Question about Altec 290 mid-range drivers

[George Roland]

I have posted and read posts about Altec mid-range drivers and horns and I have a few questions. Regarding the 290 series, which seem highly respected, the spec sheet speaks about a 15045A (or other similar number on other models) "matching transformer for 70.7V line usage". Could someone explain to me what this means? Is this transformer needed to operate these drivers in a stereo music system?

Secondly, I read in Altec's own literature that these drivers seem to be intended primarily for outdoor voice reinforcement rather than for high-fidelity music reproduction. That being the case, what strengths do they have for quality music reproduction? 

Thanks,

George Roland

[Duke Spinner]

i have some 290 E's ..

the transformer is for use in a Distributed Sound System, that operates at 70 volt's

mine had a dia change to 8 ohms..

[WMcD]

Further to what Duke said.

No, the 70 volt line transformer is not needed for home hi-fi.

Explanation of the 70 volt line transformer system:

We've read how speaker feed wire should be of greater thickness (lower American Wire Gauge number - AWG) for longer runs. This is to prevent losses. It is the voltage which is "dropped" by the resistance of the wire. Vdrop = Current x Resistance or V=IR. This is true in any resistance, of course, it is restatement of Ohms law. So the voltage drop is called IR losses. (Note here we are in the algebra just not putting in the x, it is understood that it is in the expression.)

= = = =

What do we do when we want to send electrical power over long distances? An example is a PA system with many speakers placed around a building or up the ski slope. We want to reduce IR losses. Thicker wire is one option because it reduces the R. But we can also reduce the I or current.

How? One thing we do is use a step-up transformer on the sending end. By convention, the step-up means that the voltage is increased. However, power is conserved in a transformer (except for internal losses) so V x I = Power. This means that at the output of the transformer we have the same power with higher voltage and lower current, or I.

Now we send the higher voltage and lower current off in a distribution system with some given size wire with some R. But we have lower I and thus IxR losses are lower.

This shows up how the nomenclature totally obscures the way the system works. Rather than being called a high-voltage distribution system, it should be called a low-current distribution system. One can only guess the marketing department made this up. High voltage seems powerful. Conceptually it blasts though a problem. But not so. The handmaiden of low current solves the IR problem.

Of couse on the receiving end we want to reduce voltage. Now we use a similar transformer wired backwards. It steps down voltage and increases current. That is the transformer you're reading about.

You might realize here that the a.c. power distribution system in the world works the exact same way. In both cases we can use multiple step down (voltage) transformers spaced along the transmission line to pick off power.

Getting a bit far afield: You may have read of the "current wars" between Edison (d.c) and Tesla (a.c). The main reason Tesla's system won was that transformers only work with a.c. So Edison's system could not utilize transformers.

Sorry to get pedantic.

Gil

[djk]

"Is this transformer needed to operate these drivers in a stereo music system? "

No, it isn't.

The 290 comes with a big empty tin can on the back for you to stuff your 70V transformer in for your football stadium and 1/2 mile of wire.

For home use you leave the can empty (or throw it away) and hook your wires straight to the screw terminals on the diaphragm.

"what strengths do they have for quality music reproduction? "

They're not metal, and don't ring. You need a tweeter though.

They're similar to the JBL2485 that people were raving about in another thread, but the 290 is smoother, and goes up a bit higher (with a 290 you can cross the tweeter at 7Khz, the JBL needs to be crossed at 5Khz).

http://alteclansingunofficial.nlenet.net/publications/techletters/TL_231.pdf

[George Roland]

Thank you to all respondents for such well informed and set out explanations. There is such a wealth of knowledge and experience on this forum, it's a treat to participate in it.

George Roland