4/8 Ohm Impedance Explanation

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.

Hey, @eric.kiser . . . :slight_smile:

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?

This rabbit hole can run pretty deep, @eric.kiser . . . :slight_smile:

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 . . . :slight_smile:

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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