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Old 8th March 2004, 03:45 AM   #1
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Default 4 Ohm on 25v Tap?

As some of you know I have converted a McMartin PA to a guitar amp(thanks for your help). I saw somewhere that on a Bogen PA amp you may run a 4 Ohm speaker from common to the 25V tap? Is this the case? I have a 4 Ohm speaker made in england I would like to use. My amp only has 8 Ohm, 70V and 25V taps. Can I do this as stated above or is there another way? Thanks.
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Old 8th March 2004, 04:33 AM   #2
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I think it depends on power level. Like why transformers on the other end of the 70V line have multiple taps depending on how much power you want in the speaker. But don't mind me, I'm in that purple haze (hmm I should maybe go download that song) before going to bed.

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Old 8th March 2004, 11:10 AM   #3
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Default A 4ohm speaker on 25volt tap,ykes!

You must first understand the commercial sound systems. A 25volt line is as follows: 25vX25v= 625ohms this is how the 25volt speaker line is figured. Hince a 4 ohm load into a 625 ohm would equal a load on the amp of 156watts. Your 4ohm speaker when hooked up to the 25volt leg without ANY transformer would pose a 156.25 watt load on the amp. By the same token a load on the 70 volt side is figured as 70.7vX70.7v=4998ohms. The value of both 25volt and 70volt lines are to drive quantities of speakers and a heck of a lot of wire without problems. a 25volt line would drive a maximum of 625 speakers tapped at 1watt on the 25volt transformer. Do the math and you can see the value of the commercial speaker system as opposed to trying to series parallel 8 ohm speakers.

To answer your question...NO, do not use a 4 ohm speaker on the 25volt tap or the 70volt tap either. You would be better to put it on the 8ohm tap as opposed to anything else. Better yet use an 8ohm speaker.

Tube amps are more forgiving than any SS amp would ever be when it comes to overloading.

Joe
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Old 8th March 2004, 04:22 PM   #4
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NO! Your units are all wrong. R = V/I, not V*V.

If your PA (Public Address) system is 100W, then you'll have V^2/P = R = 70^2/100 = 49 ohms. The advantages to a 70V line are lower current = lower joule heating (I^2R loss) and a slightly easier way to match multiple loads.
Indeed, even a 4 ohm output will reach 70V - around 1.2kW in fact, probably suitable for a stadium. This likely would be served by many 32 or 64 ohm loads, each tapping a hundred or two watts.

Also, you DON'T "impose a load of n watts" on an amplifier. You impose a load resistance on it. The voltage the amplifier produces determines the power output.

There is nothing wrong with connecting a .2W PC speaker to a 500W amp, nor powering your 200W cabinets off a 50mW headphones amp. The only problem is that in the first case, background noise alone might blow the speaker (that represents noise about -30dB below full power output, not impossible), let alone any useful signal; while in the second case, the amplifier will be well into clipping distortion before you can even hear anything in the speakers (without your head in front of them). A less extreme form of the second example is ideal because amplifiers usually clip with no consequences, while speakers go over limits much less gracefully. Like, 100W speaker with 50W amp. Assuming the speaker is rated for continuous power.

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Old 8th March 2004, 07:51 PM   #5
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Thanks, But I was not talking about going from the 25V alone. I was saying from the 25V+ to the 8 Ohm common ground. A little different. I heard this might work.
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Old 8th March 2004, 10:31 PM   #6
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Can't tell without knowing power. Check the equations in my above post.

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Old 9th March 2004, 12:19 AM   #7
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NO! Your units are all wrong. R = V/I, not V*V.

Relax Tim!

I may have to take another stab at this in order to satify you. First of all when calculating the load on a 25volt line the figure 625ohms is used. The 625 ohms is a figure arrived at when figuring a 25volt line/load. This figure is the product of multiplying 25X25. Tim, please feel free to check this in any commercial Altec Lansing or EV material.


Now, if a 10 watt amplifier is used on a 25volt tap the maximum impedance load would be aproximately 62 ohms load on the line. This would be 625 ohms divided by 62ohms= 10.08 watts. Do you understand?

Now if the load was 50 ohms on a 70 volt line the load would be 100watts. Now this figure would be derived by dividing the square of 70.7 by 50 ohms. Tim this would be 70.7X70.7= divided by 50.
A 100 watt load in other words.

