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post #1 of 42 Old 02-16-2012, 11:51 AM - Thread Starter
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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
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post #2 of 42 Old 02-16-2012, 11:54 AM
 
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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?

Not really, voltage and impedance are two seperate things.

Quote:
Do I have to purchase separate step down transformers to properly power the speakers?

yes

Quote:
Is there any way to bypass the transformer in the amplifier itself?

Most likely, but if you have to ask....
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post #3 of 42 Old 02-16-2012, 12:17 PM
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Quote:
Originally Posted by dragonfyr View Post

???
In a distributed 70v system, each speaker uses a small transformer.

You do not by pass anything internal to the amp!

You might want to peruse the docs at places like the Rane library...

Constant-Voltage Audio Distribution Systems:25, 70.7 & 100 Volts

I think he is asking if he can make the amp into a traditional 8ohm output amp. If the amp topology has an internal 70v transformer, then yes, it can be removed. But we still don't know the actual output impedance of the amp output circuit. If it's a 20 year old or less design, it's probably a few ohms or less just like all modern SS amps and should drive an 8ohm speaker quite well. But you really can't be sure without some research and/or bench testing.

It's possible the amps output circuits drive the 70v line directly and in that case you are stuck using transformers on the speakers which IMO, isn't worth the effort. They will be expensive for 100w units and will not have a very good frequency response.

EDIT: There is also a remote chance they may take the feedback loop off the 70v side of the output transformer (if there is one). In that case removing the transformer and modifying the feedback loop is quite an EE level exercise.

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post #4 of 42 Old 02-16-2012, 01:32 PM
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Quote:
Originally Posted by dragonfyr View Post

If that was his goal, the fact that the question even needs to be asked should provide an adequate basis that one should not be considering it.

Absolutly!

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post #5 of 42 Old 02-16-2012, 01:48 PM
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Quote:
Originally Posted by dragonfyr View Post

If that was his goal, the fact that the question even needs to be asked should provide an adequate basis that one should not be considering it.

+1

Get a proper amp for those JBL Monitors!

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post #6 of 42 Old 02-17-2012, 12:44 PM - Thread Starter
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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.
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post #7 of 42 Old 02-17-2012, 01:30 PM
 
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Anyways, upon looking into the manual for the amp I found that it apparently handles low impedance loads.

I'm not seeing that.
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post #8 of 42 Old 02-17-2012, 02:33 PM
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Once again, this is a 70.7 constant voltage amplifier. It does not have 8 ohm or low impedance outputs. At least they are not specified on the spec sheet. I have no idea what they mean by saying low impedance loads but it clearly cannot be 8 ohm loads. This is a distribution amplifer and all speakers would require stepdown transformers.

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post #9 of 42 Old 02-17-2012, 08:28 PM - Thread Starter
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Under the "Special Features" section, it says:

Quote:
Low Impedance/High Current Design

The design of the CVA-7400/CVA-7800 is exceedingly conservative with
regard to current capabilities. This ensures that the amplifier's output devices
remain within their Safe Operating Area, even under extreme conditions. As a
result, the CVA-7400/CVA-7800 runs low impedance loads safely while
retaining the high impact dynamics for which they were designed.

Auto Impedance Optimization

The High-Frequency Switch Mode Power Supply and amplifier design used in
the CVA-7400/CVA-7800 adapts to the demands placed upon the amplifier,
enabling them to run with maximum efficiency regardless of the load
impedance. Thermal management problems normally associated with
amplifiers of this capacity are therefore greatly reduced. Resources otherwise
applied toward dealing with thermal problems can be put to better use in
higher quality active components for increased reliability and improved
performance

Am I just interpreting this incorrectly?
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post #10 of 42 Old 02-18-2012, 12:51 AM
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Quote:
Originally Posted by r0ckstar View Post

Am I just interpreting this incorrectly?

Hi RockStar,

Kind-of. When they refer to low impedance, they are referring to being able to support a lot of speaker/transformers in parallel. That amp was designed more for PA use, rather than good listening (note the .1% THD and the low frequency cutoff at 35Hz).

The point of 70-volt systems is that you can add speakers at will, without worrying about impedance matching. Each speaker has the appropriate transformer attached, and as you add transformer/speakers to the system, the current draw will increase, but the power delivered to each speaker remains unchanged. As you add speakers the impedance, as it appears to the amplifier, decreases. How low an impedance the amp can accommodate determines how many speakers you can add.
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post #11 of 42 Old 02-18-2012, 08:04 AM
 
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A 70 volt amplifier maintains a constant voltage and uses sophisticated electronics to vary the current.

