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Senior Member
Username: jazzyvee

Post Number: 3777
Registered: 6-2002
Posted on Sunday, December 22, 2013 - 6:12 am:   Edit Post

I have all Mesa 8 ohm speaker cabs and love their sound. I'm just airing my curiousity here to ask if I connected my cab to my rig and set the amp output to a set volume using an SPL meter. Then set it up again but with a 4 ohm cab and again set the volume to be the same as before. What would be the difference in what I'd hear when playing through each setup?
Senior Member
Username: fc_spoiler

Post Number: 1500
Registered: 5-2006
Posted on Sunday, December 22, 2013 - 7:58 am:   Edit Post

I replaced the 16 ohm EVM speakers in my 2x10 cabs with the 8 omh versions, made no difference at all.

Maybe with some solid state amps, there's a very slight difference in tone because the poweramp is closer to its limit with a higher load? (the 8 ohm cab, at same dB level)
Senior Member
Username: edwin

Post Number: 1678
Registered: 5-2002
Posted on Sunday, December 22, 2013 - 10:26 am:   Edit Post

It depends on the amplifier, all other things being equal. If you are using a solid state amp without an output transformer, a 4 ohm cabinet will get more power out of the amp than an 8 ohm cabinet. A tube amp, with the proper output transformer taps, will be pretty much the same.

As far as the difference that you'd hear, it would debatable. Halving the impedance with a solid state amp could give you double the power in a best case scenario (but it's often less) but bear in mind that to appear twice as loud, the amp needs to be 10 times more powerful. So, you'd gain a little bit of headroom, which could make a difference if you play the amp close to the limits.
Senior Member
Username: gtrguy

Post Number: 660
Registered: 9-2004
Posted on Sunday, December 22, 2013 - 10:59 am:   Edit Post

I have several guitar amps with multiple ohm speaker output jacks and I think that some sound better than others at various ohms, as long as the amp manafacture says it is OK. Mesa and some others are pretty forgiving for that kind of thing.

As far as bass goes, it's a matter of more headroom, I think, with various ohms, rather than tone.
Senior Member
Username: keith_h

Post Number: 1940
Registered: 2-2005
Posted on Sunday, December 22, 2013 - 2:41 pm:   Edit Post

There is no connection between sound quality and impedance in the speaker itself. Identical speakers of different impedance should sound the same under the same conditions. As stated above impedance does have an effect on the output power of an amplifier. Also depending upon the amplifier you might be close to the edge of it's operating specifications with the lower impedance. Depending upon the specifications you might get a measurable increase in output but not hearable. To expand on Edwins talk about tube or solid state, you need to be sure the amp can support a single 4 ohm cabinet attachment even though it might support two 8 ohm cabinets. The two speaker configurations are not the same even though they can have the same impedance of 4 ohms.

It takes roughly a 3db increase in volume to be heard by the human ear. This equates to an approximate doubling of power needed from the amplifier. To get to twice the perceived volume requires a 6db to 10db increase and roughly 4 to 10 times the power out of the amplifier. By doubling identical speakers you will get a 3db increase or quadrupling will give you 9db. This is where the rule of thumb that you are better off adding speakers before buying a new amp comes from. A 400W amp at 8 ohms would need to be able to output 800W at 4 ohms to increase the volume by 3db. To double the perceived volume it would need 1200W to 4000W. Thus the rule is based on cost of speakers compared to amps and fact that a significant increase in power could also require more speakers adding to overall cost.

Senior Member
Username: bigredbass

Post Number: 2099
Registered: 9-2002
Posted on Wednesday, December 25, 2013 - 12:35 am:   Edit Post

OK you technical types, I need help here:

I understand the basics of how many cabinets at what ohm-rating a given amp will handle, but the one thing I don't understand, don't know how to read in specs:

For a given amp, let's say a generic bass head that is posted at "200w at 4-ohms", how do I know how much current it's passing?

In other words, I could hook up:

a) a single 15 4-ohm cab

b) two single 15 8-ohm cabs,

. . . . . or for the sake of discussion

c) two 8-10 8-ohm cabs

. . . . and supposedly be presenting the same 'load' to the amp, yet I know driving 16 10's has just GOT to be a different situation than a single 15, wouldn't it have to have more current for all those?

So how do I get a sense of the real horsepower behind a given amp?

J o e y
Senior Member
Username: edwin

Post Number: 1680
Registered: 5-2002
Posted on Wednesday, December 25, 2013 - 9:55 am:   Edit Post

200W at 4 ohms is not nearly enough information. Is it 200 watts at 1khz with 10% THD for 1 millisecond or is it continuously at full bandwidth at .0000025% THD?

But I don't think the amount of current that the amp delivers would change if the impedance doesn't change. The amp doesn't care how many speakers its connected to (this leaves out the fact that impedance is not actually a single number but changing resistance with respect to frequency, so 2 8 ohm cabinets could have very difference impedance characteristics.).
Senior Member
Username: sonicus

Post Number: 3174
Registered: 5-2009
Posted on Wednesday, December 25, 2013 - 11:52 am:   Edit Post

Yes , I concur with Edwin's statements. When I read specs I am pleased when they are stated in Watts RMS @ 8 ohms and then 4 ohms and if the amplifier will accommodate lower impedances then 4 ohms. There are many different design features from amplifier to amplifier such as if it is TUBE output with transformer coupling or transistor direct coupling or otherwise. The Mcintosh autoformer is a good example of otherwise such as the famous MC 2300 !

