Speaker Ohms!! Help me understand ....

black sphynx · September 10, 2003

I have no clue about how to match ohms with my bogner head. I have two 16 ohm cabs , so I know to run the amp at 8 ohms (this was what I was told) I dont know how to come to this conclusion so I was hoping someone here will help me out.

Thanks

bunnies=dead · September 10, 2003

use...search...button

Darth Balls · September 10, 2003

Two 16 ohm cabs = 8 ohms, as you know, so if you have two 8ohm jacks use those, or if you have an ohms selector, set it to 8 ohms, then plug in both cabs.

black sphynx · September 10, 2003

what if I want to run just one eight ohm 4x12 cab in mono? I would set the amp to eight ohms I would assume?

these are the only two scenarios I will deal with.

El Grinder · September 10, 2003

Ok, short electricity lesson.

Matching Impedance:

Impedance is measured in Ohms.

Two loads in series have the impedance added togehter: ex. 2x 8 Ohm loads in series configuration will have a 16 ohm total load, but only if they're wired like:

+ -

| |

8----8

Two loads in parallel have the impedance reduced: ex. 2x 8 Ohm loads in parallel configuration will have a 4 Ohm total load, but only if they're wired like:

+ -

| |

8----|

| |

8----'

If you check out the two diagrams, you see that in series (the top diagram) the electricity has to flow through one speaker, then a second speaker. There is more resistance to the flow of electricity.

In the bottom diagram, the electricity has two paths to travel. Imagine if it were like cars on a road. Series would have your car running through two traffic lights in a row, slow, right? The traffic lights would impede your progress. In parallel configuration, you have two routes possible to take, so there would be less traffic at each traffic light. This means that you would have less traffic to deal with at either light you go to. This equals lower impedance.

And for the last point. If something is wired as single load in a circuit:

+ -

| |

L_8-'

It will have the impedance listed on that load. A single 8 ohm speaker will simply carry an 8 ohm load of impedance.

That ends the lesson. I hope this helps you out a little bit.

El Grinder · September 10, 2003

: my drawings came out wrong... stupid auto-formatting!!!

edit: Ah, HTML saves the day!

btw, the above stuff is often referred to as Ohm's law. There are some math equations that define it, but that is the general gist of it. Let me know if you have any other questions.

H535 · September 10, 2003

To calculate Ohm loads for speakers of different ohms. First you must find the comon denominator EX1

You have (1) 4 ohm speaker cab and (2) 8 ohms speaker cabs and you want to know what is the ohm load when you hook them up to your amp. Fist write them out as a fraction-

1/4 + 1/8 + 1/8

then find the comon denominator (8)

2/8 + 1/8 + 1/8

then add your fractions

2/8 + 1/8 + 1/8 = 4/8

then invert your fraction and divide the bottom number into the top number

4/8 becomes 8/4. 8 div/by 4 = 2

answer- you now have a 2 ohm load

Lets do another one

what if you have a 16 ohm cab and a 4 ohm cab

1/4 + 1/16

4/16 +1/16 = 5/16

16/5 = 3.2 ohms

How about (2) 8ohm and a 16 ohm

1/8 + 1/8 + 1/16

2/16 + 2/16 + 1/16 = 5/16

16/5 = 3.2 ohm

Simpler example

(2) 8 ohm cabs

1/8 + 1/8 = 2/8

8/2 = 4 = 4 ohms

(4) 8 ohm cabs

1/8+1/8+1/8+1/8 = 4/8

8/4= 2ohms

Darth Balls · September 10, 2003

Originally posted by black sphynx

what if I want to run just one eight ohm 4x12 cab in mono? I would set the amp to eight ohms I would assume?

these are the only two scenarios I will deal with.

You really have only 5 possiblilities:

One 4 ohm cab = 4 ohms

One 8 ohm cab = 8 ohms

One 16 ohm cab = 16 ohms

Two 8 ohm cabs = 4 ohms

Two 16 ohm cabs = 8 ohms

All amps I've seen do not have a 2 ohms selection, so you shouldn't run two 4 ohm cabs ever.

Don't run unmatching cabs at the same time, like one 8 ohms, the other 16 ohms, because the amp selector would need to be 5.33 ohms. Might not hurt the amp, but just not good practice.

H535 · September 10, 2003

All amps I've seen do not have a 2 ohms selection, so you shouldn't run two 4 ohm cabs ever.

