Ohms FAQ

[BadExample]

Hi using a friends powered mixer with my speakers the other day whenever there was a loud passage an overload light flashed and sound would momentarily cut, wattage is compatible but on further inspection we found next to the speaker outs on the mixer there was an 4 ohm stamp i know the speaker cabs are 8 ohm would this be the problem

Are there more than two of the 8 ohm cabs?

 

[AE1]

False Dichotomy

The Merriam-Webster dictionary defines a false dichotomy is a kind of fallacy in which one is given only two choices when in fact other options are available.

What other amplifier options are available other than tube or solid state ?

 

[agedhorse]

Well, there's magnetic saturation amplifiers... not very useful for audio though.

 

[AE1]

Well, there's magnetic saturation amplifiers... not very useful for audio though.

I will admit acoustic basses do not need an amplifier at all

 

[seamonkey]

The Merriam-Webster dictionary defines a false dichotomy is a kind of fallacy in which one is given only two choices when in fact other options are available.

What other amplifier options are available other than tube or solid state ?

Solid State, Tube, Hybrid, Sousaphone Horns, Virtual, "It doesn't matter", ...,
And there's many exceptions Some tubes have OTL and hybrids of that like the Milbert Gaga, and some Solid state have transformers.

IMHO Solid State or Tube is not really a split at the top of the decision tree. There are many other attributes to steering one's decision.

 

[cyprussid]

Thanks have to investigate further

 

[Qan]

hi. sorry for my lack of ohm calculation.. i have a 500w amp with two output says min 4ohm load.. i have 3 cab..210 8ohm , 115 8ohm and 410 4ohm.. any chance i could wire them together without burning them..really apreciate. thanks in advance

 

[JimChjones]

In theory you could, but it involves making special connection boxes with non standard wiring that could go horribly wrong, and it probably would be at best no advantage over using just the 410 or the 210 and 115 together. No louder, probably worse sounding.

Better, I submit, to keep a good eye on the local secondhand market and trade gear for something that closer matches your needs.

 

[agedhorse]

hi. sorry for my lack of ohm calculation.. i have a 500w amp with two output says min 4ohm load.. i have 3 cab..210 8ohm , 115 8ohm and 410 4ohm.. any chance i could wire them together without burning them..really apreciate. thanks in advance

No, not without damaging your amp.

 

[Mr. voodoobass]

Palmer has a litte box called cabmerger or similar. If the two 8ohm cabs can be chained together(total of 4 ohm) you can connect all 3, at then a total of 8ohm. Not sure it will be any louder tho

 

[agedhorse]

Series connection of dissimilar cabinets can result in unpredictable results.

 

[AE1]

For me the key consideration in Qan's question is the minimum load permitted for the amplifier - ie 4 Ohms

Aged Horse is right in advising against a load lower than 4 Ohms in this case but I successfully mix and match speaker cabs quite regularly

A lower load than this might cause it to self-destruct or some other nasty behaviour like oscillate or turn itself off in the middle of a gig

The 500 watt nominal output power rating is neither here nor there so long as the actual power produced does not exceed the loudspeaker ratings - check the loudspeaker/cabinet specs

If this was my system I would connect all the ten inch drivers in series to give a nominal 12 Ohms load, then parallel them with the 1 x 15 - ensuring all are phased correctly so all the cones are moving in the same direction.

A series configuration of the ten inch cabs will deliver equal power to each cabinet. This will result in the 2 x 10 cab receiving 1/4 total power and the 4 x 10 cab receiving 1/4 total power

This setup will deliver half the power to the 115 and the other half to all of the tens

Therefore total power capability will be limited to double the 115 driver rating or double the 2 x ten inch cab raying - whichever is the lesser

Although only half the power is into the tens they will still most likely sound as loud as the 115 because they will be more acoustically efficient - can't say for sure because you have not identified any characteristics or specs

The cabs can be mounted side by side or one on top of the other - there will be practical difference between those configurations but space on stage might be the determiner

Side by side mounting is easier on the ears

An alternative solution is to use a stereo power amp, or a mono preamp and dual amp system

Separate amps enables each cabinet to be driven independently so in that case the difference in load is not an issue - balancing volume can be adjusted with the gain controls

Another and very common option is to use just the 4 x 10 on top of the 1 x 15. In this case parallel connection is OK - alternatively independent connection with a stereo amp

Life does not need to be complicated

 

[JimChjones]

Your description suggests all the ten inch drivers are 4 Ohms each.

They could just as easily be 16 ohm. Or even 16 ohm in one cab and 4 ohm in the other.