Now, if you impose a 8ohm speaker without a transformer on a 70.7 volt line the amplifier sees this a virtually a dead short. By the same token it is asking for trouble to connect a 4ohm or 8ohm speaker to a 25volt line.


Get real please!!!

I still stand correct in my previous post. Hooking up a 4ohm speaker without a transformer to the 25volt tap is incorrect. Hooking the 4 ohm speaker up to the 25volt tap and ground is having the amp see a load of 156watts. Tim, this figure is arrived by dividing 625by 4 ohms. I don't think the amp in question is quite large enough for this task. Do you?



Tim, hooking up a 4 ohm speaker to a 70.7 volt tap is equal to a load of 1249.6225 watts on an amp. Now Tim this is fine if the amplifier is large enough to support such a load.

Tim Wyatt

Do not hook up to the 25volt leg and ground as this WILL impose an overload to the amplifier. The less dangerous method would be to hook to the 8 ohm and ground or maybe better yet add a 4ohm resistor in series to the 4 ohm driver to arrive at an 8ohm load and then terminate to the 8 ohm leg and ground.

Joe
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Old 9th March 2004, 01:26 AM   #8
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Thanks everyone. This turned out being more trouble than I thought Transformers are still quite the mystery to me at this point.
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Old 9th March 2004, 03:53 AM   #9
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Quote:
Originally posted by burnedfingers
First of all when calculating the load on a 25volt line the figure 625ohms is used. The 625 ohms is a figure arrived at when figuring a 25volt line/load. This figure is the product of multiplying 25X25. Tim, please feel free to check this in any commercial Altec Lansing or EV material.
But your units ARE all wrong. Volts squared yields joules squared per coulomb squared (J^2/C^2), nothing more. Divided by resistance, however, it becomes joules per second. Resistance is in...joule-seconds per coulomb squared? I forget at the moment.

Quote:
Now, if a 10 watt amplifier is used on a 25volt tap the maximum impedance load would be aproximately 62 ohms load on the line. This would be 625 ohms divided by 62ohms= 10.08 watts. Do you understand?
NOW you're doing it correctly. P=V^2/R.

Quote:
Now, if you impose a 8ohm speaker without a transformer on a 70.7 volt line the amplifier sees this a virtually a dead short.
Run the numbers. 1.2kW as I said would probably fit well in a stadium.

Quote:
By the same token it is asking for trouble to connect a 4ohm or 8ohm speaker to a 25volt line.
What token? To know if it would match we still need to know the power level. That's behind the very definition of voltage-based (rather than impedance) lines.

Run the numbers, with my equations or your own. You'll find that an 80W amp would power a 25V line quite nicely with a total of 8 ohms on the end. (I say total because the whole other point of these voltage lines is distribution over either long distances or multiple speakers.)

Quote:
Hooking up a 4ohm speaker without a transformer to the 25volt tap is incorrect. Hooking the 4 ohm speaker up to the 25volt tap and ground is having the amp see a load of 156watts.
Also, assuming the amp is producing 25V on the 25V tap. Audio of a hi-fi nature will never be consistent so calling out voltages is pointless, especially without any distribution needed. Now obviously joe, JOE, JOE, the speaker isn't going to draw one hundred fifty-six watts from an amplifier which might be...30W? Where do the 126W come from?


Quote:
Tim, this figure is arrived by dividing 625by 4 ohms. I don't think the amp in question is quite large enough for this task. Do you?
Probably not. As such the impedance must not be matched correctly. So you'll have much less than maximum power output anyway. It may still be possible to operate the amplifier with the speaker on this tap but gain will be lower, distortion will be higher and clipping will come earlier (i.e. lower power).

Quote:
Tim, hooking up a 4 ohm speaker to a 70.7 volt tap is equal to a load of 1249.6225 watts on an amp. Now Tim this is fine if the amplifier is large enough to support such a load.
Huh? I thought it was a dead short?

As for the other Tim, I suspect the amplifier is...20W? 40W? Whichever method you use, output power has to be known, Ohm's law says so. What's the back panel say, Tim?

Tim
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Old 9th March 2004, 05:52 AM   #10
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I don,t know the power out. I have 475V on the plates of two EL34 in P-P Pentode mode. I am thinking about 45~50 Watts?
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