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FDP Forum / Fender Bass Guitars and Bass Amps / Why do we have different impedance ratings ?

Contributing Member


Jan 1st, 2018 03:56 PM   Edit   Profile  


In my relentless quest for knowledge this question has just occurred to me.

Why are amplifiers and speakers built/made with differing impedances ?

Is there a technical or performance reason there are different impedances ?

Is just a accident of history like using 110V or 240V ?

Are different impedances better for differnt purposes ?

Could the world get by with with a single impedance ?



Contributing Member

Whitehorse Canada

I don't get out much
Jan 1st, 2018 05:51 PM   Edit   Profile  

Different output tube configurations want to see different loads.

Contributing Member

Curled up

in the fecal position
Jan 3rd, 2018 09:41 AM   Edit   Profile  

Yeah, there was no industry standard for power amplification valves. The many makers used various materials and topologies based on the circuits the tubes were designed for.

A good mechanical cognate is the transmission on a car: there's no industry standard for how an engine makes power (i.e., RPM to torque), so the transmission is built to accommodate that. It puts the power to the wheels on the ground as efficiently as possible.

Just like how an amp's output transformer puts the power to the speaker as efficiently as possible.

Contributing Member

Who's we sucka?

Smith, Wesson and me.
Jan 8th, 2018 08:34 AM   Edit   Profile  

The number of speakers in a cab as well as the number of cabs being used alters the total load impedance.

Two 8 ohm loads in parallel will present a 4 ohm load to the amp.



Jan 8th, 2018 03:48 PM   Edit   Profile  

Yes, and 4 16 ohm speakers in parallel with lend a 4 ohm output. Two 4 ohm speakers in series will give you 8 ohms. Or put two 8 ohms in series, another two 8 ohms in series and the tie those two sets in parallel an you get 8 ohms. Fender was big on that last one.

Contributing Member


Life makes a man tired.
Jan 8th, 2018 07:27 PM   Edit   Profile  

4 crappy 32 ohm speakers in parallel will give you 8 ohms.


Bassman 10

Contributing Member


Jan 9th, 2018 05:10 AM   Edit   Profile  

Thanks but why do we have speakers with different ohms in the first place ?

Enquiring minds need to know !

Contributing Member


Life makes a man tired.
Jan 9th, 2018 01:00 PM   Edit   Profile  

So the math works in multi-speaker cabinets.

Tone happens

Jeff Scott

On a spinning rock

Jan 9th, 2018 04:40 PM   Edit   Profile  

Leftee said: "Tone happens"

Not always.

Tony Wright
Contributing Member

Stillwater, OK

I never met a calorie I didn't like.
Jan 9th, 2018 08:44 PM   Edit   Profile  

A practical application for Ohms and Watts and amplifier power rating and speaker power rating.

Let's assume that normally, you play bass guitar in a local club cover band. You play in a venue that has a fire code occupancy rating of only 200 people and honestly, you can get by with just a modest size bass rig.

So you have an Ampeg PF500 bass amp. That amplifier is rated for 300 Watts output when presented an 8 Ohm load. So you play your normal club gig with an 8 Ohm Avatar speaker cabinet with a single 15 woofer that is rated to carry around 350 Watts...so your PF500 is an adequate amp for the cabinet.

Next month, you have a larger gig. A small town festival and they tell you in advance that they do not have a lot of low frequency support, so you need to push as much as you can.

No problem, you have a buddy who will loan you his 8 Ohm Avatar 2x10 speaker cabinet that is also rated for 350 Watts.

SORRY, but your Ampeg PF-500 is not going to put out enough power to match the two 350 Watt speaker cabinet.

So, two 10s with a single 15 stack and your PF-500 on top driving 500 Watts to the three speakers, two 10s and the single 15. That Ampeg only has 500 Watts to share between the three speakers in the two cabinets.

ALL of the connections from the two Avatar cabinets are "parallel" so the math is simple. Your 500 Watts is divided by 2. Your two 10s are each rated for 16 Ohms because the cabinet is wired for 4 Ohm load. So that cabinet gets 50% of the available 500 Watts (250 Watts shared between the two 10s) and your 8 Ohm single 15 is getting an equal 250 Watts.

You would be getting "adequate" Watts to each component and thus you should benefit running the second cabinet since you have gone from the 350 Watts that the PF-500 can put out at 8 Ohms and the extra power that the same PF350 when presented with a 4 Ohm load (two 8 Ohm cabinets).

