Poweramp / cab rating question

[niceguyhomer]

I think there's been posts on this subject before but I'm still going to ask the question if you don't mind. Is it detrimental to speakers when using an amp with a higher power rating than the cabs? Even though the amp is turned down fairly low and the speakers sound happy?

The reason I ask is I used to gig with a LMII (500 watts @ 4 ohms) and two Aguilar 112's (2x300 watts @ 8 ohms each) and played some fairly big rooms with no problems at all. The volume on the amp would usually be set at 12 o clock. My current amp is 900 watts @ 4 ohms and I'm using this with two Bergantino 112s (2x300 watts @ 8 ohms each) and playing at the same sort of volumes but the volume on the amp is between 10 and 11 o' clock. Is this ok?

I use two speakons and run the amp on parallel but is there a safer way of connecting the cabs - ie; daisy-chaining?

Sorry, I can't find a smiley for thick, so this'll have to do

Edited March 3, 2008 by niceguyhomer

[Merton]

It'll be fine. The only rule, as always, is if you hear distortion, turn down!

EDIT: use both speaker outs rather than daisy chaining, that way it saves all th epower going down the first cable

Edited March 3, 2008 by Merton

[bass_ferret]

There are some good articles on the wiki about some of this stuff. What Merton says really, if it is farting turn it down.

The business about what o'clock you have on the knobs can be pretty meaningless as well cos it depends in the make, some give all the volume at the start to con people into thinking they are more powerful than they are - then dont get any louder. This is also often the case with amps with limiters protecting the output (like Peavey), the limiter cuts in when you reach say 5 on the knob and the rest is wasted.

[niceguyhomer]

Makes sense John.

[Merton]

[quote name='bass_ferret' post='150596' date='Mar 3 2008, 07:27 PM']The business about what o'clock you have on the knobs can be pretty meaningless as well cos it depends in the make, some give all the volume at the start to con people into thinking they are more powerful than they are - then dont get any louder. This is also often the case with amps with limiters protecting the output (like Peavey), the limiter cuts in when you reach say 5 on the knob and the rest is wasted.[/quote]
Yep, as I found out tonight by blowing one of the 8"s in my V-Type combo. The volume was on bloody 2. Grrr.

[david_l_perry]

HI Alan

As well as long term bass player (as well you know) I am involved in PA and bass cab speaker construction. Its become a fair old obsession...!!

Lets say your 2 drivers are rated at 300w RMS @ 8ohms
Thats 600w RMS @ 4ohms when linked together (and it [i]really [/i]does not matter if you link the cabs together or straight into the amp contrary to the second post - if it was a single 2x12 cab you would only be feeding one cable from the amp.....)

Speakers have another fact that not many people area aware of, and thats power compression.
Put simply, if you continuously put 300w into your 300w driver it will start to get hot and loose output (you may have experienced gigs where your rig sounded really loud to start with but lost output toward the end of the night ??....)

The typical figure [i]before [/i]power compression comes into play is roughly HALF the RMS rating..

Put simply anything more than half the RMS rating is just turning into heat, and is not adding to the output level...Anybody who has played a long gig with high power could have experienced power compression coming into play but simply not be aware of what is going on.....

But - and this is huge factor:- If you actually measured the voltage coming out of your power amp during playing, and worked out what the actual wattage was, I think you would be astounded how low a level your 900w amp is actually putting out. Its not to say your amp is not capable of 900w, just that due to the type of playing being dynamic, and all the other gain factors coming into play, its typically a LOT less than you expect. This is why most people don't have a problem (including myself when I ran a pre/power amp combo for a while)

Dave

Edited March 5, 2008 by david_l_perry

[Merton]

[quote name='david_l_perry' post='151507' date='Mar 5 2008, 08:25 AM']Lets say your 2 drivers are rated at 300w RMS @ 8ohms
Thats 600w RMS @ 4ohms when linked together (and it [i]really [/i]does not matter if you link the cabs together or straight into the amp contrary to the second post - if it was a single 2x12 cab you would only be feeding one cable from the amp.....)[/quote]
It doesn't matter, but it makes more sense to do it from both amp outputs. If you daisy chain, the first cable takes ALL the power down it, if you use both amp outputs, the power is split evenly across the two. There was a post by Oxblood or someone explaining this in terms of two bits of elastic and it makes sense, but I can't find it now. Basically, think of 2 equal weights, one on the end of each bit of elastic. If one bit of elastic is connected tot eh weight of the other, the first bit of elastic is holding twice the weight. If each bit of elastic is fastened to separate points (ie not together) then there is equal weight distribution between the elastic and one isn't holding both. Simple analogy, badly explained. It doesn't MATTER, but it DOES make a difference!

