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Author Topic: bridge and 70v  (Read 1347 times)

Mark Rone

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bridge and 70v
« on: April 21, 2014, 04:26:51 am »

hi,
I have a little experience with PA systems so have been asked to have a look at an old install in a bar that has a dual zone setup with one of the zones running a 70v line and the other a conventional setup with a passive inline sub.

System is Yamaha xm6150 amp x 2, various JBL control 25t and 25 speakers, sb-5 passive sub and a couple of Panasonic surround sound rear speaker!!!!!. There are a number of inputs (PC, TV, mic and ipod) running through a Numark rm6 before heading to the amp.

Some of the issues are low or no volume, inconsistent volume, poor quality and no headroom before distortion.

Having a look at the install and it is a mess and sounds terrible. I assume that the staff have been fiddling as the general volumes are being controlled by the gain controls on the mixer and the attenuators on the amps for the various channels. I can cover the basics but not an expert on 70v and there are a few peculiarities I want some advice on before I rewire it from scratch....

1. can a passive sub run on a 70v line?
2. Yamaha indicate that HPF should be set to on for all 70 channels. Why? and how would this affect a passive sub.
3. some of the channels are bridged but they have speakers connected to the -ve terminals as well as the +ve. I have never come across this and wonder if this is a mistake.
4. the amp has attenuation on each channel. is this any use on a 70v system, I would have thought that the taps would be used to set levels, with the attenuation being used to reduce the signal in general.

admittedly, have not installed a 70v system, so don't cane me, I just want to know if there is anything simple going wrong rather than some trickery on the part of the installer.

the general intention is reset the system, remove anything damaged and get the system running optimal. then looking to fill in gaps or resize the whole system if required.



thanks
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Kevin Graf

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Re: bridge and 70v
« Reply #1 on: April 21, 2014, 08:28:26 am »

Decades ago, "dB" magazine had an article on how to troubleshoot a 70V system. I wonder if anyone still has a copy?  But as I recall the number one problem is that someone hooked up a speaker without using a matching transformer.

Adding a passive sub would require a real big matching transformer and place a great burden on the amplifier.
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John Roberts {JR}

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Re: bridge and 70v
« Reply #2 on: April 21, 2014, 09:10:15 am »

Divide and conquer... disconnect the amp from the the 70V feed and then reconnect to just one speaker. If it still sounds bad try another different speaker.. Odds are against two bad speakers.

If all speakers check ok by them selves it;s the wiring.

JR
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Tom Young

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Re: bridge and 70v
« Reply #3 on: April 21, 2014, 10:28:11 am »

1. can a passive sub run on a 70v line?
2. Yamaha indicate that HPF should be set to on for all 70 channels. Why? and how would this affect a passive sub.
3. some of the channels are bridged but they have speakers connected to the -ve terminals as well as the +ve. I have never come across this and wonder if this is a mistake.
4. the amp has attenuation on each channel. is this any use on a 70v system, I would have thought that the taps would be used to set levels, with the attenuation being used to reduce the signal in general

1. Yes, but you would need a very large and high-power transformer for this sub. Better to leave this low-impedance (not 70V).

2. Setting the HPF "on" makes good sense for all the loudspeakers *except* the subwoofer. The HPF is 80Hz and this is what the Control 25's are rated for. The surround speakers are probably as, or more, unable to reproduce lower frequencies.  But (as you seem to know) the subwoofer goes lower than 80Hz (probably 30-40Hz) so you do NOT want the HPF on that signal/channel.

3.  Yamaha provides good information on wiring and setting up this amp. Download the manual.

4. The taps on the transformer on each loudspeaker are rated in watts and these are double or half the wattage of the next tap, which equals + or - 6dB.

Usually, each 70V amp channel is for a group of speakers that we would think of as a zone, or area. Usually the amp attenuators are set the same and some of the speakers are tapped higher or lower depending on how far they are from listeners or how loud the owner wants sound within that coverage area. But when you set up the system and listen or measure there is nothing wrong with adjusting the amp gains to balance the system or set one zone (relative to others) as needed. Just be aware of which speakers are being effected.

Note that 70V requires (3) amplifier channels, bridged. See the specs towards the end of the manual.