So, your only questions are:
1) Is your buddy expecting to get paid "rent"?
2) Is the extra power worth the cost?

You can relax knowing that the power to each speaker is within the limits of the speaker and the power amp.

By knowing the amplifier's Ohm ratings and the power it can produce as multiple Ohm loads will give you peace of mind knowing you will not be damaging speakers due to mismatched power ratings for either Ohms or Watts.

On the other hand, if Bubba wants to rent/loan you his 4 Ohm 2x10 cabinet that is rated to support 350 Watts....walk away. If you mate that cabinet with your 1-15 you will be placing a 2.222 Ohm load which can damage your bass amp.

IF you were a thinking man, you might look at just swapping bass cabinet for the night. One 4 Ohm cabinet will pull all 500 Watts out of that Ampeg PF-500. You have two 10 inch speakers that will receive 250 Watts each. That would be better (probably louder) than your single 15.

Does that start helping you to understand why it is important for the user to at least know what will match his power amp or provide more power when he has the capability to support a larger speaker set up, if his amp can supply the extra power at the Ohm rating presented.

Recently, I decided to buy a smaller bass rig. We play small venues...typically UNDER 100 people. I bought an Ampeg PF350 rated at 200 Watts at 8 Ohms and 350 Watts at 4 Ohms. I found a small, single 10 inch, 4 Ohm bass cabinet (Eden EX110-4). The EX110-04 is rated for 300 Watts at 4 Ohms. Since I NEVER push my bass, I was confident that I could live with the 50 Watt mis-match between amp output and the speaker rating.

PS: I used that small bass rig at our New Year's Eve gig in a local saloon/honky tonk/pool hall...it was great...never got it over 50% on the amp. PLENTY for the 75 people in that place.

Suppose I knew we were going to gig a venue that averages 250 patrons...well, I would go in the garage and grab my Ampeg SVT350Pro amp rated for 450 Watts at 4 Ohms and my Avatar B410 cabinet that can support 600 Watts at 4 Ohms. But why take something that large to that small of a venue...at 68 yr old, I do NOT need to be lifting 65 pound cabinets when a 30 pound cabinet is adequate.

And yes, I had other options, but that was the thought process at the time.

Other applications? Well, do you have a tube guitar amp? You need to match the Ohm rating of your amplifier. EXACTLY.

It is less critical to a digital amplifier or a solid state amplifier to get a perfect match of your Ohm rating.

Edit to add: That last paragraph may not be clear on my intent. It should be:

If you have an 8 Ohm solid state amp, you can support a 16 Ohm load or two 16 Ohm loads (which combine to create on 8 Ohm load) or an 8 Ohm load. NOT a 4 Ohm or 2 Ohm load. If you have a 4 Ohm load amp, you can support 16 Ohms, two 16 Ohms (which is one 8 Ohm load when combined), a single 8 Ohm speaker, two 8 Ohm speakers (which is one 4 Ohm load on the amp) or a single 4 Ohm load. But you cannot support a 2 Ohm load on a 4 Ohm amp, nor can you support 2.6667 Ohms on a 4 Ohm tube amp or solid state amp.

The purpose of this exercise (multiple cabinets and lower Ohm ratings) simply means you might end up with MORE SOUND because you have more speakers...or you might blend your sound better with different cabinets...a single 8 Ohm 15 inch speaker mated to a pair of 12 inch 16 Ohm speakers just might give you more sound that you like on that 4 Ohm tube amp head.

The rule on Ohms:

Two 16 Ohms speakers in parallel equals one 8 Ohm load.

Two 8 Ohm speakers connected in parallel equals one 4 Ohm load.

Three 8 Ohm speakers in parallel equals 2.667 Ohms

And two 4 Ohm speakers equals one 2 Ohm load.

And that's what you need to know about Ohms. This becomes IMPORTANT when you are trying to get as much sound (decibels) from your amp and speaker set up. Of course, then you need to consider the efficiency of the speakers (SPL=Sound Pressure Levels measured in Decibels). Typically, when you plan on what gear, you are looking at the Watts the amp will produce...and the Ohm rating of the amp...THEN you start looking at speaker cabinets and their Ohm rating, their power handling capabilities...AND you want those speakers to have a high SPL to get as much reach with your sound as you can get. That was the process I used when I picked up the Eden EX110-4 Ohm cabinet to mate with my Ampeg PF350. I need a small, lightweight bass rig for house parties, rehearsals, small clubs and such...I already owned the SVT3Pro and Avatar B410 cabinet. Big rig and small venue rig. To date, I have not taken the SVT3Pro and Avatar out for MY gig...sent them on gear rental gigs...But I have not needed that much power or speakers for about 3 years.

