Impedance etc

[Mr Cougar]

I've never thought about this before (I don't use multiple cabs anyway) but when hooking up your speakers in parrallel how does the wattage work.

Lets say you had 2 x 4 ohm cabs giving you and impedance of 2 ohms running in parrallel (ie:just plugged 2 speakers into the back of the amp in a straightforward manner)

The amp is at say...750 @ 2 ohms

One cab is 350w

The other 600w

clearly you have enough headroom in total but presumably this would still be a bad idea since the 350w cab is effectively being driven with a max of 375w because the 750w from the amp has been divided equally, is this correct? or is there something I'm missing.

Excellent thread.

[BOD2]

The amplifier does not "see" the power rating of a cab - it only "sees" the impedance. If both cabs are the same impedance it will split the output power equally between them.

Although on paper it looks like there might be a problem here (i.e. potentially 375W into a 350W cab) in practice this is unlikely to be a problem. It is most unlikely that you would ever extract the full power of 750W from the amp in such way as to damage anything.

As always, the most important advice is to listen. If you hear unplanned distortion, simply back off the power a little.

[noisedude]

[quote name='BOD2' post='234842' date='Jul 8 2008, 11:17 AM']The amplifier does not "see" the power rating of a cab - it only "sees" the impedance. If both cabs are the same impedance it will split the output power equally between them.

Although on paper it looks like there might be a problem here (i.e. potentially 375W into a 350W cab) in practice this is unlikely to be a problem. It is most unlikely that you would ever extract the full power of 750W from the amp in such way as to damage anything.

As always, the most important advice is to listen. If you hear unplanned distortion, simply back off the power a little.[/quote]
Are you 100% on this? Not because I doubt it but because my question was very similar.

My amp is rated 300w RMS @ 4 ohms.

I am about to start using two cabs, as follows:

1. Rated 250w RMS @ 8 ohms
2. Rated 150w PEAK @ 8 ohms.

My understanding is then that I will be running at a combined 4 ohms, and that my 300w will be 'split' 50/50 between the cabs. Is that right?

This way, if I am running my amp flat out I will be getting an average of 150w to each cab, and peaks above that. However, I don't turn the master volume up beyond seven and keep the preamp gain fairly low (around half) to keep it clean on the way in.

Am I in trouble or is this a safe enough way to get some extra air moving as opposed to my previous setup of one 300w PEAK cab @ 8 ohms?

Nik

[bass_ferret]

Alex Clabers first rule:
[b]You can use any power ouput amp with any power handling cab. If any of these combinations makes bad sounds then turn down and/or stop cranking the bass EQ excessively or damage may occur[/b]

[gilmour]

[quote name='bass_ferret' post='246559' date='Jul 24 2008, 01:05 AM']Alex Clabers first rule:
[b]You can use any power ouput amp with any power handling cab. If any of these combinations makes bad sounds then turn down and/or stop cranking the bass EQ excessively or damage may occur[/b][/quote]

and it's a great rule, and easy to follow, although for some reason we all get obssesed by the numbers.

[Merton]

[quote name='gilmour' post='246562' date='Jul 24 2008, 01:12 AM']and it's a great rule, and easy to follow, although for some reason we all get obssesed by the numbers.[/quote]
a big plus 1.

[sixshooter]

Some very interesting thoughts on this thread:

I will keep this simple because there is no need to complicate this unless you are building your own system.

Matching Power

OK, the Amp can be seen like a car engine in that it supplies power, and your speaker like the clutch brakes and tyres delivering the power to the road. If you put a bigger engine in it will work, but your clutch will fail much quicker if you use all the power and the tyres and brakes will fade and you will be replacing parts much sooner than you would like!

The same is true for the amp and speaker, drive the speaker past its limits and you will be replacing the speaker a lot sooner, so rule of thumb is make sure your speakers can take all that the amp can give plus a bit more.

Now if you put a smaller engine in the car again it will work but perfomance will be sluggish.

Using a speaker that needs a lot more power to drive it than is being supplied and it will not reach its optimal range, but it will work.

Matching Impedance

If you are running multiple speakers make sure that they are of the same impeadence (ohm's), if not your sound will not be equally divided, (ie one will be louder than the other), as the power will be split based on the resistance that each speaker gives, and if they are different then the output of each speaker will be different.

If the speakers are the same ohms ie 8ohms and 8ohms = 4ohms then the power, (watts) will be divided equally, so if your Amp says 500watts at 4ohms your lowest speaker should be rated greater than 250watts.

