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Author Topic: 70 Volt questions  (Read 11793 times)

Jeff Heart

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70 Volt questions
« on: September 17, 2007, 12:56:03 am »

I found a site that explained the use of a regular 2, 4, 8 ohm etc... amplifier for powering 70volt systems.

We do sound for a venue that has the main system on a 70v amp but they do not have a back up amp.  If it goes down, the night is over!  This led to me research which stumbled me upon this article :

 http://www.yorkville.com/default.asp?p1=6&p2=17&p_id =73

The lower half of that article details some formulas and methods to use regular power amps without the 70v transformer.  My question is to see if some of you math gurus can validate these methods and give a little further insite into what is left fuzzy still my head, due to a few vague areas in the article.

For example sake, I have 9 70v speakers in which 5 are tapped at 60watts and the other 4 are tapped at 30 watts.  420 watts total.
System impedance is 11.66 (based on formula from that link above)

Amplifier is a Crown MT 2400VZ which at 2,4 and 8 ohms stereo will NOT yield a voltage output close enough to 70v.  However, bridged mono, it's specs are 1650watts at 8ohms which will yield a bit over 114volts (again, based on math from what I read at above link)

Does this mean I can properly power the speakers with the Crown amp since my total speaker wattage is 420, the amp is at 1650 watt and the volatge calculation is above 70?  OR does the votage calc have to be right at 70v and none higher?
 The speakers have a 100v mode too which is auto sensing so I assume I would have problems since the 114v calculation is above the 100v transformer tap rating as well.

Maybe to help make any answers easier, we can keep it real simple and just say, "can I use an amp like a macrotech without a transformer as long as the voltage output calculation is greater than the 70v speaker system, and the wattage is greater than the speaker systems total wattage OR does the voltage yield have to fall on the button of either 70 or 100, just as the speaker transformers expect?"
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Mac Kerr

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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #1 on: September 17, 2007, 01:09:53 am »

Speaker transformers don't "expect" anything. The main thing you lose by using an amp that can't put out 70V into the system impedance you have is the ability to drive the system to its maximum volume. When you are playing low level background music that 70V amp isn't putting out 70V either. When it does put out 70V, each of the speakers will be receiving about the power it is tapped at.
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Jeff Heart

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70v amp question...part 2
« Reply #2 on: September 17, 2007, 01:22:46 am »

Is it true that with a 70volt amp, you do not need to calculate for headroom (other than line loss and transformer efficiency), and that you can literally load on the speakers until you have used up the entire rated wattage of the amp?

For example, a 70v amp rated at 500 watts per channel stereo.  I can tap 5 speakers at 100watts each and use up all 500 watts without worrying about the amp needing more juice for peaks, etc...? (again, taking line loss and transformer efficiency out of the equation)

I'm curious because the venue I reffered to in my other 70volt post has a system in which was just put in very recently and it had an issue the other night.  I wasn't there so I dont know exactly what they heard but... here is the scenerio :

THey have a Crown CH4 70v amplfier capable of 500 watts per side stereo.  THe one side has 9 JBL Control 266 CT's on it.  5@60 watts and 4@30 watts, with a total of 420 watts. 80 Watts of headroom to allow for line and transformer power loss.
   They called me and said that last night they heard some pops at one point and then it sounded like the first speaker in the chain was letting low frequenices through (despite crossover settings) and then randomly getting lower in volume and then louder again, along with sounding muffled.  He said the other speakers in the chain were following suit with different less abrupt degrees of issues.  He said the amp wasn't clipping ( i was on the phone with him during the incident) and the levels of the system were all in the green, no limiting or compression taking place.  I had him turn down the other amp channel and he said the problem seeemed to go away.  Considering the other channel is independent of the problem channel, I think it was just coincidence that the problem stopped.
 At my test there today, all speakers appeared to be fine with no signs of noise, pops, or any indicators at the amp.  However, I could not turn it up (it was dinner hours) to see if it was happening when the amp was serving up some real power.
 This leads me to this purpose of this post which is to ask if anyone knows what happens when a 70v amp is pushed to it's max potential or if it is loaded up with speakers that have a combined wattage that is greater than the limit of the amp channel they are on.  Even though in this case as I said earlier, it is 80 watts under the channel total.  DOes any of the symptoms above sound like anything that could happen if the amp is pushing it's limit or would that cause something entirely different?  I'm gonna check out the Crown CH4 manual as I know it will list the protection features for this particular amp but I'm assuming that it would do many things to protect itself other than the wacky issues described above.


