Ohms FAQ

[lbwdog]

Most manufacturers rate their amps at 4 Ohms. generally, the amp will put out a little more than half that power into an eight Ohm load. the power rating of the speaker is it's thermal rating (heat), so you're OK as long as the number isn't extremely low, which would indicate a cheap speaker which couldn't take much.
Just listen carefully and the speaker will tell you when it's starting to be stressed.

 

[dincz]

"For example, three 16 ohm speakers wired in parallel in a cabinet = 3/16, or 5.333333.… ohms. That’s how they get those 5.3 ohm 3x10s btw!"

I believe the fraction should be 16/3 rather than 3/16.

This quote is from 2004.

 

[Billy K]

wonder if I can get this question answered here.I have a Marshall MB 30 8 ohms. I want to wire a speakon plug into the back and run another 8 ohm cab.It seems series would draw 16 ohms off the amp (?) so can this be done? I know I know get a amp and cab. I have a 500 watt and 4x10 cab. But this little baby stack would be cool for coffee house gigs or practice acoustic with my guitar player.If it can be done

 

[MyKroberts]

Ok, in need of some help.

I play a Gallien-Krueger Bassline 600 head (300w 4 ohm) and a SWR Son of Bertha cab which is one 15" speaker and a horn (350w, 8 ohm)

I want to upgrade my stuff. I like my cab but one 15 isn't cutting it tone-wise and I want a bit of a volume boost.

My problem is that after reading the info post I still don't understand all of this Ohm business (I wasn't in charge of the purchase of my set-up and it was basically orchestrated by the people at Guitar Center)

I'm considering either going for something like the a 4 x 10 SWR Goliath or Triad, or using a Goliath Jr (2 10s) with the Son of Bertha. Some of these things come in 8 and 4 ohm versions

I have no clue how to work any of this out mathematically. Help or suggestions?

 

[44me]

Billy – series connecting two 8 ohm cabs will present a 16 ohm impedance to the amp. If the cabs are grossly different, they will not necessarily sound the same as used alone, as the series connection results in some interaction. Is you amp unable to drive 4 ohms? That’s what you’ll get if you wire them in parallel. This will make considerably more power available to the speakers while eliminating any concerns of interaction.

MyKroberts – For maximum volume, try to get a cab rated at 4 ohms. This will pull more power from the amp than 8 ohms, and all things equal, will end up a little louder. I say all things equal, because cab efficiency and tonal characteristics will have as much or more to do with volume than the incremental difference in power. You are likely to get better efficiency with cabs with more speakers (410s generally have higher efficiency than 210s, which generally have higher efficiency than 115s), but there are plenty of exceptions. That said, I’d always go for tone first.

How this all works mathematically is covered extensively in this thread.

 

[Billy K]

thanks 44, My Marshall is the MB30. It is a little practice amp with a 10 inch speaker.I thought it would be cool to have a mini stack.Maybe the hook up could be for the cab only, that would sound better I guess. I dont know I was thinking out load and hoping for some ideas from you guys. Thanks again

 

[44me]

The MB30 manual says it’s rated for 30Wrms at 4 ohms. I’m sure the internal speaker is 4 ohms since there’s no extension speaker jack. A series extension connection is your only option. A parallel connection will overload the amp. I have a Nemesis 210 combo that’s wired from the factory this way. Your extension cab should also be 4 ohms to minimize output power loss (it will drop to 15-20W with an 8 ohm total load). All said and done, I question whether it’s worth the effort and expense unless you already have the cabinet. The MB30 looks like a nice little practice amp, but I think the next step should be something with substantially more power.

 

[RobinC]

Awesome thread!

Hopefully someone can chime in on this hypothetical situation:

Say I have a head (tube pre, solid state power amp) rated for 1000 watts @ 4 ohms, 600 watts @ 8 ohms. I have two 8 ohm cabs that I plug into the head's parallel outputs.

Question 1: how many watts are going to each cab? Would it be roughly 500 watts into each?

Question 2: will there be a discernible volume difference between running one cab with 600 watts at 8 ohms and running both cabs (in parallel) with (500)? watts @ 4 ohms?

 

[GrowlerBox]

Awesome thread!

Hopefully someone can chime in on this hypothetical situation:

Say I have a head (tube pre, solid state power amp) rated for 1000 watts @ 4 ohms, 600 watts @ 8 ohms. I have two 8 ohm cabs that I plug into the head's parallel outputs.

Question 1: how many watts are going to each cab? Would it be roughly 500 watts into each?

Question 2: will there be a discernible volume difference between running one cab with 600 watts at 8 ohms and running both cabs (in parallel) with (500)? watts @ 4 ohms?

