I’m not offering an explanation. I’m looking for an explanation. If anybody feels like explaining what 4/8 ohm impedance is and how it applies to music equipment, I would greatly appreciate it.
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Hey, @eric.kiser . . . 
The impedance has to do with amplifiers and speaker cabinets. Some cabinets are rated for 4 ohms and some for 8. A cabinet designed for 4 ohms needs more power from an amp head to make the same sound level as an 8 ohm cabinet would need.
You’d have to look it up, or seek guidance from a music store for more details.
HTH and all best, Joe
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Okay, then what is the technical advantage of working with 4 ohms?
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This rabbit hole can run pretty deep, @eric.kiser . . . 
You wouldn’t need to worry about it if you use a combination amp (such as the Fender Rumble series). If you are shopping for a stack amp with a separate head and cabinets, you’d have to be careful to match the impedance properly.
I found an old video you might find instructive:
Good luck and all best, Joe
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Thanks, Joe!
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Okay. So, I had a chance to go talk to the manager at my local Guitar Center. He was very patient with all my questions and had alot of insightful answers. I’ll try and distill it down. As @Jazzbass19 said, it can turn into a bit of a rabbit hole. Please keep in mind I am not a professional and this is probably over simplified.
The wattage stuff is fairly self explanatory but I thought ohms would be easier to explain if I started with wattage to create context.
Wattage - The amount of power your amp head can put out.
Ohms - The resistance your cabinets have to that power.
Resistance generates heat. You can think of the Ohms rating on your head as how much heat your head is able to disperse before something starts to melt down, blow out, or catch on fire.
Head - If you have an 800 watt head you are able to put out 800 watts. But the wattage of your cabinets will determine how much of that 800 watts you use.
Cabinet - If you have an 800 watt head but only have a 400 watt cabinet, you are only putting out 400 watts. To get 800 watts you will need to add 400 more watts of cabinets. If you have 1200 watts of cabinets you are still only producing 800 watts because that’s the maximum for the amp head.
Back to ohms…
Here’s the progression you need to know; one 4 ohm = two 8 ohm = four 16 ohm
If the amp head is rated up to 4 ohms, it can handle…
one 4 ohm cabinet or
two 8 ohm cabinets or
four 16 ohm cabinets or
one 8 ohm cabinet and two 16 ohm cabinets
If the amp head is rated up to 4 ohms, do NOT exceed the 4 ohms or the amp head can be damaged.
Bringing it all together…
Without a physics background the ohm ratings seem counter intuitive and can be confusing. It’s really just about what can plug into whatever else. If you’re putting together a stack, it is much more intuitive if you go to the store and look at it.
Ultimately, I didn’t really need to know any of the ohm stuff and 90% of musicians probably don’t either. But I didn’t know this till I started doing research.
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Without realizing where I was going, all of this dovetails in to this other thread.
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Nice word back from a GC manager.
That’s good info, and I can attest to the quality of the info.
I played in a band that was just electric bass and drums, and I was playing a Mesa M2000 into an Eden 4x10 XLT cab.
The amp was 600W @ 4ohms, the cabinet was rated for 750W… but was an 8ohm cab.
So the resistance from the cab was keeping me from reaching max volume potential. (Something crucial to the success of this particular project.)
So.
I bought a matching Eden 4x10 XLT at 8ohms. The two 8ohm cabs - when used together - were ideal for my amp rated for 4ohms.
I had reduced the resistance and added 40" of speaker surface.
Maximum Volume acheived.
It was one of those mystical things that the music store folk (the guys over in the Pro Audio department - not us guitar and bass players) told me that didn’t make sense. Counter-intuitive, don’t you know.
That if I bought another cabinet at 8ohms, and used it with my existing cab at 8ohms, that I would actually be optimizing and reducing the resistance from the cabinets to the amp.
Crazy. Yet, true.
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@Gio Thanks for chiming in the real world experience really helps.
Based on this were you only getting 300W out of your 750W cabinet since it was 8 ohms going to a 4 ohm connection?
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Electronics courses often use a water analogy to explain resistance. So say you have a “pump” (the amp) that is rated to pump out 400 gallons of water per minute, if the “hose” (the speaker cab) will only slow the water down by 4mph.
When you connect a hose that will slow the water down by 8mph, your pump can only pump 200 gallons of water per minute, because the hose slows the water twice as much.
However, if you add a second hose in parallel, the pump can deliver 200 gallons per minute into each of them, for a total of its max rated 400 gallons per minute.
This actually works in the other direction too. If instead you connect a hose that is super fast and only slows the water by 2mph, you have to be careful with levels because your pump can pump too hard and burn up.
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I love this!! Makes tons of sense and clarifies a subject that was near-mystical to me.
So then…
Pretty much.
The full-capacity rig is a thing of decibel-crushing-glory that must be witnessed to be believed.
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@howard, that was excellent. The manager at GC did get into this but we had been talking for a while and he was talking at 4 ohms and I was hearing at 8 ohms. So, I only got half the information. Thanks for filling it out.
@Gio is it common to end up in a situation where wattage ends up being mis matched one way or another because of how the impedance works out?
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Not usually. Usually people are buying matched gear - same brand head for same brand cabinet - that kind of thing. The manufacturers work most of the math out for you on their end.
I was mixing and matching brands, and also completely oblivious to anything regarding technical details at the time.
