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Author Topic: 70 volt auxilliary zone has low volume  (Read 33344 times)

Matt Carr

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70 volt auxilliary zone has low volume
« on: January 03, 2011, 10:37:07 pm »

I recently became involved in servicing a 13 year old Church audio system that consists of a Mackie 1604-VLZPro and a 70 volt auxiliary zone that feeds the main mix to offices, basement gathering, narthex, and entry areas.  Aux is powered by an Architectural Acoustics UMA1502 amp that seems like it would have enough power to serve 10 speakers tapped at 5 watts each, but I just can't get enough output to achieve an acceptable listening level.  When I adjust the input gain the amp starts to clip.  When I adjust the output gain the amp starts to clip.  Switched the input to be controlled by sub fader 1 for more input gain flexibility but amp still shuts down after clipping, all the while never producing an acceptable output level. Tried different inputs on the amp but same outcome.  So far I have tried using the main mix output and sub-fader output with no success.  
Since the company that installed the church system is no longer around I'm trying to help out a very good customer with their issues.  Any suggestions?
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Lee Buckalew

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Re: 70 volt auxilliary zone has low volume
« Reply #1 on: January 03, 2011, 11:40:52 pm »

Matt,
Having enough amplifier power to power the speakers tapped at 5 watts tells you nothing at all about how much SPL the speakers can provide with a 5 watt input.
The critical factor is going to be the tap, at the proper voltage, combined with the speaker efficiency.

What is the speaker make and model?
Are there any volume controls in between the amp and speakers?
What are the speaker cable run lengths and what gauge are they?
(Cable run length and gauge probably won't matter unless something is way off there)

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Mac Kerr

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Re: 70 volt auxilliary zone has low volume
« Reply #2 on: January 03, 2011, 11:43:39 pm »

Matt Carr wrote on Mon, 03 January 2011 22:37

I recently became involved in servicing a 13 year old Church audio system that consists of a Mackie 1604-VLZPro and a 70 volt auxiliary zone that feeds the main mix to offices, basement gathering, narthex, and entry areas.  Aux is powered by an Architectural Acoustics UMA1502 amp that seems like it would have enough power to serve 10 speakers tapped at 5 watts each, but I just can't get enough output to achieve an acceptable listening level.  When I adjust the input gain the amp starts to clip.  When I adjust the output gain the amp starts to clip.  Switched the input to be controlled by sub fader 1 for more input gain flexibility but amp still shuts down after clipping, all the while never producing an acceptable output level. Tried different inputs on the amp but same outcome.  So far I have tried using the main mix output and sub-fader output with no success.  
Since the company that installed the church system is no longer around I'm trying to help out a very good customer with their issues.  Any suggestions?


When you measure the impedance of the speaker line attached to the output of the amp, what does it read? The amp can drive 150W worth of speakers, and a 5W tap should get reasonably loud, it seems that something is wrong. It could be a short in the line, or a very high impedance connection, or too much very small gauge wire.

The 5W tap on a speaker should be around 1000Ω impedance, 10 of them in parallel would be about 100Ω. This assumes AC volts, so it will be a little less measuring resistance with a VOM.

Mac
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Brad Weber

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Re: 70 volt auxilliary zone has low volume
« Reply #3 on: January 04, 2011, 08:48:08 am »

It probably makes sense to confirm the wiring and connectivity on the mixer/amp even if only to verify that it is correct.  The input clipping makes me wonder if they maybe have the Mackie output run into an input on the Peavey that is either intended for or set for mic level.  If the feed from the Mackie is stereo line level then there are a set of stereo RCA connectors for Input 4 on the mixer/amp.  If the signal from the Mackie is a mono line level then you have several options on the Peavey.  You could use Input 1 but because it is line level the wiring should connect to the 'TEL' connections on the input connector and the 'MIC/TEL SENS' switch should be in the TEL position.  Alternatively, you could connect to Input 2 with the 'MIC/LINE SENS' switch in the LINE position or to Input 3 or 4 In.

