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Old 19th January 2006, 01:15 PM   #1
Ovation is offline Ovation  United Kingdom
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Default Back to basics: Relationship between Amp and Impedance

Hi Guys,
Sorry to ask such a basic question, but I'm getting myself confused about something and I've not found an answer through searching previous posts:

At my local indian restaurant, the music coming out of their ceiling speakers is very distorted, even when on quietly (most noticeably at the highest frequencies). It is also quite 'tinny' - Lacking anything much at the low end.
Last night while I was there I mentioned this to the owner and then offered to take a look.

There is an unbranded 4 channel PA head, and 11 speaker cables going down behind it. I'll get a chance to pull all this out in a couple of weekends time to find out exactly what is there.

I have a nasty feeling that he has got 11 full range ceiling speakers (Guessing 8ohms each) wired in parallel!!!
I am assuming that the speakers and Amp he is using are not 100V line.

I've read loads of stuff about Impedence and Amps and seen many seemingly conflicting stories about what does what. Below is an idea of what I am thinking - I'd love to know if I've got the right idea or not:

If we take eleven 8ohm speakers an wire them in parallel we get an approximate impedance of:
(1/R)*n = 1/R
(1/8)*11= 1/1.375
R = 0.727 ohms

The wire run is quite long so perhaps we get a bit of extra impedance from that (It's standard thin cheap speaker cable running up to about 15metres per speaker I reckon).

So if we take a typical and forgiving 8ohm P.A. Amp, and only load it with 1ohm - I think from what I have read that we are going to try and exceed the capabilities of the Amp's PSU and get nasty clipping and non-harmonic distortion (and possibly burnt the amp out).

How does the maths work?
I was thinking about V=IR and P=IV but if a 100W amp will run at 40v peak on an 8ohm load for example, if I then drop the Resistance/Impedance the voltage can't go up because of the PSU - Do we then pump more current out? I assume not, as the voice coils might have burnt out if we'd done that...

Any Ideas/Suggestions you all have would be nice to hear.

Many Thanks
Jonathan
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Old 19th January 2006, 01:22 PM   #2
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Are you sure that the speakers are not '100 Volt Line' types? (75V line I believe in the USA).

In this case each speaker has its own step-down transformer with the ability to select different ratios by tappings. The amplifier has an output step-up transformer so that it can put a signal of 100V rms out. Basically you select the tapping on the transformer at each speaker to set the maximum power that is required at that particular speaker. In this system you can have many speaker/transformer units all connected in parallel and (provided that the tappings are set properly) you won't overload the amp.
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Old 19th January 2006, 01:29 PM   #3
sreten is online now sreten  United Kingdom
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Default Re: Back to basics: Relationship between Amp and Impedance

Quote:
Originally posted by Ovation

I am assuming that the speakers and Amp he is using are not 100V line.
I'd love to know if I've got the right idea or not:

Jonathan
Hi,

given your assumption yes you do have the right idea.

The eleven speakers should be rewired. Probably as 3 sets of 3
in series and 1 set of 2 in series, the sets wired in parallel. The
2 speaker set will be somewhat louder than the 3 speaker sets.

/sreten.
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Old 19th January 2006, 01:32 PM   #4
Ovation is offline Ovation  United Kingdom
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Hi Ouroboros,

I'm open to the possibility that these speakers could be 100V line types - BUT that wouldn't explain why they sound so bad..

Apparently the system was installed by builders, so there's not much chance that they had a clue what they were doing.

The Amp looks similar to some nasty Adastra P.A. heads that I've got, which have a output transformer with taps off for 4ohm, 8ohm, 16ohm, 100v (and I think 75v too!)

I would assume that if you took 100V line speakers and connected them to the 8ohm output by mistake, you would not get any sound, and if you attached 8ohm speakers to the 100V line you'd blow them up?

I'll know all this for sure when I get a chance to examine the back of the Amp, but I've just got a feeling that they may have badly installed this equipment rather than having broken something.

Cheers
Jonathan
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Old 19th January 2006, 01:44 PM   #5
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If the speakers are indeed 8 ohms or so, you may re-wire them in combinations of series/parallel to get closer to a total 8 ohms load, to better load the amp and improve sound quite a lot. Are all these speakers identical?

Jan Didden
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Old 19th January 2006, 02:08 PM   #6
Ovation is offline Ovation  United Kingdom
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Hi Jan,
Quote:
.Are all these speakers identical?
I don't know for sure if they're identical drivers and I can't get at them easily as they are up in a high ceiling - but they all look physically the same, so I have to assume that they are indeed the same types.

Cheers
Jonathan
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Old 20th January 2006, 02:48 AM   #7
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If nothing is burned out, just overloaded...
Turn down the volume.
Do they sound ok?

Put an ohmmeter on the speaker cable to check resistance.

Otherwise, pull one speaker out and check it.
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Old 20th January 2006, 08:19 AM   #8
Ovation is offline Ovation  United Kingdom
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Thanks Myhrrhleine,
That's very similar to what I was planning. My plan of action is to look at the back of the amp - confirm that the output is 8ohms and then pull all speakers out except one, and test it alone.
If it works fine on its own, I'll then arrange the speakers in a Parallel/Series combination that results in a sensible load on the Amp.

One of my main reasons for this post is actually to see if anyone can explain to me the basic principles of why a really low impedence causes the Amp to hit its PSU limits and clip the tops of the signal.

Anyone up for giving me a quick maths/physics lesson about this?

Cheers
Jonathan
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Old 20th January 2006, 08:55 AM   #9
sreten is online now sreten  United Kingdom
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Quote:
Originally posted by Ovation

One of my main reasons for this post is actually to see if anyone can explain to me the basic principles of why a really low impedence causes the Amp to hit its PSU limits and clip the tops of the signal.

Anyone up for giving me a quick maths/physics lesson about this?

Jonathan
Hi,

its not maths / physics, and the PSU does not hit its limits.

The protection circuitry in most modern amplifiers forces the
voltage rails to prematurely clip for excessive current loads.

/sreten.
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Old 20th January 2006, 12:32 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
to try and simplify the analysis, I'm going to suggest we think in terms of output current from the amp.

Into 8ohm; a 100W amp will push 5Apk. and then develop 5*8 =40Vpk

Into 4ohm the same 5Apk will develop only 20Vpk and into 1ohm only 5Vpk (=9.1W into 11 speakers = 0.83W/speaker. The missing 3.4W is lost in the cable going from 0.7ohms to 1ohm).

If the amp can only deliver 5Apk then as soon as you ask it to produce more then it will distort, whether due to current or IV limiting or a bottleneck in the current coming from the PSU.

In reality the amp is probably capable of delivering more than 5Apk but the example gives you some arithmetic basis for extracting some real numbers.

Solutions; increase the load impedance by using a transformer or rewiring the speakers or changing the speaker impedance.
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