A few questions:

What input preamp is being used in this system (make and model) ?

What, if any, processing is being used (equalizers, crossovers, etc) ?

Is there just (1) Yamaha amp ?

Is there just (1) subwoofer ?

How many Control 25's are there ? Control 25T's ? Panasonics ?

Are the Panasonic spkrs 70V (this may be indicated on the input panel or by the presence of an external transformer on each speaker) ?
« Last Edit: April 21, 2014, 02:16:42 pm by Tom Young »
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Ivan Beaver

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Re: bridge and 70v
« Reply #4 on: April 21, 2014, 12:14:52 pm »

Have you measured the IMPEDANCE CURVE-NOT DC RESISTANCE?

That would be the FIRST thing I would do.  It would give you really good starting point-so you are not having to guess.

There could be all sorts of things wrong.

I agree with the others about the sub-generally low freq and 70V do not go well together for several reasons.

An old trick (not recommended-but sometimes useful) is to simply run some pink noise at about 30V (give or take).  If somebody has hooked a normal 8 ohm speaker up to the line it will most likely die during the testing process.

So once it has burned up-it is no longer a problem-------  Until the person who hooked it up realizes it and hooks another one up.  You can't fix stupid.

The 70V speakers would be fine-due to their higher impedance.
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Mark Rone

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Re: bridge and 70v
« Reply #5 on: April 21, 2014, 07:18:10 pm »

Hi,
Thanks for the feedback, I am still in the research phase so haven't physically touched anything on site yet until I understand the components and how it is all fixed up. I have also downloaded all of the manuals and reviewed the Yamaha install guide. Another issue is that all of the speaker runs are hidden away in cable runs behind the walls and in a crawl space so also cant line each cable end with the amp binding post.

Responding to some of your questions.....


Note that 70V requires (3) amplifier channels, bridged. See the specs towards the end of the manual.


not sure what you mean here, the manual only shows 70v requiring two channels bridged and the HPF switched on with the speakers wired to the centre binding posts. I still have no idea as to why there would be additional wires connected to the remaining binding posts that shouldn't be in use if being bridged.

Also, as the amp pumps out L&R through its output they take up two channels on the amp. But as they are bridged only one of the inputs will be live, so I should look at combining the sound into one channel from the mixer into the bridged input.

What input preamp is being used in this system (make and model) ?

 the Numark RM6 mixer has a gain on each input (assume preamp stage) and then a zone level for each combined zone before heading to the amp. There is a mixture of line and level inputs going into the mixer.

What, if any, processing is being used (equalizers, crossovers, etc) ?

None I believe, other than the HPF on the amp.

Is there just (1) Yamaha amp ?

there are two amps (same model) running 12 channels. A bit more of a look identifies that the one amp is running the conventional setup (one zone) with the sub and the 25s in stereo mode (no bridge) 4 speakers (fl, fr, rl, rr) on their own channel. The other amp appears to be running the other zone with the 25t's on a 70v line. This amp has both the HPF and the bridge mode set to on.

Is there just (1) subwoofer ?

Yes one sub, As its a passive sub I assume the binding posts for the fl and fr will run into the sub before branching off to the speakers.

How many Control 25's are there ? Control 25T's ? Panasonics ?

As above regarding zone 1. Zone 2 has the Panasonics but they are the ones you get in a $200 dollar surround system.... they will be removed and binned at the first opportunity. There is a couple of 25t's but they have been disconnected as they 'don't work' and the wires switched into the Panasonics.

Are the Panasonic spkrs 70V (this may be indicated on the input panel or by the presence of an external transformer on each speaker) ?

As above, they are not 70v or even PA grade components.

I have attached a photo of the rear of the amps.... Points of note.

  • the 4 channels from the mixer (Zone 1 & 2, L&R) come into the top amp and are replicated via euroblocks to the respective channels.
    The top amp has 6 channels bridged into 3 but has additional speakers wired up to the outer binding posts. Therefore the input and attenuation of the second channel should be redundant for the bridging. but could be used to replicate to the unbridged channels?
    The bottom amp is running stereo and running the conventional setup which runs the 25's and the passive sub.
    Both amps have HPF activated for all channels, even the on front channel to the sub

I have attached a photo of the rear of the amp, what my initial plan of attack is before getting into anything serious.

