ADR DI Box

[musika]

I'd like to build this ADR DI Box just to try it out...

Do I really need the TL772 chip?  Or, could I substitute something else? 

Also, there are no markings on what I believe are zener diodes. 

 

[mjrippe]

Well, I can't find a datasheet for the LM772 (not TL772) anywhere online.  Further, neither of my vintage hardcopy substitution books (1979 & 1985) has a listing for LM772!  It is either an odd part, European only, or a typo. 

I'm sure other opamps would work in that circuit, but without specs on the original I wouldn't be able to suggest one. 

 

[analogguru]

"LM772" probably should read µA772 (Fairchild) which is an equivalent to the TL072.

Considering that the unit should run on 9V too, the zener diodes should be 3,3V.

 

[RuudNL]

I don't think it will be a problem to use a TL072.
The zeners are protection diodes, I would use 5.6 or 6.2 V. (400 mW or higher)

 

[Rob Flinn]

If you plan to use this with powering by it's phantom power facility it looks like you will need to up the value of the  2 x 100ohm resistors, because most op amps don't like 48v across them.

 

[musika]

Thanks for all input.... here's the brochure.

 

[musika]

backside with specs...

 

[musika]

So, would I need one TL072 or 2?

I see that the circuit shows ½ of the chip, but I have not done a lot of component layouts from schematics.

Is this a circuit that could be legally cloned for a project?

 

[Rob Flinn]

The TL072 is 2 op amps in one chip.

Interestingly they say it can take 48v.  Maybe the TL772 could take more V, or drew plenty of current, so it dropped plenty of V across the 2 x 100R

 

[fazer]

I built one of these back in the late 70's and i had to keep a battery in the circuit because it would not run correct on just the phantom power.  The9 volt battery actually made it run and work great.  I used a TLO72 in mine.  Seems like a lot of different amps should work.

 

[RuudNL]

The OpAmp will never get 48 V!
You will get a voltage drop over the 6.8K phantom power resistors and the 100 ohms resistors in the DI box.
As far as I know (from the top of my head) the TL072 can withstand a voltage of 36 V.
As long as the current is higher than 3.5 mA, the voltage over the 072 will be lower than 36 V.
But if you want to be sure, just connect a zener between the B+ and ground (12 V. or so.)

48 V - (3.5 * 3450) = 35,925 V,    The 3450 is 2 x 6.8 + 100 ohms parallel.

 

[musika]

Thanks,

Just want to make sure that the B+ to ground is what you mean (and that I understand).  I appreciate the math!

 

[RuudNL]

No, this doesn't work, because the phantom power will never reach the zener!
You should connect the zener in parallel with the 100 uF capacitor.
As long as the zener voltage is substantially higher than 9 volts  (>= 12 v) , the battery won't 'see' it.

 

[musika]

OK.  How about this?  I see now that pin 8 provides the +V to the TL072 opamp.  Maybe I am getting closer to understanding how the zener diode would work to protect the chip from excess phantom console voltage .??.   

But reverse the orientation (and connect all the way to the ground).

 

[RuudNL]

Now the zener diode is shorted... 
One leg of the zener to the + of the 100 uF, the other leg of the zener to the - of the 100 uF.
(And don't reverse the zener, otherwise you will get a supply voltage of 0.7 V...)
The white (or black) ring of the zener should be connected to the +.

By the way: I don't see the point of the 100 K resistor connected from the output of the left OpAmp to ground. (Low Imp Output).
I think this resistor should be connected between the - side of the 22 uF capacitor and ground, not the + side.
This way the capacitor will be charged to 1/2 Ub, to avoid loud clicks when an input is connected to the Low Imp Output.

 

[musika]

Balancing  the 100K resistors for each half of the TL072 amps seems to make sense.  Here is a drawing update based upon your comments. 

 

[Rob Flinn]

Balancing  the 100K resistors for each half of the TL072 amps seems to make sense.  Here is a drawing update based upon your comments.

The zener diode is the wrong way round. It will short the 100u cap the way you have it & will only get 0.7v supply voltage as RuudNL says.

You have moved the wrong 100k resistor & it is being shorted out in the drawing.  It should be the one connected to the output of the left op amp, not connected to the input.  Ithink both the 100k & 22u that RuudNL is talking about are the two in the lower left of the diagram near the sockets,

 

[Rob Flinn]

The OpAmp will never get 48 V!
You will get a voltage drop over the 6.8K phantom power resistors and the 100 ohms resistors in the DI box.
As far as I know (from the top of my head) the TL072 can withstand a voltage of 36 V.
As long as the current is higher than 3.5 mA, the voltage over the 072 will be lower than 36 V.
But if you want to be sure, just connect a zener between the B+ and ground (12 V. or so.)

48 V - (3.5 * 3450) = 35,925 V,    The 3450 is 2 x 6.8 + 100 ohms parallel.

According to the TI data sheet the typical current draw per amp is 1.4mA and a maximum of 2.5mA.  Therefore typically the current draw will be 2.8mA.......

 

[musika]

OK Rob Thanks to you and RuudNL.

On interpreting the schematic lines, where they cross, I am assuming there is NO connections.  Is this always the case?

 

[Rob Flinn]

Normally you put a dot on intersections to indicate a connection.  If there is no dot, then there is no connection.