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Impedance etc

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[quote name='jnrwarrior' post='987295' date='Oct 13 2010, 08:10 PM']so for this amp should i run 2 cabs that are 8 ohms each?[/quote]

Yes. 2 x 8ohm cabs or one 4ohm.

And you can't rewire the 2 x 15 4ohm to make it 8ohm as it contains 2 x 8ohm drivers in parallel. You would only be able to rewire them in series i.e. 16ohms.

Edited by dincz

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Hi folks,

I've recently discovered this forum, and think it's just great. This is my first post though, so be gentle...

I've read through all the posts in this highly useful thread, and have certainly learned a great deal. However, here's my situation:

I currently run a Trace Elliot AH200 (Series 6 GP12) into two 300W 8 ohm TE cabs (a 1153 and 2103x). I take one output from the back of the head, then daisy chain the two cabs together by XLR. So, from what I've learned from this thread, this is a parallel 4 ohm cab set-up, delivering 200W.

However, the AH200 is on its way out. I've also got two loud guitarists to cut through in the band, so I'm thinking of getting of getting an AH500 head that I've spotted on Flea Bay. The idea is to use the same speakers, so the cabs would theoretically receive a max of 500 W. However, they're 300W cabs. My question - is this a no-no because the output power of the amp would exceed the wattage of the cabs? Or do two cabs at 300W each effectively give me 600W?

Alternatively, anyone got a TE 250 or 300W SMX???

Many thanks,

BB

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[quote name='borisbrain' post='1030990' date='Nov 21 2010, 12:16 AM']the cabs would theoretically receive a max of 500 W. However, they're 300W cabs. My question - is this a no-no because the output power of the amp would exceed the wattage of the cabs? Or do two cabs at 300W each effectively give me 600W?[/quote]

The 500W will be equally divided between two cabs of the same impedance, i.e. 250W each.

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[quote name='dincz' post='1031710' date='Nov 21 2010, 07:19 PM']The 500W will be equally divided between two cabs of the same impedance, i.e. 250W each.[/quote]


Many thanks Dincz.

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[quote name='borisbrain' post='1030990' date='Nov 20 2010, 11:16 PM']Hi folks,

I've recently discovered this forum, and think it's just great. This is my first post though, so be gentle...

I've read through all the posts in this highly useful thread, and have certainly learned a great deal. However, here's my situation:

I currently run a Trace Elliot AH200 (Series 6 GP12) into two 300W 8 ohm TE cabs (a 1153 and 2103x). I take one output from the back of the head, then daisy chain the two cabs together by XLR. So, from what I've learned from this thread, this is a parallel 4 ohm cab set-up, delivering 200W.

However, the AH200 is on its way out. I've also got two loud guitarists to cut through in the band, so I'm thinking of getting of getting an AH500 head that I've spotted on Flea Bay. The idea is to use the same speakers, so the cabs would theoretically receive a max of 500 W. However, they're 300W cabs. My question - is this a no-no because the output power of the amp would exceed the wattage of the cabs? Or do two cabs at 300W each effectively give me 600W?

Alternatively, anyone got a TE 250 or 300W SMX???

Many thanks,

BB[/quote]

Problem is, with the same speakers, you probably won't get much louder even if you have more power on tap.

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Where does RMS fit into all this? A Hartke LH500 puts out 350 watts into an 8 ohm Warwick 211pro. The 211pro handles 400w RMS, what does that mean?

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[quote name='arthurhenry' post='1091120' date='Jan 16 2011, 09:41 AM']Where does RMS fit into all this? A Hartke LH500 puts out 350 watts into an 8 ohm Warwick 211pro. The 211pro handles 400w RMS, what does that mean?[/quote]
RMS (Root Mean Square) is the standard measure of wattage for musical equipment. If wattage isn't stated as being RMS be very wary - some manufacturers will state Peak Music Power or other terms in order to inflate the apparent wattage of their equipment. In your case both items are rated with RMS figures, so no attempts at deception there.

The 400w RMS figure given for your 211 pro means that the cab will handle 400w of power before the voice coil will start to melt - it's really a rating of the thermal power handling of the cab. How much power the speakers will take on the low notes will be limited by their displacement (Xmax), and will be considerably lower than it's thermal power handling.

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[quote name='arthurhenry' post='1091120' date='Jan 16 2011, 09:41 AM']Where does RMS fit into all this? A Hartke LH500 puts out 350 watts into an 8 ohm Warwick 211pro. The 211pro handles 400w RMS, what does that mean?[/quote]
RMS is the way of expressing the actual power. As Musky says, it's root mean square. What that actually means is that if you look at a sine wave, it looks like this:



The actual average power going to the speaker is the area between the sine wave curve and the horizontal axis. This is different to the power going to the speaker at the peak of the sine wave, as the average power is 0.707 (sqrt 2 / 2, or 1/sqrt 2) times the peak power. Hence "peak power" (which is most likely the figure being quoted if there is nothing to say it's RMS) will be 1.414 times RMS power.