(This message was last edited by Tony Wright at 03:56 PM, Jan 13th, 2018)

Contributing Member


Jan 15th, 2018 03:22 PM   Edit   Profile  

Thank you Tony for your reply

That has helped a great deal

I am considering getting a "small rig" head and cabinet but would the option of adding another cabinet if the need arises.
(Fender Rumble 500 head and 2x10 cab)

Now I need to learn about speaker wattage :)

Tony Wright
Contributing Member

Stillwater, OK

I never met a calorie I didn't like.
Jan 15th, 2018 10:49 PM   Edit   Profile  

The Rumble 500 head (online discount price $399.99 USD) produces 350 Watts at 8 Ohms and it produces 500 Watts at 4 Ohms.

The Rumble 2x10 cabinet (online price $349.99 USD) is rated for 700 Watts at 8 Ohms. That means the Rumble 500 head will only be giving the Rumble 2x10 cabinet 350 Watts at 8 Ohms. Not the best match. Adequate for most rooms I played in my last band.

If you decided to go with the Rumble 4x10 cabinet ($399.99 USD online price) you will find that the Rumble 4x10 cabinet is rated for 500 Watts Continuous at 8 Ohms.

When you figure the cost of the Rumble 500 amp head and the Rumble 2x10 speaker cabinet, the price is $749.98 for head amp and 2x10. The Rumble 500 amp head and the Rumble 4x10 cabinet price is $799.98 which makes MY choice of those two systems simple....two more speakers and because the cabinet is rated for 8 Ohms meaning it gets the same power as the 2x10. So...why spend the extra?

And while we are spending extra, we could simply buy the Fender Rumble 500 Watt 2x10 Bass Combo Amp (online price $599.99) and produces 350 Watts at 8 Ohms but can also support an 8 Ohm extension should you opt to buy one at some point.

Combo amp 2x10 $599.99
Head amp and 2x10 $749.98
Head amp and 4x10 $799.98

In the case of the combo 2x10 amp and the head amp with 2x10, then your two 10 inch speakers will each get 175 Watts...350 Watts total per cabinet.

The head amp with 4x10 would send 87.5 Watts to each of the internal speakers. A total of 350 Watts to the 8 Ohm cabinet...EXCEPT the Rumble 500 cannot produce 500 Watts at 8 Ohms. It is restricted to 350 Watts when presented an 8 Ohm load.

"I" would opt for the budget priced Rumble 2x10 combo amp for $599.99 (budget is in the value of the equipment and the eye of the beholder).

NOW...there are dozens of other options, but if you follow the math and money and Watts and number of speakers....well, you can find what is a bargain for you in your current situation.

Going back to my Ampeg PF350 head (350 Watts at 4 Ohms) and my Dean EX110 speaker cabinet (rated at 300 Watts at 4 Ohms) You will see that my small rig is very efficient in power and speaker output. I have a 2x10 Ampeg SVT210AV should I need more, but I also have lots of other stuff around the house...and there is always the opportunity to go direct into the PA system with my Sans Amp Bass Driver DI preamp.

So look at your options and learn by reading the owner's manuals and asking questions from local guys you trust and then guys like me who act like we know what we are talking about...and sometimes we do know something. Other times? Not so much.

Contributing Member


Jan 16th, 2018 03:36 PM   Edit   Profile  

Thanks again Tony for such an informative reply.

The criteria is

First the background,it's for an old mans blues band playing small rooms of around 100, same as your small rig example.

Being on the other side of the world and off the beaten track a bit, choice is limited so Fender is the first option. (also prefer to buy local where possible).

Would rather err by having too much power than not enough.

now specifically -

The 4x10 cab is very good value but it is simply too big to physically handle.

Combo vs Head and cab

-The head and cab allows for easier carrying and loading.

-The head or cab could be replaced some time in the future if something better comes along.

- The height of a 2x10 cab would get the sound "off the floor", so to speak, that would be the case with a single speaker cab.

-Could add an extra 2x10 cab if the occasion arises

That's how the thinking has gone so far.

Would adding another 2x10 cab reduce the load to 4 ohm ?

Fortunately, after a lifetime of only buying what could be afforded,I can now buy what I want.
It's matter of determining what is appropriate then I can chase the best price for it.

