V and  mic, on a normal phantom power circuit, it would only lift the ground by around 20mV.

[Ale]

What do you think of this version? the xlr pin 1 is disconnected from GND, but connected to the battery. Is it possible to do the same using a second power supply, used only for the phantom and "ground lift" (perhaps ground separation)?

[Zero999]

What do you think of this version? the xlr pin 1 is disconnected from GND, but connected to the battery. Is it possible to do the same using a second power supply, used only for the phantom and "ground lift" (perhaps ground separation)?

Looks sensible to me.

[mansaxel]

One question, why do you think you need to lift ground?

In my experience (I spent the better part of the 90s in pro sound fighting the audible and sometimes unaudible artifacts of ground loops) ground lifts are the wrong solution to a problem barely understood.

The reasons hum occur are several, but the most common are:

* either a strong magnetic field induces hum (rare),

* mains-related current flows along signal wires between mains connected pieces of equipment.

In the first case, screening will help. As long as the screening enclosing the circuit is given a path to discharge the induced current; because the current is still induced, only this time in the screen, where it supposedly won't harm anything.  Here, a ground lift will subvert the screening, at least at audible frequencies. Like hum.

In the second case, the problem is that the signal path between the two pieces of equipment is (in comparison) the best path for equalizing  the potential difference occurring from the fact that mains ground is not at exactly the same potential at all points in a local mains grid. Removing a non-audio-conductor from this set of wires (by means of said ground lift switch) will not exactly improve the situation, unless it is combined with a galvanic separation through optics or more commonly a transformer. This is the passive DI solution.

Since you are proposing a microphone input, exactly how do you envision a mains connection to a SM57? Because, if you do not, the ground lift switch will only sabotage your P48 feed (I'd make a silly argument about SM57 and phantom, but I'll refrain, since you obviously think about using other microphones too) and remove the EMI screening.

My advice would be:

1. Connect Pin 1 of the female input XLR by means of a _very_ short and stout jumper directly to the metal chassis. Do NOT connect it to the PCB.

2. Run Pins 2 & 3 to the PCB, using a twisted wire pair.

3. Make certain that the chassis and the circuit 0V are well connected, but at one point only.

4. The output ideally should be balanced, but if that is impossible, and the output -ve is equal to 0V,  use a metal 1/4" jack, and only connect the tip to the pcb, instead deriving the -ve from the chassis connection of the 1/4" jack.

5. Save the money from not installing a ground lift.

[Ale]

Wow, thank you very much for your advise. I made a prototype, slightly different from this. I will post the schematic in the next days. I removed the ground lift and phantom for now.