What you also need to consider is energy - that is to say, power being made available during a period of time. This is why you may see speakers (generally PA) being quoted with a "programme" rating as well as a constant RMS rating. If you put bursts of sound into a speaker, the voice coil will have a chance to cool down between the higher volume parts. If you put a constant sine wave in, at some point the speaker will be unable to dissipate the energy in the voice coil as quickly as it is fed in, and the voice coil will overheat and physically distort or burn out. Hence music with reasonable dynamics will have its volume limited by the Xmax of the speaker (ie. how far the cone can travel), but a constant wave (sine, square, triangular, whatever) will be thermally limited before you can drive the cone to Xmax.

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There's actually no such thing as RMS power. RMS values of voltage and current can be used to calculate power in watts - and there's only one kind of watt. What manufacturers call "RMS" power is normally the continuous power rating rather than the peak, or short burst capability.

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I was gonna ask the same question as Dincz, but thats opened another can of worms for me! I understand the Ohmage side of things... Im looking at buying a new rig... but want to get the most efficient poissible. After reading this topic i've discovered that your cab should be around about half more than ur amp... so for example, if I had a 500W head, i'd need a 750W cab OR 2 x 375W cabs. And if this is the case, then you wouldn't be able to add or take away any cabs and would have to use the rig as it is... Is this correct??... Actually, sod it, i'm gettin a combo!!! lol

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Hey coop,

Try not to get too frustrated here! The general thinking is that use what you like, as long as the Ohm ratings match, and use your ears to tell if it's not working out. e.g. you plug your 500W amp into a 300W RMS cab and it farts out when you boost the bass too much. Just back it off until it doesn't fart out.

Hope that helps,
Jon

P.S. Alex's Barefaced tech notes are just what you need to read: [url="http://barefacedbass.com/technical-information/mythbusters1.htm"]CLICK HERE[/url]

Edited by jonthebass

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Hi guys,

I own an Ampeg SVT-15E cab which is rated at 8 ohms. Originally, I was going to get an SVT head but actually, I quite fancy the Marshall VBA400 which is all tube and has selectable output of 1, 2 or 4 Ohms.
I know it's probably a question which has been asked a thousand times and I do know that, in theory, I should be ok running the 8 Ohms cab with this head; however people seem more cautious about impedance mismatch when it comes to all-tube heads. Do you think I would be ok running my SVT15 with the all-tube VBA400 ? If not, has someone got an idea why ? Is it because of resonance issues due to the inductive load, or something else... ?

Thanks

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[quote name='Zeuhl' post='1338395' date='Aug 13 2011, 07:34 AM']Hi guys,

I own an Ampeg SVT-15E cab which is rated at 8 ohms. Originally, I was going to get an SVT head but actually, I quite fancy the Marshall VBA400 which is all tube and has selectable output of 1, 2 or 4 Ohms.
I know it's probably a question which has been asked a thousand times and I do know that, in theory, I should be ok running the 8 Ohms cab with this head; however people seem more cautious about impedance mismatch when it comes to all-tube heads. Do you think I would be ok running my SVT15 with the all-tube VBA400 ? If not, has someone got an idea why ? Is it because of resonance issues due to the inductive load, or something else... ?

Thanks[/quote]
You should definitely NOT run an amp with a valve output stage at a higher impedance than intended.

With transistor amps you're fine to run a cab with with a higher impedance than the amp's minimum load (typically 4 ohms). All that happens is that the amp will output a lower wattage into the cab. Valves are very different - they have a very high output impedance (thousands of ohms), and need an output transformer to match them to the comparatively low impedance of a speaker. They also run very hot and at high voltages. Running them into a higher impedance than intended will mean the valves will produce a higher voltage to drive the load and get even hotter. Not good for valves at all. If the voltage gets too high you can expect arcing to occur anywhere along the high voltage path. Result? Destroyed valves/valve bases/output transformer. The worst case scenario would be running the amp with no load connected (or a broken speaker lead) which presents the amp with an infinite load. The magic smoke will very quickly appear.

Depending on the tolerances of the valves and design of the amp you might get away with running the amp on the 4 ohm tap into an 8 ohm load for a while, but the valves will run hotter and have a shorter lifespan. And as I'm sure you know, revalving a big amp can be an expensive business.

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[quote name='Musky' post='1338423' date='Aug 13 2011, 08:45 AM']You should definitely NOT run an amp with a valve output stage at a higher impedance than intended.