Zone 1 - Lower amp - conventional set-up
confirm spec of speaker and ensure that 25's are in place rather than 25t's
stereo mode for all four channels.
HPF switched on for rear speakers.
HPF switched off for front speakers which run through passive sub.
reset gain structure and attenuation

Zone 2 - upper amp - 70v
confirm spec of speaker and ensure that 25t's are in place rather than 25's. remove Panasonic jobbies
bridge mode for four channels into two. need to review combined channels to mono rather than stereo
HPF switched on for all channels (as per Yamaha guidance)
remove cabling wired to redundant binding posts and check all terminations at speaker locations are not shorting.
reduce taps on speakers, fire it up and see what is working.
in this zone I would be looking to run the internal 2 restaurant speakers off one bridged channel and run the two outer deck speaker locations off another bridged channel (no speakers connected a present)
reset gain structure, attenuation and taps

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Ivan Beaver

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Re: bridge and 70v
« Reply #6 on: April 21, 2014, 08:10:10 pm »

It does not appear as if any of the outputs are actually wired bridged.

That means that you have about 8dB less capability than if you were running at 70V.

And that amp can't actually drive a 70V line-only about a 50V line-so each speaker is not getting the power it is tapped at.

If the bridge switch is pressed-then that means that the "opposite" channel is actually out of polarity.  This can cause all sorts of cancellations.

So it appears as if there are some serious wiring issues.

As I said before-before you change anything- you REALLY NEED to do do an impedance sweep and determine what the actual impedance is on each line.  THEN you can figure out how to wire the amp.

Without that basic information-it is a real shot in the dark getting good usable answers.

Short of that exact number and taps are required for each run.

It sounds as if there is a number of different problems.  Each one needs to be addressed separately
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Tom Young

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Re: bridge and 70v
« Reply #7 on: April 22, 2014, 08:07:12 am »

You are right about needing (2) amp channels for 70V. I misread the manual.

A few comments:

You need to be careful with the low-impedance (non-70V) speakers when connected to the bridged amp outputs. I would wire them so the combined load is not lower than 4-ohms. You do this by employing series *and* parallel wiring.

The JBL SB 5 appears (from photographs and memory) to have (2) onboard passive crossovers and is fed stereo signal from the amps. So you feed stereo fullrange signals (L&R) from (2) bridged amp outputs (with HPF off) to the subwoofer inputs and then feed the L&R 25's from the high-passed outputs on the subwoofer. Good luck trying to balance the subs with these 25's, BTW. 

If you do not use the subwoofer's crossovers plus you have the amplifier HPF's off, you may blow the 25's when the system is run loud.

You're nuts if you do not trace every speaker cable and measure what's on the other end (as Ivan has advised), before you proceed. Logically, you would also ID what's on each end.
« Last Edit: April 22, 2014, 09:53:47 am by Tom Young »
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Tom Young
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Keith Broughton

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Re: bridge and 70v
« Reply #8 on: April 22, 2014, 08:35:11 am »


You're nuts if you do not trace every speaker cable and measure what's on the other end (as Ivan has advised), before you proceed.
There is no easy way to solve this kind of problem.
I have run into similar situations and a complete investigation and mapping of the wiring is critical.
Take your time and it will be easier in the long run.

As for the amps, the top ones are not wired correctly if they are bridged.
A bridged output uses the left red and right red connectors, not any black ones.
I have done installs using a bridged amp as a 70v source but ALL speakers on that line must have transformers to work correctly.

Good luck and keep us posted on your progress.
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Mark Rone

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Re: bridge and 70v
« Reply #9 on: April 22, 2014, 05:11:29 pm »

I have run into similar situations and a complete investigation and mapping of the wiring is critical.

yep, I  was hoping not to need to do this as the place is a rabbit warren. I had hoped that as the system has been in place a for a number of years that that the main install would be OK and that the issues they were having were people fiddling with the front end. Oh well....

will be in there early doors on Sunday morning to go over it in more detail.
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