Anyone purchasing gear with their ohm-antennae up will do just fine and if you’re buying matched brand gear, it’s almost guaranteed to be optimized for its own stuff.
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That’s a pretty good analogy, @howard . . .
For example, I know that the Rumble 200 only puts out 140w and you won’t get the full 200w unless you attach another cabinet to it.
All best, Joe
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I feel like you should really look up how to add parallel and serious resistors to really understand what is going on… ohms is “resistance” or electrical resistance. 4 ohms is half of 8 ohms (half the resistance) so you can get twice the current with the same voltage. Two equations need to be sort of memorized so you can break out the calculator and figure stuff out if you are ever left to solve this stuff on your own. the voltage : v = (i)®
voltage (volts) = current (amps) times resistance (ohms)
and the power equation (there are two) :
p=iv and p = i^2 * r (i squared r)
power (watts) = current (amps) times voltage (volts)
power (watts) = current squared times resistance (ohms).
with these two equations you can see that 4 ohms being half of 8 ohms will instantly double the amount of current out of your amplifier for any given volume setting (voltage output level setting). WHY? because v=ir
Assuming you don’t touch your volume, and the resistance is cut in half, then the current MUST double to maintain an equation!!!
Just remember that voltage is the “force” that pushes electrons through your speaker wire. lower ohms is like a “thicker” wire in the voice coil. In general, any amp will make twice the power if you cut the resistance in half. This is not “always” the case because ultimately, an amp has a max current output-- which can cause voltage sag in the rail lines and you could just get more distortion… many times though, as long as your amp can handle the lower ohm rating, it will double the power output. Twice the power is not twice the volume heard. Volume heard is more related to speaker excursion or distance traveled… and with low frequency it has a bit to do with volume of air moved. Where things get tricky is when you hook up two speakers to the same amp. USUALLY, this makes them a “parallel load” which means common ground, common positive terminal— and you can not add the ohms together-- you should have matching cabinets (ohms) if you do this or else the cabinet with the lower ohms will take more current and diminish the one with more ohms. It can get really hairy. I personally would not hook up more than one “bass” cab to an amp because low frequencies draw more current and current causes “heat” and “heat” burns up transistors. With tubes-- its a bit different… some tube amps are designed for higher current and if you stick in their rated range, you’ll be fine… Tube amps do not do well without a load on them-- which is the opposite of solid state which you can power up without a load no problem. Trying to understand why requires mastery of this topic. anyhow… Series would mean the electrons flow out of the amp’s ( - ) terminal into the ( - ) terminal of the first speaker (thru the speaker) then OUT the ( + ) terminal of that speaker then into the ( - ) terminal of a second speaker (through the 2nd speaker) then out the ( + ) terminal of the second speaker then back to the amp’s ( + ) terminal. THIS is how most people “try” to think of the math but obviously a speaker cable has a + and - terminal and you can’t do this because if you look closely, in series the electrons go from the amp to the FIRST speaker… then to the 2nd speaker THEN LAST, back to the amp-- how is that possible??? when you use speaker cables that have both + AND - ???
you can’t do that-- you’d have to use individual wires and make special connectors to hook them up to do two cabs in series!!! ( believe me, people out there do this in some cases to increase “ohms” and squeeze more power out of fewer high power amps without burning them up)… high current burns up amps, not high watts necessarily which is always relative. The goal should be to get a big enough amp that one channel can supply the peak amp power (tops) to each cab without running the amp below 4 ohms for solid state and for tube, below manufacturer recommendations which I have seen 4 or 2. I don’t care what they say, I won’t run subs or bass cabs in a way where the amp sees 2 ohms. Current levels just run too high. I like to run bass cold and have more power than necessary and just not turn it up loud and then you never work anything too hard and don’t run out of juice when you hit those extended low notes.
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Thanks for the in depth explanation.
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Good technical thread on amps that I feel like bumping!
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Actually you do need to worry about this if you wish to expand your combo amp to get more wattage, and sound out of the limits put on the combo amp.
Like the case @Gio stated in his first post, needing another cabinet to get the full rated wattage from his 600w head.
I know there has been a more accurate and technical explanation posted in this thread, but I was in a situation where this mattered, and lead me ultimately to my decision when I purchased my SWR Combo. It is 160w at 4ohms. It CLEARLY STATES. To expand DO NOT USE A CABINET LESS THEN 8OHMS.
My SWR WorkingMan 15 Combo rated at 180 RMS is 200 @4ohms with with addition of an 8ohm Cab, but if I were to add a 4ohm cab, I risk burning up the internal amp head.
The reason it mattered is that I have a Hartke 8x10 cabinet already, and it has not sold yet. When I was researching Combo’s I was hoping to find something that I could use with that Cab nstead of getting, a cab with the combo amp now or in the future, however the Hartke is 4ohms, and almost every combo amp I looked specified expansion cabs must be no less then 8ohms.
I think a lot of expansion cabs are 8ohms. I think the reason the Hartke cab is 4ohms is because it is 8x10, a full stack, and may typically not be used along with other cabs on the same head. And / or because it’s 800 watts, and any head that is going to put out enough power for two of them will likely be rated down to 2ohm output.
Just thoughts, and posting in a dead thread for conversation
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Parallel resistance equations… this brings me back to my Navy days when I learned all this stuff. Glad there are already some real solid explanations in this thread already so I don’t need to jump into it. Great info in here. Howard’s two hoses one pump analogy is the simplest way to wrap your head around it without more complex electrical theory.
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