Balancing an unbalanced output?

I?m trying to modify a Sherlock that has an unbalanced line level output. I?ll be running the Aux (line level) output through a patchbay and into a mixing board. As it is, this output is unbalanced. Given how I plan to use it, it seems sensible to have a balanced output instead. What do I need to do to balance it?

:?:

Basically, you cant - but don't worry too much about it.

The aux output impedance is probably too high to drive a line balancing transformer properly, so you're stuck with unbalanced (or the mic-level balanced output). Which should not be a problem in a studio? I don't think you'll have any benefits from running this simple circuit balanced?

Jakob E.

The only problem with unbalanced is that I don?t like to run those through the patchbay. I?m a bit confused by the whole thing, but I think that it?s better to run a balanced connection through a balanced patchbay. I may be wrong about this?.

I did find an article on the Jensen site, where they said I can make a ?true? balanced output by sending the signal to pin 2, pin 1 to ground, and the signal to pin 3 with an ?impedance matched passive resistor network to ground for the - output,? as Jensen calls it. The have a schematic too. Is there any reason NOT to do this? Is it a waste of time?

http://www.jensen-transformers.com/an/an003.pdf

(page 3)

Waste of time depends on whether or not you're feeding long lines in a noisy environment...

It can help cancel noise... but ONLY if the receiveing device is fully differential/balanced. Otherwise, yes, you're wasting your time!

:green:

Keith

"Is there any reason NOT to do this? Is it a waste of time? "

Not a good reason---you add a teensy bit of thermal noise but it is almost certainly negligible compared to where you are already. Meanwhile you get the advantages of ground noise rejection.

(and yes as Keith says, gotta go into a true balanced input for the benefits).

Note that this doesn't work well in single-conductor-under-shield ("RCA") style situations since the shield impedance is raised and selectively exposed to electric fields, usually making noise pickup way worse.

Sweet - I'll go ahead and do it. I'm trying to get all the "prosumer" gear out of my studio, and as it is, none of the gear I'd be interfacing the Sherlock with is unbalanced. So balanced it is.

Thanks!

Hm. I'm reading an article on the Jensen Transformers site that says to match the impedence of the output, and goes on to describe it in terms of the output from an op-amp, but I don't think the Sherlock/Level-loc has an op-amp....so I'm not sure what impedence to match. Is there a way I can just measure it somehow with my DMM? I have a schematic, but I can't make heads or tails of it. I'm new to DIY and schematics are still largely greek to me.

To measure output Z you can load the output with a given resistor and measure the a.c. voltage with your meter before and after. Don't make the signal level way high or you may run into current limiting effects rather than what you want to measure.

Also do this at a frequency high enough so you are not seeing the low frequency cutoff of any output coupling cap, but low enough where your a.c. meter works well.

The simplest case: load the output with a resistance equal to the output impedance and the signal should drop in half.


PS: or if you can scan and post the schematic there are people here who can tell you the answer.

Hm. I scanned the original schematic, the schematic with the mods I?m trying to make, and the bit from the Jensen page. I had to add some things to the schematic as it only included the circuit that on the PCB, and none of the off board stuff. Hopefully I did it right (or close enough for you to understand). On the one with the mods, I tried to draw those as best as I can, but if anyone is confused about what?s there, please let me know and I?ll try to explain. As I?m new to DIY, I?m still trying to learn how to draw (and read) schematics. Basically, I?m trying to have a mic input, mic output (both XLR and TRS), and line input and line output. The input is supposed to be switchable from ?mic? to ?line.? I added a power LED. I also added a "power filter" because the unit was making this weird buzz and that's what was suggested to help it. Any suggestions about how to do this, or better ways to do it, etc, would be a really big help.

The only problem is that I don?t have a site to uploads the scans to. Is there a public site you guys use (yahoo briefcase or something?) or is anyone willing to upload these?

Thanks, David.

> post the schematic

The Drewtronic Sherlock is an unauthorized interpretation of the Shure Level-Loc:

http://www.shure.com/pdf/discontinued/m62v.pdf



Without a deep study of it, I'd say it will drive 10K to a Volt or so, but not lower Z or higher V. That is in fact what Shure claims.

I do not know how the Sherlock is same/different from the Shure.

PRR:

The Sherlock schematic is exactly the same for that part, but I don't really understand what that means. How do I add an "impedence matched passive network to ground for the - output (as recommended per Jensen)" based on that?

Tricky. But you could duplicate the stage shown and feed it from an a.c. grounded signal if you really wanted to preserve the balance. You could even try to feed that dummy stage from the output of the active stage since it is inverting polarity. You would wind up with a net +6dB gain in the process.

Ah, this should do it.

http://groupdiy.twin-x.com/albums/userpics/10059/Scan.pdf

:grin:

> How do I add an "impedence matched passive network to ground for the - output (as recommended per Jensen)" based on that?

Dammit: it is a Level-Lock, a cheap (overpriced) speech-PA tool, NOT Pro-Audio stuff.

Yes, it is effective, not dirty, and notoriously used by Led Zepp (I think?) to mash wild drums into heavy metal.

Still: don't gild the sow's ear.

If you have a true Shure Level-Lock (I got one in the bottom of a pile), the instant "Pro" interface is to run the Mike Out to a Mike In. This is true floating on an XLR, and you have XLR mike inputs around.

If you must take that AUX Out, just run the RCA jack to a board's 1/4" TRS jack. Yes, the line is unbalanced and will radiate a bit. But not generally near enough to crap-up other signals in a patchbay, not when tracking and certainly not when working with material that calls for a rude limiter like the Level-Lock. (And that "balanced impedance" trick will not reduce radiation.)

If you find you must balance-float the output: use a low-price 10K:10K transformer rated 1V or +2dBu.

I'm trying to get all the "prosumer" gear out of my studio

Click to expand...

Well, I don't think you can ever remove all the unbalanced/prosumer/-10dBV gear from a studio- most clients will bring in some form of "DJ" or "Groovebox" Rolandy-type gear that is inherently unbalanced RCA, and most synths are the same.

Long term it's best to build some form of multi-channel bal>unbal, unbal>bal converter with variable gain and pad. Kev has his version here:

http://www.celestial.com.au/~rosswood/diy/balancingbox/balance.htm

...this will give you a low-noise bal>unbal converter with a "pad", and unbal>bal converter with gain (this allows you to get from -10dBV to +4dBm worlds)

There are lots of other designs out there too- e.g. the input and output stages from most DIY gear.

You can add a 1:1 10k input transformer and 1:1 600R output transformer to most of these designs for fully floating balanced inputs and outputs.

Build a few channels of this in a 1u case, bang it in the rack above the patchbay, and it's smiles all round.

Mark

Westwick audio used to manufacture just this sort of thing: the "MILLI"

(Multiple Instrument-to-Line Level Interface)

Useful as all get out, and easy to DIY.

Keith