sizing resistor ratings?

trans4funks1 · Apr 4, 2018

Hello,
I need to build a cable that will "sum" a stereo output to mono to facilitate interconnection of two pieces of gear I want to use.

I found some instructions at the Rane website describing the layout, but I would like to figure out the wattage rating that is appropriate for my application.

I made this drawing to explain what I hope to assemble:

I will use it with either a line level TRS output and or a Headphone out output and send it to a device that can accept a line level signal.

The primary use is for a device with an output rating as follows:

Output: 100ohm outputs, unbalanced stereo for use with greater than 6kOhm inputs

connected to another device with an input rating as follows:

Input impedance: 1kOhm

Can someone help me figure out the wattage ratings appropriate for the resistors?

I see that the impedance bridge between the two devices is not ideal. Is it possible to select another resistance value that helps with that specific issue?

Thank you.

ruffrecords · Apr 4, 2018

0.25W types should be fine. Remember, +26dBu into 600 ohms is 400mW

+20dBu is only 100mW and you are feeding it into about 1K rather than 600 ohms so the power will be even less.

Cheers

Ian

trans4funks1 · Apr 4, 2018

Thank for being both helpful and merciful. I know that I should be able to make these calculations but always get hung up.

Is there any merit to my idea that raising the resistance value of the 470 ohm resistors to a higher value will help with the impedance bridge.

The output I specified is an "alt" output that I suspect was an after thought. The spec that suggest it should see a >6K load makes me think it isn't very robust.

As I understand it, the 470 resistors are meant to prevent the legs of the stereo output from dumping into each other. Do these values also effect what the output sees with regards to the intended connection?

Thank you!

john12ax7 · Apr 4, 2018

Are you trying to sum to a balanced or unbalanced connection?

Yes you would want to raise the value of the 470 resistors.

trans4funks1 · Apr 4, 2018

Thank you for the reply and thank you for asking about the balanced input.

Yes, I am connecting the unbalanced TRS stereo signal to a mono balanced input. The layout in the drawing may not be ideal but I don't see a practical way to make the signal balanced with the form factor of a cable.

I have been referring to this web page: http://www.rane.com/note109.html and I am beginning to understand that it does not describe the particular connection I need to accomplish.

I'll appreciate any suggestions that may be offered.

Thank you.

john12ax7 · Apr 5, 2018

Ok, your original drawing would work then. But the issue is really the 6k load requirement. You would need to use 5.1k or 5.6k in each leg depending on how the 1k input is defined. Can you tolerate the loss of this? Might be up to 20 dB.

ruffrecords · Apr 5, 2018

John is right. I missed the 6K load part in your original post and I missed the following input impedance was 1K. LOL I am getting old! So the loss seen by each output is more or less the series resistor plus 1K. So for this to look like K or more you definitely need a series resistor of 5K or more.

Cheers

Ian

trans4funks1 · Apr 5, 2018

Thank you to both for the helpful suggestions.

The TRS output I am patching from is an extra "pair" of outputs available on a portable field mixer, which seem like an engineering afterthought.

The mixer provides two pairs of balanced line level outputs that can see 600 Ohm, but I have already dedicated these to other routing.

The connection I am trying to make with this cable is for a talkback/cue feed via a wireless system. I have learned that the input of the radio transmitter uses an XLR connector, but has mini switches that will let me configure the input circuit for an unbalanced connection, so the cable configuration will be more appropriate than I had anticipated.

I will try your suggestions and see if the level reduction is problematic. I would rather struggle with make up gain, and a high noise floor on the cue feed than load down the outputs on the mixer.

Thank you!

sizing resistor ratings?

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