[david_l_perry]

[quote name='Merton' post='151521' date='Mar 5 2008, 09:13 AM']...it DOES make a difference![/quote]

Not even in the wildest of dreams does it make any difference at all.....If you can [i]hear [/i]a difference then its in your head, or a faulty lead

As long as the cable is capable of transmitting the current then its fine.

Its standard practice on [i]much [/i]higher power PA cabs to do this.

Check out any 4x10 cab and you will see they effectively use nothing more than bell wire to connect the drivers together.....

Dave

[Merton]

I didn't say it made a difference to the sound, it makes a difference to the amount of power going down the first lead.

But I did also say that it doesn't matter, you can connect how you want!

[gilmour]

[quote name='Merton' post='151521' date='Mar 5 2008, 09:13 AM']It doesn't matter, but it makes more sense to do it from both amp outputs. If you daisy chain, the first cable takes ALL the power down it, if you use both amp outputs, the power is split evenly across the two. There was a post by Oxblood or someone explaining this in terms of two bits of elastic and it makes sense, but I can't find it now....[/quote]

Here you go, different forum though

[quote name='Oxblood']...in the vast majority of modern cabs: if the cab has two sockets on the back, they'll be wired in parallel too. You feed your amp's signal into one socket (doesn't matter which) and then use the other one to pass the signal on to the next cab - it's called 'chaining'.

Let's imagine our amp has two output sockets as described above, and we've got two cabs. If we run one cab off each socket, the cabs will be wired in parallel. If (for whatever reason) we decide to use only one socket of the amp and 'chain' our two cabs together, they are still wired in parallel.

The only practical difference between the two setups has to do with the current being drawn by the speakers, and whether your speaker cable is hefty enough to handle it. Running one cab off each of the two amp output sockets will share the current drain equally between the cables. As soon as you chain two cabs together (to make life easy, we'll assume that both cabs are electrically identical in terms of impedance etc.), the length of cable from the amp to the first cab has to carry all the current drawn by both cabs. The cable from the first cab to the second one only has to carry half the current.

Here's a way to envisage it: imagine a piece of string tied to a hook in the ceiling. Hanging from the string is a 1 kilo weight (that's analogous to the first cab in the chain). If we then attach another length of string to the bottom of that weight and hang another 1 kilo weight on the end of it, what happens? The string from the ceiling to the first weight has to be strong enough to carry 2 kilos without breaking, but the string from the first weight to the second one only has to be capable of carrying 1 kilo. Geddit?

With modern heavy-duty speaker cable, however, such considerations should not be a major worry - unless you're planning an Entwistle-style multi-thousand Watt rig![/quote]

Basically you are both right - just gotta make sure that your cable is thick enough (which it should be)

Edited March 5, 2008 by gilmour

[Merton]

Cheers dude!

[david_l_perry]

Perhaps if you are using bits of cheap bellwire or string to transmit the current, that may be true....
....The sort of power rating Alan is talking about here it makes absolutely no difference...go and chastise yourself Rob

Having said that a good pal of mine and seasoned player, was still using a signal lead as a speaker cable.....

..as long as you are using any standard speaker cable it does make no audible difference at all.....

Dave

[Merton]

Absolutley. I think I kinda missed that point with what I was trying to say so it came across slightly wrong, apollogeez. I suspect Alan (correct me if I'm wrong) is using OBBM cables so all hunky dory

[i]EDIT 2: to stop me being a pedantic asshole![/i]

I love going to rehearsal rooms where an amp is connected to a cab with a patch lead. That's fun.

Edited March 5, 2008 by Merton

[gilmour]

[quote name='david_l_perry' post='151586' date='Mar 5 2008, 10:49 AM']..as long as you are using any standard speaker cable it does make no audible difference at all.....[/quote]

That's what I took Oxbloods post to mean, basically as long as you use a proper speaker cable it's fine. But good to understand the mechanics behind it.

I think I caused confusion where I was trying to hlep *doh* - I am suitably chastised!