Next the speaker paradigm ! With some 8 /10 cabs other the years I believe there has been other then straight parallel wiring such as series parallel . They did not all use 32 ohm speakers if I remember correctly . Personally , I prefer just parallel speaker connections . Then we have Thiele-Small parameters of the speakers in relation to their given enclosures. ____ TOO MANY VARIABLES ... ... Yikes

There is also test equipment that will give you finite answers, BUT then you might need an Anechoic chamber ----

Given all these variables that unless you are prepared and enjoy to do lots of math to see what your various cabinets sound like in combination , just jump in the water to find out if you like it ______ Listen ______!

Just make sure that you are NOT loading the amplifier lower then the specs require, do not load it @ 2 OHMS if it specs are not lower then 4 ohms or you might fry it !
Also make sure that you do not cross the polarity on multiple connections ( often referred to as out of phase)
This should also be paid attention to at the input of the signal chain !

Experimentation can be fun ____ It is one approach of many and can be safe ___a way that anyone can have fun !


(Message edited by sonicus on December 25, 2013)
Senior Member
Username: keith_h

Post Number: 1941
Registered: 2-2005
Posted on Wednesday, December 25, 2013 - 5:22 pm:   Edit Post

All things being equal there will be no difference in current output between your examples. They all present the same approximate load to the amp. If you are talking power distribution between cabinets then:

In "a" the single 15 would see the full 200 watts.

For "b" & "c" each cabinet would see 100 watts (200 watts total).

In the case of "b" it is the full 100 watts per speaker. For "c" the 100 watts is distributed across the eight speakers in each cabinet.

If we start intermixing cabinets of different impedance in the same connection the the power distribution would be asymmetrical. If we go with things like horn loaded cabinets the load seen by the amp could actually be lower than nominal speaker impedance (i.e. An 8 ohm speaker in a reflex box could be 6 ohms in a horn loaded box). To calculate this we would need more information on the amplifier, specific cabinet design and speaker as it requires math to calculate.

Senior Member
Username: dfung60

Post Number: 597
Registered: 5-2002
Posted on Sunday, December 29, 2013 - 2:35 pm:   Edit Post

Amp stuff is complicated because the audio signal is AC. DC circuits have simple resistance and power calculations (but are awfully dull sounding!). With AC signals, you have power flowing through wire coils which creates a magnetic field, and those coils are attached to cones and suspensions whose physicality affects the electrical characteristics. Impedance (sort of the AC analog of resistance) is a complicated calculation that turns out to vary a LOT depending on frequency. The formal term for a device like a transducer is a "reactive" load.

When you see an impedance figure for a speaker, it's a nominal number, which can be much higher or lower depending on frequency and even on how you design the cabinet it's in. Out of the cabinet, the impedance of dynamic drivers like speakers gets higher at the free-air resonance frequency. If you take the same speaker and put it in two different cabs, you may read different impedance response because the air in the cabinet changes the stiffness of the speaker's suspension. That may or may not be an intentional part of the design.

Tube amps have a higher internal impedance than solid state amps. When an amp drives a speaker, it works pretty efficiently when the amp's output impedance is much lower than the speaker's input impedance. Solid state amps are often a fraction of an ohm, so they are very forgiving with different loads. If the speaker impedance is lower than the amp's output impedance as is often the case with a tube amp, the transmission is very inefficient - a lot of power literally gets reflected back into the amp. So, tube amps usually have a transformer that matches the amps output to the speaker. If you mismatch the connection, it can be very hard on your tube amp.

If impedance is the AC analog of DC resistance, then this kind of explains what's happening with power, which is sort of the product of voltage and current. When the resistance/impedance is lower, then more current can flow and you'll get more power ever if the voltage is the same. This is what's happening with solid state amps that give double the power when the load impedance is halved - the voltage of the output stays the same, but the current increases inversely proportional to the impedance. This is also why you dont want to run at 2 ohms in most cases - the amp is pushing a LOT of current out at this low impedance, which equates to heat. If there's not enough cooling available, you'll cook the output transistors. When you see an amp that doesn't double power between 8 ohms and 4 ohms, it's probably because the power supply or output transistors can't push this much power.

The physical difference between an 8-ohm and 16-ohm speaker may just be much more wire on the coil, but the magnet and suspension may differ as well. Having all these individual speaker impedances is a by product of the tube years, when you really want to be able to make parallel/series connections of multiple speakers to get to a specific value that's good for the amp. The changes that influence impedance are probably relatively minor compared to the other things that speaker manufacturers are modifying between models, but it's possible that they may sound different or have different levels of reliability (I've never seen anybody comment that a 16-ohm speaker is more reliable than 4 ohm or vice versa).