I appreciate the time you have taken to respond

Thanks again

(This message was last edited by windmill at 05:43 PM, Jan 16th, 2018)

Contributing Member

North Florida

A Friend of Bill W.
Jan 16th, 2018 03:49 PM   Edit   Profile  

I use three 8 ohm cabinets, well I use one or two at any one time. For just the reasons Tony mentioned above. My amp will supply 500 watts to 8 ohm and 900 to 4 ohm. I’ve got an old SWR Goliath 4 X 10”, a recent SWR neo 2 X 10” and a Hartke 15” all rated at 500 watts.

Options are good.

Contributing Member

Wichita, Kansas

Drums = pulse, Bass = heartbeat
Jan 16th, 2018 05:01 PM   Edit   Profile  

Taking arguments that a "4x10 is too big to move" and "I'm able to afford what I need, I think I would go with a separate amp like the Rumble 500 and and 2 matching 2x10 8 ohm cabs. This would make a very modular rig.

For small gigs, use 1 cab. You get 350 watts and 2 10" drivers at 8 ohms.

For large gigs, use 2 cabs. You get 500 watts and 4 10" drivers at 4 ohms.

If you need anything bigger than that, it's time to have PA support.

BTW, set the 2x10 cab vertical. If you use both cabs, stack them vertically. This "column" will result in speakers up higher off the floor. It will also increase horizontal dispersion and lessen the "beaming" of a normal 4x10 cabinet.

Tony Wright
Contributing Member

Stillwater, OK

I never met a calorie I didn't like.
Jan 17th, 2018 06:19 AM   Edit   Profile  

Windmill: I would note that according to Musician's Friend, the weight of the Rumble 2x10 combo and the weight of the Rumble 2x10 speaker cabinet is exactly the same and the exact same size. So amp and head would be two things to carry (albeit very light weight) rather than just one thing (the combo amp).

I have NO personal experience with the Rumble products but a friend owns one of the smaller combos (I think the Rumble 200 1x15 combo) and it works fine for his needs...bass in a "classic big band jazz ensemble". They perform at the venue where I work...and while I do take his signal via a DI, I do not recall ever needing to reinforce his stage volume thru the PA...the room has good acoustics and the amp is adequately "loud enough" for our 700 seat theater.

Which leads me to the concept of head and cabinet for versatility of adding a second 2x10 cabinet. Just to keep the playing field equal...you should note that the Rumble 210 combo amp has an 8 Ohm speaker out so you COULD stack the combo 2x10 on top of the 2x10 cabinet for those times you feel you need the extra. And only take the 2x10 combo when you are in smaller rooms and at rehearsals.

BUT IN THE END...it is your sound and your band and your money...get what makes you happy. I have several times, and while I still swap, buy and trade...I love every new toy I get, for the honeymoon...but eventually get wandering eyes.

(Sshhh, don't tell my current wife. Well, actually, after 43 years, my one and ONLY wife.)

Contributing Member


Jan 17th, 2018 05:11 PM   Edit   Profile  

Thanks again for the replies.

Thanks Tony for pointing out the weight.

The 2x10 cab actually weighs more than the Rumble 500 2x10 combo. (20kg vs 16.5kg)

Considering the price difference and that there are no 2x10 cabs left for sale in the country,(next shipment arrives at the end of February/early March), the combo is now the obvious choice try

Also if I buy an extra cab any future purchase of a seperate head would be able to be used with the cab !! Genius !!

Thank you to everyone for their advice


Tony Wright
Contributing Member

Stillwater, OK

I never met a calorie I didn't like.
Jan 17th, 2018 07:06 PM   Edit   Profile  

It is what we do....



Jan 19th, 2018 11:57 AM   Edit   Profile  

I happen to have the Rumble 500combo and a Rumble 210 cab. I love them and also can plug my Genz Benz Streamliner into the 210 when I feel like something different. Love the tone and the power of the rumble stuff and it looks awesome when sitting the combo on top of the matching cab( always a plus). I would recommend this set up to anyone. I also went into my local GC store and told them that if they gave 20% off I would order one right then, as they didn't have one in stock. They happily obliged. I went back at a later time and did the same with the cab. Happy camper here.

Contributing Member

North of Philly

Solid state = solid sound+light weight
Feb 19th, 2018 09:02 PM   Edit   Profile  

I haven't found a need yet for anything larger than my Rumble 500 combo. But I did replace the ceramic drivers with neo's, and now it weighs less than my friends Rumble 100 combo!!!

FDP Forum / Fender Bass Guitars and Bass Amps / Why do we have different impedance ratings ?

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