Please explain ohms with an XLR DI output

[lowend range]

I know ohms is a measure of resistance. As is the case with mathematics, a known constant doesn't change...

Take a Mesa Boogie 400+ head that has a 600 ohm balanced line out. Is this the same ohm reading as if plugging a head into a cabinet? Does this just mean it has an incredible amount of resistance as to not overload a mixing console? Can someone explain this whole concept of ohms when referring to an XLR line out on a head?

I just wanted this topic de-mystified for a while.

Any info or explanation would be appreciated.

[jazzblade]

"600 OHM" BALANCED LINES are used almost universally in recording, broadcasting and most large installations where analogue signals need to be fed from one piece of equipment to the next. The balancing virtually eliminates ground noise and unwanted signals which can be induced into cables. Transformers are a very effective means for converting unbalanced signals to balanced as well as providing almost complete isolation from ground noise. A further benefit of using balanced lines is the elimination of unwanted "pops" and "bangs" when signals are switched. Many HiFi enthusiasts are now using 600 ohm lines as their equipment becomes more complex.

WHY 600 OHMS? There is a lot of history and some confusion here. The ideal impedance for driving an audio line is zero ohms and the ideal input impedance for connecting to it is infinity. The actual wire does not need to have any particular "characteristic impedance" nor does it need to be terminated like radio frequency cable. 600 ohms is a nominal value used to describe an environment.
In summary it could be said that a line driver should have an impedance no more than 600 ohms and a receiver should have an impedance no less than 600 ohms. If both driver and receiver were both 600 ohms however, half the signal would be lost in the driver.

[mikeddd]

I'm no amp designer and I'm certainly NOT a mathematician, but I know this: The DI-out of an amp has nothing to do, power-wise nor ohm-wise with it's speaker out(s).

The DI-out is taken after the pre-amp section of the amplifier. Better quality amps let you select "pre" or "post" tone controls and may offer a ground lift switch. The signal that gets passed from the preamp to the power amp (where your speakers, if any , live) has no idea whether or not there's a DI there or not.

[okcrum]

600 ohms is the input impedance of most pro audio gear inputs. Using this value allows long cables to be used without severe high frequency rolloff. Think of it as a 600 ohm resistor at each end of the cable in the case of DI.

Your telephone works on the same principle.

[BassmanPaul]

It's much the same idea as with speakers. A speaker has an impedance, typically 8Ω usually measured at 1KHz if my old head remembers correctly. Similarly a DI has an output impedance. 600Ω is a standard from way back into the early days of radio and recording. The signal level is much reduced from that fed to a speaker. It is usually fed into a line level input of a mixing console although most mixers nowadays can handle both line and mic levels.

Paul

[jazzblade]

Remember its called a 'Balanced" line which basically means the two 'HOT" leads run inside a shielded cable where the Shield is grounded to reject outside noise. It allows for very long cable runs while rejecting noise and hum.

[lowend range]

Very interesting. Thanks for all that info.

[Sten]

And to explain the "balanced" thing a bit more: the same signal is sent into two wires, but it is inverted in one. In the other end (mixer) the other signal is inverted again and the signals are added back together. Any noise that was picked up is the same in both wires, (they are so close to each other, that they really cannot pick up different noise) is therefore cancelled out.

[bongomania]

And in case it wasn't clear, impedance and resistance are not quite the same thing, even though they are both rated in ohms. They're related, but it's useful to think about impedance as a relationship between two devices, rather than as a value by itself. The 600 ohms of output impedance from the XLR jack will have an ideal relationship with a mixer input that has at least 6 Kohms of input impedance. An input impedance as high as 100K or 1M is not uncommon, in order to relate well with a wide range of output impedances.

[James Judson]

To get the maximum signal transfer you need to match output impedance to input impedance. Ohm's law.

[bongomania]

Ohm's law doesn't say anything about matching impedance.

For an amp-to-speaker connection, you need impedance matching in order to maximize power transfer. For line audio, you are not trying to transfer power, but voltage. To maximize voltage transfer, you use impedance bridging instead of impedance matching. In impedance bridging you need z load >> z source, and the rule of thumb is that z load should be at least 10x z source.

[Mark Reccord]

Originally Posted by James Judson

To get the maximum signal transfer you need to match output impedance to input impedance. Ohm's law.

For maximum power transfer, yes. However, in audio things are usually set up for maximum voltage transfer, in which case a low impedance output driving a high impedance input is exactly what's required. Also Ohm's law.

[GreggBummer]

Originally Posted by jazzblade

"600 OHM" BALANCED LINES are used almost universally in recording, broadcasting and most large installations where analogue signals need to be fed from one piece of equipment to the next. The balancing virtually eliminates ground noise and unwanted signals which can be induced into cables. Transformers are a very effective means for converting unbalanced signals to balanced as well as providing almost complete isolation from ground noise. A further benefit of using balanced lines is the elimination of unwanted "pops" and "bangs" when signals are switched. Many HiFi enthusiasts are now using 600 ohm lines as their equipment becomes more complex.

WHY 600 OHMS? There is a lot of history and some confusion here. The ideal impedance for driving an audio line is zero ohms and the ideal input impedance for connecting to it is infinity. The actual wire does not need to have any particular "characteristic impedance" nor does it need to be terminated like radio frequency cable. 600 ohms is a nominal value used to describe an environment.
In summary it could be said that a line driver should have an impedance no more than 600 ohms and a receiver should have an impedance no less than 600 ohms. If both driver and receiver were both 600 ohms however, half the signal would be lost in the driver.

Jazzblade, you should consider writing a book. This was well written, easy to understand, and interesting. I'm not joking.

To the OP and others,

This is a great thread. I want to know more about live sound/recording topics like this. Is there a good book that someone can recommend me? It might take Jazzblade a while to write his book.

Thanks.

[bongomania]

Well, there's this: http://www.sowter.co.uk/pre-amp-line...ansformers.php
Some good sites to read:
http://www.geofex.com/index.htm
http://www.rane.com/note110.html
http://www.rane.com/note124.html
(all of Rane's tech notes are pure gold)
http://www.prodigy-pro.com/diy/
http://www.pmillett.com/tecnical_books_online.htm
http://www.ovnilab.com/faq.shtml

[Rick Auricchio]

The history of all of this comes from the phone company. They pioneered long-line analog connections. The whole 600-ohm, +4dBu system was developed by Bell Labs in the 1920s.

[GreggBummer]

Originally Posted by bongomania

Well, there's this: http://www.sowter.co.uk/pre-amp-line...ansformers.php
Some good sites to read:
http://www.geofex.com/index.htm
http://www.rane.com/note110.html
http://www.rane.com/note124.html
(all of Rane's tech notes are pure gold)
http://www.prodigy-pro.com/diy/
http://www.pmillett.com/tecnical_books_online.htm
http://www.ovnilab.com/faq.shtml

Excellent links, all of them now bookmarked. Thank you.