[quote name='Merton' post='151588' date='Mar 5 2008, 10:53 AM']I love going to rehearsal rooms where an amp is connected to a cab with a patch lead. That's fun.[/quote]
I'm ashamed - I have actually done this - Again I chastise myself

R

Edited March 5, 2008 by gilmour

[niceguyhomer]

Thanks for the help fellas and yes - I use OBBM's cables and always will.

[Toasted]

There's an audible difference between my rig with a brand new Stagg speakon and an OBBM speakon.

That's not imagined, I've done a double blind test. I could always tell the difference between the two with 100% accuracy. The OBBM cable sounds [i]that much better[/i].

[Bill Fitzmaurice]

[quote name='Merton' post='151559' date='Mar 5 2008, 05:12 AM']I didn't say it made a difference to the sound, it makes a difference to the amount of power going down the first lead.

But I did also say that it doesn't matter, you can connect how you want!

[/quote]+1. You probably won't hear it, but daisy chaining does result in the cable connected to the amp carrying twice the current than otherwise.
[quote]There's an audible difference between my rig with a brand new Stagg speakon and an OBBM speakon.

That's not imagined, I've done a double blind test. I could always tell the difference between the two with 100% accuracy. The OBBM cable sounds that much better.[/quote] 'Better' is the wrong choice of words, technically speaking. No cable can make anything sound better. OTOH, a poorly built cable can make something sound worse.

[tauzero]

[quote name='david_l_perry' post='151586' date='Mar 5 2008, 10:49 AM']Perhaps if you are using bits of cheap bellwire or string to transmit the current, that may be true....
....The sort of power rating Alan is talking about here it makes absolutely no difference...go and chastise yourself Rob[/quote]

Paralleling rather than daisychaining does also mean the current to each speaker is only going through two jack/socket interfaces. All still pretty trivial though.

Should I mention that I built my 2x10 cab with two jack sockets so I can run it as either a single 4 ohm speaker or 2 x 8 ohm speakers? There is a serious reason for that...

[quote name='Merton' post='151588' date='Mar 5 2008, 10:53 AM']I love going to rehearsal rooms where an amp is connected to a cab with a patch lead. That's fun.[/quote]

I do that.

Not when I'm using anything seriously, of course, just when testing stuff.

[alexclaber]

[quote name='david_l_perry' post='151507' date='Mar 5 2008, 08:25 AM']The typical figure [i]before [/i]power compression comes into play is roughly HALF the RMS rating..

Put simply anything more than half the RMS rating is just turning into heat, and is not adding to the output level...Anybody who has played a long gig with high power could have experienced power compression coming into play but simply not be aware of what is going on.....[/quote]

Power compression is power compression, not power limiting. So as any voice coil warms up its resistance will increase and thus the impedance the amp is driving, thus reducing current flow and thus the power going into the voice coil. But it isn't a simple on/off state, and to say that once you're above half the thermal power rating that any additional watts will make no difference in output is incorrect. I understand that Bill F has written about this regarding his subs and in those cases more caution is needed due to the combination of the acoustic low pass of the horn hiding any over-excursion distortion and typical compressed program material having greater average power and thus more risk of overheating the voice coil but for live bass guitar use I can't see power compression causing many problems.

Here's some power compression data for a midrange driver I'm using:

Power Compression @-10dB (12 W) 0,7 dB
Power Compression @-3dB (60 W) 1,2 dB
Power Compression @Full Power (120 W) 1,6 dB

So 12W input to this 102dB sensitivity driver produces 112.1dB when it should produce 112.8dB output, 60W gives 118.6dB when without compression it should produce 119.8dB, and 120W gives 121.2dB when without compression it should produce 122.8dB. Note that going from half to full power gives you an increase in output of 2.6dB, versus the 3dB increase you should get without power compression but it's still 2.6dB more output which is a lot more than 0dB extra output.

[quote name='david_l_perry' post='151507' date='Mar 5 2008, 08:25 AM']But - and this is huge factor:- If you actually measured the voltage coming out of your power amp during playing, and worked out what the actual wattage was, I think you would be astounded how low a level your 900w amp is actually putting out. Its not to say your amp is not capable of 900w, just that due to the type of playing being dynamic, and all the other gain factors coming into play, its typically a LOT less than you expect. This is why most people don't have a problem (including myself when I ran a pre/power amp combo for a while)[/quote]

Exactly - far far less! And if you bear in mind that the speaker impedance is rarely as low as the nominal impedance the power involved is even less.

Alex

[niceguyhomer]

Thanks everyone