? for Jack Stoner

Greg Derksen · Post by **Greg Derksen** » 15 May 2000 7:51 am

Jack, I have been running my signal from
my Pro-Fex II directly into the board in the (line in) with good results, however this
last gig there was a ground hum, is the
outputs on the profex Balanced? or are they
unbalanced? I was using a unbalanced cord.
Its funny how it popped up all of a sudden.
Thanx, Greg

Jack Stoner · Post by **Jack Stoner** » 15 May 2000 9:24 am

The Left/Mono and Right outputs are unbalanced. It's been a while since I had my Profex II and I don't remember if there is a balanced (XRL) output on it. If not, you may want to consider an adapter that will convert it to a balanced signal.

Bill Crook · Post by **Bill Crook** » 16 May 2000 5:11 am

There is no balanced output on the ProfexII.

You can purchase the "line balance adaptors" tho and put then in line with the outputs of the ProfexII, if needed. I've never had a problem with my unit of this nature.

Keith Hilton · Post by **Keith Hilton** » 20 May 2000 8:47 pm

Bill, I'm confused. I thought XLR plugs had 3 pins. The outputs on the back of a ProFexII have 1/4 inch jacks. I always thought these were two conductor guitar cord type.
I think the word "balanced" means a inverted and non-inverted signal with signal grounding. I don't know how you could get that out of a two conductor guitar cord.
Greg, if it worked before, I'll bet there was something different. You may of hooked up the same, but something may of been in a different location. Lots of things can cause hum.

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[This message was edited by Keith Hilton on 20 May 2000 at 09:48 PM.]

Jack Stoner · Post by **Jack Stoner** » 21 May 2000 2:03 am

The 1/4" outputs out of a Profex II are unbalanced (signal and ground). You can convert the signal by using a DI box with a transformer in it or an inline device that has a 1/4" plug on one end and an XlR connector on the other and a transformer between the connectors. These devices convert the signal into a "balanced" 600 ohm three conductor (two hot and a shield ground) signal. In this particular reference "balanced" has nothing to do with balancing the levels on the two channels.

Keith Hilton · Post by **Keith Hilton** » 21 May 2000 7:17 am

Jack, I agree with you as usual. I might add that all my reference books say, to be a balanced signal ,one of the hot signals must be inverting and the other non-inverting. I don't see how that can happen with a isolation transformer. Jack, I am not saying you are wrong. I may be confused. I always understood the "industry standard" for a balanced signal to mean two hot lines, one non-inverting and the other one inverting. Jack, can you clear this up?

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Jack Stoner · Post by **Jack Stoner** » 21 May 2000 12:28 pm

Keith, I'm not sure what you're looking for. In relation to the secondary on a transformer - any transformer, power, audio, isolation or whatever - you have positive and negative on opposite ends of the secondary (180 degrees)depending on where the AC signal is in it's cycle.

Also, at the receiver end of the signal (termination) there will be another transformer that will convert it back to an unbalanced (signal and ground) signal so it can be processed by the preamp. Since you effectively have a closed loop with transformers on each end there is no need for a ground (except to shield the signal while it's being transferred in the coax cable).

You're forcing me to go back 40 years to basic electronics

Keith Hilton · Post by **Keith Hilton** » 21 May 2000 2:03 pm

Jack, you take a signal, turn it upside down, or invert it, and then you have two signals. You have the original signal ,and the one turned upside down. It is my understanding that you must have this to have a balanced line. This is really easy to do with OP-AMPS. I'm pretty sure this is what it takes to make a true balanced line. Lots of guys, especially sound guys at theaters, simply hook two hots up, no matter how the form, inverted, or non-inverted. I really don't think this is technically correct. It may work, but the definition of a balanced line is not just two hot lines in a XLR fitting. This always bothered me, because I have books that say to get a balanced line you have to have a inverted and non-inverted signal. Again Jack, I have the highest respect for your knowledge and ability. I am not saying you are wrong. I just understand it differently.

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Jack Stoner · Post by **Jack Stoner** » 21 May 2000 3:03 pm

Here's something I got from one of my old ARRL (amateur radio) handbooks.

"Single ended and balanced Circuits
With reference to ground, a circuit may be either single ended (unbalanced) or balanced.
In a single ended circuit, one side of the circuit (the cold side) is connected to ground. In a balanced circuit, the electrical midpoint is connected to ground so that the circuit has two hot ends each at the same voltage above ground."

It goes on to show some sample (basic) circuits but basically in a single ended (unbalanced) type with a transformer one end of the secondary is hot and the other is ground. In a balanced output circuit the secondary of the transformer has a center tap that is grounded and the two secondary ends are hot.

I looked at the Peavey Session 500 schematic, which has a balanced XLR output, it shows an op amp driving a "single ended" primary of a transformer and the balanced secondary it is not center tapped.