Quality solid state amps can run at 2 ohms and have 2 ohm selector switches. Some even dynamically sence the ohm load for you and set them selves up. You can safely run combinations of 8, 16, 4 etc as long as the ohm load of your speakers is higher than the ohm load setting/rating on your amp. This is found quite often in monitor setups. For example an amp setting of 4 ohms could run any load 4 ohms or higher. IE an 8 ohm cab or a 16 ohm cab or a 4 ohm cab. Cheap power amps might give you a 2 ohm rating but that does not mean they are stable at 2 ohms and wont clip or power down or smoke under peak performance.

jimfowler · September 11, 2003

"Some even dynamically sence the ohm load for you and set them selves up."

crest audio, for one. down to 2 ohms of blisteringly hi-fi sound...whoops...must not change subject.

over and out.

-jim

El Grinder · September 11, 2003

My Strategy 500 (all tube) can go down to 2 Ohms, but Mesa was wise enough to HIDE the brown wires tied off inside the chassis. Basically, this beast is capable of going to 2 Ohms, but they set it up so only an experienced user would make use of that feature for the safety of the amp...

Darth Balls · September 11, 2003

Originally posted by H535

Quality solid state amps can run at 2 ohms and have 2 ohm selector switches. Some even dynamically sence the ohm load for you and set them selves up. You can safely run combinations of 8, 16, 4 etc as long as the ohm load of your speakers is higher than the ohm load setting/rating on your amp. This is found quite often in monitor setups. For example an amp setting of 4 ohms could run any load 4 ohms or higher. IE an 8 ohm cab or a 16 ohm cab or a 4 ohm cab. Cheap power amps might give you a 2 ohm rating but that does not mean they are stable at 2 ohms and wont clip or power down or smoke under peak performance.

OK, I shouldn't have said all amps. I was referring to current production tube amps like the Bogner, Marshalls, Mesas, ect, ect, ect. They don't have 2 ohm outs.

gambit · December 27, 2003

i'm looking at cabs right now. but my head only has 4 ohm speaker outs (its solid state)

i've read several times that it IS safe to run an 8 or 16 ohm cab with only 4 ohm amp head, but some of the statements talk about "a degree of power loss".

what does this mean in real terms?

a 300w cab running at 4 ohms instead of its 8, means i only get half - 150watts with my head?

if anyone can answer this question asap, i'll be very grateful, coz i might be egtting an amp as early as tomorrow or monday.. or this afternoon..

cheers

chrisaxeman · December 27, 2003

Originally posted by black sphynx

I have no clue about how to match ohms with my bogner head. I have two 16 ohm cabs , so I know to run the amp at 8 ohms (this was what I was told) I dont know how to come to this conclusion so I was hoping someone here will help me out.

Thanks

Hey Black,

Lets simplify this,it's getting out of hand.My job is an electrician,so I'll handout the no BS approach to amp and speaker impeadence before you get confused and break something..........

Rule 1.your amp of x impeadence can run cab of x impeadence or higher.

Rule 2.don't do the opposite of rule one unless you like smoke shows from your amp.

Suggestion 1.If you use multiple cabs,for amp purposes use the same impeadence cabs so that they will be of equal volume-in a real world they will not be spot on but the difference will be trivial.

Rule 3.Most amps use a parallel outs for multi cabs.Check your manual,it should show the connections,and will be marked clearly on the back panel-I have a Laney GH50L that has dedicated multi out's(1x16,2x16/1x8,2x4/1x8).My TSL100 has a 1x16,1x4/2x4,1x8/2x8,with a selector switch.

Your pair of 16 ohm cabs will indeed = 8 ohms,you just have to connect them to the appropriate outputs.I'd have a suspicion that the Bogner would have dedicated jacks for connection in full stack mode(2x16ohm quads).......

Rule 4.Impeadences in series are added together-eg 8ohm + 8ohm =16 ohms.

Impeadences in parallel(if the same value)-divide the value by the number of speakers.eg 2x16 ohms = 16/2 = 8ohms

4x16 ohms = 16/4 =4 ohms

16 ohm cabs most often use 4 x 16 ohm speakers wired in series/parallel.eg a pair of speakers each are wired in parallel,so you end up with,Pair 1-2x16ohms=8 ohms

Pair 2-2x16ohms=8ohms.

Pair 1 is wired in series with pair2,

Pair1(8ohms) + Pair2(8ohms)=16 ohms

Cabs are sometimes switchable between 4 and 16.The 4 ohms comes from all of the speakers being wired in parallel,16/4=4 ohms.

Rule 5.If you do use cabs of different impeadences,the parallel rule works like this-divide 1 byeach value (ie,make 16=1/16,8=1/8 etc),add the fractions together,and then invert the fraction.

eg.16 ohm in parallel with 8= 1/16+1/8=3/16

invert=16/3=5.33ohms

Rule 6.Look at the example in Rule 5 and notice that when different values are used in parallel,the total impeadence is always smaller than the smallest resistance-5.33ohms

Chris........

okavipra · January 26, 2011