Volume Controls

How much output, (watts) being delivered to the speaker, (not to be confused by being generated by the amp) will depend on where the volume control is set, and there is no set relation to the number on the dial, (so just because it goes up to 11 does not mean that it is louder than one that goes up to 10, remember Spinal Tap).

So, bottom line is if your Amp is more powerfull than your speaker keep the volume down, and your speaker may just last out, (although I would not recomend this as a long term arrangement).

I am sure that this will generate a bit of further discussion as in this case fiction is greater than fact!

[Merton]

[quote name='sixshooter' post='264679' date='Aug 18 2008, 04:05 PM']OK, the Amp can be seen like a car engine in that it supplies power, and your speaker like the clutch brakes and tyres delivering the power to the road. If you put a bigger engine in it will work, but your clutch will fail much quicker if you use all the power and the tyres and brakes will fade and you will be replacing parts much sooner than you would like!

If the speakers are the same ohms ie 8ohms and 8ohms = 4ohms then the power, (watts) will be divided equally, so if your Amp says 500watts at 4ohms your lowest speaker should be rated greater than 250watts.[/quote]
Hmmm.

[b]Alex Clabers first rule:
[i]You can use any power ouput amp with any power handling cab. If any of these combinations makes bad sounds then turn down and/or stop cranking the bass EQ excessively or damage may occur [/i][/b]

[alexclaber]

[quote name='sixshooter' post='264679' date='Aug 18 2008, 04:05 PM']Matching Power

OK, the Amp can be seen like a car engine in that it supplies power, and your speaker like the clutch brakes and tyres delivering the power to the road. If you put a bigger engine in it will work, but your clutch will fail much quicker if you use all the power and the tyres and brakes will fade and you will be replacing parts much sooner than you would like!

The same is true for the amp and speaker, drive the speaker past its limits and you will be replacing the speaker a lot sooner, so rule of thumb is make sure your speakers can take all that the amp can give plus a bit more.

Now if you put a smaller engine in the car again it will work but perfomance will be sluggish.

Using a speaker that needs a lot more power to drive it than is being supplied and it will not reach its optimal range, but it will work.[/quote]

Although this analogy is full of holes it does make some sense given a bit of tweaking. If you take a normal car onto a race track and try to achieve consistently fast lap times then your brakes will give out, guaranteed. Take that car back on the road and those same brakes will be more than capable of decelerating the car as required for many thousands of miles, even if you like to drive fast.

Consider the amp the engine and the speakers the brakes - in normal use your amp can have significantly more power than the speakers can handle and you will never ever have a problem. Push your amp to the point where things are not sounding nice, just as driving a car fast round a track will seem rather brutal compared to driving quickly on the road, and you risk speaker damage. And just as a low powered car has enough power to wreck its brakes on a track, so too can a low powered amp wreck a speaker if abused.

[quote name='sixshooter' post='264679' date='Aug 18 2008, 04:05 PM']How much output, (watts) being delivered to the speaker, (not to be confused by being generated by the amp) will depend on where the volume control is set, and there is no set relation to the number on the dial, (so just because it goes up to 11 does not mean that it is louder than one that goes up to 10, remember Spinal Tap).[/quote]

The wattage delivered to the speaker depends on the voltage output from the amp and the speaker impedance at that frequency. The voltage output from the amp depends on the voltage input to the amp from your bass and the gain within the amp. Play hard and use a hot bass with the volume on the amp set at 2 and more voltage will be output than with a quiet bass and a light touch but the volume set at 11. See the other thread about gain vs volume for more information.

Alex

[sixshooter]

As I said guys, I tried to keep it simple!

If you want more theory then do what I did 35 years ago and go to college or what appears to happen nowdays spend your life surfing the interblog :brow:

[Merton]

[quote name='sixshooter' post='264706' date='Aug 18 2008, 04:37 PM']As I said guys, I tried to keep it simple!

If you want more theory then do what I did 35 years ago and go to college or what appears to happen nowdays spend your life surfing the interblog :brow:[/quote]
Alex's first rule is even simpler

But yes, your analogy with a bit of added input from Alex makes sense to anyone who needs to know

[Spikeh]

A bit of blurb I found on another forum in relation to solid state amps:

