RCA to XLR balanced/unbalanced converter

[suryabeep]

Hi everyone,
Some Background info:
I've been using one of these to put my turntable through to a pair of active speakers: https://www.amazon.com/Talent-SXC-Balanced-Unbalanced-Converter/dp/B00MATWKFW
I only ever use the RCA to XLR channel, and have no need for the second channel. As of late, I'm finding the little beeps and boops it makes occasionally rather annoying when I'm listening to my music.

Here are my questions:
1) How does the unbalanced-balanced conversion thing work?
2) Is it possible for me to roll my own?

I'd appreciate any and all answers, because I really do want to get my hands dirty in a field of audio that isn't pedals 
Thanks!

[highwater]

Basically a direct-inject box? Something like this: http://sound.whsites.net/project35.htm

If you're only using it for a turntable, though, you might be better-off building a phono preamp (unless your turntable already has one built-in). There's a couple fairly simple transistor-based ones here (figures 7-9), and few more here.

None of those have balanced outputs, so you might still need a DI box (which could be built-in).

If you want to get into non-pedal DIY audio, a DI box and a phono pre-amp are definitely a good place to start. A headphone amplifier is another good one.

[rankot]

If you need balanced to unbalanced only, you can make something like that using INA2134 - http://www.ti.com/product/ina2134/description - take a look at the datasheet.

If you need unbalanced to balanced, then this one: http://4.bp.blogspot.com/-LOlP3knMzWM/T-4WckFIAeI/AAAAAAAAAGA/riZmcsnDplE/s1600/BALANCED_LINE_DRIVER_DRV134.png.

[suryabeep]

Thanks for the links! I'm slightly confused about something, I see in the Bipolar Transistor Cookbook that the examples given are for a mono signal. If I'm to implement this for left and right rca channels, do I need to make two of each circuit and use dual gang pots everywhere?

[rankot]

Yes, you need double everything

[ElectricDruid]

There's some relevant stuff over at THAT Corp too, since they make a load of balanced line driver chips, both sender and receiver:

http://www.thatcorp.com/THAT_IC_Selection_Guide.shtml#LineDrivers

TBH, you might find some of these overkill. But they make some pro-quality chips, if that's what you need.

Tom

[suryabeep]

rankot- what power supply should I use when building the unbalanced to balanced converter?

ElectricDruid - yeah man, I think those might a bit overkill for this one 

[suryabeep]

So I drew up a preliminary schematic. I pretty much followed the block diagrams from the BJT cookbook and tacked on that unbal-bal converter on the end with a choice to switch between RCA out and XLR out.
Does it look ok? Did I do any dumb stuff?

[rankot]

This looks OK, you just need transformer and rectifier with smoothing capacitors to provide unregulated ±18-20V DC for feeding those 7X15 regulators.

[Steve.mg]

Hey Suryabeep,
Over here in Aus. there is an electronics magazine called Silicon Chip that ran an article and developed a project that you could buy a kit for
just the thing you're talking about. The local Electronics Suppliers carried the Kits that weren't very expensive at all. Plus the article 'explained
exactly [and in you and I language, tinkerer's!] what happens. I can look deeper into this avenue if you wish or if you are happy with what's
been stated to follow. Let me know...

Steve

[suryabeep]

Steve- That'd be great, thank you! I saw that issue of Silicon Chip cited as a source in one of the links provided above, I found this but it obviously doesn't have the full article http://www.siliconchip.com.au/Issue/2008/June?res=nonflash .

Rankot - I'm not 100% comfortable with using transformers quite yet (also I don't know where to begin). I found some examples online using 25.5V transformer and a full rectifier, and also a center-tapped 36V transformer, but I'm not sure where I can buy these or how I should implement them. How would you recommend I proceed?

EDIT: found this Tamura center tapped transformer: https://www.mouser.com/productdetail/tamura/3fd-336?qs=sGAEpiMZZMvwUzoUXIIvyVgtLw924glFGl3MjHQOrWI%3D .  Datasheet: http://www.tamuracorp.com/clientuploads/pdfs/engineeringdocs/3FD-3XX.pdf
Seems like it should work, right?

EDIT 2: nope, found this instead, seems to be far simpler and cheaper too.