Balanced XLR Effects

[zpyder]

Recent interest in using effects on vocals has led me to investigate balanced inputs/outputs.  It seems to me that any effects circuit could be used in many many ways for vocals on stage if we are able to maintain a balanced signal.

A couple approaches:
1) It seems that in general (and this may be a gross oversimplification), to make any effect "balanced" we essentially make two of the same circuit and use one across XLR pin 1 in and XLR pin 1 out and the other across XLR pin 3 in and XLR pin 3 out.  This way we maintain both the hot & cold signals and don't have to dump our balance to ground.  We may want to take into consideration the tolerances of the components we use since the balancing of the signal is dependant upon pins 1 & 3 carrying the same exact signal, only with 3 inverted... Also, is there a ground loop problem here?  IE would we have to isolate the grounds of the 2 "identical" circuits?

2) Perhaps it would be better/easier/cheaper to dump the XLR pin 3 to ground on input, apply our effect to the "hot" pin 1 signal, and then create an inverse image of that.  For starters we then only have to buy 1 circuit's worth of components, and we should have a more identical balancing signal.  I've recently read that the DRV134 chip (http://www.mouser.com/search/refine.aspx?Ntt=DRV134) will accept one input, and output both that signal, plus an inverse copy of it...


For reference:

XLR pinout-
1: Hot Signal
2: Ground
3: "Cold" Signal (The exact inverse of the pin 1 Hot Signal - When 1 & 3 are recombined at the PA, any interference is canceled out when 3 is re-flipped and added to 1)

cheers!
zpyder

[Mike Burgundy]

usually (studio) effects gear use balanced lines for the hookups between different bits of gear - internally they are unbalanced. There's no real bonus in building two identical (well, if they are single-end sensing, one would have to be the exact opposite of the other) compressors, and using closely matched dual-gang pots on everything if you can get a completely clean and uninterfered result with some care in circuit pcb, and housing design.
The best way of doing this (unless you really want to get into discrete transistor designs) is using a run-of-the-mill but very good TL07x opamp, and hooking hot up to the + input and - to the inverting input (via some good-housekeeping parts, such as pull down resistors, DC-blocking caps, etc).
This instantly yields a nice and hot signal at the opamp's output with all of the pick-ed up radio, hiss, whatever nicely cancelled out. This way you still use one opamp stage, but also benifit from noise cancelling the signal line before the opamp. If you clamp the - line to ground, you also dump noise cancelling.
If the circuit is neatly laid out and put in a shielded box, you'll be fine.
All you need afterwards is a regular output buffer with a parrallel inverting amp at 1:-1. This will leave you with a balanced signal again, and since the shielded enclosure and neat design did not allow anything to contaminate your signal, you're perfectly fine. This is how even totally pro gear (Neve, Meek, Oram, SSL,DNR, DDA, Avalon, you name it) works. If you want to get really high-end, a good mic pre stage (2217, unfortunately out of production, but ESP - see later - has something on that) is good, but not really necessary since you'll be using stuff that's definitely not designed for subtle vox processing.

Have a look around at Rane corp (loads of schems there) and the Elliott sound products (ESP) pages, there's a lot of rack and mic gear that will give you ideas.
Having a loop-box with balanced in/out and gain-tweakable buffers but an unbalanced loop (for guitar pedals) is a cool addition to any toybox, IMHO.

hih

Edit: You may know this already, but it looks like you got the pinout mixed up. Here's an easy way to remember an XLR's hookup: Xternal, Live, Reverse.
so:
1)shield (external) or ground
2)Live, hot, + or send
3)reverse, cold, - or return
Keep in mind the pin numbering is not left-to-right (3 in the middle) and ofcourse mirrored for male vs female

For stereo Jacks, the Ring is Right or Return.

There *are* other standards (actually a lot of US manufacturers used a pinout that yielded zero signal when combining with this pinout in single-line gear) but nowadays 99.something% of the world uses this.
again, hih

[zpyder]

Is this what you mean for the input?  I'm not sure of the TL072 pinout off the top of my head, but I think I'm close...

I'm still not clear on the output method you mention... Are you saying to use another TL072??? Or are you talking about a transformer.  If transformer, what do you suggest?  42TM018?

Also, could you point me to useful schems on Rane/ESP, I've never used their sites and they're a little daunting.

Thanks a bunch..

cheers
zpyder

[Mike Burgundy]

Okay, quicky. If you want deeper info, ask again, but this should get you in a ballpark.
Yes, opamps usually have a + and - input, meaning one input is in phase with the output, the other is inverted. This is why you can have inverted gain stages, and negative feedback by hooking up the output to the - input via an appropriate resitor.
Use both of the, and it's a differential input - with a balanced signal, you get "double" the output plus noise reductio. You got that.
Output: there's several ways to do that. The rough way is to make a 1: 1 inverting amp and parallel that to the output - but this will yield all kinds of impedance and mismatch issues. Have a look at the links though... You may not get a direct schem for what you want, but you'll get a pretty good idea of what's involved which in some ways is worth more

Oh yeah,I meant the 2017 preamp, not 2217. anyway.
Input:
http://sound.westhost.com/project13.htm

Output:
http://sound.westhost.com/project35.htm
http://www.rane.com/pdf/old/mlm82asc.pdf
(just picked one that has balanced in and out)

also see if you can get hold of one Graig Anderton's PAIA projects: the TubeMicPre I think it was called. Balanced input, tube stage, balanced out.

If anyone wants to chip in, I seem to remember a "fake" but working well pseudo-balanced output stage at ESP but can't find it readily.

Also, search you behind off on opamp circuit theory - there's loads on the net, and it's very, very helpful.
hih
.

[Pushtone]

Having a loop-box with balanced in/out and gain-tweakable buffers but an unbalanced loop (for guitar pedals) is a cool addition to any toybox, IMHO.

Bingo, I said the same thing in the other thread on this topic.

I would add,
Transformer isolation is called for whenever interfacing unbalanced gear with a pro PA system.


This document shows the concept using transformers
http://www.jensen-transformers.com/an/an003.pdf#search=%22balanced%20input%22
Here is another
http://www.jensen-transformers.com/as/as075.pdf


I found these transformers, cheap, small and a 200Hz - 15kHz freq response
http://www.alliedelec.com/Search/SearchResults.asp?SearchQuery=967-2540&Submit.x=0&Submit.y=0
there is a data sheet at this link.

They would need an active buffer stage following the transformer to boost mic level.
Since the transformer is doing the unbalanced and impedance conversion the
buffer can be a basic non-inverting single opamp (ie +9vDC supply).
From there you could use another buffer stage to lower the level and transformer to balance and isolate ground
between the pedalboard power supply and the mixer supply.


Think of balanced signals as an encoded transmission system.
All signal is unbalanced while running around on the circuit board.
The signal is ENCODED balanced before leaving the chassis.
XLR cables are one connector standard, so is DB25
The encoded signal is transmitted via a twisted pair with shield cable.
Upon arriving at the next chassis the signal is DECODED back to unbalanced and passed to the circut board.

[Gilles C]

I thought I already posted this link, but didn't find it in my posts.

http://www.tangible-technology.com/power/Phantom_Power_connect.html

I like the picture with the signals added.

Gilles

[zpyder]

Thanks for the info guys - I'm getting somewhere!! 
I'm gonna read this stuff over, and then probably repost with more specific questions 

Think of balanced signals as an encoded transmission system.

I like that... it's always good to have a useful perspective

cheers
zpyder