I am posting this information with the suggestion that this thread be made sticky. The question often seems to come up how to handle balanced-unbalanced conversions, and the answer is always the same.
When I asked about this a while ago, I was referred to this document. It explains several easy and in some cases dirt-cheap ways of adding balanced outputs to a device that only has RCAs. Another good use for this thread might be to collect reports on the different methods and see what's worked for people.
The simplest one (2.1 in the doc) involves just tying pin 3 to ground. That will work and provide (according to the document) 30dB CMRR at 60Hz, but not much at high frequencies. This can even be done just with an 'adapter cable'. Take a balanced cable and put the usual XLR male jack at the destination. Put an RCA at the source end and tie whatever wire will end up at pin 3 to ground (actual output to pin 2 and ground pin 1, of course). Voilá!
The next simplest (2.4) involves putting an impedence matching network that's tied to ground in front of pin 3 at the output. This involves two resistors and one cap per channel (and a piece of perfboard, if you want), so it's still pretty cheap (unless you've used exotic caps). Performance will depend upon tight matching of components (as someone emphasized to me in another thread). I'm about to try this in a B1 Korg, and I'll report the results here once that's done.
This solution may not be applicable in all cases, though. Direct coupled outputs wouldn't have the same network, but maybe there's a similar solution in that case? Surely someone here can tell us.
The 'better' option (2.2) is to use an output transformer to invert the signal for pin 3. This is not so cheap. The Jensen JT-11-FLCF is probably the cheapest option, and they go for $45 each at Parts Express, and you need two (for stereo). This is said to do better with hum (55dB at 60Hz) but not so much for buzz (30dB at 1kHz, which isn't nothing, though), I've tried it---I used JT-11-BMCFs, which are about $100 each---and it's very effective.
What I did was to make a general purpose box with RCA inputs and XLR outs; the box has nothing but the two transformers in it. So I can use that with any preamp, and it can sit nearby or on top. Doing it that way does add one set of mechanical connections, but I doubt that will be audible, even in my system, which is extremely revealing, IISSM. (No, I'm not quite crazy enough to have tested that.) I used a 10"x6"x2" Hammond enclosure (about 25x15x5cm, if you're metric), such as this one, so there'd be plenty of room, but you could get away with 8"x4", I think. So you could do it for $90 (transformers) plus $25 (enclosure) plus whatever you want for the RCA input jacks (personally, I am in love with these) plus the XLR jacks, but you would need those anyway unless you just used the adapter cable.
The last option (2.3), which is described as 'best', involves putting an input transformer in front of the destination component (typically, the power amp) and using an adapter cable. I think this could be implemented again as a simple box, XLR to XLR, in this case, that would just have the input transformer in it. (It's claimed that this will improve CMRR even with components that have balanced inputs, but I do not know if that is true.) But the JT-11P-1 is even more expensive: $120 each at Parts Express. I'm intending to try that, too, though, at some point.
I think this same transformer could be used, differently wired, to do balanced to unbalanced conversion, as described in 3.2 in the doc, but without the attenuation from the 4:1 step-down input transformer they describe. (See the data sheet for the JT-11P-1.) That would add a 'balanced input' to amps that don't have balanced inputs.
I haven't sat down and worked out the circuit, but it seems pretty clear that one could add a switch that would swap between XLR and RCA outputs, and even optionally add a 'pad' to do attenuation. Anyone have thoughts about that option? That would give you a single box that would serve both to add a balanced output (in effect) to a pre-amp and to add a balanced input to an amp, depending on the switch, and that would be one super-useful box for those of us who live in high-noise environments and have long runs to our amps. (My cables are 40ft long!)
When I asked about this a while ago, I was referred to this document. It explains several easy and in some cases dirt-cheap ways of adding balanced outputs to a device that only has RCAs. Another good use for this thread might be to collect reports on the different methods and see what's worked for people.
The simplest one (2.1 in the doc) involves just tying pin 3 to ground. That will work and provide (according to the document) 30dB CMRR at 60Hz, but not much at high frequencies. This can even be done just with an 'adapter cable'. Take a balanced cable and put the usual XLR male jack at the destination. Put an RCA at the source end and tie whatever wire will end up at pin 3 to ground (actual output to pin 2 and ground pin 1, of course). Voilá!
The next simplest (2.4) involves putting an impedence matching network that's tied to ground in front of pin 3 at the output. This involves two resistors and one cap per channel (and a piece of perfboard, if you want), so it's still pretty cheap (unless you've used exotic caps). Performance will depend upon tight matching of components (as someone emphasized to me in another thread). I'm about to try this in a B1 Korg, and I'll report the results here once that's done.
This solution may not be applicable in all cases, though. Direct coupled outputs wouldn't have the same network, but maybe there's a similar solution in that case? Surely someone here can tell us.
