Impedance balancing / unbalancing for digital console outputs

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Potato Cakes

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I have a small 1RU mixer than I am building to handle specific needs for live audio monitoring. There will be a set of stereo input XLR's that will directly feed a Neve style summing amplifier that will need to unbalanced. The source for this connection will almost always be a digital console of some sort. I am looking to perform the unbalancing via resistors is how I would like to go as cost and chassis real estate is becoming an issue, but I want to make sure to prevent any possible situation where strapping pin 3 and 1 together might not make the source very happy. I found this layout for doing stereo XLR to single TRS (600R to 10k) but I did not see the math involved. And since the buss resistors the type of summing circuit being employed historically has been 10k (I think so anyways), I was wondering if R3 in the diagram would become the buss resistor. And since these connections points will be a digital console analog output card most impedances I've read on their spec sheets are about 75 ohms and I don't know if that should be the value I use for any conversion calculation or if using 600R as the incoming value would be sufficient.

The other thing I would like to be able to determine is the value of the resistor needed to impedance balance a connection from a BA283AV style card that will be in this unit for the insert. The board being used has a dedicated unbalanced tap separated from the TX out connections, so I would like to use that for the insert send but am not sure where to look or meter to find that value. I was shown how to do this for IC op amps with the output limiting resistor and I would guess there is something similar for a discrete transistor design, but nothing seems apparent to me from looking at the schematic. And it would probably be fine with just keeping the input and out traditionally unbalanced but as before I would like to play it safe incase someone else not familiar with the design is using this.

For the insert send, it is returning to a BA283AM style amplifier and would like to use the same resistor method for unbalancing that signal, but I am not sure what resistance value to use to match the card's input. On the schematic there is a 10uF tantalum capacitor followed by a 2k2 resistor and then the signal feeds the rest of the circuit. Do I need to measure the resistance of the capacitor plus the resistor?

For the summary, I'm looking to balance and unbalance some of the connections via resistors in this project. The main input and outputs are all transformer balanced, but I for the additional inputs direct to the summing amp and the insert connections I'm looking for a solid way to do this without transformers or active components as I won't really be able to accommodate either.

Thanks!

Paul
 

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Potato Cakes

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Taking a closer look at IC op amp designs for unbalanced outputs, I can see the similarities in the design with the capacitor followed by a resistor to ground as found on the BA283AM card. On the schematic with the IC they've added a 47R limiting resistor before it leaves the unbalanced output connection. The BA circuit I'm using does not have this resistor. My thoughts are to just add a 47R resistor to Pin 2 of the unbalanced output then I can strap another one to Pin 3 and ground like I have done on other projects, but I don't know if can just use these values or if there is another calculation I would need to make to determine this resistor value.

I did find an old post from Mr. Tanner stating that older Neve consoles used 15k for the buss resistors, but they also used transformers at the input of the summing amplifier, so I don't necessarily know if I can use 15k since I won't have transformers at before the summing circuit.

I'm still a little hung up on handling the input unbalancing. Aside from the diagram in the first post, I still have not been able to find the math associated with it. Internet search engines seem to think that what I want is a pad calculator or to buy some sort of attenuator product, but I can't seem to find a suitable formula to calculate balanced to unbalanced resistances. For the time being to move forward with testing the overall build I could just tie Pins 1 and 3 together, but I know there's a smarter way to do this passively that doesn't involve a transformer. I just don't know what magic key phrase to use to get the internet to give me that for which I am searching.

Thanks!

Paul
 

ruffrecords

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I am not sure how you are interpreting the three transistor amplifier that is the ouput stage in the BA283M card but there are no ICs in it, just discrete transistors. The 47R resistor you refer to is the one connected to pin A on the card. This is not an unbalanced output. This is the emitter load for TR3 and is normally connected to pin J (0V). The unbalanced output comes from pin F.

Cheers

Ian
 

Potato Cakes

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Hi, Ian,

Yes , the BA card does not have transistors. The ICs I was referring to was in a completely different circuit that I was using as reference for how to determine the value needed for impedance balancing the output. The BA283AV PCB that I have has an unbalanced connection on it that is Pin F on the original AV schematic. I did see the 47R resistor at R7, but it wasn't in series with the output connection, which is what I thought was needed to match resistors for impedance balancing. I didn't know if I needed to add a limiting resistor in series if picking signal off from this particular spot.

As for the unbalancing via resistors to better accommodate 75ohm analog audio signals from digital console outputs, I'm going to need to do more digging. I'm not sure what I'm looking for as far as a formula goes that pertains to the diagram attached above. I may just use the 600 to 10k setup that is shown in the example and see if there are any audible problems. For now I have some small utility boards to do the unbalancing via INA134s. I would like to still figure this part out as it will be useful for some upcoming projects.

Thanks!

Paul
 

Matt Syson

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As a first thought, the schematic of resistive balanced out to unbalanced input may not work particularly well depending what the characteristics of the balanced out really are as the two input resistors and 3K3 will produce a 'sum that is not in itself referred to ground.
You should also be aware that 'balanced outputs' can have several actual meanings and as such can give quire different results. for a cheap 'impedance balanced' out can simply be an unbalanced signal with say 50 Ohms resistance (possibly a real resistor) and the cold side being another 50 Ohm resistor thus being 'impedance balanced. a cross couples 'electronic output will give erroneous results if the 'cold' side doesn't go into a balanced input of the same impedance hot and cold, and loses a lot of level if the cold is not taken to ground. Therefore you have to either KNOW what the output of your digital thingy is OR use either an transformer or a proper 'unbalancing stage (even the classic 4 resistor op amp version would do).
Op amp circuits usually have a 47 Ohm or thereabouts on the output to help protect against shorted output lines and to isolate the capacitance of connecting cables that might load the output at high frequencies thus impairing stability. The compromise being a mild 'low pass filtering' which is preferable to the possibility of the op amp oscillating wildly.
The BA283M could be used in a number of ways with either an output transformer (often used as step up) or a choke and as I think Ian said, the 47 Ohm resistor is there as simply a place to put it for when that board was used in certain configurations. Putting components on the edge connectors is so messy!
 
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