What Is An Ohm?

[Paul Smith 0]

An ohm is the unit used to measure resistance. It is also used to denote impedance (as in loud speakers) and shows the amount of resistance (load) a speaker will offer to an amplifier.

 

So an 8ohm speaker connected to an amplifier will offer approx 8ohms of load to that amplifier (I say approx because the leads connecting the 2 together will also add to the resistance but unless you have long runs or are using an inferior cable this won't have a large effect)

 

It's quite common though to run more than one speaker from each output of the amplifier so how will this effect the load offered and the performance of the amplifier?

 

There are 2 ways to link speakers together - series & parallel. The method adopted (or you can use a combination of both) will dictate the final output performance of the amplifier.

 

Parallel:

 

http://i223.photobucket.com/albums/dd258/5StarDisco/speaker2parrallel.jpg

 

In this example the cables from the amplifier connect to both terminals of the 1st speaker and additional cables then run from these to the next speaker (and so on).

 

How does this effect the resistance load?

 

Here's the formula: Ohms Total = (Ohms Speaker 1 X Ohms Speaker 2) / (Ohms Speaker 1 + Ohms Speaker 2)

 

So if you had two 8 ohm speakers the calculation would be (8 x 8) / (8 + 8) = 64/16 = 4 Ohms

 

If you joined an 8 ohm and 4 ohm in parallel the load presented to the amplifier would be (8 X 4) / (8 + 4) = 32/12 = 2.66 ohms.

 

Some amplifiers will happily work with a 2 ohm load but others won't so it's worth checking the spec before adding additional speakers in this way.

 

Series:

 

http://i223.photobucket.com/albums/dd258/5StarDisco/speaker2series.jpg

 

In this example a cable from the positive output of the amplifier is connected to the + terminal of the 1st speaker. A wire is connected to the - terminal on the 1st speaker and taken to the + terminal of the second (& so on for more speakers). Finally a cable is taken from the - terminal of the last speaker in the chain back to the negative output on the amplifier.

 

How does this effect the resistance load?

 

Here's the formula: Total Ohms = Ohms Speaker 1 + Ohms Speaker 2 etc.

 

So if you had two 8 ohm speakers wired in series the total load presented to the amp is 8 + 8 = 16.

 

What difference does all this make?

 

Firstly if you get it wrong and present an impedance to the amplifier that is too low it will either go into protection mode or blow something (usually the output stage amplifiers)

 

Secondly amplifiers perform better when the load is at a premium. For instance an amplifier rated at 1000 watts RMS at 4 ohms may only provide 600 watts RMS at 8 ohms.

 

One final point on connecting speakers - they need to be in phase. Each speaker has one of the terminals marked with either a + symbol or a red dot. In parallel it is important to connect the same phase terminals together or one cone will be pushing out whilst the other is moving in.

 

Mobile DJ Knowledgebase info here

 

Please do not hesitate to add to this article and help a DJ :Thumbup:

Edited June 15, 2008 by Paul Smith

[superstardeejay 0]

It is also used to denote inductance

 

Well, inductance is measured primarily in Henries. I think what you mean is impedance..which is the combined effect of resistance, inductive and capacitive reactance.

 

But I suppose I'm splitting hairs...

 

 

 

[Robster 0]

A speakers impedance is not fixed and varies when signal is played through it .

speaker ratings are nominal impedance.

you will see figures stating the nominal impedance of a speaker (most speakers have a nominal impedance of between 4 and 8 ohms). This is the speaker's average resistance to the flow of current through the circuit going from the amplifier into the speaker and back to the amplifier. However, this figure is only the average impedance rating across all frequencies into which the speaker produces output. In reality, the impedance may drop at some points to 2 or 3 ohms and may reach 40, 50 or even 60 ohms at other frequencies.

Edited June 15, 2008 by enquirer

[superstardeejay 0]

A speakers impedance is not fixed and varies when signal is played through it .

 

Indeed..which is why speakers' impedance, rather than resistance, is quoted...a speaker being inductive. The same goes for anything inductive subject to AC, whether it's a coil or solenoid or whatever. However, for the purposes of calculating how many cabinets you can connect in a particular way to a particular amplifier, the amp designers have taken all this into account when they rated your amp, so nominal impedance calculations still stand.

 

Since inductive reactance is equal to 2 x pi x f x L where L is the inductance and f is the frequency of the current, the reactance (which makes up part of its impedance) will vary with frequency. As the frequency goes higher, so the impedance goes higher and less current flows. As the frequency goes down, so more current flows and this is why a faulty amp going into DC (zero frequency) has the potential to ruin a speaker because the inductive reactance is effectively zero...and the few ohms resistance is futile to a 110V DC blast from said amp.

 

I havent mentioned back EMF and damping factor but that's maybe a topic for another thread!

 

 

 

 

 

 

 

[Robster 0]

 

 

I havent mentioned back EMF and damping factor but that's maybe a topic for another thread!

 

 

indeed !!!