No, Ohm would disagree, it simply means that the output voltage is stepped up by a transformer, resulting in lower current through the speaker cables. The voltage is reduced, and current increased, by the step down transformers on each speaker.

Quote:


A 70 volt amplifier, because of its sophisticated internal configuration, can power many more speakers, each at a selectable volume.

The "sophisticated internal configuration" is called a transformer.

Quote:


The standard 2 - 8 ohm amplifier-speaker combination provides a lot more power per speaker.

No necessarily, and has nothing to do with output voltage.

Quote:


There is also a moderately high voltage involved,

Anything over 48V is considered high voltage.
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post #12 of 42 Old 02-18-2012, 12:49 PM
 
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Quote:


one can only imagine the point of the post.

Indeed.
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post #13 of 42 Old 02-18-2012, 03:23 PM
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You can also make any HiFi amplifier into a 70v amp. All you have to do is place a 70v transformer on the amps speaker outputs backwards, that is the amp drives into the secondary and the primary becomes the 70v line. Radio shack used to sell a 100w rated transformer to do just this.

As I said earlier, many modern 70v amps drive the line directly. We have a 150w at work in the phone room and I looked at it. There is no way they have a 150w output transformer in that box. It's too small. It also probably uses a switching power supply. Further proof these are not HiFi amplifiers at all.

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post #14 of 42 Old 02-18-2012, 04:16 PM
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There seems to be some confusion about what the "70-volt" part of the circuit is supposed to do or what it is connected to. 70 volts is the approximate maximum voltage the amplifier puts out with the volume control turned all the way up. It DOES NOT always put out a constant 70 volts. If it did, there would be no way to control the overall PA system volume.

It puts out lower voltages at lower amplifier volume settings. It DOES put out a fairly constant voltage between zero and 70 volts for any given volume setting.

The bottom line is that the speakers you have are not going to be readily usable with that amplifier; let's let it go at that.

If you want to pursue the theory further find a good book on public-address amplifier systems.

The basic concept, though, is that the system is designed for the amplifier to be set at some appropriate level so that it puts outs a fairly high voltage, allowing the total current to be small and the wire used to be small, even for runs of hundreds of feet. Each speaker has a step-down transformer mounted on it with several taps on the primary side and the secondary hooked to the speaker.

The transformer tap of each speaker's transformer that is hooked to the amplifier feed wire determines the turns ratio and therefore the volume of that speaker. In this way many speakers can be hooked in parallel without drawing excessive amplifier current, and each speaker can be at a different volume level; one can be very loud (more Power) for a factory floor and another can be much quieter (less power) for an office, all on one pair of wires from the amplifier. Each transformer has as many as 8 taps to set various power levels as needed for that speaker's usage.

To use a standard speaker with such an amplifier, you would need a very large high-quality transformer rated at 100 watts or so, with the correct step-down ratio to match the speaker impedance to the amplifier's higher output impedance.

This is somewhat analogous to the commercial AC power distribution system, where 12,000 volts goes to local areas and is stepped down to 480V, 240V, or 120V through various-sized transformers having different voltage ratios and power ratings as needed.



Quote:
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

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post #15 of 42 Old 02-18-2012, 05:33 PM - Thread Starter
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Thanks guys, I've received the information I was looking for.
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post #16 of 42 Old 02-18-2012, 07:05 PM
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Quote:
Originally Posted by dragonfyr View Post

Yes, your interpretation in incorrect.


The difference between a 70 volt amplifier and a 'normal' (2-8 ohm) amplifier is that normal amplifiers change their voltage to produce a varying current in the speaker. A 70 volt amplifier maintains a constant voltage and uses sophisticated electronics to vary the current.

.

That is one of the most misunderstood "things" in audio. The 70V is NOT constant.

There is no "sophisticated electronics" involved. Just an very old fashioned transformer. One of the earliest "componets" in electronics.

To vary the current and produce sound, that would mean that the loads would have to be varying their impedance in association with the volume control.

The main reason is that 70V is most common is the highest voltage that can be considered "low voltage". Above that-you need a high voltage license to wire up. 100 and 140V system are also used.