 

[AE1]

You are right Jim, but what matters in practice is the effective load at the terminals of each cabinet

Qan's question refers to his specific system but others may have whatever mix and match setup they prefer

To decomplicate the post I have deleted the comment "Your description suggests all the ten inch drivers are 4 Ohms each" because it detracted from the overall explanation.

 

[JimChjones]

Series connection of dissimilar cabinets can result in unpredictable results.

This is something I've been wondering about. I understand that quoted impedances are nominal, and the real impedance varies greatly around the resonant peak. If I have two drivers with significantly different resonant values, isn't that going to mean that instead of getting an equal share of the amp output one and then the other will be taking a very much greater share of the output? Another reason not to mix drivers? Or am I overthinking and in practice unless you feed pure sine waves the overtones and complexity of the signal will tend to even things out?

 

[agedhorse]

Lots of things no longer work as expected. Impedance is one of these, and yes it's made worse by real impedance differences,

 

[AE1]

This is something I've been wondering about. I understand that quoted impedances are nominal, and the real impedance varies greatly around the resonant peak.

Real (ie actual) load impedances from electro-dynamic speakers vary across the full range - see attached graph below for typical characteristics of a bass woofer speaker. This curve should be regarded as typical only because speaker characteristics may vary widely between makes and models. The lower line is impedance at the corresponding frequency. The nominal impedance usually occurs at the lowest point on the curve, which means your speaker will normally present a load to the amplifier of more than that at typical operating frequencies - for bass guitar useful fundamental tone range is between about 40 and 320 Hz. Furthermore the mix of fundamental tones and harmonics will also vary the actual load seen by the amplifier.

Consider a 6 string guitar playing a chord - what will be the effective load ? It is definitely not the resonant frequency

If I have two drivers with significantly different resonant values, isn't that going to mean that instead of getting an equal share of the amp output one and then the other will be taking a very much greater share of the output?

Theoretically yes but the audible effect will depend upon the difference between the resonant frequencies and the SPL of each speaker. But also consider that if both speakers are of equal loudness and resonance the effective SPL will increase by 3 db, but if the resonant frequencies differ significantly - eg 40 Hz and 60 Hz, then each speaker will only deliver normal SPL so will not appear to be louder at resonance.

Consider also a stereo amplifier for home hi-fi where the speakers will not be exactly identical.

But then if you look at the graph you will see the load impedance increases dramatically at resonance, which results in substantial reduction in audio power from the amp. This translates into a lower SPL (loudness) with a solid state amp but maybe some boominess with a tube amp.

Another reason not to mix drivers? Or am I overthinking and in practice unless you feed pure sine waves the overtones and complexity of the signal will tend to even things out?

IMHO there is no reason not to mix drivers - size, make, model - the more speakers the better the overall effect. The Ampeg 8 x 10 system proves this. Have seen them used in a multiple cab setup for large shows - 24 or 32 x 10 inch drivers side by side is an awesome system. Then there are those popular setups having 4 x 10 on top of a 1 x 15 underneath - they work well.

In the case of resonant frequency there will be variation between identical units of identical make and model because there are always manufacturing tolerances at work. The natural cone resonant frequency will also change between dry and wet weather and through loosening (less stiff) of the suspension due to wear. Then there is also cabinet resonance, which adds to the complexity.

If you approach music as a science then you may end up never satisfied - enjoy the moment

Obviously professional sound systems will be engineered but for the typical bass player who supplies his or her own gear then it is a case of doing the best we can with what we have

Do not forget to ensure that after mixing and matching cabs, the actual load presented to the amplifier is never less than the manufacturer's rated load.

 

[AE1]

hi. sorry for my lack of ohm calculation.. i have a 500w amp with two output says min 4ohm load..

Minimum means minimum

The two outputs are there to allow you to connect two separate speakers but only if the averaged load is not less than 4 Ohms

On the other hand if your amp has two separate amplifiers - ie stereo - with an output socket for each, then each amp may be loaded down to 4 Ohms

Suggest you study the User Manual for the amp so you have a clear understanding of what the amp will do for you

 

[agedhorse]

Obviously professional sound systems will be engineered but for the typical bass player who supplies his or her own gear then it is a case of doing the best we can with what we have

While some (or even many) bass cabinets are not "engineered", there certainly are manufacturers that place a lot of emphasis on solid engineering behind their products. By choosing one of these manufacturers, many of these issues disappear in practice.

 

[AE1]

You are entirely correct in your observation but the OP is mixing cabs - as many of us do
Even if we use mixed cabs from the same manufacturer there will still be mismatch in characteristics
We are stuck with using what we have so have to make the best of it