 

speakers rated the same ie one rated at 8 Ohms and another rated at 8 ohms nominal impedance can often provide very different loads to an amplifier when music is played though them ,one rated the the same can be difficult and another easy for the amp to deal with better quality amps can often deal with more difficult loads but i obviously agree that for the purposes of calculating how many cabinets you can connect in a particular way to a particular amplifier, the amp designers have taken all this into account when they rated your amp, so nominal impedance calculations still stand.

 

and again i am moving away from the original point of the post...sorry ,ill shut up now... all my kids have bought me bottles of booze for fathers day so i am getting drunk and rambling...lol

[Paul Smith 0]

Well, inductance is measured primarily in Henries. I think what you mean is impedance..which is the combined effect of resistance, inductive and capacitive reactance.

 

But I suppose I'm splitting hairs...

You are right - it should have read impedance (& now does smile icon )

 

I agree that the post only covers the real basics and is a simplistic way of looking at this subject but that was the plan - most DJs without technical know how (or the wish to know it) look for a straightforward answer to 'Can I connect more speakers to my amp?'

 

I'm sure the tech-heads would be interested in further more detailed posts on the topic though ...

[supersound dj 0]

I have a 4 ohm amp and 8 ohm speakers ,I know im not getting full power from my amp but it sounds better and runs cooler with 8 ohm speakers.I reccon that you get 3/4 power doing this ,roughly.

 

 

Paul

[Andy Westcott 0]

That's the method of connection I would recommend; It means you may not be getting the full possible power out of the amp, but sometimes reliability is more important and if you can afford to buy a bigger amp to provide the necessary output into 8 ohms, then do it!

 

"I havent mentioned back EMF..."

This is what causes a speaker to exhibit an impedance of upwards of 60 ohms (at certain frequencies) when a multimeter test of the motor reads 6 ohms. A speaker motor is a coil of wire in a magnetic field, and you may recall from your physics lessons that if you move a coil in a magnetic field in such a way that the turns cut across the lines of force, an EMF (electromotive force) is generated. Basically, a speaker will generate lekky if you vibrate the cone. (this is how many microphones work) It follows that the movement of the cone in response to the signal applied also generates a voltage in opposition to that applied, and tends to oppose it to a lesser or greater degree. If the cone is free to move (for example not in a cabinet) that opposing voltage will be greater, and highest at resonance. Resonanace is where... oh just Google it!

 

About ohms and current for those not learned on the subject:

Imagine the electricity supply (the amp in this case) can rovide an almost unlimited amount of power. If you short out the output with a bit of wire (zero ohms), an extremely large current will flow because there is nothing to hold it back except the tiny bit of resistance in the amp itself. The effect of a huge current passing through a resistance is to heat it up - much like friction really - and this is when amps start to smoke and cost money. Don't do it...

 

If the amp is trying to force power through a higher resistance - 8 ohms say - far less current will flow because of that relatively high resistance, and that current will not cause the tiny resistance within the amp to heat up so much - a far greater proportion of the energy is used up in the speaker. Amps are designed to deal with a certain amount of heat and to get rid of it, but too much and it'll be damaged. This is why the manufacturers stipulate a minimum load resistance (impedance). This isn't the recommended load, it's the absolute minimum, below which the amp will be unable to get rid of the heat without damage.

 

Combining speakers:

If you connect two 8 ohm speakers on parallel, each is drawing the same current as a single unit so therefore the total current is doubled, (in a pefect amp) increasing greatly the heating up within the amp. (maybe 4 times as much) This doubling of current suggests that the resistance is now half of what it was with just one speaker connected, and this is indeed the case - 4 ohms instead of 8. Lower resistance means more current and heat. If the power has to pass through one resistance and then continue on through another before completing the circuit, (series connection) the resisances add together, and higher resistance to current means less of it.

 

I'm waffling and I've forgotten the point I was trying to make...

Edited June 15, 2008 by Andy Westcott

[superstardeejay 0]

The point you make about two dissimilar cabinets of identical nominal impedance (eg 8 ohms) is a particularly valid one regarding DJ setups. I would be disinclined to mix different makes and models of speaker in parallel (eg mixing a pair of Skytronic and a pair of Warrior all on the one amp) for this reason. I know many people who do this routinely but it's not a textbook way since the different cabinets will react differently (electrically as well as acoustically) and it's possible to over-run one or not get the full benefit of the other so to speak.

 

 

 

 

[Robster 0]

The point you make about two dissimilar cabinets of identical nominal impedance (eg 8 ohms) is a particularly valid one regarding DJ setups. I would be disinclined to mix different makes and models of speaker in parallel (eg mixing a pair of Skytronic and a pair of Warrior all on the one amp) for this reason. I know many people who do this routinely but it's not a textbook way since the different cabinets will react differently (electrically as well as acoustically) and it's possible to over-run one or not get the full benefit of the other so to speak.

 

very eloquently put , I have experienced this a few times. Matched speakers do work better from my limited experience. Occasionally you can get mixed systems which are brilliant but predicting the end sound quality of mixed systems is hit and miss.

I think audiophiles call is system synergy., when mixed components end up with a sound quality above the sum of the individual parts ..whoops rambling on again the hangover from last nights booze is still in full force