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Author Topic: Speaker amp question  (Read 888 times)

Seth Thompson

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Speaker amp question
« on: February 05, 2019, 08:11:57 pm »

In our foyer we have three Tannoy 4" CVS4 160 watt speakers fed by a single channel of a BiAmp MXA75 75 watt 6 channel mixer/amp.  We have 2 problems with this setup. The first is that it's never loud enough.  The second is that if we turn the amp up half way (6 out of 12), the amp will inevitably stop working and not work again until powered off and back on.  I do not know enough to say if we need more amp or more speakers.  I don't know if the amp is shutting off because it's trying to protect the speakers or if it's just failing.  Does anyone have any insight they can share?
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Peter Tran

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Re: Speaker amp question
« Reply #1 on: February 08, 2019, 03:23:47 pm »

In our foyer we have three Tannoy 4" CVS4 160 watt speakers fed by a single channel of a BiAmp MXA75 75 watt 6 channel mixer/amp.  We have 2 problems with this setup. The first is that it's never loud enough.  The second is that if we turn the amp up half way (6 out of 12), the amp will inevitably stop working and not work again until powered off and back on.  I do not know enough to say if we need more amp or more speakers.  I don't know if the amp is shutting off because it's trying to protect the speakers or if it's just failing.  Does anyone have any insight they can share?

I'm not a professional by any means, but
there are several variables that need to be considered in this scenario:
1. Are the speakers wired with transformers
2a. If not with transformers are they wired in series or parallel (i.e. resistance/ohm reading)
2b. If with transformers (I can't help you because I am not familiar with voltage systems)
3. How large is the room?
4. Is this for ONLY speech or are you trying to play music through it?
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Mike Caldwell

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Re: Speaker amp question
« Reply #2 on: February 08, 2019, 07:28:38 pm »

What are the speaker wattage tap selectors set at and on the amps output terminal strip what terminal positions do you have the speaker line connected to.

Erik Jerde

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Re: Speaker amp question
« Reply #3 on: February 08, 2019, 11:27:16 pm »

In our foyer we have three Tannoy 4" CVS4 160 watt speakers fed by a single channel of a BiAmp MXA75 75 watt 6 channel mixer/amp.  We have 2 problems with this setup. The first is that it's never loud enough.  The second is that if we turn the amp up half way (6 out of 12), the amp will inevitably stop working and not work again until powered off and back on.  I do not know enough to say if we need more amp or more speakers.  I don't know if the amp is shutting off because it's trying to protect the speakers or if it's just failing.  Does anyone have any insight they can share?

Your “single channel” comment is confusing as this amp is mono output and by definition is only a single channel.  We need to know how the amp is wired and how the speakers are tapped.
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Chris Grimshaw

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Re: Speaker amp question
« Reply #4 on: February 09, 2019, 10:31:08 am »

My bet is the amp is going thermal (or overcurrent) and shutting down.
Check wiring at both ends, and look at the amp to see if there's any dust/fluff/rubbish built up in there.

Chris
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Ken Webster

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Re: Speaker amp question
« Reply #5 on: February 10, 2019, 05:01:04 am »

Found a PDF data sheet for the speaker:
Impedance 6 Ohm

Power:
40 W average
80 W program
160 W peak

Maximum SPL:
103 dB average
109 dB peak

Recommended amp 80 W into 6 Ohm


Mixer amp has 6 input channels.

MXA75   
Output power, 4 ohms:  75W  RMS
Power consumption:  100W  Maximum
Weight:  8.6kg


Ken
« Last Edit: February 17, 2019, 04:12:41 am by Ken Webster »
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Mike Caldwell

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Re: Speaker amp question
« Reply #6 on: February 10, 2019, 10:58:43 am »

My bet is the amp is going thermal (or overcurrent) and shutting down.
Check wiring at both ends, and look at the amp to see if there's any dust/fluff/rubbish built up in there.

Chris

My thought as well as to why it's shutting down.

Most likely caused by wrong speaker tap settings and the wrong connection on the amp's output terminals.

My guess is the speakers are all set on the direct tap setting put somewhere around a 2 ohm load on the amp.

Ken Webster

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Re: Speaker amp question
« Reply #7 on: February 10, 2019, 03:36:14 pm »


I Deleted my post as too many erroneous and misleading assumption made.

Ken
« Last Edit: February 17, 2019, 04:14:01 am by Ken Webster »
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Seth Thompson

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Re: Speaker amp question
« Reply #8 on: February 10, 2019, 04:06:55 pm »

Thanks for all the responses. 

Here's a link to the electrical drawings they left with us, but when I look at them I can't tell if series or parallel.  https://drive.google.com/open?id=0B1vK_TvhbNg9ZjJjMDMxOTgtZDk2MS00OWIyLTk4OTctOWQwYWQwZmJkNDNi
I will try and do the "only one speaker" test soon.

We use it for both music and speaking and typically it will shut down during the music (much louder) and we can reset for the speaking and it stays on until music starts back. 

I will blow out the dust on the amp before the service tonight and see if that helps too.
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Erik Jerde

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Re: Speaker amp question
« Reply #9 on: February 10, 2019, 04:40:40 pm »

Thanks for all the responses. 

Here's a link to the electrical drawings they left with us, but when I look at them I can't tell if series or parallel.  https://drive.google.com/open?id=0B1vK_TvhbNg9ZjJjMDMxOTgtZDk2MS00OWIyLTk4OTctOWQwYWQwZmJkNDNi
I will try and do the "only one speaker" test soon.

We use it for both music and speaking and typically it will shut down during the music (much louder) and we can reset for the speaking and it stays on until music starts back. 

I will blow out the dust on the amp before the service tonight and see if that helps too.

That drawing is useful.  It shows that the speakers should be being driven off of the 70v output of the amp.  This will be easy to confirm by just looking at the connections on the back of the amp.  Verify that positive is connected to the 70V terminal (second from left) and that negative is connected to com.

The speaker tap setting is accessed by pulling the grill.  If they are not all set to 30w/70v then start by getting that set and test.  If they are set to 30w then step them down to 15w and test.  That amp is not up to a 90w continuous load so if you’re really hitting it then tapping down the speakers may help.
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Mike Caldwell

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Re: Speaker amp question
« Reply #10 on: February 10, 2019, 05:58:19 pm »

As Eirk said double check the tap settings and connection to the amp's output terminal strip.

How loud are you trying to get in the lobby, those three little speakers will only do so much!!



Also there may not be a proper high pass filter on the line feeding that amp.
There should be a steep high pass filter of at least 100hz on those speakers.

Jamming too high of a level of music with a lot of low frequency would saturate the speaker's transformers and overload the amp.

Depending on how the outputs are configured on your mixer and stage box you may be able to do it in the mixer, does a separate mix out just feed the lobby?

The ideal place to apply a high pass would be in the DSP unit, you would need the software and most likely a password to get into it and a Bi Amp DSP is not something you just jump into, make a quick change and be done unless you have done a few of them.

At the very least you could turn the bass control on the amp down, maybe way down and see what that does for you.

Ken Webster

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Re: Speaker amp question
« Reply #11 on: February 17, 2019, 04:26:43 am »

I apologise for my previous erroneous and misleading assumptions.  The data I had on this led me to think it was a simple direct amp speaker connection.  I now realise the amp and speakers are transformer connected, isolating the amp from the low speaker impedance load.  Found this rather simple outline of what is going on that may be helpful.  I made too many errors on this one and will leave it in Mike's capable hands.

https://www.redbackaudio.com.au/understanding-100v-line-distributed-speaker-systems/

Regards, Ken
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