4 Ohm on 25v Tap? 4 Ohm on 25v Tap? - Page 2 - diyAudio
Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

4 Ohm on 25v Tap?
4 Ohm on 25v Tap?
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 9th March 2004, 11:31 AM   #11
Burnedfingers is offline Burnedfingers  United States
diyAudio Member
Join Date: Nov 2002
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. WoW you impressed me again!

Please check the formulas for figuring 25volt and 70.7 volt speaker lines.
The figures are as I stated. The impedance of 625 ohms for the 25volt line is correct as is the figure of 4998 ohms for a 70.7 volt line.
Throwing a 4 ohm speaker on a 25volt line is imposing a load of 4 ohms
on the amplifier or 625 divided by 4 = a load of 156 watts. This means it would take a 156 watt amplifier using the 25volt tap to drive this load.


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.

Tim, it would take a 1200watt amplifier to drive this load when tapped at 70.7 volts.


Run the numbers. 1.2kW as I said would probably fit well in a stadium.

Tim, first of all you don't run main horns,(clusters)on the 70.7 volt tap.

The amplifier would run a 70.7volt line with a load of 1200watts. What I was trying to say was the 8 ohm speaker on the 70,7 volt tap will impose a load of 1200watts on the amplifier.


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.)

Tim, I have no problem with this as it is correct. the 80 watt amplifier would drive a 7.8125 ohm load. Do the math Tim, a 4 ohm speaker would still require an amplifier of 156watts. Yes, Tim I do understand the concept of line distribution and multiple speakers.


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?

Did I say the speaker would draw 156watts? No, the amplifier will see the 4 ohm speaker as an overload. This would be too little impedance.

The amplifier will see the load as an overload unless the amplifier is large enough. Telling some one that is ok is just plain crazy. Unfortunately, it seems the logic you posess seems to be running free because I run into numerous know it alls at factories, grocery stores, and the like that seem to feel it is ok to attach a 8ohm speaker to the 70.7 volt system.

They wonder why it is distorted the amp runs hot and goes into thermal overload and why that 8 ohm speaker they added is the loud one and the
others can bearly be heard.

The idea of the step up transformer is to step up the voltage to either a 25volt line or a 70.7 volt line. This concept also requires that a step down transformer be attached to the speaker/s that are the load.
  Reply With Quote
Old 9th March 2004, 11:48 AM   #12
Burnedfingers is offline Burnedfingers  United States
diyAudio Member
Join Date: Nov 2002
One of my cheaper impedance meters that is capable of figuring the load imposed on an amplifier.
Attached Images
File Type: jpg p3080015.jpg (76.2 KB, 114 views)
  Reply With Quote
Old 9th March 2004, 11:55 AM   #13
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
EC8010's Avatar
Join Date: Jan 2003
Location: Near London. UK
Default Oil on troubled waters, or oil on the flames?

Gentlemen, it is conventional for 100V and 70V line systems to talk about a loudspeaker imposing a load of "x" Watts. If you look at the matching transformers (for the loudspeaker end), you find that the secondary may have 4 or 8 Ohm taps, but the primary is marked in terms of the power it will draw from the amplifier. This makes it easy for the user of a PA system (probably not a physicist) who knows that they have a 100W amplifier to avoid overloading said amplifier when they connect lots of loudspeakers in parallel. It's also handy for controlling loudspeaker directivity. By driving different proportions of power to identical loudspeakers in a line array, directivity can be adjusted at will.

However, V x V does not equal Ohms. The units are wrong, and it doesn't matter how many loudspeaker manufacturers make the claim.

Getting back to the original point, a 4 Ohm loudspeaker connected directly across a 70V line would only be correctly matched if the amplifier was rated at 1225W. Similarly, connecting to 25V line would require the amplifier to be rated at 156.25W. A push-pull pair of EL34 can stretch to 50W, precluding use of the 25V line output. Unfortunately, even loading the 8 Ohm output with 4 Ohms will cause the output power to fall drastically, and distortion will rise. You really need a true 8 Ohm loudspeaker.
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
  Reply With Quote
Old 9th March 2004, 05:44 PM   #14
Sch3mat1c is offline Sch3mat1c  United States
diyAudio Member
Sch3mat1c's Avatar
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
Thank you.

If it's 50W, since that's "stretching it", 40W is a good estimate (although...being a PA amp, the 50W might not be too far off); that 25V output might be 16 ohms or so... Then yes I think there would be 4 ohms between the 8 ohm and 25V taps. Although I can't seem to get the math to work out (1.4 ohms, um, no).

Seven Transistor Labs, LLC
Projects and Resources / Electronic Design and Consultation
  Reply With Quote
Old 9th March 2004, 08:47 PM   #15
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
EC8010's Avatar
Join Date: Jan 2003
Location: Near London. UK
If you look at voltages instead, then the 8 Ohm output produces 20V for 50W, whereas the 25V output produces, gosh, 25V. The difference between them is 5V, so to absorb the full 50W by connecting between those two taps, you would need a 0.5 Ohm load. Nice thought, though.
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
  Reply With Quote
Old 9th March 2004, 10:11 PM   #16
sreten is offline sreten  United Kingdom
diyAudio Member RIP
Join Date: Nov 2003
Location: Brighton UK
hi all,

I'll refer you to this link :


By implication a 25V line output is 25V RMS at max amplifier power.

For a 4ohm load to be ideally matched to this the amplifier
power output would need to ~ 160W.

This seems to be high and this the case - line systems are
high impedance - this the whole point - speakers are tapped
off along the line with various power and hence volume settings
without resistive losses.

For 4 ohm to match a 100V line it would need to be a 2.5Kw

I can not make head nor tale of your posts.

A 100V line amplifier is arranged to have maximum output at 100V.
The amplifier output is specified - say 35 Watts.

You can then load this line to the total of number of Watts specified.

The speakers have 100V transformers that allow you to choose
the number of Watts drawn from the line and hence the relative
speaker volume.
Total number of watts drawn should not exceed the amplifier power,
otherwise you will start to get an impedance mismatch, drawing less
power causes load impedance to be higher which is not a problem.

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.
Simple fact here is for an 80W amplifier the 8 ohm output
and 25V line output would be one and the same.

If your amplifier was 150W describing the 4 ohm output
as 25V line would be very bizarre.

So it it likely output is less than 50W and no output will drive 4 ohms.

  Reply With Quote
Old 9th March 2004, 11:13 PM   #17
Burnedfingers is offline Burnedfingers  United States
diyAudio Member
Join Date: Nov 2002
Default Formula

I think the formula P=EXE/Z should explain my point of view. This is Power equals voltage squared over Impedance. Therefore, E squared or 25X25=625. If the load imposed is 4 ohms this would be 625 divided by 4 or 156 watts.

By the same token, 70.7X70.7= 4998.49 over 4 ohms = 1249.6225 or P=1249.6225

To answer the Question. No, do not hook up to the 25Volt leg with the 4 ohm speaker. The reason as I have proved above using this formula.


In the case of a tube amplifier, the tube amplifier is a lot more forgiving that a SS amplifier would be. You could expect a SS amplifier to go belly up in a short amout of time. I have personally seen tube amplifiers run way overloaded for years and keep on ticking like a Timex watch.

I rest my case.

  Reply With Quote


4 Ohm on 25v Tap?Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

New To Site? Need Help?

All times are GMT. The time now is 06:39 AM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright ©1999-2017 diyAudio