70v amp, 8-Ohm speakers - Page 2 - AVS Forum
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post #31 of 42 Old 11-07-2013, 10:52 AM
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The "volume control" on a power amplifier is an input pad. A power amplifier can only be driven by a pre-amplifier. There is no input for a mic or guitar on a power amp.) The "volume control" changes the sensitivity of the input. It is not a gain stage and it is not a limiter. Setting the "volume control" to 20% just means that it takes more voltage on the input to drive the amp to full output. Setting the "volume control" to 100% means that it will take less voltage at the input to drive it to full output. Many manufactures still use the old standby sensitivity of .77v at the input will drive the amp to full output.

There are lots of ways to hookup a pile of equipment and get some sound out of it but if you hear distortion THERE IS SOMETHING WRONG.

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post #32 of 42 Old 11-07-2013, 11:15 AM
 
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The "volume control" on a power amplifier is an input pad.

It's a simple voltage divider.
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post #33 of 42 Old 07-27-2014, 04:59 AM
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I saw this thread and figured I would try to answer some questions. With respect to driving your JBL speakers with a 70 volt amplifier.... rated at 200 watts. I don't believe you mentioned the brand/model of amplifier in question, If the amplifier isn't setup in other words doesn't have a terminal strip showing various output various impedances then the 70.7 volt transformer needs to be taken out of the circuit because the amplifier is only capable of running a load of 25 ohms minimum. The amplifier is rated 200 watts @ 70.7 volts and will not run the 8 ohm nominal load of the JBL's. When figuring the impedance a 70.7 volt amplifier can run the following is used. First the 70.7X70.7=4998 ohms and the industry rounds this to 5000 ohms. So, 5000 divided by 200= 25 ohms. The 25 ohm load is the maximum load the amp can run. Now, not all amplifiers are capable of running a add on transformer because their power supply simply isn't large enough.

Seventy volt and 25 volt transformers and systems.

Generally schools intercom systems are 25 volt systems like Bogan for example. There is NO set rule saying that 25 volt only systems can be used in schools. In todays world some schools are going to a simple 70 volt system with just an all call page.

Most of the older commercial systems put in the 1950's and earlier were 25 volt.

The 100 volt system are common in Europe are are not a norm for say stadium systems. Been doing this type of work for many many years all over the country and have not run across one 100 volt system yet.

How 70 volt systems work...

The amplifier has been designed to run an internal 70 volt transformer or is hefty enough current wise to do so example Altec 9446 that will run two 600 watt external 70 volt transformers one per channel. Other commercial rated 8/4 ohm amplifiers are capable of running external transformers.

The 70 volt transformer steps the voltage up to 70 volts on the line. Each speaker is equipped with a step down transformer to lower the voltage and restrict the power to the given speaker.

Using a 8ohm speaker on a 70 volt line or using a 8 ohm speaker with a commercial transformered volume control.

Trying to use an 8 ohm speaker on a 70 volt line will result in an overloaded speaker line. Checking the load with an impedance meter will result with a load of 8 ohms and a meter reading in excess of 900 watts. It is easy to find because it will be the only speaker that is screaming.

Have had numerous times when I have pulled a 8 ohm speaker off a 70 volt transformered volume control. When wired this way the load is still in excess of 900 watts. The speaker still needs a transformer on it.

So, my suggestion is this....

I would purchase a power amplifier to use with a preamplifier to run the JBL speakers. By using a pre-amp and a power amp it allows you to upgrade later to a higher power amplifier.

Do NOT believe the speaker ratings...... I have seen more damage to speakers with amplifiers rated at less than the manufacturers ratings than I ever have with amplifiers that are larger. Why? Simple. Its because the small amplifiers clip easier and produce a distorted signal. When this happens damage is done to the drivers in the speaker. I currently run amplifiers that are rated 4 or 5 times the power rating of the speakers without a single problem.

Hope this helps.
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post #34 of 42 Old 07-27-2014, 05:18 AM
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Originally Posted by r0ckstar View Post
Thanks for your responses. As I mentioned, these were spare equipment laying around and I was merely asking if I could use one with the other. There was no "goal". Anyways, upon looking into the manual for the amp I found that it apparently handles low impedance loads. Can anyone verify this for me: the amp is a Stewart CVA-7400 ([www].stewartaudio.com/manuals/CVA%207400-7800%20-%20Manual.pdf) Sorry for my apparent lack of know-how in this department.
Actually this amplifier WILL drive an 8 ohm load because it is a current drive amplifier. I happen to have one that I got out of a commercial 70 volt system. It is one of the few out there that will drive a 8 ohm system or a 70 volt system.
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post #35 of 42 Old 07-27-2014, 05:31 AM
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Originally Posted by wall-of-sound View Post
old thread but worth reviving

you can run the amp as is wtih 70 volt line, just put 4 speaker cabs in series on it, 8 ohms each, total 32 ohms, and it will play. Get a mixer and 2 of those amps with 4 cabs on each one, and you have a nice little home PA system to play through. It's not going to be no high end concert system but it will work. The downside is you'll lose bandwidth, but I see a lot of recommendations based on theory here, but no one got off their lazy asz and actually tried it and hooked it up, except the OP. And he proved the damn thing will play. The world is full of inventions that people said would not work but did. If God meant us to fly, we'd have wings, right ? A 70V line, IS a 32 ohm line, for a 150 watt amp. Plug it into Ohms law and Watts law. Volts output squared, divided by total speaker ohms load, should be equal to rated amp output in watts, and it will work.