[quote]Most solid state power amps and a small amount of amp heads are “stereo” containing 2 separate power amp sections. This allows you to connect more cabinets to the amp, and of course gives you more wattage. Most of these stereo amps can be run in “bridged mode”. This connects the two amplifiers together and requires you to hit a switch when the amp is off, and to hook up your speaker cable differently, (usually by connecting a banana jack to the positive terminal of both 5 way output jacks on the back of the amp. When running in bridged mode each amp “sees” ½ of the total impedance of the cabinets connected to it. Therefore, an amp rated at 4 ohms minimum per side that will put out 300 watts into 4 ohms per side can only be safely bridged into an 8 ohm load, and will put out 600 watts into the 8 ohm cabinet. Most amplifiers only safely power a minimum of 4 ohm loads, however, more and more amplifiers have been produced that handle 2 ohm minimum loads, which is good news for us, as we can run a good number of cabinets in different combinations. As a last point, you may have noticed that the ratings for our “typical amplifier didn’t double each time the impedance of the load was cut in half. Why not? Well, in a perfect world, an amplifier that put out 200 watts into 8 ohms would put out 400 watts into 4 ohms and 800 watts into 2 ohms. In the real world, other consideration, (such as thermal limits), limit the actual number of watts an amplifier will safely produce at a given impedance.[/quote]

OK, so, this pretty much explains my current situation - I run an Ampeg SVT-4 PRO, which has the following output power ratings:

1600 Watts Mono-Bridged @ 4 Ohms (1200 Watts Continuous)
1200 Watts Mono-Bridged @ 8 Ohms (900 Watts Continuous)
2 x 900 Watts @ 2 Ohms (600 Watts Continuous)
2 x 625 Watts @ 4 Ohms (490 Watts Continuous)
2 x 350 Watts @ 8 Ohms (300 Watts Continuous)

I currently run it in to a single Ampeg 8x10 running @ 4ohms through the single mono-bridged output, whice gives me the maximum output I can get with the amp, which is great.

However, I'm looking at "downgrading" and getting two new cabs to replace the single 8x10 and its too damn heavy.

Looking at getting a 4x10 @ 4ohms (EDEN D410XST) and a 1x18 at 8ohms (EDEN D118XL). I'll be running them in stereo mode in order to take advantage of my crossover control.

Am I right in my calculations here:

Total cab impedance: (4/2)+(8/2) = 6ohms
Total "visible" impedance to the amp in stereo mode: 6/2 = 3ohms

So the mimimum load on the head would be 3ohms, which puts me between 625W and 900W output per side?

The EDEN D118XL is 500W output, and the EDEN D410XST is 1000W output. Does this mean that the D118XL will be overloaded? Or will the wattage be added together too? I don't know how this bit works - I'm afraid one of the bi-amps will blow the D118XL?

[Thunderhead]

[quote name='Spikeh' post='296644' date='Oct 1 2008, 02:16 PM']A bit of blurb I found on another forum in relation to solid state amps:



OK, so, this pretty much explains my current situation - I run an Ampeg SVT-4 PRO, which has the following output power ratings:

1600 Watts Mono-Bridged @ 4 Ohms (1200 Watts Continuous)
1200 Watts Mono-Bridged @ 8 Ohms (900 Watts Continuous)
2 x 900 Watts @ 2 Ohms (600 Watts Continuous)
2 x 625 Watts @ 4 Ohms (490 Watts Continuous)
2 x 350 Watts @ 8 Ohms (300 Watts Continuous)

I currently run it in to a single Ampeg 8x10 running @ 4ohms through the single mono-bridged output, whice gives me the maximum output I can get with the amp, which is great.

However, I'm looking at "downgrading" and getting two new cabs to replace the single 8x10 and its too damn heavy.

Looking at getting a 4x10 @ 4ohms (EDEN D410XST) and a 1x18 at 8ohms (EDEN D118XL). I'll be running them in stereo mode in order to take advantage of my crossover control.

Am I right in my calculations here:

Total cab impedance: (4/2)+(8/2) = 6ohms
Total "visible" impedance to the amp in stereo mode: 6/2 = 3ohms

So the mimimum load on the head would be 3ohms, which puts me between 625W and 900W output per side?

The EDEN D118XL is 500W output, and the EDEN D410XST is 1000W output. Does this mean that the D118XL will be overloaded? Or will the wattage be added together too? I don't know how this bit works - I'm afraid one of the bi-amps will blow the D118XL?[/quote]It's much simpler than you think!

The two sides of a stereo power amp work totally independently when they are not bridged, and only deliver the amount of power that each can into the impedance that is connected to it. So one side will deliver 625W into the 4-ohm 4x10", and the other will deliver 350W into the 8-ohm 1x18". Both are well within the ratings of the cabs, and everything will be fine. There will be less power going to the 1x18", but since one 18 is likely to be a little bit more efficient than four 10s, it will probably work out about right overall - you should be able to balance it with the crossover anyhow.


