Balanced to single-ended converter (XLR to RCA / BAL to SE)

Discussion in 'DIY' started by purr1n, Jun 27, 2020.

  1. purr1n

    purr1n Finding his inner redneck

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    I've been asked this a few times now so I figured I would share with how I constructed my balanced to single-ended converter using line input transformers. The sonic reasons for doing so are to provide an alternative sound for DACs which have a balanced (XLR) output where you have an amplifier or preamp that only takes unbalanced or single-ended (SE) inputs (RCA). Some DACs such as the Schiit Modius have a balanced output which is a different circuit from the SE outputs, so this method provides another option. I will not guarantee that using the converter will sound better, only possibly different. For example, if it sounds better, then all the power to you. In some instances, such as with pro audio gear DACs, we will have no choice because pro gear usually does not come with SE outputs.

    IMG_20200627_200649.jpg

    The technical reasons for doing this are well explained by @atomicbob here: https://www.superbestaudiofriends.o...o-unbalanced-input-a-brief-visual-guide.7012/

    This "guide" explains what I did specifically to get the results that I wanted, down to the model number of the transformer and the wiring. I happened to use the Cinemag CMLI-15/15B. We can get the specs here: https://cinemag.biz/line_input/line_input.php. This is a 1:1 transformer and will not reduce hot pro audio gear signals to a lower voltage. So be sure that your consumer gear will be able to take the hot input. I believe the Radial Engineering Twin ISO uses a similar 1:1 transformer, but from Jensen. Note that Jensen and Cinemag have a shared heritage from Reichenbach and that Jensen's equivalent model the JT-11-p has the same specs and even uses the same color coding for the wires. @atomicbob measured the Radial Engineering Twin ISO here: https://www.superbestaudiofriends.o...sen-transformers-technical-measurements.7017/

    There may be other suitable transformer models that may work, but these discussions are outside the scope of this post. I will say however, that the commonly available and seen Jensen Iso-Max PC-2XR is not the same thing. I only point this out because some people have asked. The PI-2XR uses the JT-10KB which is a 4:1 step down. I do not think this line input transformer sounds as transparent and clear as the Cinemag 15/15B. It's a little bit warmer and hazier, but we may not have much of a choice if we need signal attenuation from pro gear into consumer RCA inputs.

    The only thing I will provide is a wiring diagram that may clarify some questions from the data sheets. It's up to you guys to solder the wires, connectors, and assemble the chassis. Note that I left the white wire from the transformer floating and that pin 1 from the balanced output side is not used. The orange and black wires are tied together for signal ground on the RCA side. The XLR connector diagram is from rear, the soldering side.

    upload_2020-6-27_20-43-47.png
    • We can call Cinemag directly for the transformers, for sometimes, eBay may be selling some used ones for cheap: https://www.ebay.com/sch/i.html?_nkw=CMLI-15
    • This ain't exactly a cheap solution, even if you decide to DIY, so you may want to reconsider doing this for a cheap DAC.
    • I didn't have any issues with the hot outputs from the Convert-2 DAC into an EC tube amp.
    • Be aware of the fine strands of wire from the transformer leads. With one connection, I left a tiny strand which was shorting to ground. Took me an hour to figure out what was wrong.
     
    Last edited: Jun 27, 2020
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  2. crenca

    crenca Friend

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    I think it is the PC-2XR, and not the PI-2XR which is the step down (4:1) model...
     
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  3. purr1n

    purr1n Finding his inner redneck

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    Fixed
     
  4. monacelli

    monacelli Friend

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    Cool post, @purr1n. I noticed the Radial Twin-Iso has XLR outputs, so you would want to make a custom cable that shorts pins 1 and 3 on the XLR end to match your wiring diagram. Is that how you would do it?
     
  5. atomicbob

    atomicbob dScope Yoda

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    there are three things that once acquired become a permanent fixture in the audio arsenal:
    1. high quality 1:1 transformers for Bal to SE or SE to Bal conversion
    2. high quality stepped attenuator
    3. Dante digital delivery such as RDL SF-ND2 - USB nervosa assassinator - 88.2K rules

    With these three one can configure systems to handle balanced -> SE nervosa, adjust signal levels without fear, kill USB nervosa and never look back.
     
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  6. Melvillian

    Melvillian Friend

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    If the “hot” signal is an issue, can a resistor be used to reduce the voltage?
     
    Last edited: Jun 28, 2020
  7. Hrodulf

    Hrodulf Prohibited from acting as an MOT until year 2050

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    Looks nice! I'd still recommend connecting PIN1 from XLR and the white wire to the chassis ground. Probably won't change anything audibly, but it's free shielding!
     
  8. Walderstorn

    Walderstorn Friend

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    Very interesting and it's good to have the perspective of finally having something 1:1 for my needs. Unfortunately this is beyond what i can dream of doing but this type of knowledge sharing makes everyone grow.

    One small doubt that i want to clear out though, using a 4:1 from consumer gear to consumer gear (i ask this because i would imagine that many of us only have consumer gear) would be actual "detrimental" (lack of a better adjective) while using this 1:1 would be ideal, right?!
     