thanks as usual,
jeff
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Jeff Heart
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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #3 on: September 17, 2007, 02:05:43 am »

Not to side track my original questions but, based on what you said, it led me to one other question.  Since the transformers have 70v and 100v settings, but only 1 input....how does the transformer compute which setting I want?  Does it use the 100v setting after the amp is putting out more than 70v?  When the selector on the speaker is pointing to say 60 watts on the 70v side, it is also simultaneously pointing to 30 watts on the 100v side (i'm not certain about those numbers, it's not infront of me).  SOmeone told me in the past that it knows what is being sent to it and uses the setting of the supplied voltage.  WHich is why I referred to thinking it "expected" something.  
  Maybe the selector is not pointing to both sides at the same time and rather just looks that way.  Even still, it seems redundant then to even have a 100v 60 watt setting AND a 70v 60 watt setting.  Why would I ever need the 100v setting if I can reach the same max power output at 70v?
    Again, this is a bit side tracked so I hope any responses don't neglect my original questions but perhaps just add this one to the mix.

Mac Kerr wrote on Mon, 17 September 2007 06:09

Speaker transformers don't "expect" anything. The main thing you lose by using an amp that can't put out 70V into the system impedance you have is the ability to drive the system to its maximum volume. When you are playing low level background music that 70V amp isn't putting out 70V either. When it does put out 70V, each of the speakers will be receiving about the power it is tapped at.

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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #4 on: September 17, 2007, 10:14:40 am »

A quick rough calculation of the total system impedance is 11.9 Ohms. That makes the short answer to your question, Yes, you can use an amp like the Macrotech without a transformer.

This is based on a 30w 70V tap being 163.3 Ohms and a 60w 70V tap being 81.6 Ohms

Check my math before you hook anything up. I don't have a lot of confidence in my own math.

These things are nice for guys that don't work with the formulas enough to know them by heart:
index.php/fa/11365/0/


I am not so sure about those "auto sensing" speaker taps. I'd take a closer look at that.

A 25V, 70V, or 100V speaker distribution system is basic Ohms Law stuff, and not nearly as mysterious as what some people make it out to be.
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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #5 on: September 17, 2007, 10:45:29 am »

Hey Jeff,

This is not rocket science, but does require some math as Dave noted above.  I direct drive 70 Volt all the time.  I often explain this little slice of audio as a lawn sprinkler system, which works just fine as long as you keep the pressure (voltage) up and don't draw too much water (watts) from the system.  Our rule of thumb is to use an amp that can generate more than 700 watts into an 8 ohm load.  For large scale systems, you do need to figure in the wire as part of the load that the amplifier is seeing.  If you do not, your calculations for actual delivered SPL may be off.

Ken
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Jeff Heart

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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #6 on: September 17, 2007, 12:56:16 pm »

Ken Freeman wrote on Mon, 17 September 2007 15:45

Hey Jeff,

This is not rocket science, but does require some math as Dave noted above.  I direct drive 70 Volt all the time.  I often explain this little slice of audio as a lawn sprinkler system, which works just fine as long as you keep the pressure (voltage) up and don't draw too much water (watts) from the system.  Our rule of thumb is to use an amp that can generate more than 700 watts into an 8 ohm load.  For large scale systems, you do need to figure in the wire as part of the load that the amplifier is seeing.  If you do not, your calculations for actual delivered SPL may be off.

Ken


Ok, so for purely example sake, could I take an amp that is waaaay over rated such as an Itech 8000 and then, make sure the total wattage of all speakers on the channel do not surpass the total wattage of the connected amp channel at the 8ohm spec, send a pink noise signal through the system and then measure voltage on the output.  Slowly turn up gain on the amp until voltage is about 70v and then call it a day and be sure the amp is not "supplying too much water" as you put it?
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Bob Lee (QSC)

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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #7 on: September 17, 2007, 01:31:14 pm »

No, transformers don't "compute" anything and can't change ratings around. Basically, a transformer changes an input voltage to an output voltage. The ratio of output voltage to input is equal to the ratio of the number of turns of wire in the secondary and the primary windings, respectively.

A 70-volt amp can put out a maximum RMS voltage of about 70 volts. (How much current it can put out at 70 volts determines its 70V power rating.) A power amp rated at about 600 to 650 watts into 8Ω, or about 1200 to 1300 watts into 4Ω, will fit this description.

A 100-volt amp, similarly, can put out a maximum RMS voltage of about 100 volts.
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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #8 on: September 17, 2007, 02:13:38 pm »

Yes, that would be watering the garden with a Fire Hydrant.

