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Confused about impedance


Jono Bolton
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I was looking at the Ashdown ABM 410 T, which is an 8 ohm cab. However, the ABM head (the 500W one at least) is 4 ohms. Does this mean the head and cab are not compatible with each other? Or, does it mean that I need to run another 8 ohm cab to get it to work? I'm not really sure how impedance with heads and cabs works.
Cheers
Jono

Edited by jono b
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[quote name='jono b' post='118143' date='Jan 10 2008, 08:26 PM']I was looking at the Ashdown ABM 410 T, which is an 8 ohm cab. However, the ABM head (the 500W one at least) is 4 ohms. Does this mean the head and cab are not compatible with each other? Or, does it mean that I need to run another 8 ohm cab to get it to work? I'm not really sure how impedance with heads and cabs works.
Cheers
Jono[/quote]

Not that they're not compatible, but if you run two 8 ohm cabs together, the overall ohmage is reduced to 4 ohms, and therefore you'll get full power output from your amp. An 8 ohm cab will just reduce the maximum output from the amp, that's all.

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[quote name='jono b' post='118143' date='Jan 10 2008, 08:26 PM']I was looking at the Ashdown ABM 410 T, which is an 8 ohm cab. However, the ABM head (the 500W one at least) is 4 ohms. Does this mean the head and cab are not compatible with each other? Or, does it mean that I need to run another 8 ohm cab to get it to work? I'm not really sure how impedance with heads and cabs works.
Cheers
Jono[/quote]

You should run it with another 8 ohm can to get the maximum output from the head. You can run it with an 8 ohm cabinet however you will get approximately half the output. Impedance is a funny thing. The more cabinets you have, the lower the impedance drops. I have given you a link.

[url="http://www.sputnikmusic.com/lesson.php?lessonid=97&page=1"]http://www.sputnikmusic.com/lesson.php?les...d=97&page=1[/url]

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This idea that 8Ω+8Ω=4Ω only applies when the loads are wired in parallel, of course - if wired in series, it would be 8Ω+8Ω=16Ω as you'd expect. The manual should say which it is, but parallel is more common.

The formula for calculating parallel loads is the one at the bottom right of this diagram, so in your case it's 1 / (1/8Ω + 1/8Ω) = 1 / (2/8Ω) = 4Ω.

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OK, "impedance" is technically more correct for speakers, because they contain coils, but for this subject "resistance" is good enough IMHO.

When you specify impedance, you should specify the frequency too e.g. 8Ω @ 80Hz - but you don't see that on the backs of amps or cabs, do you? The impedance of a speaker & cab varies as the frequency varies, so a 8Ω label on a cab is an approximation. I'll spare you the geeky discussion on inductive reactance and phase vectors. :)

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[quote name='Jack' post='118542' date='Jan 11 2008, 09:17 AM']I think Bill was more about 'ohmage' than 'resistance'.

Hey, at least youre aware of 'it', more than a lot of people.[/quote]
The problem with the quasi-word 'ohmage' is that it could refer to impedance, could refer to resistance, could refer to inductive or capacitive reactance. Being non-specific makes it useless in an engineering context, so while it does appear in the vernacular it is not used by the engineering community. Resistance is only used with respect to speakers in its context as a component of impedance.

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Just to make it really simple for those who haven't any knowledge of Electrical technology. If you're looking to buy a new amp have a look at the spec' given for it. It will probably say something like 500watts rms at 4 ohms. All you need to do is make sure that you plug in one 4 ohm cab or two 8 ohm cabs to get your full 500 watts. If you plug in one 8 ohm cab you'll get something like 250-350 watts. if you use one 8 ohm and one 4 ohm you'll need to check the spec' on your amp as you will lower the impedance to 2.66ohms. This is obviously all assuming parallel connection. don't worry about how or why it works this way it just does !!. For a full technical explanation read the "sticky" info.
cheers
Al.

Edited by Al.B
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[quote name='bass_ferret' post='119011' date='Jan 12 2008, 10:49 AM']The manual should say, maybe even the labelling on the sockets - otherwise assume parallel. Only some Eden amps have the speaker outs in series for some strange reason.[/quote]
And 'daisy chaining' cabs should be parallel as well?

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Yup, head->cab 1->cab 2 is still parallel if thats what you mean.

I think the reason Eden have series on some of their cheaper combos is so a beginner can plug in ANY cab and not have to worry about impedance.

Edited by Jack
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[quote name='Jack' post='119059' date='Jan 12 2008, 06:31 AM']Yup, head->cab 1->cab 2 is still parallel if thats what you mean.

I think the reason Eden have series on some of their cheaper combos is so a beginner can plug in ANY cab and not have to worry about impedance.[/quote]
More likely the internal driver(s) is already at the amp's minimum load rating to maximize output. For example, all the Nemisis save the 15 are 4 ohm rated with 4 ohm speakers.

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