balanced/mic inputs on stompboxes?

[cedric]

how can i incorporate an XLR input (balanced) into the AMZ mosFET boost?
i'm not just talking using an XLR for input and output, but making it suitable for balanced signals?

i've tested the boost with a modded balanced (now unbalanced w/ a mono jack) mic. and it sounds WONDERFUL!!! hence the dream of making the mosFet boost into a "micpre"/balanced mic boost.

please help.
Jack? anyone?

sincerely,
- ced

[niftydog]

well... a balanced input is usually converted to an unbalanced signal very early in the signal chain.  You need a difference amplifier.  This will give you the full signal strength (ie; the difference between the two out of phase signals)

Unbalancing by shorting one of the balanced lines will result in half the full signal, due to the fact that you're only getting ONE of the balanced lines, as opposed to the difference between them!

Confused?!  Didn't explain that too well.  Basically what I'm saying is, be aware that proper unbalancing may cause overload of the input stage if you havn't compensated for it!

I've got a fantastic gif here, but I can't access my ftp from work!   :x

Try this.

[Bluesgeetar]

Good luck cedric!  I have been all through this forum trying to find info on this that a regular feller can understand.  Forget it.  This is 1/4 inch jack heaven here but anything else and your SOL.  I wanted to to put xlr inputs on a SHO circuit for vocal and acoustic hair.  I dream that someday this forum will diversify into other things but I don't think it will happen.  I am about fuzzed and distortioned to death.  I wish some more was created and talked about in reference to acoustic guitar and effects with xlr so we can use with mics and stuff.  I guess no one here records vocal or acoustic tracks.  

But on the bright side this place is with out a doubt the single best place in the universe for electric guitar effects.  But if your a vocalist or mic'd acoustic recording artist then begone with you go back to your cave and never enter here again.

I hope somepeople prove me wrong.  I would gladly put my foot in my mouth for some help with stuff other than electric guitar.

[brett]

There's lots of information available about balanced inputs, both here and elsewhere on the web.  The concept of balancing is so simple that my 8 year old understands it.  One line has the signal, another has no signal.  They both pick up noise (hopefully the same amount).  So when they get to an amplifier, you can deduct the noise on the dummy line from the signal+noise on the active line, and you are left with just the signal.  Simple.

So how do you do the noise deduction thing, I hear you ask.  That's simple, too.  A differencing amplifier.  All those op-amps (TL071/072/741/NE5532/RC4558 etc) have two inputs (+ and -).  If you connect the live and dummy lines into an op-amp, you just get the signal at the output.  Any noise going into both the + and - inputs is cancelled.     Op-amps are natural-born differencing amplifiers.

By the way, I'm no expert on this, it's just what I've learned from the forum and building a Direct Injection box a while ago.  Actually, there's a good site here http://www.sound.westhost.com/ with several DI and balanced mic DIY projects and schematics.  If you are familiar with schematics, you'll probably see what I mean straight away.

Have fun and stay cool  

[Mike Burgundy]

unless you use a Jensen transformer on both in- and output, parts count will be a LOT higher than just the one booster. Transformers (with a center tap) work fine for converting line-to-balanced and vice versa, although they need to be very high quality and therefore are expensive.
(hook up the + to one side, 0 to center tap and - to the other end of the primary. The secondary now sees *twice* the signal just as with a summing amp. This is a summing passive, so to say)

What's keeping you from using *two* boosters between two XLR connectors? One for the regular signal, one for inverted. It's a simple enough circuit.

[cedric]

...............
What's keeping you from using *two* boosters between two XLR connectors? One for the regular signal, one for inverted. It's a simple enough circuit.

i've actually thought about that.. didn't know if it could be done. but i guess now, that it can! sweet.

that WOULD mean me, making 16 mosfet boosts, just to have an 8ch mixer...
hoped it would be simpler. but hey... i'll give it all a shot.

mighty expensive DIY mixer though..

[Paul Perry (Frostwave)]

The concept of balancing is so simple that my 8 year old understands it.  One line has the signal, another has no signal.  

You might want to check with your 8 year old, I think it is more common to have two signals, 180 degrees out of phase, so at the differential amp input, you are subtracting a negative signal and noise from a positive signal and noise. Which is why you have a shielded twin conductor cable.

[ejbasses]

use op amps for a summing amplifier

i have a schematic for a simple differential amplifier using opamps. i havent verified it yet but i think it will work. PM me about it

[RedHouse]

A very low noise (and simple) circuit can be made from LM833's look at page 9 of the LM833 data sheet:

http://www.national.com/ds.cgi/LM/LM833.pdf

May need some minor tweaks but it's mostly there for you.

[niftydog]

hmm, well...

key thing to remember, guys, is that phase is all important.  If you run each leg of a balanced signal into it's own circuity you may upset the phase relationship of the signals and you'd no longer have a balanced signal.

Balanced lines have TWO SIGNALS 180 degrees out of phase, then a shield.

The difference amplifier ONLY amplifies the DIFFERENCE between the inputs.

Noise picked up by the cable is THE SAME on both signal lines, hence it is ignored by the difference amplifier.

Thus, if you do a shonky unbalancing act by grounding one signal line, the difference between them is halved... this is what I was getting at with my first post.

It's easy to do with a simple op amp difference amp.  No black magic!  Google "difference amplifier" or follow the link I've provided.

I would gladly put my foot in my mouth for some help with stuff other than electric guitar.

Hey, I'm here to help!  Can't know everything, but I and some other groupies here are electronic engineers... bounce a question of us!  Although technically there are more suitable forums for this type of thing...

just none of them are as friendly as this place!!!    

[ikiru-]

Transformers seem to be the simple way to balance something...most people seem to like the transformer sound too... You could try the OEP transformers. they are cheap, and many people say they have a good 'vintage' sound.. there is also lundahl, they are very nice, and moderately priced. There is also edcor, they are dirt cheap!
good luck! let us know how it turns out...

[niftydog]

Found a some good images.  Ignore the extraneous guff and strip it down, you've got the basic idea of unbalancing and balancing amplifiers.



[/img]

[cedric]

thanks for the gifs nifty. they look great. now what opamps can i use for this? TL071's? (wayyy off!?)

- ced

[Gilles C]

The 5532, for lower noise...

[niftydog]

what opamps

take your pick... they'll mostly all work, just depends what you have on hand and like Gilles said, noise figures etc...