Far and away, my favorite 70 Volt amp is an EX-4000....or MLA-1100 in a another form.

Will you build me 12 more on a special run?

Ken
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Jeff Heart

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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #9 on: September 17, 2007, 02:29:17 pm »

Ken Freeman wrote on Mon, 17 September 2007 19:13

Yes, that would be watering the garden with a Fire Hydrant.


Ken


But I could do it right?  Despite Niagra Falls power, as long as I measure the output of the amp with pink noise and turn it up until it reaches 70v, and no more?
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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #10 on: September 18, 2007, 02:25:31 pm »

Another thing to consider, and this may be ancient history by now, but some of the older solid-state power amps didn't deal very well with the fact that a constant-voltage system is a more reactive load, with all those transformer primaries in parallel.

Bob Lee might be a good resource on this, but I remember in the old days blowing up some solid-state amps because they would go into HF oscillation on such loads.

It might be worthwhile to throw a scope on the line and watch for any spurious oscillation that't not related to the signal.

I suspect that the newer Class D stuff would be immune from this sort of thing, though.

Just a thought.
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Re: 70v amp question...part 2
« Reply #11 on: September 18, 2007, 03:04:48 pm »

Jeff -

I spent 17 years with a large systems contractor here in St. Louis, and we did a lot of constant-voltage sound system installations - 25 volt systems for educational and healthcare, 70 volt systems for commercial/industrial/retail, and even 200 volt systems for large places like racetracks and airplane factories.  That being said, I'd be happy to share my $ .02 with you.  I'll apologize up front if I'm telling you things you already know.

In a constant-voltage system, the rated voltage is the maximum voltage that the amplifier can cleanly produce at the rated load.  Bear in mind, "clean" audio for PA/background music probably is around 2-5% THD, and the dynamic range of this material isn't nearly as wide as concert audio or any type of live performance.  If you're feeding this system with live music, you're going to want a good quality limiter feeding the input to ensure that you don't overdrive the system beyond the 25/70/200 volts that it can produce.

Headroom is achieved by tapping speakers (selecting the appropriate wattage secondary winding on the speaker) to provide a good level when nominal audio signals are played, and then ensuring that you never exceed the rated voltage on the primary sides of the speakers/amplifier output. In no case do you want to put more than the rated voltage into the primaries of the transformers;  it may or may not hurt the speakers, but would most likely damage the transformers themselves.  (I've seen many of them fried over the years.)

Any of today's commercial ampflifiers should be able to run the full rated load with no problem;  there is no derating for transformer loss.  The transformer just causes some of that power to be turned into heat instead of sound.  Of course, it never hurts to run any amp below it's rated capacity.

One common problem with constant-voltage sound systems is when someone has improperly wired an L-Pad to control a local speaker.  The most common error is that it's wired so that it puts a dead short on the secondary of the transformer, which reflects right back to the primary/amplifier output side.  On some occasions, I've seen them improperly wired across the primary side itself. These can often be difficult to locate, as the users may change the setting before you come out to troubleshoot, so check them for proper operation and wiring.

Also, check for non-transformer coupled speakers that someone may have added to the system - they can be loads of fun.

If you're going to do much work with constant-voltage systems, I'd highly recommend you invest in a commercial Z-meter, which measures the impedance of the load and is calibrated directly in wattage ratings for ease of use.  The "easy" math for 70.7 volt systems is to remember that power = voltage squared/impedance, and conveniently, 70.7 squared is 5000.  So a 500 watt 70 volt amplifier doesn't want to see less than a 10 ohm load.  The Z-meter will "see" through the transformers and show you the true load, helping to find those shorts, bad speakers, and miswired controls, as well as knowing the real load on the system.

I know the McMartin and Toa both used to make these;  I've been out of that business for 15 years, so I'm sure there are others by now.

Hope this helps!
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Re: 70v amp question...part 2
« Reply #12 on: September 18, 2007, 05:38:24 pm »

Jerry Turnbow wrote on Tue, 18 September 2007 12:04

The "easy" math for 70.7 volt systems is to remember that power = voltage squared/impedance, and conveniently, 70.7 squared is 5000.


I learned that same shortcut from Ted Uzzle.

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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #13 on: September 18, 2007, 05:44:06 pm »

Perhaps even more important than load reactance is that if the speaker transformers saturate, they may put a voltage spike on the line when the cores un-saturate.

