Yet another thread on heads, cabs, watts and amps

[billleeee]

Apologies if this has been addressed, but after much searching, I’m not finding the answer, so I figured I’d ask the very specific question and hope some smarter minds than I can shed some light (and based on 2025 knowledge).

Have played for about 30 years but always had combos. Looking to move to separate head and cabinets and want to make sure I’m making the right pairing. I’m leaning toward the GK Fusion 500s head, which indicates ratings of 180 Watts at 8 ohms, 300 watts at 4 ohms and full power (450 watts) at 2.7 ohms, which they describe as a 4 ohm and 8 ohm setup. There is a 3 position switch on the head that allows you to change it from 8+8, 4+8 and “4 or 8”

Since we’ll be playing a range of rooms with potentially different needs, I was thinking two cabinets could give me 3 potential options that I could use as space allows. My current combo is a 1x15, and I really like how that sounds, so I found a 1x15 400 watt 8 ohm cabinet and a 2x10 400 watt 4 ohm cabinet. The thought was that if I use the 15 alone, I’m at the lowest power rating from the amp, if I use the 2x10, I’m in the middle, and if I combine them, I get the full power of the amp, plus the full range of sound from mixing 15 and 10” speakers.

Flaws in this logic? Concerns about damage to the amp or speakers? Appreciate any advice as this is a fairly large step up in my gear investment.

 

[Wasnex]

Flaws in this logic?

You only seem to be considering power. You also need to consider the sensitivity of the speakers.

The rest is TLDR. If your curious, please continue reading.

SPL is a result of usable power and the sensitivity of the speakers. Usable power is the lower value between how much the amp and deliver and how much the speakers can handle. For example if you have an amp that can deliver 400W and a speaker that can only handle 200W, then usable power is 200W. Usually sensitivity is expressed as the SPL measured at 1 meter with 1 Watt applied.

The SPL goes up 3dB every time you double usable power.

Let's assume an 8 ohm cab rated for 300W with a 97dB 1W/1m Sensitivity rating.

1W = 97dB
2W = 100dB
4W = 103dB
8W = 106dB
16W = 109dB
32W = 112dB
etc


If you know the usable power and the 1W/1m Sensitivity rating, you can use this formula:

• Calculate SPL = 10Log(Usable Power) + Sensitivity

For example calculated SPL for 32W in a cab with 97dB 1w/1m Sensitivity

• 10Log(32) + 97 = 112.05dB--essentially same as we got with the 3db rule.

At 300W

• 10Log(300) + 97 = 121.77dB

The other thing you need to know is what happens when you add a cab. Usually cabs are connected in parallel. If the cabs match and they are connected in parallel, combined impedance is 1/2, combined power handling is 2X, combined sensitivity is +3B. This happens every time you double the cabs.

So if we have a pair of our 8 ohm cabs rated for 300W with a 97dB 1W/1m Sensitivity

• Combined impedance is 4 ohms
• Combined power handling is 600W
• Combined Sensitivity is 100dB 1W/1m
  • If you add a cab and the power stays the same, you get +3dB from increased Sensitivity.
  • If usable power doubles you also get another +3dB from doubling the power. So net increase is 6dB.
  • If usable power increases but does not double, you get +3dB from the increased Sensitivity plus whatever change is produced by the extra power.
    • One cab with 180W
• Calculated SPL = 10Log(180) + 97 = 119.55dB
  
  • Two cabs with 240W
• Calculated SPL = 10Log(240) + 100 = 123.8dB

If you are curious the decibel difference between 240W and 180 = 10Log(240/180) = 1.25dB

Assuming Sensitivity is expressed at 1W/1m, the formula for Sensitivity Gain for matching cabs is 10Log(number of cabs)

Sensitivity Gain 1 to 2 cabs = 10Log(2) = +3.01dB
Sensitivity Gain 1 to 3 cabs = 10Log(3) = +4.77dB

So if we have three of our 8 ohm cabs rated for 300W with a 97dB 1W/1m Sensitivity

• Combined impedance is 8/3 = 2.67 ohms
• Combined power handling is 300 x 3 = 900W
• Combined Sensitivity is 97 + 4.77 = 101.77dB 1W/1m

450W total into three cabs with Sensitivity of 97dB 1W/1 each and 101.77dB 1W/1m combined.