The 'better' option (2.2) is to use an output transformer to invert the signal for pin 3. This is not so cheap. The Jensen JT-11-FLCF is probably the cheapest option, and they go for $45 each at Parts Express, and you need two (for stereo). This is said to do better with hum (55dB at 60Hz) but not so much for buzz (30dB at 1kHz, which isn't nothing, though), I've tried it---I used JT-11-BMCFs, which are about $100 each---and it's very effective.
What I did was to make a general purpose box with RCA inputs and XLR outs; the box has nothing but the two transformers in it. So I can use that with any preamp, and it can sit nearby or on top. Doing it that way does add one set of mechanical connections, but I doubt that will be audible, even in my system, which is extremely revealing, IISSM. (No, I'm not quite crazy enough to have tested that.) I used a 10"x6"x2" Hammond enclosure (about 25x15x5cm, if you're metric), such as this one, so there'd be plenty of room, but you could get away with 8"x4", I think. So you could do it for $90 (transformers) plus $25 (enclosure) plus whatever you want for the RCA input jacks (personally, I am in love with these) plus the XLR jacks, but you would need those anyway unless you just used the adapter cable.
The last option (2.3), which is described as 'best', involves putting an input transformer in front of the destination component (typically, the power amp) and using an adapter cable. I think this could be implemented again as a simple box, XLR to XLR, in this case, that would just have the input transformer in it. (It's claimed that this will improve CMRR even with components that have balanced inputs, but I do not know if that is true.) But the JT-11P-1 is even more expensive: $120 each at Parts Express. I'm intending to try that, too, though, at some point.
I think this same transformer could be used, differently wired, to do balanced to unbalanced conversion, as described in 3.2 in the doc, but without the attenuation from the 4:1 step-down input transformer they describe. (See the data sheet for the JT-11P-1.) That would add a 'balanced input' to amps that don't have balanced inputs.
I haven't sat down and worked out the circuit, but it seems pretty clear that one could add a switch that would swap between XLR and RCA outputs, and even optionally add a 'pad' to do attenuation. Anyone have thoughts about that option? That would give you a single box that would serve both to add a balanced output (in effect) to a pre-amp and to add a balanced input to an amp, depending on the switch, and that would be one super-useful box for those of us who live in high-noise environments and have long runs to our amps. (My cables are 40ft long!)
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If Wayne or Nelson should read this, I wonder if they could tell us how this is handled in Pass's commercial offerings (if that's not a trade secret). How, for example, are the XLR outputs provided in the phono stages? The service manual for the Aleph Ono describes a 'single-gain inverter'. It'd be interesting to make up some boards for that, or something similar.
Something to watch out for with many transformers is the loading. It can change the high frequency behavior quite a bit. Also, be mindful of the low frequency saturation -- some saturate early enough to introduce significant LF THD at line levels. Be sure to check the transformer's spec sheets carefully to see if either of these will be an issue in your use case. Active circuits, whether made using opamps, purpose-designed ICs, or even tubes, will be essentially immune to both of these. I'm not asserting that active circuits are better than transformers -- only specific use cases can determine that. I just want to point out a couple potential transformer pitfalls that may prevent you from having the best experience.
One thing to watch out for with approaches 2.1 and 2.4 is whether what you are driving is fully balanced or merely differential. The latter is an increasing trend in "balanced" power amps in the consumer market: use two single-ended power amps in a bridged configuration, with the hot input driving one amp and the cold driving the other. If you use a 2.1 or 2.4 adapter with these, you'll only be driving one of the two amps with audio; the other will be sitting at ground (except for noise picked up along the cable). This will limit the maximum output power as well as eliminate the even-order nonlinearity cancellation that was carefully engineered into the amplifier.
One thing to watch out for with approaches 2.1 and 2.4 is whether what you are driving is fully balanced or merely differential. The latter is an increasing trend in "balanced" power amps in the consumer market: use two single-ended power amps in a bridged configuration, with the hot input driving one amp and the cold driving the other. If you use a 2.1 or 2.4 adapter with these, you'll only be driving one of the two amps with audio; the other will be sitting at ground (except for noise picked up along the cable). This will limit the maximum output power as well as eliminate the even-order nonlinearity cancellation that was carefully engineered into the amplifier.
I will repeat: the "cold" input has to have inverted signal when bridging amps. If it's ground, it will not work as imagined...
Dead link. It's here https://www.jensen-transformers.com/wp-content/uploads/2014/08/an003.pdf and also attached for prosperity:
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In this balanced to unbalanced schematic, what is the 2k37 resistor for after the trannie, across the positive and ground before the RCA out. What would happen if it's omitted? If you have time to answer, please assume you are explaining to someone who knows nothing! 🙂
proper loading of xformer itself, resulting in flat freq/ response
Those are not physical opamps, it's a functional circuit of the chip, which is in a DIL08 case - 2 x 4 pin opam-type chip.
BTW, the best way to go from balanced to single ended with preserving astronomical CMRR is a THAT1200 Ingenius chip. Also DIL08 ;-)
Jan