Similar on the Peavey output, clipping there could indicate a mismatched load or improper wiring.  For a 70V system there should be a metal jumper in place between the 'XFMR' and 4Ohm/25V terminals with the speakers then wired with +/red to the '70V' terminal and -/black to the 'COM' terminal.

If all the connections and switch settings on the back of the Peavey mixer/amp are correct then you may need to start looking at the speaker wiring.
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Matt Carr

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Re: 70 volt auxilliary zone has low volume
« Reply #4 on: January 04, 2011, 10:46:44 am »

Good morning everyone and thanks for the fast responses. I'll try to answer all the questions but I'm on my phone so bear with me here.
What is the speaker make and model?  Didnt record the make but they look like an old $10 radio shack wallmount from 20 years ago. Only the narthex is a new 8" atlas sound ceiling speaker.
Are there any volume controls in between the amp and speakers? Yes. Each speaker has it's own volume control. I bypassed a few of them with no improvement in spl.
What are the speaker cable run lengths and what gauge are they?  About 100-200' on what seems to be 20-22awg. Definitely not 18awg. This seemed small to me.
On my last visit I made a new jumper from subfader out 1 to input two of the amp. Input two is set to line.
Verified the amp has the jumper in place on the output. All speaker transformers are connected to the 70v side.
The real tricky side to this is the church claims the aux zone used to operate real loud until they had a guy come and adjust the eq settings on the inputs and the aux send outs that are being used for monitor amp inputs. Nothing physical was changed. I was a witness to the eq adjustments.
I'm considering running a temp piece of 16awg to the closest speaker and eliminating the rest of the speakers down the line to see if spl improves and then connect one by one to see if I can pinpoint the problem.
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Gary Creely

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Re: 70 volt auxilliary zone has low volume
« Reply #5 on: January 04, 2011, 11:21:45 am »

One thing that comes to mind is the possibility of a volume control buried somewhere.
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Jonathan Johnson

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Re: 70 volt auxilliary zone has low volume
« Reply #6 on: January 04, 2011, 11:52:51 pm »

If you are getting clipping/distortion you might try inserting a 20dB pad at the input. Just a thought.
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Matt Carr

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Re: 70 volt auxilliary zone has low volume
« Reply #7 on: January 05, 2011, 07:21:41 am »

thanks guys.  I will be there saturday and update any new developments.

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Sidney.Pilien

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Re: 70 volt auxilliary zone has low volume
« Reply #8 on: January 05, 2011, 08:52:44 pm »

Matt Carr wrote on Tue, 04 January 2011 10:46


The real tricky side to this is the church claims the aux zone used to operate real loud until they had a guy come and adjust the eq settings on the inputs and the aux send outs that are being used for monitor amp inputs. Nothing physical was changed. I was a witness to the eq adjustments.



If the system was loud and nothing was physically changed, the system must have been installed properly (correct wire size, speakers, connections, etc) so the only fault has to be at the changed settings, that is, if the system lost power immediately after that.

The speaker wire size sounds correct. The system runs higher voltage-low current  at high impedance for long wire runs and less heat so increasing the wire size may worsen the problem.

Hope this helps.
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Mac Kerr

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Wire size
« Reply #9 on: January 06, 2011, 01:20:33 pm »

Sidney Pilien wrote on Wed, 05 January 2011 20:52

The speaker wire size sounds correct. The system runs higher voltage-low current  at high impedance for long wire runs and less heat so increasing the wire size may worsen the problem.


Huh? Care to explain that bit?

Mac
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Matt Carr

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Re: 70 volt auxilliary zone has low volume
« Reply #10 on: January 06, 2011, 06:44:13 pm »

Sidney Pilien wrote on Wed, 05 January 2011 19:52The speaker wire size sounds correct. The system runs higher voltage-low current  at high impedance for long wire runs and less heat so increasing the wire size may worsen the problem.


