XLR (Balanced) to RCA (single end) converter

[Comzee]

Hi all!

I recently ordered some parts to make my own XLR (balanced) to RCA (single end) converter.
I ordered a pair of Cinemag CMLI-15/15B2 and some XLR and RCA chassis terminals, as well as an aluminum box to put it all in.

I found this helpful tutorial by @fullheadofnothing for something similar using the CMLI-15/15B (This forum won't let me post the link to it tho)

I have a few questions about this project.
I noticed @fullheadofnothing uses just a resistor in his modification, but I was wondering if a capacitor might also be helpful.
I ask this because I have an amp already that has the CMLI-15/15B built in for this purpose, and that topology uses a capacitor and a resistor.
Capacitor was: CBB21 104J400V
The resistor value I'm not sure of, I couldn't seem to lookup the right values based on the bar codes: Green -> Brown -> Black -> (nothing empty space no bar?) -> Gold (edit: using an online calculator appears to be this value "51 Ohms 5%")

One more thing I'm trying to do is build this conversion box with a potentiometer for voltage control. In theory I want my 4.0v balanced out -> Cinemag conversion box with voltage control -> single end out.
I was thinking of using a 10k ohm pot on the SE side for this, which means I probably don't need the resistor, and maybe just a capacitor?

Thoughts on this would be appreciated. I can give more info if anybody asks.

Thanks!

[Paul Joppa]

Google "Cinemag CMLI-15/15B2" and download the data sheet - you will see that they show it with a 10K or 15K resistor load. Take the manufacturer's advice.

[Paul Birkeland]

I think it would help if you were a whole lot more specific about what you're actually doing.  What two components are you trying to connect together and how long is the cable run?  Are you currently experiencing noise issues without your proposed adapter box?

These questions would need to be answered before we could say much about what you're proposing to do. 

The loading resistor in the modification you're proposing is simply to make our attenuator look more like a 10K load for the secondary of the input transformer.  You would not want to put a 50 ohm resistor across one of the windings on a 10K input transformer.

[Comzee]

I think it would help if you were a whole lot more specific about what you're actually doing.

I'm basically trying to recreate this product "Radial j-iso" with the Cinemag transformers.
The radial uses Jensen 4:1 stepdown transformers, and I've done some research that the Cinemag transformers are more transparent and less warm sounding.

Another way of putting it, is I want to make a converter that transforms XLR balanced outputs to single ended RCA outputs, using those Cinemag transformers.
I've already ordered the transformers, so I'm all in on this. I could have just ordered the Radial J-Iso, but I feel like this is a good starter project for myself. I've been wanting to get into DIY for years. This project seems simple enough to start me off.
I'd rate my experience level as very beginner.

This is the chain of ideally how I would want to to go: DAC -> XLR output (4.0V 75ohm) -> XLR cable -> Converter box [XLR Input -> Resistor/Pot - Capacitor? -> Cinemag transformer -> RCA single end output -> Amplifier single end input (100k ohm).

The secondary objective, is to figure out if I can integrate a potentiometer within this conversion box I'm making for variable voltage output on the single ended output side. This isn't absolutely needed, but I believe it's possible, since the Radial J-Iso has this feature.

I also emailed Radial and asked them two things, what was the impedance of the pot they used, and what was the output impedance on the single ended output side.
They said pot was 10k ohm, and SE output was 580ohm. That super low ohm output on the SE side is generally also what I'm aiming for.

Sorry if anything I said isn't clear enough, I can add more details, just ask.
Also, the inclusion of the pot isn't a deal breaker for me, I could just use a single resistor. But I do need to step down the balanced 4.0v to somewhere around 2.0v single end.

[Paul Birkeland]

I'm basically trying to recreate this product "Radial j-iso" with the Cinemag transformers.
The radial uses Jensen 4:1 stepdown transformers, and I've done some research that the Cinemag transformers are more transparent and less warm sounding.

The step-down would help if you plan to put at pot at the output.

Another way of putting it, is I want to make a converter that transforms XLR balanced outputs to single ended RCA outputs, using those Cinemag transformers.

Why?

This is the chain of ideally how I would want to to go: DAC -> XLR output (4.0V 75ohm) -> XLR cable -> Converter box [XLR Input -> Resistor/Pot - Capacitor? -> Cinemag transformer -> RCA single end output -> Amplifier single end input (100k ohm).

Is this the amplifier that already has the input transformers?

The secondary objective, is to figure out if I can integrate a potentiometer within this conversion box I'm making for variable voltage output on the single ended output side. This isn't absolutely needed, but I believe it's possible, since the Radial J-Iso has this feature.

With very short cables on the output side, you could probably get away with this.

Does your DAC not have an unbalanced output?

[Comzee]

Why?
---
Does your DAC not have an unbalanced output?

To answer both those questions, yes the DAC only has XLR balanced 4v output.

The amp doesn't have input transformers, that was a different amp I was talking about.
The reason I'm taking this strategy is my other amp already has Cinemag CMLI-15/15B. The new amp I'm getting only has singled end (RCA) inputs.

So the strategy, is to get my DACs 4.0v balanced output to my new amps SE inputs. (using the Cinemags I bought)
With the two main things, step down to at least < 2.0v on the SE side, and low ohm output.
Pot is only optional theory I had, can scrap that if it's not possible.

[Paul Birkeland]

I would use the CMLI-10/600B.  I would send full output of the DAC to the 10K primary and put a 1K pot at the output.   That's just me though, YMMV.

You could also get the unbalancer.

[Comzee]

I would use the CMLI-10/600B.  I would send full output of the DAC to the 10K primary and put a 1K pot at the output.   That's just me though, YMMV.

You could also get the unbalancer.

Ok, well I already bought the CMLI-15/15B because that's what I've read works.
If anybody wants to help me out with this I'll leave this thread up.

Maybe I'm wrong, but I assumed this wouldn't be that hard to wire up. I think I just need a capacitor and resistor.
This is how I think the conversion box would be built:

XLR input terminals -> Capacitor -> Resistor -> Cinemag Transformer -> RCA output terminals.

I just need to figure out the capacitor/resistor values I need.

edit: (Oh also, the CMLI-15/15B is also the transformers @fullheadofnothing used in his guide for XLR -> RCA conversion. I just can't direct message him on this forum to ask him any question unfortunately)

[Paul Birkeland]

Again, things could go south with a 1:1 if you have too much capacitance (in the form of cables) on the output.  I do not see the purpose of adding the capacitor as you describe.  As PJ mentioned, the manufacturer gives recommendations on loading resistors.

FYI - fullheadofnothing did the photography and photo editing for that guide.  I specified the transformers, resistors, and wiring instructions. 

Also, Paul Joppa is our transformer designer and would have the most expertise on this subject.

[Comzee]

Again, things could go south with a 1:1 if you have too much capacitance (in the form of cables) on the output.  I do not see the purpose of adding the capacitor as you describe.  As PJ mentioned, the manufacturer gives recommendations on loading resistors.

FYI - fullheadofnothing did the photography and photo editing for that guide.  I specified the transformers, resistors, and wiring instructions.

Ahh, ok. Well, maybe I need to modify my plan, and literally integrate the transformers into the amp chassis, for super short output cables.
Thanks for the info. I'll get back to you on what I decide.
I'm also going to return the current CMLI-15/15B because they are not the "single 3/8″ stud" version as you suggested.

[Paul Birkeland]

The 3/8" stud is just how the transformer mounts, it doesn't have any affect on performance.

The 15/15B would make a lot of sense as an input transformer in an amplifier.  If you want to do the whole step-down and volume control thing, then a 4:1 makes a little more sense.