On Joey's question about different speaker cabinets, the math says that things should sound the same, but that's not been my experience in real life. The amp may be producing the same amount of power and it's being proportionally divided between the speaker per their impedance, but what's really having a big effect is the amount of speaker cone in motion. A 15" speaker has about 175 sq in of cone area. This is about the same as two 10" speakers which are 78.5 sq in each. But the 15" speaker cone is heavier than the cones in the 10" speakers and has much less high end response. So, it will probably sound pretty different. An 8x10" has 625 sq in of speaker, double the moving area of a 2x15", so the 2-15" has to work a lot harder to move the same volume of air and do so as cleanly as the 8x10".

Everything else equal, more speakers sound better to me, as long as I'm not the guy who has to load them all back into the van after the gig!

David Fung
Advanced Member
Username: willie

Post Number: 230
Registered: 11-2003
Posted on Thursday, January 09, 2014 - 10:20 am:   Edit Post

A lot of interesting perspectives on this.
Regarding your question of current. It is true that impedance is different than resistance, and that the impedance of inductive loads such as the voice coils of speakers varies with the frequency of the voltage applied to them. But you can use ohm's law to calculate the approximate current for a given situation. It actually allows you to figure out any one variable, (i.e. voltage, current, resistance, power), if you know 2 others.
For current in your question, divide the power by the resistance and then take the square root of that number. For example, it takes 5 amps of current through an 8 ohm load to generate 200 watts of power. 200 divide by 8 = 25. Square root of 25 is 5. Look up OHM's law for the circle and you can figure out a lot using it.
It is not always true that an amp will put out more power through a lower impedance load. My Crown Macro Tech I series amps rail voltage to the output devices stays at 200 volts. If you put a 2 ohm load on it, it would try to drive it with 100 amps of current risking damage to amp or speaker. It ramps down internal limiters to prevent that and actually decreases it's own power output to protect itself. Not all amplifiers have such capabilities though and will try to drive lower loads with more current and can be damaged by that.
Wow, continuous full bandwidth power at .0000025 % THD. Show me that amp.
Yes, using different sized drivers does affect the sound you hear from a given amp for various reasons. Cone area but also the frequency response of different sized drivers is different. That's why I use 3 different sized drivers amplified separately with the proper frequencies going to the drivers that can reproduce them the best with the least distortion.
And I don't use 10" woofers. I use 10" mid bass drivers. They reproduce the midrange better and since the lows go to 15" drivers, the 10"s will never see and have to reproduce them. The high's go to high frequency compression drivers. With up to 15000 watts of power available to this speaker system, it can reproduce the full audio range of the human ear, crystal clear at full concert volume. Something I find very helpful in reproducing the frequency and dynamic range an Alembic Bass is capable of.
Enough of my rambling. Hope that answers your current question.
Senior Member
Username: edwin

Post Number: 1695
Registered: 5-2002
Posted on Thursday, January 09, 2014 - 11:02 am:   Edit Post

"Wow, continuous full bandwidth power at .0000025 % THD. Show me that amp."

Yeah, that was hyperbole to make a point. But my QSC CX 1202v does do 1100 watts per channel at 4 ohms, 20 Hz-20 kHz, 0.05% THD. Which is a lot better information than the Crest Pro-Lite 2.0 gives you: 540W per channel at 4 ohms @ <0.1% (both channels driven, continuous sine wave). They don't give you a bandwidth, so it's probably at 1khz. When you get it to perform within the same parameters as the QSC, it's probably a lot less than 540 wpc. So, I think you get my point.

In any case, where can I see/hear your rig? It sounds awesome!
Advanced Member
Username: willie

Post Number: 231
Registered: 11-2003
Posted on Friday, January 10, 2014 - 5:44 am:   Edit Post

Hi Edwin
You got me with the THD thing. It seemed impossible. With a spec like that, I was waiting to hear that Alembic made a power amp that I somehow didn't know about. In which case, I would have to get SEVERAL. A Crown MA 90000I which is a fully Ethernet capable touring sound amplifier is rated at 3000W per channel at 4 ohm's. Normally that would mean bridged into 4 ohm's would be 6000W, but it monitors the load it sees and drops it's output to 5000W to protect itself. I haven't tried QSC amps but if so many people here use them they must be good. I mentioned the 10" drivers because people were comparing different sized drivers based on cone area. There is a lot that will determine how a particular driver sounds. The material and mass of the cone, the material and flexibility of the surround and spider along with the type of wire and how it is wound in the voice coil and the magnet structure, the "engine" of the driver. I prefer ferrite magnets over neo but the neo magnets are much lighter. My 10" have 14.5 lb. magnets, the 15's have 23.5 lb. magnets and the bullet tweeters have 5 lb. magnets. A lot of weight but the sound is incredible. I have 2 brothers who are master carpenters and I design my and build my cabinets with them. I also modify other cabinets.
As to where to hear it, my neighbors might have a thing to say about that. Hopefully it will be heard live in the near future. I have played Alembic basses for over 30 years and worked on this system for 35. I have several pre-amps but the heart of this system is an F-1X. It has the best sound.
I do know one thing. It is a lot of fun to play the best basses in the world through that system.

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