Sidney, why would it be worse?  Is it because in a sense I'm switching it from a high impedance to a low impedance system if I use 16awg wire, therefore, making the amp work harder to push current through that much wire?  I've always been taught that undersized wire meant more resistance and more heat which is exactly what you don't want your amp to have.
I certainly don't consider myself a pro sound guy so I'm just asking the question.
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Lee Buckalew

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Re: 70 volt auxilliary zone has low volume
« Reply #11 on: January 06, 2011, 08:29:51 pm »

Matt,
Larger wire is not worse for a 70 volt (or 25 volt or 100 volt) system.  Different rules are applicable that allow for the use of a higher gauge (smaller conductors) but it does not require the use of higher gauge wire.


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Frank DeWitt

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Re: 70 volt auxilliary zone has low volume
« Reply #12 on: January 06, 2011, 10:28:32 pm »

There is a lot of confidence that nothing is wrong (nothing was changed, it just got quiet)  This reminds me of the joke we told while working on cars.  Has it got gas, yes, spark, yes compression, yes,  GET OUT OF THE WAY it must be running. Of course it never was.

So, something changed, or the gain structure is messed up.  If it was me, I would disconnect the speakers and put one 8 ohm on the 8 ohm output and listen Sounds good at any volume, amp and mixer and gain structure are OK  Doesn't, then I would disconnect the input and try a input from my tone generator or I-pod to check the amp.  

Once you have a good output then reconnect the speakers.  Still bad, Take one down and try that, if it is OK reconnect, and try to brake the line in the middle, ETC until you find it.  BTW  I know everyone says nothing changed, but is there one speaker that is nice and loud?  perhaps someone decided to help out and added or changed a speaker and put one in with no transformer. That will mess it up.

It looks like you are down to divide and conquer.

Frank

Jonathan Johnson

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Re: 70 volt auxilliary zone has low volume
« Reply #13 on: January 07, 2011, 12:08:56 am »

So what would the effect on the system be if an individual speaker or its transformer experienced a short-circuit failure, creating an extreme low-impedance situation, but not low enough to trigger protection in the amplifier?

This could be tested for by disconnecting all the speakers and connecting them one by one until the system dumps out. To make things quicker, you might be able to disconnect groups of speakers.

Just a thought to throw out there.
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Sidney.Pilien

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Re: 70 volt auxilliary zone has low volume
« Reply #14 on: January 07, 2011, 12:09:50 am »

Matt Carr wrote on Thu, 06 January 2011 18:44

Sidney Pilien wrote on Wed, 05 January 2011 19:52The speaker wire size sounds correct. The system runs higher voltage-low current  at high impedance for long wire runs and less heat so increasing the wire size may worsen the problem.


Sidney, why would it be worse?  Is it because in a sense I'm switching it from a high impedance to a low impedance system if I use 16awg wire, therefore, making the amp work harder to push current through that much wire?  I've always been taught that undersized wire meant more resistance and more heat which is exactly what you don't want your amp to have.
I certainly don't consider myself a pro sound guy so I'm just asking the question.


Like telephone lines, like balanced lines, the smaller wire=higher line (not speaker) impedance= longer runs.

It's like this: blow air through a 1/4" straw 20' long and another through 1" at 20'.
The amount of air(current) required to flow through the wider straw (lower resistance or impedance) is higher than the smaller (higher impedance) at the same rate and requires more effort thus producing more heat. And the air will travel farther w/the smaller straw(smaller lines).

This is not your typical 8 ohm amp in that it is designed for long runs typically paging systems and so lowering the size increases current and thus heat, reducing the life of the amp since it remains in use for the entire business day.

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Frank DeWitt

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Re: 70 volt auxilliary zone has low volume
« Reply #15 on: January 07, 2011, 09:36:11 am »

At audio frequencies bigger wire is always better.  There is a point where making it bigger doesn't improve things enough to be worth more money, it but it never hurts.  