• Calculated SPL = 10Log(450) + 101.77 = 128.3dB

You could also figure the SPL of each cab.

• Each cab receives 450/3 = 150W
• Calculated SPL of one cab = 10Log(150) + 97 = 118.76dB
  • Theoretical SPL gain 1 to 3 cabs producing the same SPL = 20Log(3) = +9.54dB
    • 118.76 + 9.54 = 128.3dB--same answer as above.

The +9.54dB is made up of +4.77dB from the sensitivity gained by tripling the number of cabs and +4.77dB gained from tripling the power: 4.77 + 4.77 = 9.54.

 

[el murdoque]

Flaws in this logic?

Yes, one:
A 115 and a 210 are usually considered a good enough match, because the 210 and the 115 are in the same ballpark concerning speaker surface area.
However, you picked a 4 ohm 210 and an 8 ohm 115, so if you combine them, a lot more power from the amp will be directed towards the 210, creating an imbalance.

On top of that, there is always the issue of "the full range of sound from mixed drivers". Both cabinets will get a full range signal and do their best to replicate it all. They absolutely CAN help each other out - if one sounds a bit muffled but has huge bass while the other sounds crisp up top but lacks down low, they can help each other out. But there is a list of possible issues, like phase, comb filtering, cancellation and so forth that can make two mixed cabs sound worse than you planned. If cabs are constructed to be played with each other (by solid engineers), they obviously should work. If you mixed different brands, there's no guarantee that the result will be to your liking.

 

[ardgedee]

The thought was that if I use the 15 alone, I’m at the lowest power rating from the amp, if I use the 2x10, I’m in the middle, and if I combine them, I get the full power of the amp, plus the full range of sound from mixing 15 and 10” speakers.

@Wasnex brings the science, but in brief:

Your goal is not power. Your goal is being loud. Power is one of the components in creating that volume. A lot of power into very inefficient speakers can give you less volume than a little power into highly efficient speakers.

As an aside, speaker size does not dictate tone. But you didn't ask about it so I'll leave it at that.

 

[DeepHz]

Where else can you get info like this from real time users/engineers? I love Talk Bass!

 

[shoot-r]

Appreciate any advice as this is a fairly large step up in my gear investment.

I'll only add about your comment, "fairly large step up in my gear investment."

Unless you're opposed and want to go with new equipment for warranty reasons, have you considered going used?

Since you've done your research and have figured out what amp head and combination of cabinets will probably work for you, I'd suggest your watching the T.B. Classifieds, Facebook musical classifieds, Craigslist, ect. for the particular equipment your considering.

With patience and a watchful eye you can find just what your looking for, faster than you might imagine, just by watching the T.B. classifieds.

Your gear money will go much further by considering used but not abused.

 

[michael_t]

I found a 1x15 400 watt 8 ohm cabinet and a 2x10 400 watt 4 ohm cabinet. The thought was that if I use the 15 alone, I’m at the lowest power rating from the amp, if I use the 2x10, I’m in the middle, and if I combine them, I get the full power of the amp, plus the full range of sound from mixing 15 and 10” speakers.

Flaws in this logic?

Those two cabinets you're talking about... Are they the same brand with the same type of drivers? As already mentioned, mix and matching different cabs can be hit and miss. Also, the whole idea that you get the "full range of sound" by mixing drivers of different sizes is rather outdated.

 

[S-Bigbottom]

With this low power of a head It could work. An expensive gamble that you will like it together.

At 2.67 Ohms and 450 watts total, the 15 cab's speaker would get 150 watts while the 2x10 cab would get 300, but that will be shared to each speaker for 150 watts each. No big danger in power.

Sound... that is another story not knowing what these cabs are.

 

[dbsfgyd1]

I'll only add about your comment, "fairly large step up in my gear investment."

Unless you're opposed and want to go with new equipment for warranty reasons, have you considered going used?

Since you've done your research and have figured out what amp head and combination of cabinets will probably work for you, I'd suggest your watching the T.B. Classifieds, Facebook musical classifieds, Craigslist, ect. for the particular equipment your considering.

With patience and a watchful eye you can find just what your looking for, faster than you might imagine, just by watching the T.B. classifieds.

Your gear money will go much further by considering used but not abused.

Amen to this! I haven’t bought a new piece of sound gear (amp or cab) since the 70’s and have saved thousands.

