Sorry to interrupt when grown-ups speak 
But could one of you guys briefly explain what “pad” means in this context?
Much obliged!
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that’s just a signal attenuation to make preamps compatible with both active and passive instruments, @joergkutter
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Electronic circuit, usually resistors, that drops the signal by so many dB. The mic/line switch on a mixer engages a pad to pad down the line level to mic level so it doesn’t overdrive the preamp. The active/passive switch on an amp head is the same concept.
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Or as I like to call it the active/passive button.
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Good bit of info.
So, I have an 8ohm cabinet, it is part of my set. It is a 400w 4x10 cabinet that is made to accompany the matching SWR WorkingMan 15 combo amp.
But having this as my cabinet, if I choose to get a head to power it alone, and not use it with the combo, based on what you said, I would really need to go big on the amp head. If I get a 250w head, I am limited to 125w? And if I get a 500W head 250 it what I will push, or will push it, and if I wanted to use the whole 400w it is rated for that i would need to get an 800w head?
Or
If I wanted to use 500W I would need to get the 500W head and a matching 8ohm speaker cabinet to run with it?
Is that all correct?
Thanks
Is there instances where there would be an AMP HEAD rated for 250w @8ohms? or is that ONLY a 500w head (@ 4homs)
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It depends on what the amps are rated at. The amp specs should give the rated power output at a specific cabinet impedance, i.e. “500W at 4 ohm”. If it doesn’t, the power rating will be for the specified minimum impedance, which should definitely be noted. The minimum impedance will probably be on the back of the amp as well. Note that some amps may have higher minimum impedances (8ohms, etc).
If you are driving an amp rated at 500W at 4 ohms into an 8 ohm cabinet, the amp will only deliver 250 watts to the cabinet. Driving the same amp into two 8 ohm cabinets in parallel will deliver 250W to each, or 500 total. Driving the same amp into two 8 ohm cabinets in series will deliver 125W to them.
See this topic:
this is discussed in detail there.
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Thanks, but the question was specifically related to what @DaveT said in the header of my question.
I get all the math you laid out, but it seems to be an industry standard the way Dave said the only reason to get a 8ohm cabinet is if you want to run two on an amp.
Specifically the question is are ALL amps rated towards 4ohms. And by All, I don’t mean the exceptions, cuz there will always be some, so then the better way to ask is are MOST amps rated based on 4ohms.
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I checked the manual for your Workingman 15. It has the option for an extension cabinet. This is why they ship it with an 8 Ohm initially, so you can parallel two cabinets. Their spec sheet says it’s only a slight power drop when using only the 8 Ohm. I don’t know why they are able to claim this.
The 4 Ohm Power number is almost always the advertised one because it’s bigger.
It’s possible to design amp heads that drive down to 2 Ohms. I’ve seen a couple.
It’s possible to see an amp spec where the 8 Ohm power and the 4 Ohm power are the same. It’s unusual (maybe none) for bass gear. I think it was a Crown PA amp I saw this.
The thing that’s hard to believe is that there’s very little difference in level when using 250 Watts or 500 Watts. It sounds big, but it only gets you 3dB. If someone buys a 500 Watt amp and connects it to an 8 Ohm speaker, they will be happy to the end of time at how good their 500 Watt rig is, probably never missing that 250 Watts. Otherwise, they are probably about to graduate to a whole new category of huge.
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Weird. Some kind of internal load resistor when operating at 4 ohms? 
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tube power amps have different outputs for different cab impedance, wired to different secondary output transformer coil tap, so they have the same power under 2, 4, 8, 16 ohm , depending on the outputs available but you get the idea.
I’ve already seen solid state amps rated at the same power under 4 and 8 ohm, but it’s rare. don’t know how it works.
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Yes, I have the combo amp AND the extension cabinet. I have the whole set.
What I was talking about was that you said the only reason to get an 8ohm is if you want to run two speakers together on one amp.
So
I am asking, because i have one 8 ohm cabinet. I may want to run another amp thru it, I don’t want to get another cabinet to use with it. This was really the only question.
Not about the math, not about my gear, but about the industry gear if you have an 8ohm cabinet and want to use it. I assume there are amps out there that may be rated for 8ohm cabinets, and I know they go down to 2ohm, I had one for a little while. Is there specific amps to look for to run a single 8ohm cabinet, or just get any amp and 1/2 the power to know what i am getting?
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What is the difference between 125w and 500w.
If I were to get a 250w and run it thru 8ohm cab, I would get 125w.
Is the difference between 125w and 500w thru the same cabinet staggering, or is it again, not too big? This would be very helpful information. I do not really know the math about decibel and loudness, power is easier for me to understand, but what sound level it creates is not so easy for me to grasp without hearing it.
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6db.
Every three decibels of volume requires the power output to double.
in contrast, a perceived doubling of volume is something like 10db (I forget exactly how much).
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Yes
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Thanks, that helps
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+1
It’s 10
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Is there any way to guesstimate something like:
125w amp full power would be equivalent to 250w at 7/10 and 500w at 5/10, or are there too many variables to make a generalization like that.
and I don’t mean 7/10 and 5/10, just using numbers to fill in, if you know, what would they be
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I wouldn’t be able to guess it.
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Here is what I used to use for stereo amps. It is a generalization. It also assumes some things. One is you have to know the sensitivity of your speaker. A speaker that is 85 dB sensitivity (average home speaker) means 1W will give you 85 dB with your head 1 meter in front of it. Not realistic in the real world, sound diminishes quickly with distance.
Watts to dB (see explanation below)
1W = assume 85dB/w/m = 85dB
2w = +3 dB = 88dB
4w = 6
8w = 9
16w = 12 = 97dB (2x loud)
32w = 15
64w = 18
128w = 21 = 106 dB (2x loud)***
256w = 24 = 109 dB***
512w = 27 = 112 dB***
1024w = 30 = 115 dB (2x loud)
2048w = 33
4096w = 36
8192w = 39 = 124 dB (2x loud)
some sample spl readings at performances:
New age: 60-70 dB
Folk: 75-90 dB
Jazz: 80-95 dB
Pop: 90-95 dB
Classical: 100 dB
Rock: 95-110 dB
Heavy metal: 110 dB
It’s kind of accepted that the smallest increase in dB that can be easily heard is +3 dB (i.e. increasing 5 dB to 8 dB sounds “a little louder”). In order to get this you need to DOUBLE the amp’s watts. A +10 dB increase is perceived as twice as loud. So as you can see as amps get bigger you get insane diminished returns. Which is why I think buying huge amps is a waste of money. Some people will argue that it gives you added “headroom” (peak transients/loud short bursts) but the usefulness of this seems to be controversial amongst engineers (and basses don’t have nearly as much peaks as guitars). This makes sense because as we just said, an amp would have to be able to momentarily double it’s continuous power to give you a +3 increase in dB (a little louder). To me, for most people the sweet spot zone for amps is *** above, which is why most bass amps are between 100-500w.
tldr: get a 500W amp. if you need a super monster amp, get a 1000W amp. Crown amplifiers has a great article on this.
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@itsratso I read the whole article.
Now I want to play a coffee shop with 10,000 watts and a double stack of 105 db sensitivity speakers. 
Seriously though, that was a great article.
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