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Impedance etc

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The wattage rating of the amp isn't really useful as a limiter to keep your cab safe, cabs can break with much less, listening and turning down is the only thing that keeps cab safe. More speakers will generally help being louder, but mixing cabs can give tone fun.

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I'm a bit confused too. I have a Hughes & Kettner BC115 rated as 300W at 8ohms. I want a small light weight bass amp for rehearsals and pub gigs and I like the look of the TC Electronic BH250. The speaker out states 4ohm minimum load. Hence I still get the full 250W with my 8ohm rated cab. Or do I?

I've tried revisiting Physics O'level for an insight but it's not any clearer :unsure:

Edit: can someone post up the equations to calculate the change in output power for the change in speaker impedance?

Edited by paulconnolly

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Hello,
I have a question that I still haven't found the answer for...and probably still won't haha!

I have a 600w 8ohm 4x10 cab.

If I were to run, say, an Ashdown Mag 300 head into this, I would get 175 watts (according to the back of the Mag) into it from the amp because the cab is 8 ohms.

If the resultant sound is loud enough for my use at about half (or maybe a touch more) the master volume, will this in any way decrease the life of the amp?

As it's trying to power 4x10 speakers that is rated at nearly 4 times the wattage at what it can deliver at 8 ohms?

A 2x10 at 150 watts was not quite loud enough for me, so I'm hoping to keep the cab (so that I can upgrade head in future when I have more money), and use the 175 watts from an old Ashdown Mag head. I'm hoping that double the speakers at a bit more wattage will do the trick for now.

Matt

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Running an amp at less than full power won't damage it. Quite the opposite.

The speakers' wattage rating is just the maximum amount of electrical power the voice coils can handle for a sustained period without melting. That's it. Your amp isn't trying to push that amount of power into the cab; that's determined by the impedance of the speakers in conjunction with the resistance provided by the volume control.

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Hi bassists!

Been a combo user for last few years since picking up the bass, but have just purchased an Ashdown MAG 300 head 307W and am running it into an Ashdown 115 which I belive is rated at 8 ohms off top of my head.
I have seen a Hartke 2100 cab for sale and was wondering if this would be able to run side by side with the 115 even if its rated at 250W at 4 ohms.
Can you mix speakers with different ohm ratings into same head etc etc????

Bit of a minefield this ohmage thing and dont want to end up blowing anything up unnecessarily!!!

Cheers guys, great forum btw!!!

Steve

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Most amps give all their power into 4 ohm, and approx half power into 8 ohm. They will tell you what the minimum ohm is on the back of the amp. That is most likely to be 4 ohm.

So, 1 cab at 8 ohms into an amp if fine. If you run 2 cabs they should be 8 ohm each, as 2 8 ohm cabs equals 4 ohms.

You can also run 1 4 ohm cab but you could damage your amp if you run an 8 ohm and 4 ohm cab together (makes 2.6 ohms) or 2 4 ohm cabs (makes 2 ohms).

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Hi Chris,
Thanks for that, thought that would be the answer just wanted to check as new to this head/cab mullarky!
Cheers

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Hi all,
Another newbie to cabs and heads as I was previously a combo man. I've just made a purchase off of a Trace AH350SMX head off of a top guy who is a member on this forum.
Its an early 90s one with only one speaker output on it (350w at 4ohm). If I was to get a 4x10 (800w at 8ohm) would this be a suitable match and then I can add a second 8ohm cab at a later date if I do so wish. This impedance stuff is frying my brain. :/ Many thanks.

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Ωs wise, that's what I'd do (& I do do too). Your amp will put out up to 350 watts at 4Ω or @ 200 at 8Ω.
As for the cab, it depends. Try it. If you don't like it, you can always sell it on.

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[quote name='xgsjx' timestamp='1373221277' post='2134968']
Ωs wise, that's what I'd do (& I do do too). Your amp will put out up to 350 watts at 4Ω or @ 200 at 8Ω.
As for the cab, it depends. Try it. If you don't like it, you can always sell it on.
[/quote]
Good call, and thanks that makes sense. :)

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[quote name='28mistertee' timestamp='1373220935' post='2134961']
....with only one speaker output on it....
[/quote]

If the amp only has one speaker output and you get an 8 ohm cab with the intention to add another 8 ohm cab you'll need to daisy chain the cabs. Make sure the cabs have 2 speakers sockets in them.

If you are getting a 410 then I'd just get a 4 ohm cab. If you need any more than 350 watts into a 410 you’re looking a more powerful amp, IMO.

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[quote name='chris_b' timestamp='1373274706' post='2135437']


If the amp only has one speaker output and you get an 8 ohm cab with the intention to add another 8 ohm cab you'll need to daisy chain the cabs. Make sure the cabs have 2 speakers sockets in them.

If you are getting a 410 then I'd just get a 4 ohm cab. If you need any more than 350 watts into a 410 you’re looking a more powerful amp, IMO.
[/quote]

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Not necessarily more power but just the option of adding say a 1x15 in at a later date just to give a bit more bottom end should I want it. Just didn't want to buy one 4ohm then want to add at a later date.

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[quote name='28mistertee' timestamp='1373281902' post='2135553']
Not necessarily more power but just the option of adding say a 1x15 in at a later date just to give a bit more bottom end should I want it. Just didn't want to buy one 4ohm then want to add at a later date.
[/quote]

Don't be misled into believing that adding a 1x15 to a 4x10 is a good idea or will add any bottom end.
Find a cab that you like the sound of & if you need more volume, get an identical cab.
If there's a 4Ω & an 8Ω version of a cab, go for the 8Ω unless you plan to never add another cab or plan on getting a 2Ω amp. The volume difference between the 2 would not be noticed & watts don't determine decibels, speakers do that.

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[quote name='xgsjx' timestamp='1373283113' post='2135575']


Don't be misled into believing that adding a 1x15 to a 4x10 is a good idea or will add any bottom end.
Find a cab that you like the sound of & if you need more volume, get an identical cab.
If there's a 4Ω & an 8Ω version of a cab, go for the 8Ω unless you plan to never add another cab or plan on getting a 2Ω amp. The volume difference between the 2 would not be noticed & watts don't determine decibels, speakers do that.
[/quote]
An 8ohm 4x10 then for the best all round sound?

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You can add a 15 if you like but I think (know) that these days you'll get better low notes out of a good modern 4 ohm 212.

A good 4 ohm 410 will give you a ton of low mids which is more important for low punchy bass than actual bass. Bass can get lost in a busy or very loud mix. Low mids will punch your notes through with more force and clarity.

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Thats ok then, didn't know if it made a difference to sound. I'm after a Trace cab and every 4x10 I have found is 8ohm so far.

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Why a 4x10? Like Chris says, a good 2x12 is a better option, though I'm not sure if Trace do a 2x12. But then, why Trace?

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[quote name='xgsjx' timestamp='1373305482' post='2135936']
Why a 4x10? Like Chris says, a good 2x12 is a better option, though I'm not sure if Trace do a 2x12. But then, why Trace?
[/quote]
No reason really apart from the good stuff I've heard about them, my mate plays a 4x10 Hartke and it sounds great. Trace because I wanted to keep it the same brand as my head. Will look at the other options thanks.

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That's the fun part. Trying things & finding what You like. It's good getting the wonderful advice that is available on this forum, but it's your ears (& your back) that should make the decision.
I tried loads of rigs & ended up with a 2x10. :)

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