Basically a 70V (or other high voltage system) is a high impedance system. By using high impedance, you can parallel many loudspeakers (that are high impedance due to the use of transformers). A 1 watt tap is 5000 ohms-10 watts is 500 ohms 100 watts is 50 ohms etc.

This allows smaller wired to be used for a particular wattage. Less voltage drop across the wire for the same total power.

In fact you do not need a a step up transformer to get the 70V level. Any amp that can produce 600 watts into 8 ohms is considered to be a direct drive for 70V lines.

By doing it that way- you don't have all the problems associated with an output transformer that large.

I do this all the time to drive 70V lines. Works great. A favorite amp to do this with is the QSC RMX850 in bridge mode.

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post #17 of 42 Old 02-19-2012, 11:29 AM
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Quote:
Originally Posted by dragonfyr View Post



And let's see. In a traditional system we utilize a pot to vary output levels. We just turn a single knob and the levels of ALL of the speakers vary directly. As opposed to, for a given output level of the distribution amp, individually controlling the output of each speaker by the use of a stepdown transformer allowing all sorts of relative output levels on the same distribution line without ever touching the master output level.


:

And if you reduce (or increase) the level on the amplifier-or the source of a "70V" amp (by whatever means you want to), the output voltage goes up and down-just like a regular amp. NO difference.

Again the ONLY difference between a 70V system and a regular amp is JUST the transformer.

In fact many 70V amps have "regular 4-8-16ohm etc) outputs as well as a 70V output. The transformer is simply hooked to the regular output (which is a normal amp) and steps up the voltage.

Now there are a few exceptions -such as the Crown CTS amps-but that is a different topic.

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post #18 of 42 Old 02-19-2012, 11:50 AM
 
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You still beating that horse?

Sure the gain varies! DUH! ...Otherwise it would be rather stupid to incorporate a gain control and/or an on-off switch!

Yes, the ABSOLUTE gain CAN vary!
But, at whatever gain the 'master gain is set, the relative gain differential between the various drivers in the distributed system DO NOT. So you adjust the master gain of the source, and then modulate the relative gain levels of the various sources with a transformer - while the master gain remains 'constant' at whatever level that it is set!

And yes, I am aware that this COULD be done with a traditional system by combining various nominal impedance speakers, but aside from their integration into a single loudspeaker for some reason, it is neither a recommended nor common practice. Or are you now going to now raise anomalous examples such as a Bessel array and cite combining drivers in and out of polarity and in series and parallel as well as an example of a mainstream traditional topology!?

You seem to be the only one here who thinks that's an issue and wants to argue semantics while failing to understand the use of a term with respect to its absolute and relative application.
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post #19 of 42 Old 02-19-2012, 12:48 PM
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Quote:
Originally Posted by Ivan Beaver View Post

The main reason is that 70V is most common is the highest voltage that can be considered "low voltage". Above that-you need a high voltage license to wire up. 100 and 140V system are also used.

And don't forget there is a 25v version as well. The 25v is actually the second most popular version leaving the 100v+ systems for large stadiums.

I read somewhere that the 25v systems are required in schools but I question if that's an NEC mandate.

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post #20 of 42 Old 02-19-2012, 12:53 PM
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Quote:
Originally Posted by dragonfyr View Post

You have to love this forum. Rather than simplify issues from the POV of the OP, we complain when simplified generalizations are used that the explanation is not complex enough, and when the explanation is complex, that the answer is not simple enough.

The OP's question is well addressed in this topic. But there's nothing wrong with others expanding the topic and clarifying the subject in greater detail. This forum also serves as a future reference. And there are frequently posts regarding 70v systems. So the inclusion of accurate information does no harm.

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post #21 of 42 Old 02-20-2012, 04:31 PM
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The amplifier mentioned by the OP is indeed sophisticated. It is a Class H amplifier which means that the supply voltage for the output rails is continuously variable. The amplifier senses the needed voltage for the given signal and constantly adjusts that voltage to a few volts above the required level. This results in a very high efficiency amplifier in terms of power consumption for delivered power output. The tradeoff is typically in distortion as mentioned earlier.
I think one of the things that is important when discussing Distribution Amplifiers, whether they be 25,70 or 100 volt systems is to remember that as Commsysman mentioned, these systems can drive speakers that are hundreds of feet from the amplifier and the IR loss in the those wires does not appreciably affect the performance of the system. This is not the case with a low impedance output amplifier where the resistance of the speaker wires can become significant if those lines are really long.