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  9. Taverius

    Taverius Smells like sausages

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    All transformers have a 'sound' - practically speaking, without scifi magimaterials this is simple fact - step-down have more pronounced sound.

    Provided your gear can handle the hot inputs, the 1:1 does less to the sound and is 'better'; everything else being equal ofc.

    IME, 1:1 + attenuation (via stepped attenuator/fixed resistor) is better than 1:4 for gear that can't take the hot inputs as well.
     
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  10. purr1n

    purr1n Finding his inner redneck

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    There is supposed to be a gray wire from the CMLI 15/15B, but out of several units that I have, they didn't have it. Maybe the "B2" or earlier have this. This gray wire would lead to the "earth" ground which the XLR pin 1 is supposed to be connected to, either directly or with a RC (50-ohm and 0.1uF) circuit in between. Another thing to do would be to disconnect the black wire from signal ground and connect the white and black together to chassis ground, with the option of the RC circuit in between.

    If I had run longs (and only on the XLR side), I'd probably disconnect the black from signal ground and tie that into the white with then an RC to chassis ground. And then pin 1 to with an RC to chassis ground. Given the short distances and intended use in a consumer environment, I didn't bother because I wanted to avoid any chance of ground loops and because I put everything into an external box, not within a chassis of an amp or DAC. It's hard to say which would be the best way. I put what I did above on the analyzer and was happy, at least in my environment.

    Lots of cables like that around. I believe most XLR to RCA cables are that like. The Radial Twin ISO will allow you to lift the ground on the RCA side. The Jensen PI-2XR boxes have dip switches where we can run through all of the above grounding options. For short distances, I'd just float everything. An audio analyzer could help to see which combination of grounding schemes yielded the lowest noise or best shielding.
     
    Last edited: Jun 28, 2020
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  11. batriq

    batriq Probably has made you smarter

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    I acquired a pair of WE 111C ~5 years ago with the intention to build something like this, but never got around to. It can be used as 1:1 or 4:1: https://www.westernelectric.com/static/library/specifications/transformers/02-repeating-coils.pdf

    Edit: forgot to mention that the schematic shows the transformer electrostatic shield connected to audio ground as Marv did (I'm assuming that's what the black wire is)

    This thread is encouraging me to get around to building it.

    [​IMG]
     
  12. Armaegis

    Armaegis Friend

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    Feels like a variation of the "pin 1 problem". Here is Rane's note on it: https://www.ranecommercial.com/legacy/note165.html

    edit: and an interesting google link that says similar things: https://books.google.ca/books?id=2-...kQAQ#v=onepage&q=xlr shield capacitor&f=false
     
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  13. wormcycle

    wormcycle Friend

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    I am trying to understand how the end to end connection would work, and compare it to the simple XLR to RCA cable. For example the RME ADI-2 manuals says: " XLR line outputs do not operate servo balanced! When connecting unbalanced equipment via XLR, make sure pin 3 of the XLR output is not connected."
    How would that requirement change the Cinemag CMLI-15/15B wiring, or, in the case of Radial Twin-Iso, the XLR side of connection?
     
  14. atomicbob

    atomicbob dScope Yoda

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    The transformer presents a balanced load to the ADI-2 output, there are no issues. XLR to XLR on the balanced side of the transformer. The other side is configured for Single Ended output. That is the beauty of the transformer which transforms Balance to SE while satisfying the load / source requirements for each.

    I use a Radial Twin ISO 1:1 for this duty with my ADI-2 Pro regularly, no issues. Internally it is wired similarly to @purr1n's diagram when used with an XLRF to RCA plug adapter cable.
     
    Last edited: Jun 28, 2020
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  15. mitochondrium

    mitochondrium Friend

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    That is one of the things I like about my music first audio passive magnetic attenuator, plug in source SE or balanced plug in amplification SE or balanced, the transformer does the trick. Even if it is rather pricey.
     
  16. gridmaster

    gridmaster Acquaintance

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    I've looked into this before. Excuse my ignorance, this stuff is over my head, but I do have a question I wanted to get some input on. I was told the CMLI-15/15B isn't that good for external boxes because of the output capacitance, you'd want extremely short cables on the output end. I was told it was more meant to be integrated within an amp where the output wires would be extremely short. Is there any merit to that?
     
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  17. Hands

    Hands Overzealous Auto Flusher - Measurbator

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    So just use really short interconnects to be safe?
     
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  18. m17xr2b

    m17xr2b Friend

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    So right, I'll take an opportunity and build an external enclosure for mine. Designed a PCB to take all three types of sowter transformers in a neat small little box that can be used internally as well.
    [​IMG]
     
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  19. Andre Y

    Andre Y Friend

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    Yes, because those are input transformers, but as Hands suggests, keep your interconnects short and well under 100pF. The Blue Jeans LC-1 for example is only 12.5 pf/foot, so a foot of that should be fine. I don't know if the Cinemags also need a damping network like the Jensens do (and implement in the PI boxes).
     
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  20. Aklegal

    Aklegal Almost "Made"

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    @m17xr2b, if that pcb ever becomes a group buy thing then count me in.
     
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