With transistor amps you're fine to run a cab with with a higher impedance than the amp's minimum load (typically 4 ohms). All that happens is that the amp will output a lower wattage into the cab. Valves are very different - they have a very high output impedance (thousands of ohms), and need an output transformer to match them to the comparatively low impedance of a speaker. They also run very hot and at high voltages. Running them into a higher impedance than intended will mean the valves will produce a higher voltage to drive the load and get even hotter. Not good for valves at all. If the voltage gets too high you can expect arcing to occur anywhere along the high voltage path. Result? Destroyed valves/valve bases/output transformer. The worst case scenario would be running the amp with no load connected (or a broken speaker lead) which presents the amp with an infinite load. The magic smoke will very quickly appear.

Depending on the tolerances of the valves and design of the amp you might get away with running the amp on the 4 ohm tap into an 8 ohm load for a while, but the valves will run hotter and have a shorter lifespan. And as I'm sure you know, revalving a big amp can be an expensive business.[/quote]


Thanks for the detailed explanations, Musky. :)
Is it the same for hybrid valve/transistor heads or are their valves totaly isolated from the output ?
I suppose I might have to get another 8 Ohm cab and wire the two in parallel if I want to stick to the VBA400... or get another head (or a low impedance cab...)

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[quote name='Zeuhl' post='1338437' date='Aug 13 2011, 09:07 AM']Thanks for the detailed explanations, Musky. :)
Is it the same for hybrid valve/transistor heads or are their valves totaly isolated from the output ?
I suppose I might have to get another 8 Ohm cab and wire the two in parallel if I want to stick to the VBA400... or get another head (or a low impedance cab...)[/quote]
It only applies to valve output stages. Amps with only a valve preamp are not affected.

It might be worth having a word with Marshall - it's possible the output transformer might have an 8 ohm tap which isn't used. Unlikely, but you never know. :)

Edited by Musky

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[quote name='Musky' post='1338442' date='Aug 13 2011, 09:15 AM']It only applies to valve output stages. Amps with only a valve preamp are not affected.

It might be worth having a word with Marshall - it's possible the output transformer might have an 8 ohm tap which isn't used. Unlikely, but you never know. :)[/quote]


Thanks, I will try to contact them. Shame that electric diagrams of amps are not easily accessible...

Now, it might sound stupid but what if I connect a dump load in parallel with the cab, I mean a big chunky resistor such as this one: 500W, 10 Ohm: [url="http://uk.rs-online.com/web/p/high-power/7013943/"]http://uk.rs-online.com/web/p/high-power/7013943/[/url] It would bring down the impedance to 4.5 Ohms. Now, it's a purely resistive load so I'm not sure what the overall effect of the RL circuit would be ... Don't hesitate to tell me if I'm being an eejit or if doesn't sound too bad an idea. :)

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It would work, but you'd be dissipating plenty wattage in the form of heat. A lot of it - the spec sheet gives the max temperature as 358 C. :)

Some sort of fan might be in order. Or just get a second 8 ohm cab.

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[quote name='Musky' post='1338512' date='Aug 13 2011, 10:28 AM']It would work, but you'd be dissipating plenty wattage in the form of heat. A lot of it - the spec sheet gives the max temperature as 358 C. :)

Some sort of fan might be in order. Or just get a second 8 ohm cab.[/quote]

That's right: big heat sink, thermal compound, casing, fan, psu for the fan... it might end up being cheaper buying another cab... Would be a funny project though: an amp ideal for cold long winters. :)

Thanks for all the help anyway, Musky.

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[quote name='Musky' post='1338442' date='Aug 13 2011, 09:15 AM']It only applies to valve output stages. Amps with only a valve preamp are not affected.

It might be worth having a word with Marshall - it's possible the output transformer might have an 8 ohm tap which isn't used. Unlikely, but you never know. :)[/quote]

Hey Musky,

Just for your information, I have asked Marshall and there is no 8 Ohm tap on the output transformer of the VBA400; which makes sense, otherwise they would probably have put a 8 Ohm output to the cabs too.

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[quote name='Musky' post='1346303' date='Aug 20 2011, 01:25 PM']Shame.

Have you started working on that dummy load then? :) :)[/quote]

Lol. No, I have ditched this idea (I have many crazy ones! :lol:) and I am looking for another head now. I would quite like a Mesa 400+ actually (it has a 8 Ohm output) but they seem a bit hard to find... I'll keep on looking.

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[quote name='jcater' timestamp='1327568664' post='1513353']
....If my amp goes down to 2 ohms, EBS hd350, can i use four 8 ohm cabs linked together?....
[/quote]
Yes.
2 x 8 = 4
3 x 8 = 2.6
4 x 8 = 2

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