Answer 1: Yes

Answer 2: assuming the cabs are identical, then yes, but because you have more speakers, not because of the power difference.

 

[44me]

Answer 2: assuming the cabs are identical, then yes, but because you have more speakers, not because of the power difference.

Actually, the volume boost from the increased power 1000W vs 600W (2.22dB) and doubling the number of speakers (3dB) is pretty close. The noticeable difference in volume you typically get from doubling the number of cabs is from the combined effect. By itself, a 2 to 3dB increase from cutting the impedance from 8 to 4 ohms, or from doubling the driver efficiency is discernable, but small. However when you combine the two, and you get a 5dB+ increase, that’s enough to notice on a gig - at least until your guitar player turns up again.

 

[Matthew_84]

Sorry, I hate asking this because I think I know the answer and I'm sure it's been asked to death...

I just got a Crown XLS 1000, in bridged mode it runs 8 Ohms @ 700 watts, and 4 Ohms @ 1100 watts. I have ordered a GK 410MBE (800 watts @ 8 ohms). So I can run this cabinet fine. If I decide to get another GK 410MBE, I would decrease the impedance to 4 Ohms, but would the wattage double? Could the speakers handle the 1100 watts @ 4 ohms, or would 800 Ohms @ 4 Ohms be their limit?

 

[44me]

The 1100W splits evenly between the two cabs, so you only end up with 650W in each –a little lower than when you run one cab by itself.

 

[Matthew_84]

The 1100W splits evenly between the two cabs, so you only end up with 650W in each –a little lower than when you run one cab by itself.

Oh right... forgot about that... I knew that but completely forgot it. Thanks a lot man

 

[Jaco who?]

The 1100W splits evenly between the two cabs, so you only end up with 650W in each –a little lower than when you run one cab by itself.

Actually 550 watts into each. Unless there's some new math I'm unaware of.

 

[44me]

Actually 550 watts into each. Unless there's some new math I'm unaware of.

Hmmm. . . yes, just checking to be sure there was someone else paying attention.

 

[csmithbass]

Very very helpful! i've always been quite confused by all this ohm/impedence stuff.

I'm currently looking for a new head (last time i did was like 8 years ago) so it's good to finally know what i'm looking at.

 

[Fendbassfan]

If you hook up a 4 ohm cabinet and an 8 ohm cabinet to an amp that can power a 2 ohm load, the 4 ohm cabinet will get twice the wattage as the 8 ohm one. "

I have an ADA Bassline VX. Quoting from the manual.
'The Bassline provides both Series and Parallel jacks to allow the most flexibility in designing your speaker array'.

So I understand if you connect 2 8ohm cabinet in series you will have 16ohms. Right? so if you connect an 8ohm and a 4ohm would you have 12 ohms?

 

[Fendbassfan]

Most cabinets are designed to be “daisy chained” together. On the back of the cabinet you’ll probably see 2 ¼ inputs labeled “parallel inputs”. This is the most common method of “daisy chaining” cabinets together. Almost always this results in the cabinets being run together in parallel. You may want to check with a particular brand of speaker, but I’ve never run across a cabinet that daisy chains in series.

I have an ADA Bassline VX Bass Cabinet. It is 2/10's and 2/5's and it does have a series jack also. Here is what the Owner's Manual says. "The Bassline provides both Parallel and Series Extension Jacks to allow the most flexibility in designing your speaker array". So could I run 8ohm and a 4ohm Cab with my amp (which says no lower than 4ohms) It is my understanding two 8ohm cabs run in series gives you 16ohms so if 2 8's give you 16 wouldn't 1 8ohm and 1 4 ohm give you 12ohms?

 

[44me]

Yes although I’m not a big fan of series connecting dissimilar cabs. Rated impedance is just a nominal. It actually varies quite a bit over the frequency range, and the changes can be very dramatic in the lowest octave or two. When the cabs are the same design, the impedance variations track reasonable well, and the voltage applied to each stays balanced. On the other hand, if the cabs are grossly different and the impedance variations don’t track well, neither cab will be driven as designed. If you are pushing power limits, this potential overdrives each cab in certain frequency ranges (drivers may bottom out earlier than expected). Also, damping of ported cabs is likely to suffer, which may be good for your Reggae sound but may kill your finger funk articulation. This is all very situational dependant. There are no steadfast rules, but it’s something you should be aware of with series connected cabs.

 

[Fendbassfan]

I'm selling it all anyway except the Bassman Tube head and the Avatar B212 I just got and my old GK 400RB (always my backup). Getting rid of the Ampeg SVT-350H and SVT-15E and ADA 210/-2/5's.