i.e. 70v x 70v = 4900/32 ohms = 153 watts

if the amp isn't rated that high, add more speaker load and ohms, and it will work

you don't need no damned transformers. That's only if you're piling 100 or 200 speakeres on the line and amp, in a stadium or something

there's nothing mysterious here to be afraid of, the most that can happen is a cheap PA amp and free speakers get blown up

and that's how you learn

don't believe me ?

what's on the back of this old Altec PA amp

ask yourself why the Altec engineers had that printed on there- it plainly states
"25/32 ohm (70v)" meaning the only thing that amp needs to see on that 70v line, is around 25 to 32 ohms, to function

The picture is of a Altec 1590 commercial amplifier. If you examine the output possibilities I believe you will find its capable of driving a 25 volt line. This would drive an 8 ohm speaker without a problem.

With respect to comments on distortion in some past threads. The human ear starts hearing distortion when it reaches 1%. Your not going to hear distortion of an amplifier rated at .1 as opposed to an amplifier rated at .001
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post #36 of 42 Old 08-01-2014, 05:42 PM
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Originally Posted by r0ckstar View Post
Hey everyone, this is my first post on these forums so be nice to the noob lol =)

I have a question regarding some equipment I have laying around. I have a 70.7v distribution amplifier (200 watts) and a pair of passive JBL 4408 studio monitors (8-ohms, 100 watts each). I have read that connecting these speakers directly to the amp would blow the amp due to the output voltage of the amp not matching the speaker impedance. Is this the case? Do I have to purchase separate step down transformers to properly power the speakers? Is there any way to bypass the transformer in the amplifier itself? Basically what I'm trying to get at here is can I use the JBL's with the amplifier without having to buy anymore equipment.

Thanks in advance,
Dave
This amplifier supports 8-Ohm Loads, by means of bypassing the transformers - you were misinformed - a truth delivered late is better than not at all; I guess?
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post #37 of 42 Old 08-03-2014, 12:04 PM
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I wanted to hop on an existing 70v thread and ask a question as to avoid the "n00b posting a new thread" controversy. The most important question is: Are 70v systems the best solution for multi-zone (2), music only, commercial audio setups? Most of the material I've been researching says something along the lines of "70v is used mostly for voice only applications." The only problem I've come across is the limitations in the length of wire runs in other setups. If anyone has any insight, and maybe a link for a newbie like me to research the basics, it would be much appreciated.
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post #38 of 42 Old 08-03-2014, 01:31 PM
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Quote:
Originally Posted by botopz View Post
I wanted to hop on an existing 70v thread and ask a question as to avoid the "n00b posting a new thread" controversy. The most important question is: Are 70v systems the best solution for multi-zone (2), music only, commercial audio setups? Most of the material I've been researching says something along the lines of "70v is used mostly for voice only applications." The only problem I've come across is the limitations in the length of wire runs in other setups. If anyone has any insight, and maybe a link for a newbie like me to research the basics, it would be much appreciated.
This link might help a little. http://www.toacanada.com/techtip-zm104a.html
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post #39 of 42 Old 08-04-2014, 06:34 AM
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Originally Posted by Garidy View Post
This amplifier supports 8-Ohm Loads, by means of bypassing the transformers - you were misinformed - a truth delivered late is better than not at all; I guess?
As I said early in this thread, you can remove the output transformer provided:

1) The output stage is sub ohm impedance. In a solid state amp, it probably is but you can't be sure without more research.

2) The feedback point is not taken from the secondary of the transformer. If it is it will have to be re-compensated for direct connection - not a trivial task.

I would highly discourage a novice from simply removing the output transformer in a PA amp without some expert analysis of the circuit.

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post #40 of 42 Old 08-04-2014, 06:50 AM
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Quote:
Originally Posted by SAM64 View Post
Who are you responding to?
Quote:The "volume control" on a power amplifier is an input pad.


It's a simple voltage divider.
...which is a kind of pad.
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post #41 of 42 Old 08-05-2014, 05:43 AM
 
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Originally Posted by arnyk View Post
...which is a kind of pad.

Arny, are you a little desperate for attention?
Despite being 9 months late with your reply, I'm sure you know that a pads attenuation depends on the source and load impedances, while a voltage divider is typically used to bridge a low impedance source.
If you're going to nit pic, at least be correct.
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post #42 of 42 Old 08-05-2014, 06:37 AM
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Originally Posted by SAM64 View Post
Arny, are you a little desperate for attention?
Yup, I set out some fly paper.

Quote:
Originally Posted by SAM64 View Post
Despite being 9 months late with your reply, I'm sure you know that a pads attenuation depends on the source and load impedances,
Which is true of a voltage divider as well.

Quote:
Originally Posted by SAM64 View Post
while a voltage divider is typically used to bridge a low impedance source.
The above seems to be conflating the conditions of the use of something with the use itself.

IOW any four terminal circuit's operation depends on Z(source) and Z(load)

Pads of all common kinds are four terminal circuits.

Therefore any pad's operation depends on Z(source) and Z(load)

Quote:
Originally Posted by SAM64 View Post
If you're going to nit pic, at least be correct.
Yup. ;-)
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