Keith Hilton · Post by **Keith Hilton** » 21 May 2000 8:24 pm

Jack, how do you find the electrical midpoint? That don't make a bit of sense to me. Also, just because the Peavey Amp is using a XLR adapter does not mean that the line is balanced. It may be just a way to convert a 1/4 inch jack to where a person can run a XLR plug to it. I ask Hartley Peavey about this very question one time, and he also beat around the bush. So what makes a line balanced? As I said, I always thought it was a inverted signal, a non-inverted signal, and a ground. Maybe we can get some guys like Mark or Rich to help us with this problem.

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[This message was edited by Keith Hilton on 21 May 2000 at 09:29 PM.][This message was edited by Keith Hilton on 21 May 2000 at 09:30 PM.]

Jack Stoner · Post by **Jack Stoner** » 22 May 2000 1:49 am

Keith, I think you're trying to read more into it than what is there.

Another example of a "balanced" circuit is your telephone. A telephone line uses basically the same circuit that a balanced audio line does (then that's what the telepnone line is) it uses the red/green wires. Open up a telephone and you'll see some small isolation transformers inside which do the same thing.

Bill Terry · Post by **Bill Terry** » 22 May 2000 6:17 am

My understanding of balanced was that it described any circuit where some cancellation of common mode signal was acheived, whether balanced by transformers or solid state devices. The only difference was how well balanced the circuit is, measurable as a common mode rejection ratio? Maybe that's oversimplification????

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bterry.home.netcom.com

Keith Hilton · Post by **Keith Hilton** » 22 May 2000 6:28 am

Bill, that makes a lot of sense! I can understand that. Jack don't get me wrong, I'm not disagreeing with you. Everyone talks about balanced lines, and I have wanted to pin the exact definition down.

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Michael Maese · Post by **Michael Maese** » 22 May 2000 8:33 am

Keith,
A balanced line provides a pair of connections that are both floating above chassis and earth ground. This could be the output of a push-pull or similar type circuit or a transformer. The common XLR-to-phone adaptors often contain a transformer to get both lines above ground.
A balanced input circuit utilizes the positive and negative inputs of an op-amp. The advantage of a balanced input is common-mode rejection. This means that any signal that is found on both of the balanced lines with reference to ground (like a hum or electrical noise) is cancelled when the inverted signal meets the normal signal in the op-amp. The audio signal that the balanced lines are carrying goes into the op-amp without this cancellation and is amplified normally. You could think of the signal as moving down one of the balanced lines and returning via the other. One of the balanced lines acts as signal ground, but it is not actually connected to that part of the circuit we normally think of as ground.
A balanced cable provides a pair of wires (like in telephone lines) or a pair of wires enclosed within a shield (like the cable with XLR or 3-conductor phone connectors at each end).
Hope this helps.

Blake Hawkins · Post by **Blake Hawkins** » 22 May 2000 11:23 am

Couldn't resist adding my $.02. 'Cause you are all correct.
So I may be restating what has already been said.
In 40 years of audio work I have used both balanced and unbalanced lines.
Balanced audio existed many years before practical Opamps were made.
The Opamp just takes the place of the transformer at one end of a balanced line.
A transformer with a single primary and a center tapped secondary will properly convert an unbalanced line to a balanced line. In this case the two "hot" wires from the secondary each contain audio of opposite polarities. If you send a sine wave, on one line the peak will be positive going and on the other the peak will negative going with respect to ground.
On the two conductor shielded cable the positive going would be connected to the red wire and the negative would go to the black with the shield to the center tap. Audio engineers call the positive going "Hot" or "High" and the negative either "cold" or "low" with the ground refered to as "Ground" or shield.
Sometimes, especially in circuits with opamps, the inputs to which you connect the wires are "+" (non inverting) and "-" (inverting). This causes some confusion because the "-" in this case is not ground.
In audio work it the way the center tap is connected varies depending upon the installation. On microphones there is a transformer inside the mike....it's centertap is connected to the shield of the cable.
Usually on the input of the amplifier the shield is connected to chassis ground, which may or may not be the signal ground.
In many cases, to prevent ground loop hum, the center tap is not connected to ground but left floating on lines other than microphones.
The line is still considered balanced as long as there are equal potentials in it's two conductors.
There is, however still a ground and it is run in a separate wire.
There is a concise description of this in
"Sound System Engineering Second Edition"
on page 382. Published by Howard Sams.
If you do any serious audio work you should have this book. (Or the older "Audio Cyclopedia.)
If you want a trouble free audio installation
you make all the lines balanced.
This is not always done in music because balanced lines are much more expensive than unbalanced..so many effects boxes and processors have unbalanced inputs and outputs.
Folks, it's a real art to get all that stuff connected together in a recording studio with patch panels and not get hum in some signal lines...but it can be done.
Blake

Keith Hilton · Post by **Keith Hilton** » 22 May 2000 3:28 pm

Michael and Blake, thanks for the "wonderful" explanation of balanced and unbalanced lines. Everything is very clear to me now. Everthing ended great as we were all correct!

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