Connecting Gear with Balanced & Unbalanced Inputs

[bob p]

Is there an easy way to connect an unbalanced signal to an amp that has balanced inputs? I'm trying to decide whether or not its possible to hook up some standard unbalanced home audio gear to an amp that has balanced XLR-type inputs. I just thought I'd ask before I go and do something stupid. TIA!

[Enzo]

The worst thing that could happen would be a lot of hum and a weak signal.

The typical balanced input jacks on a commercial power amp will also accept unbalanced signals. Of the two signal leads going in, ground one and apply the unbalanced signal to the remaining pin.

I always have some XLR to 1/4" adaptors in the kit. I keep a pair wired pin 2 hot and pin 3 ground and another pair wired pin 3 hot and pin 2 grounded.

WHich ones get grounded usually don't matter, unless you are driving a system with other amps and phase matters.

Watch the levels though, you didn't state what you are connecting. The hot line out from a mixer/receiver might overdrive a mic input on something else.

[bob p]

Thanks, Enzo. I was thinking about hooking up a hifi preamp to a commercial power amp. In that case I think the signal levels would be ok.

Now this has me thinking -- if I were to build a tube type preamp to feed a commercial power amp for bass (IIRC Alembic did something like this with a Fender preamp ciruit), how would you design the balanced output? Are the (+) and (-) signals on the XLR connectors in pro sound gear 180-degrees out of phase with one another?

[Brad1]

Maybe this will interest you?

http://rane.com/note124.html

Brad1

[Enzo]

The point of balanced cables is that you can make a lengthy run with them and not pick up a lot of excess noise and high end losses. but if you are only running a couple feet, why bother with it? Or are you running down a snake?

Rane has a lot of very interesting papers on file, that is a good one. DOwn in the middle of it somewhere it tells you the secret - turn the knobs until you get the gain you want.

The signals on the two hot pins of an XLR at the end of a balanced line would be 180 degrees out of phase if a steady sine wave were flowing through, But for a constantly changing signal like music, the two are simply the same signal with one reversed in polarity.

[bob p]

The signals on the two hot pins of an XLR at the end of a balanced line would be 180 degrees out of phase if a steady sine wave were flowing through, But for a constantly changing signal like music, the two are simply the same signal with one reversed in polarity.

thanks, Enzo -- that answers the question for me.

but i still have another question relating to what's been discussed in the article -- how to create the "pad" between the unbalanced signal source and the balanced amplifier? is there any practical way to fix the 6 dB loss problem, or do those 6 dB just get thrown away?

thanks Brad for the link.

[Enzo]

May I suggest that you connect the two together, and IF you find something lacking, THEN we can solve the problem. One very simple solution is an impedance adaptor - the cigar shaped thing with XLR on one end and 1/4 on the other and a transformer inside. Lo Z to Hi Z in a flash. A few of those live in the tool box of every sound man I know. it is the thing one uses to connect Lo Z mics to mixer inputs that are all 1/4" Hi Z.

[Steve Conner]

What pad? I've always just hooked balanced and unbalanced gear together without thinking about it much. In particular I have a stereo power amp with balanced XLR inputs that I connected CD players and suchlike to. I made a RCA to XLR adaptor lead for the purpose. Core to pin 2, screen to pin 3, and if the unbalanced piece of gear isn't grounded (many hi-fi separates aren't) link pin 1 to pin 3.

In this kind of situation I often put a 10 ohm resistor between 1 and 3 instead of just linking them, to make a cable that will sort of ground a floating chassis, but won't aggravate ground loops too much, when you decide to use it with something that doesn't float one day.

The only problem I ever had with the 6dB issue is that my mixer's direct outs (which are unbalanced) can't drive the ADCs in my Delta 1010 (with 1/4" TRS balanced inputs) beyond 50% of full scale. The mixer clips 6dB before the ADCs do.