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I want to design an impedance matching transformer to convert an audio line from unbalanced ( stereo phono jack type) to a balanced type ( XLR jack with 600 ohms). What can be the easiest way around ?

What if I need to handle other audio jack types ( may need variable impedance matching) to XLR, how to design in these circumstances?

EE already answered similar query in the past but not for variable type.

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  • \$\begingroup\$ Why would you want to do this? Low impedance audio lines (via a 1:1 transformer) work very well driving a 600 ohm input. \$\endgroup\$
    – Andy aka
    Jul 13, 2018 at 9:51
  • \$\begingroup\$ And basically nobody in audio does matched impedance these days, it is all bridging voltage connections as far as the eye can see, so low Z outputs (~100R or so typically) and high Z inputs (10k or so typically). This pretty much means that impedance is mostly only a concern with instrument outputs that can be high resistance and very reactive (Think guitars), if doing a DI box intended for such just do a jfet follower as an input stage. \$\endgroup\$
    – Dan Mills
    Jul 13, 2018 at 13:39
  • \$\begingroup\$ Sometimes I have to do live recording from venues where they allow tapping their outputs only from their audio consoles. Some have phono type with banana connector, others have XLR, mini XLR etc and in spite of have a break out box connection of these different connectors,frequently the impedance mismatch caused much pain. \$\endgroup\$
    – seccpur
    Jul 13, 2018 at 14:46
  • \$\begingroup\$ @seccpur From your wording, it seems like you are assuming different types of connectors represent different source/input impedances or something. Please correct me if I'm misunderstanding your question. A source or input will have/expect the same signal level and impedance regardless of the type of connector. Don't overthink the connector itself, pay more attention to the characteristics of the source and load. Is your "XLR jack with 600 ohms" on a device intended for microphones or line level sources? The issues you are having may have nothing to do with impedance. What symptoms do you see? \$\endgroup\$ Jul 13, 2018 at 20:00

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You say "frequently the impedance mismatch caused much pain" - that is odd. Any half decent console output will have an output impedance around the 50-100 ohms range, and you should not be presenting a load that looks like less than 10k or so. If you are having problems in this situation, it is more likely that :

(a) the connection is balanced and you are sticking an unbalanced connection onto it, which could make the other connected inpuit level suddenly drop by 6dB, or ...

(b) you are running into grounding issues in an unbalanced system (these are all too common, and they can be a bit fragile).

However there is a reason that old school pro (read : broadcast) consoles insisted on good quality transformers all over the place, and that is that they were frequently connecting to equipment which they neither knew nor trusted. Transformers have the great twin virtues of providing galvanic isolation (goodbye grounding problems and hello to not worrying about badly behaved systems or electromagnetic phenomena blowing up your gear) and being capable of being connected to balanced/unbalanced gear more or less freely.

So if you are frequently in this situation the nice thing to do is (as suggested above) to respect your hosts system and have some boxes with good quality (e.g. Lundahl/Sowter/Jensen) audio line input transformers (the input side connectors to the host, the output to your gear) and a variety of connectors (which just saves a bit of hassle).

(This would have been a comment but a bit too long for that.)

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  • \$\begingroup\$ I'm wondering if the XLR input described in the original question is intended for mic level signals. That would explain certain types of issues. It's hard for us to say what the problem is without knowing the symptoms. But @dmb has listed some common issues here which are important to be aware of. Also, if you'd like to save some money on transformers, Edcor makes some really nice, cheap stuff (first link in my answer above). \$\endgroup\$ Jul 13, 2018 at 22:27
  • \$\begingroup\$ that is also possible, although not so likely as these are inputs on the desk, and the OP states he is taking taps on outputs. \$\endgroup\$
    – danmcb
    Jul 15, 2018 at 1:53
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You should be able to use a 1:1 or maybe a step down transformer for what you're describing. Maybe something like this.

To go from unbalanced to balanced (or vice versa), you could use a circuit like this: Schematic from Jensen

[You could use a TS or RCA jack at the input for unbalanced connections, or just plug a TS connector into the TRS jack as shown]

Choose whatever transformer you need based on the signal levels and impedances you are dealing with (although impedance should be less important for line level audio). 1:1 should work fine if everything is line level (which is how the original schematic is shown).

If you need to step down from line level to mic level (if your XLR input happens to be mic level), use a 15:1 or 12:1 transformer (basically a DI). A pad could help you with that too. If you need to go from line level to instrument level, use a 4.1:1 transformer (basically a reamp box).

You can choose whatever connectors you need too, doesn't really matter. If you want a few different connector options for input and output, you can mult them together and only use one at a time. Combo jacks may be available too depending on what you need.

Jensen has a lot of great schematics like this on their site, you just have to make an account to view them. Very useful resource.

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