Impedance match in summing box chain

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jonathanak

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Hi!

I'm building a summing box with 4 stereo inputs (four left and four right). Each channel, a total of eight, will have a Sowter 7026 (200:200) on it. The summed stereo signal will then go into a Studer line amp (input impedance >10k, output impedance <50ohm).

I'll send the signal directly from my Antelope Orion 32+ Gen3 into the summer and then into the interface again. The output impedance of the interface is 56 ohms and the input impedance is 11.2k ohms according to their website.

I'm a rookie when it comes to impedance and stuff, so I'll just ask here - will this work? I'm mostly thinking about the 200:200 transformers. When I'm looking around it's mostly 600:600 or 10k:10k that's recommended, but I got these 200:200 from an old Soundcraft Delta board, so I thought I could use them for something.


All the best,
Jonathan
 

ruffrecords

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200:200 is certainly too low. Their inductance will be low and even really low output impedacne sources may suffer bass loss and/or distortion. For a balanced passive summing mixer you do not need a transformer on every input (unless you specifically want extra transformer mojo per input. Most summing mixers just use a pair of 5K resistors, one in the hot and the other in the cold to feed the bus. The resultant mix is fed to a transformer and often the bus is slugged to reduce interaction between inputs so you might have about 300 ohms or so across each bus. However, since you are feeding into a Studer line amp with a 10K input impedance you, whose gain may be limited, you might dispense with the bus slugging resistor. (is the studer balanced in an out?)

Antelope input and output impedances look OK for this application.

Cheers

Ian
 

jonathanak

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200:200 is certainly too low. Their inductance will be low and even really low output impedacne sources may suffer bass loss and/or distortion. For a balanced passive summing mixer you do not need a transformer on every input (unless you specifically want extra transformer mojo per input. Most summing mixers just use a pair of 5K resistors, one in the hot and the other in the cold to feed the bus. The resultant mix is fed to a transformer and often the bus is slugged to reduce interaction between inputs so you might have about 300 ohms or so across each bus. However, since you are feeding into a Studer line amp with a 10K input impedance you, whose gain may be limited, you might dispense with the bus slugging resistor. (is the studer balanced in an out?)

Antelope input and output impedances look OK for this application.

Cheers

Ian
I suspected that would be the case. The transformers would only be for some extra color. So I guess I'll look for some with higher numbers! Any idea what I can use the 200:200 for? Mic inputs?

The studer line amp is capable of -2 to +28db gain! The model is 1.914.501. Both the input and output is balanced!

Best,
Jonathan
 

john12ax7

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Not familiar with the Studer line amp, but if it has good current drive capability you could use the 200:200 on the line outs.

Edit: looks like the Studer line amp already includes a transformer on the output.
 

ruffrecords

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I suspected that would be the case. The transformers would only be for some extra color. So I guess I'll look for some with higher numbers! Any idea what I can use the 200:200 for? Mic inputs?
Good question. I am not sure I have a good answer. If they are input transformers then the answer is probably not much. Which make/model are they?
The studer line amp is capable of -2 to +28db gain! The model is 1.914.501. Both the input and output is balanced!

Best,
Jonathan
The you definitely should avoid using a slugging resistor or use a relatively high value one. Without a slugging resistor, the bus loss in a passive mixer is equal to the number of inputs. In your case each one has 4 inputs so the loss is 12dB and that's the gain the Studer will need. To make the bus work properly it is a good idea to ensure that unused inputs have their hot and cold shorted together. THis is easy to do with TRS connectors.

Cheers

Ian
 

jonathanak

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Good question. I am not sure I have a good answer. If they are input transformers then the answer is probably not much. Which make/model are they?

The you definitely should avoid using a slugging resistor or use a relatively high value one. Without a slugging resistor, the bus loss in a passive mixer is equal to the number of inputs. In your case each one has 4 inputs so the loss is 12dB and that's the gain the Studer will need. To make the bus work properly it is a good idea to ensure that unused inputs have their hot and cold shorted together. THis is easy to do with TRS connectors.

Cheers

Ian
The model of the transformers is Sowter 7026!

So 5k resistors on the channels hot and cold will be enough? How does it affect the impedance that the Studer will see?

Best,
Jonathan
 
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ruffrecords

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As long as all the inputs are fed from low impedance sources then with 5K i hot and cold, the bus impedance is 10K/N where N is the number of inputs to the bus. So if you have 4 inputs, the bus inpedance will be 10K/4 = 2K5. This is the source impedance that will be " seen" by the Studer. Presumably the Studer has a bridging input impedance of about 10K so it should be perfectly happy with a 2K5 driving source impedance.

Cheers

Ian
 

jonathanak

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As long as all the inputs are fed from low impedance sources then with 5K i hot and cold, the bus impedance is 10K/N where N is the number of inputs to the bus. So if you have 4 inputs, the bus inpedance will be 10K/4 = 2K5. This is the source impedance that will be " seen" by the Studer. Presumably the Studer has a bridging input impedance of about 10K so it should be perfectly happy with a 2K5 driving source impedance.

Cheers

Ian
Thank you for your replies and knowledge!

I think I understand how it works now. But I have one last question about this;

What is the best solution for the ground/shield? Is it best to tie them all together or connect them all to chassi at the inputs?
The unit will have a PSU inside, if that matters.

Best,
Jonathan
 

ruffrecords

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Thank you for your replies and knowledge!

I think I understand how it works now. But I have one last question about this;

What is the best solution for the ground/shield? Is it best to tie them all together or connect them all to chassi at the inputs?
The unit will have a PSU inside, if that matters.

Best,
Jonathan
Shields should be connected directly to chassis at the connector.. There is no 0V. A balanced signal exists only between the hot and cold pins..

Cheers

ian
 
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