 

[KevInDaBassment]

All good advice up above ^

About the only companies I would trust to mix speaker sizes on would be the (newer) GK Neo stuff and the Mesa Subway cabs. Some other brands would probably mix ok too, but mixing drivers like that is a crapshoot. Keeping in mind that it may sound great in 60% of places you play, but hot garbage in 40% of them. Safest bet if you can't do extensive testing is two identical cabs, and usually adding more on makes the rig sound better in general

 

[micguy]

Apologies if this has been addressed, but after much searching, I’m not finding the answer, so I figured I’d ask the very specific question and hope some smarter minds than I can shed some light (and based on 2025 knowledge).

Have played for about 30 years but always had combos. Looking to move to separate head and cabinets and want to make sure I’m making the right pairing. I’m leaning toward the GK Fusion 500s head, which indicates ratings of 180 Watts at 8 ohms, 300 watts at 4 ohms and full power (450 watts) at 2.7 ohms, which they describe as a 4 ohm and 8 ohm setup. There is a 3 position switch on the head that allows you to change it from 8+8, 4+8 and “4 or 8”

Since we’ll be playing a range of rooms with potentially different needs, I was thinking two cabinets could give me 3 potential options that I could use as space allows. My current combo is a 1x15, and I really like how that sounds, so I found a 1x15 400 watt 8 ohm cabinet and a 2x10 400 watt 4 ohm cabinet. The thought was that if I use the 15 alone, I’m at the lowest power rating from the amp, if I use the 2x10, I’m in the middle, and if I combine them, I get the full power of the amp, plus the full range of sound from mixing 15 and 10” speakers.

Flaws in this logic? Concerns about damage to the amp or speakers? Appreciate any advice as this is a fairly large step up in my gear investment.

"Getting the full power of the amp" is WAY over rated. I don't know why that comes up here so often. In an era where you can get 800 watt heads, you don't need to make sure you get all that power. The loudest band I was ever in, I had an amp that had a whopping 90watts of power, and it was LOUD. Granted, the speakers I had were very efficient (and big), but still.... you don't really need al that much power, if the speakers are decently efficient. Also, 8 ohm drivers generally have better efficiency than 4 ohm drivers, so, in many cases, using an 8 ohm speaker gets you about as much acoustical output for half the power of a 4 ohm one. Yes, that depends on cab configuration - a 210 may use 2 8 ohm drivers and be a 4 ohm total cab. Anyway....

I'll suggest an alternative strategy - let's call it "making sure you have a working amp at the end of the night" A 2 ohm capable head with 2 (or 3) 8 ohm cabinets is a great strategy. You can bring as many cabs as you need, nothing gets stressed (everything has some margin) and if you really need it , 3 cabs will likely give you a whopping amount of acoustical output.

Operating an amp under its rated power makes it more reliable - when you push a lot of modern amps, they can overheat, and shut down to protect themselves. "Getting as much power as you can from an amp" is precisely what you want to avoid if you want to stay away from thermal shutdowns - more power means hotter, which mans more chance of a shutdown.. it's always better to operate an amp with some margin.

 

[billleeee]

You only seem to be considering power. You also need to consider the sensitivity of the speakers.

The rest is TLDR. If your curious, please continue reading.

SPL is a result of usable power and the sensitivity of the speakers. Usable power is the lower value between how much the amp and deliver and how much the speakers can handle. For example if you have an amp that can deliver 400W and a speaker that can only handle 200W, then usable power is 200W. Usually sensitivity is expressed as the SPL measured at 1 meter with 1 Watt applied.

The SPL goes up 3dB every time you double usable power.

Let's assume an 8 ohm cab rated for 300W with a 97dB 1W/1m Sensitivity rating.

1W = 97dB
2W = 100dB
4W = 103dB
8W = 106dB
16W = 109dB
32W = 112dB
etc


If you know the usable power and the 1W/1m Sensitivity rating, you can use this formula:

• Calculate SPL = 10Log(Usable Power) + Sensitivity

For example calculated SPL for 32W in a cab with 97dB 1w/1m Sensitivity

• 10Log(32) + 97 = 112.05dB--essentially same as we got with the 3db rule.