This phenomenon tends to happen mostly on lightly loaded lines, and it used to frequently take out some older direct-drive 70-volt solid-state amps that either didn't have flyback diodes to absorb the spike or had inadequate ones.
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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #14 on: September 18, 2007, 05:56:07 pm »

Bob Lee (QSC) wrote on Tue, 18 September 2007 22:44

Perhaps even more important than load reactance is that if the speaker transformers saturate, they may put a voltage spike on the line when the cores un-saturate.

This phenomenon tends to happen mostly on lightly loaded lines, and it used to frequently take out some older direct-drive 70-volt solid-state amps that either didn't have flyback diodes to absorb the spike or had inadequate ones.



That's why the old tube-type paging/intercom systems had a relay contact that actually shunted the output transformer of the all-call amp when the system was idle.  That flyback voltage could actually damage those amps!  Not to mention give the service guy a good jolt if he happened to be across the terminals when the page was completed.  (I blew up several early JBL and McMartin solid state amps before I found that shunting relay!)

(Yeah, I'm that old!)
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Jeff Heart

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Re: 70v amp question...part 2
« Reply #15 on: September 18, 2007, 07:41:34 pm »

That definitely helped some.  Thanks.  Also, I just noticed after revisiting the specs for the Crown CH4, that it is not rated at 500 watts at 70v.  Rather it is rated for 70volt mode to deliver 1200watts per channel.  4.16ohms minimum based on the calculation method you gave me.
 The channel the speakers are on which had the problem is loaded down to approx. 11.9ohm (9 speakers - 5 at 60watts and 4 at 30 watts = 420 watts)
  The amp is ran after a DBX Driverack with limiting in place that is successfully protecting the amp from going into clipping and a crossover is in place with a high pass set to somewhere above 90hz.  I wired the system so I can personally assure proper cabling (with 16 awg wire and no more than 75 feet) and can rest assured there are no other speakers on the line.
 This leads me to believe it is something other than the amp reaching it's limit.  Hmmm, maybe I tapped one of the speakers at 8ohms and didnt even notice it.  It has a selector switch so it's possible I turned it too far.  Very doubtful but I will check.  They havent had the issue since that first time but I won't assume that it is not going to happen again.  It's bound to resurface sooner or later unless they just have no clue what they were talking about.  As far as I know, it could have even been a problem in the DJ's crappy MP3 burnt discs.  Maybe some bad recording in the one channel.
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I heard the problem the owners were talking about
« Reply #16 on: September 23, 2007, 01:44:59 pm »

I was at this venue the other day when the 70v speakers did indeed reveal an issue just as the owner said.  It wasn't quite like he described but what is happening is that all the speakers on channel 1 lose all the top end, sounds very muffled, and they get real low in volume.  Every few seconds, they exhibit a popping sound as they cut in and out, sounding like static as if someone was taking either the plus or minus connection and repeatedly rubbing it against the speaker contact and then removing it.    It happened twice the other day at low volume so I now know it wasn't the amp clipping.  It's not the dj's recordings as it happened on a a few input sources.

One thing I did notice was that the driverack 260 had the outputs to the 70v amp crossed over at 44Hz - 18Khz.  I had it set at 70+ Hz when the system was first tuned so apparently someone messed with it.  Could that be the culprit?  Does the problem described above sound like a transformer saturation problem?  I know they don't like frequencies below 70hz too much.

Also, on Channel 1 there are 7 speakers.  5 of which are fed directly from the amps channel 1 output and the other 2 are fed from the same channel but are first fed through a 100w Atlas AT-100 attenuator.  Those two speakers are tapped the 30watts each.

My best guess at this point is that it was saturation from the lower frequencies.  My second guess is that perhaps there is a shorting issue with the attenuator or perhaps on the feed to the 5 other speakers. The issue effects all speakers on the channel.

Only thing that wouldn't make sense to me as far as the saturation issue is concerned, is that channel two was fine.  Both channels were receiving the 44Hz - 18khz passband so I would  think all the speakers should have had the problem if saturation were the case?   I don't have enough experience with 70v speakers to know what saturation issues sound like.

Since it is intermittent, it's hard to trouble shoot as it doesn't last long enough for me to run tests.  It did it for about 5 minutes the other day and then once again for less than a minute a few hours later.

Since the 70v system runs 80% of the bar/club, it is the primary sound source and if the amp dies, it's not going to be pretty.  So I am a bit concerend about finding the issue instead of sitting back and saying, "well, it seems to be gone now".  I'm wondering if the problem is a pre-warning sign to some approaching full failure. I'm thinking about removing the x-over and letting all frequencies through at low volume to see if it recreates the issue.  If it does, then I know it was only the low frequency issue and I can rest easy.  However, can that test damage the amp or speakers for using intentional saturation or is that an OK test as long as I do for a few seconds only at low volume?