Sometimes as these topics come up, there is a lot of give and take in terms of technical detail and toes can get stepped on. This at times can be more confusing to the OP but the fact that these are widely read threads, I agree with Glimmie that it is important to ensure that there is technical accuracy in the posts.

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post #22 of 42 Old 03-05-2012, 09:40 AM - Thread Starter
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So I decided to bite the bullet and connect the amp to the speakers, I've got nothing to lose considering these pieces of equipment almost got thrown out by someone else. And....well everything seems to be working fine minus a tiny bit of distortion. Follow ups?
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post #23 of 42 Old 03-05-2012, 01:26 PM
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They are your speakers and your amp. You can do with them as you like. If they are working in this situation for you fine. It would be interesting to know what "a tiny bit of distortion" really is and where you have the volume control set.

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post #24 of 42 Old 03-06-2012, 09:22 AM - Thread Starter
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I have the volume on the amp set at around 50% and tweak it with my mixer. At 100% there is a noticeable hiss. This is all connected to my computer using an m-audio delta 44 interface.
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post #25 of 42 Old 05-10-2013, 08:57 AM
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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

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post #26 of 42 Old 05-17-2013, 06:37 AM
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I have spoken to (2) technicians who were heavily involved with building and servicing PA system type amps, and one of them also was an installer in large stadiums, hospitals, churches, etc. for 70 volt systems. What we are talking about here is a transformer coupled output. These PA type amps are not direct coupled, and there isn't 70 volts at the output taps at all times.

The voltage ratings for the various taps are for MAXIMUM VOLUME and output from the amp's output transformer on the secondary side- when it's turned all the way up. If there are 4, 8, 16, 32 ohm taps, the maximum output voltage of the secondary also goes up as well- that's why many of them are labelled both with ohm rating and votlage rating.

What happens is, at the lowest ohm rated taps, the full voltage output of the amp is not available. That's why you will see a "25 volt" or "35 volt" tap on an amp with lower ohm ratings like 4 ohm, 6 ohm, 8 ohm, but the maximum voltage tap will be 70 volts.

all those taps refer to voltage output AT FULL VOLUME.

I recently obtained a "70 volt" PA amp that also had 6 ohm/8 ohm, 25 ohm/32 ohm taps, that were also labelled 35 volt and 70 volt correspondiingly.

I set it up with a stereo tape input, and Sony pre-amp from a mini system, and ran stereo into the pre, pushed the "mono" button on the pre, and ran mono from one channel of the pre (now containing the stereo mix albeit in mono) to the amp.

First I connected an 8 ohm speaker and played it via the 35 volt tap, then connected (2) 8 ohm ohm speakers in parallel for about 4 ohms and played it from the 35 volt taps.. Then I connected two 8 ohm speakers in series for a total of 16 ohms and played it from the 35 volt taps, then finally played the 16 ohm speaker load through the 70 volt taps.

The amp played strong and loud from all taps no matter what speaker load was on it, but the fidelity was best with the 16 ohm speaker load on the 70 volt taps- which only makes sense, being with those connections the amp is harnessing its full output potential.

Many of these amps were designed to be connected to a long line of speakers and turned up to MAXIMUM VOLUME the full 70 volts, then the local speaker volumes controlled remotely.

Nothing will blow up, short out, etc. if it is transformer coupled on the outputs. The old PA amps with the HUGE output transformers like above in the pciture, were designed to keep right on playing and not damage the amp and protect it, even if a cable got frayed, shorted, cut, a bird or squirrel chewed through it at a stadium, etc. That's why they were built that way. They were made to keep playing through a cut up damaged line and not damage the amp, and protect it, hence the large transformer coupling on the speaker output. Much more forgiving, much safer.

A direct coupled amp will have better sonics but one touch of the wrong speaker wire and "POOF" it takes out all the final amp transistors in a row. There's no protection.

so to answer the OP, yes you can run an 8 ohm speaker on the 70 volt line, but it will sound a LOT better if you run the ohm rating needed for the 70 volt line, to attain the output in watts the amp was rated for. The field tech told me that right out, volts x volts/ohms = watts so adjust the ohm load of the speakers to attain the equal the watts output rating of the amp- I was already doing that because it was only common sense- per my previous posts.

