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TRS stereo to XLR mono summing cable
Old 5th November 2015
  #1
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TRS stereo to XLR mono summing cable

I'm trying to build a stereo to mono summing cable. Following Lotus 7's schematic, I've drawn out the wire with resistors (using slightly different resistor values), and wanted to get a second eye on it to make sure my drawing looks right.

The other question I had is if the cable affects balancing/unbalancing the signal, e.g. will a unbalanced signal remain unbalanced and a balanced signal remain balanced? Thanks!
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TRS stereo to XLR mono summing cable-screenshot-2015-11-02-15.10.56.png  
Old 5th November 2015
  #2
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Richard Crowley's Avatar
 

The basic schematics look correct. But the resistor values seem odd.
It would help to know what this was designed for.
What is the level and impedance of the source? Is it a typical headphone output?
What is the level and impedance of the destination? Mic level? Line level?

I don't understand the question about balanced and unbalanced?
This cable is NOT suitable for any kind of "normal" balanced signal.
There is nothing balanced about it from end to end.

The source is not balanced, and without a transformer or active circuit,
there is no way to present a balanced signal to the XLR output.
Old 6th November 2015
  #3
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Yes, I should clarify: The intended application is to connect a consumer wireless DAC to a powered monitor. For example, I'd like to connect a Chromecast Audio (1/8 TRS, line level) to a single Meyer HD-1 monitor (balanced XLR, 10k ohm impedance).

The resistor values in the drawing I took from Figure 2 here: Why Not Wye? ... 475ohm, 20k. Are other values more suitable for my application?


Quote:
Originally Posted by Richard Crowley View Post
The basic schematics look correct. But the resistor values seem odd.
It would help to know what this was designed for.
What is the level and impedance of the source? Is it a typical headphone output?
What is the level and impedance of the destination? Mic level? Line level?

I don't understand the question about balanced and unbalanced?
This cable is NOT suitable for any kind of "normal" balanced signal.
There is nothing balanced about it from end to end.

The source is not balanced, and without a transformer or active circuit,
there is no way to present a balanced signal to the XLR output.
Old 6th November 2015
  #4
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Think of your drawing in two stages.
1] Unbalanced wye stage.
2] Unbalanced (wye output) to balanced stage.
Yes, it's OK.
Old 6th August 2018
  #5
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danvales82's Avatar
Hi, how did you calculate the resistors values?
I'm trying to build an audio summing cable from stereo-RCA to mono XLR.
Thanks,

Daniele
Old 6th August 2018
  #6
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Richard Crowley's Avatar
 

Quote:
Originally Posted by danvales82 View Post
Hi, how did you calculate the resistors values?
I'm trying to build an audio summing cable from stereo-RCA to mono XLR.
Thanks,
The resistor values are not all that critical. This isn't a first-class solution, but it probably doesn't need to be.

The original series resistances (330 ohms) seem quite low considering the load impedance of 10K. And the 10K load resistor seems rather high if the actual load impedance is already 10K.

I would have used something like 1K ~ 5K summing resistors, and just eliminate R3 altogether. Don't sweat it.
Old 1st May 2019
  #7
Quote:
Originally Posted by Richard Crowley View Post
The resistor values are not all that critical. This isn't a first-class solution, but it probably doesn't need to be.

The original series resistances (330 ohms) seem quite low considering the load impedance of 10K. And the 10K load resistor seems rather high if the actual load impedance is already 10K.

I would have used something like 1K ~ 5K summing resistors, and just eliminate R3 altogether. Don't sweat it.
I know this post was a while ago, but I am currently looking to DIY my own summing cable and would love to know why you suggest ditching the 3rd resistor- the one connecting the ground wire to the mono signal?

I have been looking at the diagram from the Rane article and it has three resistors too.
Old 1st May 2019
  #8
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Richard Crowley's Avatar
 

A problem with all passive solutions like this is that they are highly dependent on the source impedance(s) and the load impedance(s) which are completely outside the influence of the passive circuit. And furthermore typically unknown and varying between different gear.

So, leaving out the parallel load resistor just uses the input impedance of the destination and results in a higher-level signal (less attenuated). OTOH, including the parallel load resistance makes the math more predictable, but results in significantly more signal loss.

If your situation is some fixed installation, then you can measure the source impedances and load impedances and optimize the summing circuit series resistances. But if you are making something for use out on the road for connecting to random, unknown, unpredictable gear then you may need something more "self-contained" with an internal parallel load resistor.

Like most things in life, it is a toss-up, this-vs-that decision. And ultimately, only you can make the final decision how much of your life to spend on something rather trivial and of minor importance in the larger scheme of things. The conventional wisdom is that you make a passive summing circuit with a fixed, internal load resistance and live with the signal loss. Mine is admittedly the "minority opinion".
Old 1st May 2019
  #9
Thank you for taking the time to clarify, Richard.

My case, like a number of others who seem to post about this on forums, is of wanting to take a signal from a stereo output into a single mono speaker.

Specifically for me, this is an installation of a cable from the output of my behringer HA400 headphone amp (output impedance approx 80ohms) into a single active avantone mixcube (can't find input impedance spec for it). So I don't mind tailoring resistor values or layout to this task. It is fairly confusing though, for someone with fairly basic electronic understanding to figure out which component values do or don't matter let alone which components.
Old 1st May 2019
  #10
For example, how strict should I be when following the schematic, to stick to the resistor values, % tolerances, etc?
Old 1st May 2019
  #11
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IMHO, in the original circuit here, those 330 ohm resistors seem quite low and the 10K load resistor seems ridiculously high (depending on the input impedance of the destination). If I were making that circuit, I would use a pair of 1K resistors and leave out the parallel load resistor. It isn't nearly as critical as you seem to think. Likely any resistance value from 500 ohms to 5000 ohms (5K) will work just fine. And I wouldn't bother with the parallel load resistor at all.

The tolerance and power rating are of essentially no importance for a casual circuit like this. Line-level (or headphone-level) audio is measured in milliwatts (thousandths of a watt) Any resistor that is big enough to handle without tweezers will be more than enough. And even a mismatch of 10% will make no significant difference for casual monitoring. Two resistors from the same batch will be closer than that even if they are rated at 20% tolerance (which is almost impossible to find something that terrible in modern times.) The critical factor here is how well the two resistors are matched with each other. Their absolute resistance is not anywhere near as important.

Last edited by Richard Crowley; 1st May 2019 at 06:55 PM..
Old 1st May 2019
  #12
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Personally, I think I’d choose approximately the same resistor values (I built summing cables using 200R), and maybe also the parallel resistor. Here’s my rationale:

Most Stereo TRS outputs will be designed to drive headphone outputs. If it’s a mobile phone/mp3 player or notebook they will very likely have DC blocking capacitors in series, and sometimes those are a little bit small. Going higher then, say 32Ohms therefore is a good idea to get more bass/low end.

If I have to decouple additionally with a DI box/line transformer because of ground loops, I want to have a relatively low impedance driving that transformer, 1:1 line Transformers rarely work well with kiloohm output impedance. Two 330R in parallel give me 155 Ohms, which is fine for that.

The parallel resistor (10k) is insignificant for the signal, but can discharge any series capacitors on my signal source. So I get less pops when plugging in.

And I understand that, with such a cable, I’ll overload older high impedance line outputs. But I never met those for this usecase.

Of course, as you’ve written, almost everything from a few 100 to a few 1000 will very likely work just fine.
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