I haven't heard the issue since I changed the highpass crossover back to 70Hz and that was three days ago.
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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #17 on: September 23, 2007, 02:38:06 pm »

Bob Lee (QSC) wrote on Tue, 18 September 2007 14:44

Perhaps even more important than load reactance is that if the speaker transformers saturate, they may put a voltage spike on the line when the cores un-saturate.

This phenomenon tends to happen mostly on lightly loaded lines, and it used to frequently take out some older direct-drive 70-volt solid-state amps that either didn't have flyback diodes to absorb the spike or had inadequate ones.




Yes, as Master Bob has noted, it can get a little squirrelly if you don't give the amplifier something to drive.  I have always found that putting a 1000 feet of 14 gauge cable (about a 4 ohm load) seems to settle things down.  Due to the odd venues that I end up in, I often build these systems while they are hot so we can check them as we go...Then just remove the dummy load and continue to the next system or branch.

Ken
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Re: I heard the problem the owners were talking about
« Reply #18 on: September 23, 2007, 03:00:03 pm »

Lock down the DriveRack.  I had a club install where they kept blowing up 15s.  The rig was for syndicated foreground music.  They kept it pretty loud, but not pushing it hard enough to clip, or even make it sweat hard...  it turned out the cleaning crew would crank the system far louder than the bar crowd would tolerate.  All the way up...

They shouldn't have been anywhere the controls were located, but they did anyway.  We put in a locking rack and disabled the remotes behind the bar... the maintainence issues went away.

Someone thinks those little speakers can do car stereo bass.  Lock down the DR or call a clergyman for an exorcism if the lockout doesn't work.

Have fun, good luck.

Tim Mc
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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #19 on: September 23, 2007, 03:10:07 pm »

Ken Freeman wrote on Sun, 23 September 2007 19:38


Yes, as Master Bob has noted, it can get a little squirrelly if you don't give the amplifier something to drive.  I have always found that putting a 1000 feet of 14 gauge cable (about a 4 ohm load) seems to settle things down.  Due to the odd venues that I end up in, I often build these systems while they are hot so we can check them as we go...Then just remove the dummy load and continue to the next system or branch.



I'm a bit confused by your response.  Is "yes" answering my question about damaging the amp if I do the saturation test, or is it "yes" about the 40 - 70 Hz bands being able to cause the problem,  OR about something earlier in the post?
  The amp is definitely driving something....7 speakers on channel 1 which is perhaps a light load? additively 360 watts.
The cable is 16 gauge with a total length of approx. 250ft to all speakers on the channel (250 ft. total, NOT 250 between each speaker).  So, does it seem possbible to exhibit the problem mentioned with that scenerio due to saturation?  THe amp is rated at 1200watts per channel in 70v.    360watts would indeed seem light.  IF that were the case, I could move over some speakers from channel 2 but then channel 2 would be light.  Both channels are already "light" so I am assuming that is not the issue.
Ken
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Re: I heard the problem the owners were talking about
« Reply #20 on: September 23, 2007, 03:13:04 pm »

Tim McCulloch wrote on Sun, 23 September 2007 20:00

Lock down the DriveRack.  I had a club install where they kept blowing up 15s.  The rig was for syndicated foreground music.  They kept it pretty loud, but not pushing it hard enough to clip, or even make it sweat hard...  it turned out the cleaning crew would crank the system far louder than the bar crowd would tolerate.  All the way up...

They shouldn't have been anywhere the controls were located, but they did anyway.  We put in a locking rack and disabled the remotes behind the bar... the maintainence issues went away.

Someone thinks those little speakers can do car stereo bass.  Lock down the DR or call a clergyman for an exorcism if the lockout doesn't work.

Have fun, good luck.

Tim Mc



Yup, done that.  I usually always lock it once set but I didn't do it this time. It's set back to 70Hz HPF and password protected now.  If that was the issue, it's fixed now but I still need to know for sure.
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Jeff Heart
Wiggle Productions LLC
A/V, Lighting, Photo, Install, Rentals, Sales

Ken Freeman

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Need more info...time for a drawing of your plan.
« Reply #21 on: September 23, 2007, 04:32:24 pm »

Hey Jeff,

Please make a drawing of your set-up.  Either post it, or PM it to me.  Include speaker models, taps you propose to use, cable diastances and sizes and the make and model of your amplifiers.  I think this could be really easy, but don't want to lead you down the part of bad audio by mistake.

Ken
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