Schematic for balanced audio input

[electrospeaker]

I'm working on a preamp circuit with both mic and line inputs (1/4" jack) but want to add balanced XLR sockets to this. Does anyone have or know of a suitable circuit for this?

[blue hell]

Like this?

( http://www.mediacollege.com/audio/connection/images/XLR-monojack2.gif )

[electrospeaker]

Thanks, that might come in handy for later but I didn't mean to convert a balanced cable into an unbalanced one, but rather: I want to add balanced audio to my existing preamp (currently with 1/4" mic and line inputs).

[blue hell]

Ah ok, do you have a schematic of what you have now?

[electrospeaker]

Sure, here it is.
I want to keep the existing inputs as they are but add a balanced XLR connecter to the mic input (maybe the line input as well).

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[LFLab]

SSM2141?

Maybe a bit too high end for your purposes, but very nice. There was a write up in Elektor magazine on a balanced receiver using this IC.

(bottom schematic)

[L´Andratté]

Look at figure 2 for the most simple implementation (works ok),

http://sound.whsites.net/project51.htm

There are also 2 and 3 opamp versions, look for balanced line receiver...
That corp. has some nice off the shelve chips, that are easy to get, if you´re looking for high performance-

[electrospeaker]

Thanks for those suggestions.
I'm not sure how "high end" I need it to be. This will be for the speech and excitation inputs (two input amp PCBs that is) for an ETI vocoder I'm building.

A simple solution such as using one of the THAT chips with just about everything built-in would be the easiest and most tempting, but the Elektor article might be better?

So I suppose I just add the line driver circuit's output to one of the existing preamp inputs (and have it switched (yes, there are XLR sockets with built in switches) so that it'll disable the other inputs whenever an XLR plug is inserted), but does it output a line-level signal (hence the name "balanced line receiver")? In that case, for the mic input; where should I connect the line receiver's output in the preamp circuit?

[L´Andratté]

That chips are state of the art, you can´t get easily any better regarding noise/distortion, why not use That mic input with That line input, there are app notes with pretty much everything being done for you already..

OTOH don´t let yourself be irritated by micpre discussions on the internet, it´s either very sophisticated stuff or very dumb. For vocoder use the simple circuit I posted above should do very well with your circuit (which should be checked for errors btw., pinout if ic1b is swapped +/- terminals etc.).

You could couple the diff.in to -in of ic1b(the swapped one) via 100k for unity gain.

[electrospeaker]

[LFLab]

It's convenient that they mentioned the XLR connector, but a balanced connection can be on a stereo jack as well.....

Regarding THAT 12xx, it could be that these are the same functionally as SSM2141.
I've only looked into balanced outputs myself, but SSM2142 is the same as THAT1646 and DRV134.

Apart from the fact that quality is very good, the limited number of components is convenient as well, so that might push the decision towards specialised silicon, despite the price.
But it is a lot more expensive than a sole TL071 (I think the SSM2142 is about 4,50 euro or so).

Elektor also did PCB layouts, didn't post that part of the article, if you are into self etching, I can send you that section as well (don't recall if the layout was double sided, as I rolled my own for the balanced output).

[electrospeaker]

Yes, I just checked and the TL071 is indeed a lot cheaper than both the SSM2141 and the THAT ICs. I haven't done a side by side comparison of the schematics, but getting high quality audio and less components (though the TL071 circuit doesn't use many external components anyway) is much preferred despite the higher costs (I won't be using so many of these anyway). The THAT schematics I found also seem worth going for as they have input protection too.

Thanks for offering to scan the PCB layout but I'll likely redo it in Eagle as I don't have the equipment or room for etching my own PCBs. While at it I'll see if I can include the input sockets on the same board as well, then use a cable and connector to attach it to the existing input amp PCB (also redone in Eagle).

[LFLab]

Stopped etching myself, so much easier and convenient to get boards readymade (ah, double sided, silkscreen, plated holes, solder resist, swooonnnnn).



[electrospeaker]

Too true
Are there sites out there with Eagle PCB layouts for various projects? Perhaps someone else has built a THAT-based balanced receiver so I don't have to reinvent the wheel.

Another question: I plan to add a compressor/limiter to the input amp boards, but I suppose it has to be placed before the balanced receiver circuit? Or doesn't it work in the same way as a preamp and can be clipped by a strong mic (or line) signal?

[LFLab]

The first thing happening in most outboard studio equipment is conversion to unbalanced. I would place your compressor/ limiter after the balanced receiver, else you'll need extra circuitry. Balanced connection is needed for transport of signal only

[electrospeaker]

That's good to hear -makes things easier indeed
So you're saying the balanced receiver simply passes on the electrical signal as it is, so there's no chance of distortion like within the preamp circuit itself?

[LFLab]

Distortion and noise are a fact of life, but will be very small, especially when using specialised receiver IC's.