Just by the way - cab power ratings are [i]input[/i], not output. Your calculations for combined impedance would also be wrong, if the cabs were actually working in parallel from the same amp. Total impedance in parallel = 1/(1/Impedance1 + 1/Impedance2), so a 4-ohm and an 8-ohm cab in parallel is 1/(1/4 + 1/8) = 2.67 ohms. 2/3 of the total power will go to the 4-ohm cab, and 1/3 to the 8-ohm. Hopefully that makes sense, even if you don't need it!

Edited October 9, 2008 by Thunderhead

[colin100]

I need some help from you guys.

I have an SWR 350x which gives me 350 watts at 4ohms. I use a 4 ohm cab and get 350 watts. The manual for the amp says that at 2ohms I will get 450watts but not to run the amp continuosly like this as it will get really hot and may get damaged. So if I was to get another 4 ohm cab ill be running at 2ohms and risk smoking my amp? Is that right? Will this happen even at quarter volume or even half volume? Can I rewire the cabs to run in series and therefore get 4ohms and not heat my amp up to much?

Ive been scratching my head about this and am going bald now!

Colin

[stevie]

Could one of the moderators please sort out the spelling of the header, for crying out loud?

[bass_ferret]

Never noticed that! Doh!

[stevie]

Thank you. That's much better. [center] [/center]

[stevie]

[quote name='bass_ferret' post='370349' date='Jan 4 2009, 09:05 PM']Never noticed that! Doh![/quote]

You clearly don't have my anal tendencies.

[stevie]

Maybe this will make you feel better, Ferret.

[V4lve]

Hi. Forgive the stoopid question but...... am planning to get a 15" cab to go with my Peavey TB RAXX /Classic 120 head. The Peavey Classic 120 is a 120W mono valve power amp with 2, 4, 8 and 16 ohm outputs. Thought I would go for a Peavey cab (seems good value for money) but am wondering whether to get the 4 or 8 ohm version. Am assuming the 4 ohm option makes more sense as it will draw more watts from the amp. Am I right? Is this a good basis for my decision. Ta

[Musky]

[quote name='V4lve' post='404140' date='Feb 9 2009, 11:29 AM']Hi. Forgive the stoopid question but...... am planning to get a 15" cab to go with my Peavey TB RAXX /Classic 120 head. The Peavey Classic 120 is a 120W mono valve power amp with 2, 4, 8 and 16 ohm outputs. Thought I would go for a Peavey cab (seems good value for money) but am wondering whether to get the 4 or 8 ohm version. Am assuming the 4 ohm option makes more sense as it will draw more watts from the amp. Am I right? Is this a good basis for my decision. Ta[/quote]

Nope. Valve amps always put out the same wattage - that's what the output transformer does. You just need to match the transformer tap to the impedance of the cab as it it can be disastrous for the health of your amp if you don't. There might be other benefits to using a particular impedance (which maybe someone more qualified could comment on), but you won't get a higher wattage from it.

[V4lve]

[quote name='Musky' post='404145' date='Feb 9 2009, 11:37 AM']Nope. Valve amps always put out the same wattage - that's what the output transformer does. You just need to match the transformer tap to the impedance of the cab as it it can be disastrous for the health of your amp if you don't. There might be other benefits to using a particular impedance (which maybe someone more qualified could comment on), but you won't get a higher wattage from it.[/quote]

Ooo. Thats interesting. Thanks. So it doesn't matter whether I pick an 8 or 4 ohm, it will still deliver 120 watts? I guess I just need to be careful to plug it into the appropriate output.

[Noisyjon]

Hello Folks,
A question for you those of you with a wealth of amp knowledge:

On my Mesa Boogie Bass 400+ (all valve amp) I have 3 pairs of speaker outputs - 2x 2 Ohm, 2 x 4 Ohm and 2 x 8 Ohm. All fair and good.
I was looking at Alex's 'Big One' speaker box and noticed that it is a 6 Ohm unit.
What speaker output would I need to use or is it not worth entertaining such an idea with a valve amp?

All and any help much appreciated,
Cheers,
Jon.

[Noisyjon]

Question bump

[elfabyanos]

[quote name='jonthebass' post='453355' date='Apr 3 2009, 01:53 PM']Question bump[/quote]

I wouldn't risk using an impendence that doesn't appear as an option on a valve amp. I guess guess that choosing either 4 or 8 ohms could be close enough, but if it doesn't blow up immediately you could still be shortening the life of the valves in the output transformer. Due to the lack of forgiveness in valve technology, and given the vast array of speakers available with the right impendence for your amp, I wouldn't bother with a 6 ohm cab.