A 70 volt system doesn't need very big wire because the voltage is high and therefor the current is low but big wire won't hurt it.

Ray Rayburn who has a extensive resume including designing audio systems for the US congress has published a couple of spread sheets to help select the best wire for speakers.

for low impedance systems.
http://www.soundfirst.com/LZ_DF_calculator.zip

And for constant voltage systems.
http://www.soundfirst.com/CV_DF_calculator.zip

His web site is here
http://www.soundfirst.com/technical.html

Frank

Matthias Heitzer

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Re: 70 volt auxilliary zone has low volume
« Reply #16 on: January 10, 2011, 10:39:32 am »

If there really is a short somewhere in the line, bigger wire could make things worse, but that's the only situation i can think about.

If we are talking about AC, the electrons aren't acutally travelling through the wire, they just move back and forth.
That's a good thing, because they are as slow as 0,3 mm/s.

Let's take Sidney Pillen's straw and fill it with water.
Now we fill our mouth with orange juice and pump the juice through the water-filled straw. What will come out of the straw? It's water, and if we beginn to draw back the liquid fast enough, the juice won't drip out of the straw. (if we exclude that water and juices blends in real life)
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Gary Creely

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Re: 70 volt auxilliary zone has low volume
« Reply #17 on: January 10, 2011, 11:05:18 am »

Matthias Heitzer wrote on Mon, 10 January 2011 10:39

If there really is a short somewhere in the line, bigger wire could make things worse, but that's the only situation i can think about.

If we are talking about AC, the electrons aren't acutally travelling through the wire, they just move back and forth.
That's a good thing, because they are as slow as 0,3 mm/s.

Let's take Sidney Pillen's straw and fill it with water.
Now we fill our mouth with orange juice and pump the juice through the water-filled straw. What will come out of the straw? It's water, and if we beginn to draw back the liquid fast enough, the juice won't drip out of the straw. (if we exclude that water and juices blends in real life)


Ok, I need to just say all this back and forth nonsense about cable size is just very unlikely to have anything to do with the problem at hand. If it was 18ga or 10ga it should not be causing a dramatic loss of volume.
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Matt Carr

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Re: 70 volt auxilliary zone has low volume
« Reply #18 on: January 10, 2011, 01:40:18 pm »

Well, it has been interesting and informative reading all the replies. Thank you.
I discovered Saturday when I disconnected the speaker run at mid point I was able to identify a problem with the first section. I discovered one speaker connected to the 25V side of the transformer in an old office that's not currently being used. The church didn't even know it was in there. Once it was connected to 70V everything worked great, even on the 22awg wire.
Thanks for all the feedback.
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Brad Weber

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Re: 70 volt auxilliary zone has low volume
« Reply #19 on: January 10, 2011, 03:28:51 pm »

Glad you found the problem and took care of it!
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Jonathan Johnson

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Re: 70 volt auxilliary zone has low volume
« Reply #20 on: January 11, 2011, 01:10:40 am »

Matt Carr wrote on Mon, 10 January 2011 10:40

Well, it has been interesting and informative reading all the replies. Thank you.
I discovered Saturday when I disconnected the speaker run at mid point I was able to identify a problem with the first section. I discovered one speaker connected to the 25V side of the transformer in an old office that's not currently being used. The church didn't even know it was in there. Once it was connected to 70V everything worked great, even on the 22awg wire.
Thanks for all the feedback.


That makes sense. I would expect the 25V winding to present a lower impedance to the system, which could overload the amp to the point of bringing down the voltage supplied to the rest of the system.

I have seen 70V systems with fuses in the speaker line where it leaves the amp, to protect against such problems as this. I'm not sure how to calculate what size of fuse is needed. Maybe one of the experts on here can enlighten us?
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Re: 70 volt auxilliary zone has low volume
« Reply #20 on: January 11, 2011, 01:10:40 am »


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