The amp will sound better on the 70 volt line with higher ohm speaker loads- and it will play much louder without distortion. I have taken a few videos of the tests on the various taps and may upload them to Youtube if need be, and post the links here. You will be amazed at how good this mismatch sounds, using an old tape as a source.

70 volt line ? That is a misconception- that is merely the maximum rating with full input and full output through the amp. It's just a label- because in reality I measured the LINE VOLTAGE while playing the amp VERY LOUD at over half volume on the amp, and about "4" on the pre-amp- , and there was only 2 to 4 VOLTS measured coming from the output to the speakers using the 70 volt output tap line and 16 ohm speaker load, measured with my Fluke digital voltmeter in AC mode. I was filling a 28 x 14 listening room with about 90 db with only about 1 watt or less.

the guys that said this wouldn't work, just haven't tried it themselves. Yes you can play a PA amp through home audio speakers on the 70 volt taps. But you're going to need at least 16 ohms on each amp, and they are usually mono amps- so you will need 2 amps to get stereo.

It's just a lot easier to use a dedicated consumer stereo amp. But if you get the PA amps for free or dirt cheap, go for it.

FWIW, the Sony pre-amp also has a MIC IN jack and mixer knob to adjust the mic level. I plugged my Fender Strat into it, and the damn thing CRANKS. I plugged it into a tube pre and then into the PA amp and really cranked it, overdrive and sounded really cool with a mellow tube distortion.

just like a guitar amp.

amps are amps, nothing mysterious about them
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post #27 of 42 Old 05-17-2013, 06:56 AM
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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


put the (2) 100 watt speaker cabs in series, connect to the 70 volt line, and input your signal via a pre-amp that has tone controls, filters, loudness button, etc. and the amp will play. If it's transformer coupled you won't blow up anything. Yes it will play and you may even be amazed at how well it plays. If you want to use these old PA amps in stereo, you'll need 2 of them, one for each channel.

you'll find that it will be quite loud and clear with only about 3 or 4 on the pre-amp volume, and 5 on the main PA amp. Enough to go deaf with.

The line PA amps prefer and are designed for higher ohm rated speakers. To get the most out of that setup, you should have 24 ohms of speaker loading on it.

using a 70 volt line and 200 watt output, this is the formula

70 x 70 = 4900 / 24.5 ohm speaker load = 200 watts output

back into the speaker load needed by using the voltage and watts output numbers

4900/200 - 24.5 i.e. add one more 8 ohm cab in series

it would also play with a 32 ohm speaker load i.e. 4 cabs, each 8 ohms
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post #28 of 42 Old 06-26-2013, 08:29 AM
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update on this thread, here's a video mpeg, pair of Altec Lansing PA amps playing through 8 ohm speakers, using a vintage 8 track tape cartridge deck as a source.

http://www.youtube.com/watch?v=j54bjapp_5U


those of you who stated that a PA amp won't work for home hi-fi, simply NEVER TRIED IT.

next time do the R&D, before posting a reply
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Let it go, pumpkin...the question was answered 4 months ago.
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post #30 of 42 Old 06-26-2013, 01:02 PM
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Originally Posted by wall-of-sound View Post

update on this thread, here's a video mpeg, pair of Altec Lansing PA amps playing through 8 ohm speakers, using a vintage 8 track tape cartridge deck as a source.

http://www.youtube.com/watch?v=j54bjapp_5U


those of you who stated that a PA amp won't work for home hi-fi, simply NEVER TRIED IT.

next time do the R&D, before posting a reply

I don't see where anybody outright said it would not work, that is produce sound that may in fact be quite acceptable. But it's not the ideal nor the proper way to interface 8ohm speakers to 70/25v amplifier circuits.

There are basically three types in the electronics world as I see it:

1) The EE. This person has studied the theory, mathematics, and physics of how electrical circuits work. Some never leave the research library and only postulate ideas but never design or produce anything. Then there are those that take that learned theory plus new ideas and go onto to create viable products. The world needs both types here.

2) The professional technician. This person does not have the deep scientific theory but does listen to those that do and who designed the equipment. They also learn why things are done a certain way and often acquire just as much or more knowledge over their career as a formally educated engineer.

3) The hack. This person knows nothing about electrical circuits nor do they wish to learn. They simply hook things together and see what happens. Sometimes they are successful. Other times they get smoke and flames. Their limited success record tends to make them "experts".

Now where do you fit in? I'm thinking #3!

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