One for the Ohmage Experts.....

[garethox]

I have an Ashdown Mag 300 with two speaker outputs on the rear stating '4 ohm Max. Impedance'. I want to connect to it a Hartke 4x10 (400W at 8 ohms) and a Hartke 1x15 (150W at 8 ohms) thereby giving 4 ohms.

So, if I have this right, that gives me 550W speaker wise and the amp will split 300W equally between the two cabs when in parallel... and as I'll never have to max the amp out I should have enough headroom in each cab.

Does this sound OK?

Cheers.....

[warwickhunt]

[quote name='garethox' post='826236' date='May 3 2010, 10:07 AM']I have an Ashdown Mag 300 with two speaker outputs on the rear stating '4 ohm Max. Impedance'. I want to connect to it a Hartke 4x10 (400W at 8 ohms) and a Hartke 1x15 (150W at 8 ohms) thereby giving 4 ohms.

So, if I have this right, that gives me 550W speaker wise and the amp will split 300W equally between the two cabs when in parallel... and as I'll never have to max the amp out I should have enough headroom in each cab.

Does this sound OK?

Cheers.....[/quote]

The only bit that matters is that you have a 4ohm load, which you do if the cabinets are each 8ohm!

The matter of 550w of speakers is unfortunately a bit of nonsense. I'm not trying to offend/upset you but the quoted figures for your cabs mean very little in the real world and it would take too long to go into why those figures and the distribution of power is as much use as a chocolate fire-guard BUT as your impedance is correct your amp will operate quite safely with those cabs and you would have to be cloth eared not to hear when the drivers started to complain.

Some folks will tell you that you need speakers rated 1.5/2/10 times what your amp is and others will say that your amp needs to have more headroom than your speakers. I can [s]tell[/s] advice you that both can be correct and I've personally driven 200w cabs with a 1000w amp and conversely 2000w cabs with a 400w amp and neither will come to any harm IF you take care not to drive your amp into clipping and listen to what the drivers are doing.

[garethox]

Thank you for taking the time to answer so thoroughly!

Regards,

Gareth.

[flyfisher]

+1

Ohms is the important bit, wattage is the 'black art' bit. Look after the ohms to protect your amp and your ears should protect your speakers, unless you like really horrible noises.

I use the same amp with 300W 1x15 and 250W 2x10 cabs, both 8 ohms and thus 4 ohms in parallel. Works fine.

[Count Bassie]

That was about as good an answer as I've ever read on the subject (I know of a forum where this same topic becomes a great inflammation 3 times out of 5). I especialy like the bit about the 'chocolate fireguard'. You Brits are so funny...

[Ancient Mariner]

When you connect 2 or more cabs in parallel, the max wattage of the whole system is always the wattage of the lowest power handling cab X the number of cabs. Thus 150W X 2 = 300W.

In the 'bad old days' of valve amps you needed to at least match the max output of the amp with the cabs, and if you intended to dime it then preferably allow 1.5X in power handling. Valve amps wattage is normally their clean rating, and so will often produce a bit more when pushed hard. Solid state amps appear to have all sorts of clever trickery built in (sometimes) and may be safe even when your speakers theoretically shouldn't cope - just don't bank on it!

[Count Bassie]

[quote name='Ancient Mariner' post='826575' date='May 3 2010, 11:13 AM']When you connect 2 or more cabs in parallel, the max wattage of the whole system is always the wattage of the lowest power handling cab X the number of cabs. Thus 150W X 2 = 300W.

In the 'bad old days' of valve amps you needed to at least match the max output of the amp with the cabs, and if you intended to dime it then preferably allow 1.5X in power handling. Valve amps wattage is normally their clean rating, and so will often produce a bit more when pushed hard. Solid state amps appear to have all sorts of clever trickery built in (sometimes) and may be safe even when your speakers theoretically shouldn't cope - just don't bank on it![/quote]

^^ Here's some of that 'dark arts' part: Rules are made to be broken, so there's a certain level of treachery in the whole deal. It does all boil down to paying attention to [i]knowing what you're plug in to where[/i], and using common sense.

[Ghost_Bass]

Here's the thing: your amp is rated as 307W máx! Doesn't matter how powerfull your cabs will be, it'll never put out more than 307W of power when working at 4ohms. So you won't feel the diference between the 400W speaker and the 150W one. If you were to connect just one of those speakers to the amp you'll be getting something close to 180W@8ohm of output. In that case you should be careful with the 150W speaker to not max out it's power handling.

in sum:

the amp says how much power can be putted out by it
the speakers say the max power output they can handle

normaly you'd get speakers with more power handling than the amp can produce so they won't be harmed by it but big differences in handling power to the output can be harmfull for the amps since they will force the amps to produce excessive heat.

In your case i think you would be more than ok with your rig.

Cheers