At 300W

• 10Log(300) + 97 = 121.77dB

The other thing you need to know is what happens when you add a cab. Usually cabs are connected in parallel. If the cabs match and they are connected in parallel, combined impedance is 1/2, combined power handling is 2X, combined sensitivity is +3B. This happens every time you double the cabs.

So if we have a pair of our 8 ohm cabs rated for 300W with a 97dB 1W/1m Sensitivity

• Combined impedance is 4 ohms
• Combined power handling is 600W
• Combined Sensitivity is 100dB 1W/1m
  • If you add a cab and the power stays the same, you get +3dB from increased Sensitivity.
  • If usable power doubles you also get another +3dB from doubling the power. So net increase is 6dB.
  • If usable power increases but does not double, you get +3dB from the increased Sensitivity plus whatever change is produced by the extra power.
    • One cab with 180W
• Calculated SPL = 10Log(180) + 97 = 119.55dB
  
  • Two cabs with 240W
• Calculated SPL = 10Log(240) + 100 = 123.8dB

If you are curious the decibel difference between 240W and 180 = 10Log(240/180) = 1.25dB

Assuming Sensitivity is expressed at 1W/1m, the formula for Sensitivity Gain for matching cabs is 10Log(number of cabs)

Sensitivity Gain 1 to 2 cabs = 10Log(2) = +3.01dB
Sensitivity Gain 1 to 3 cabs = 10Log(3) = +4.77dB

So if we have three of our 8 ohm cabs rated for 300W with a 97dB 1W/1m Sensitivity

• Combined impedance is 8/3 = 2.67 ohms
• Combined power handling is 300 x 3 = 900W
• Combined Sensitivity is 97 + 4.77 = 101.77dB 1W/1m

450W total into three cabs with Sensitivity of 97dB 1W/1 each and 101.77dB 1W/1m combined.

• Calculated SPL = 10Log(450) + 101.77 = 128.3dB

You could also figure the SPL of each cab.

• Each cab receives 450/3 = 150W
• Calculated SPL of one cab = 10Log(150) + 97 = 118.76dB
  • Theoretical SPL gain 1 to 3 cabs producing the same SPL = 20Log(3) = +9.54dB
    • 118.76 + 9.54 = 128.3dB--same answer as above.

The +9.54dB is made up of +4.77dB from the sensitivity gained by tripling the number of cabs and +4.77dB gained from tripling the power: 4.77 + 4.77 = 9.54.

Thank you for the very diligent and well thought out reply. I suppose the problem with asking for opinions from “smarter minds than I” is that I may lack the processing power to fully comprehend your response. Can you please clarify what I should be taking away from your calculations? Is it that the difference in realistic output (ie, volume) is not really meaningful from one of the configurations to the other?

Just to be clear - there wouldn’t be a 3 cab configuration. It’s either the 1x15, the 2x10 or both cabs together. The speakers I was looking at were the Markbass Pure, so they’re matched to each other if not specifically the amp. The sensitivity rating is 101 for the 2x10 and 100 for the 1x15.

 

[HolmeBass]

Thank you for the very diligent and well thought out reply. I suppose the problem with asking for opinions from “smarter minds than I” is that I may lack the processing power to fully comprehend your response. Can you please clarify what I should be taking away from your calculations? Is it that the difference in realistic output (ie, volume) is not really meaningful from one of the configurations to the other?

Just to be clear - there wouldn’t be a 3 cab configuration. It’s either the 1x15, the 2x10 or both cabs together. The speakers I was looking at were the Markbass Pure, so they’re matched to each other if not specifically the amp. The sensitivity rating is 101 for the 2x10 and 100 for the 1x15.

So the 4 ohm cab is a bit more sensitive than the 8 ohm cab. When you use them together the 4 ohm cab will dominate the sound produced. Which may be ok?

Every doubling of power is roughly 3 dB louder. The 4 ohm cabinet will always pull twice the power that the 8 ohm one pulls, so when you use them together it will always be 4 dB louder than the 8 ohm one. Which may work ok, or you may not get the sound you were hoping for.

So that 210 will get 300 watts used alone, or in conjunction with the 8 ohm 115. If you can try it out before hand I would. Just to see what adding the 115 to the 210 actually brings to the party. Might be a lot less than you are imagining.

The three 8 ohm cabinet idea is much more flexible - when you add the third cabinet it really will make a big difference.

 

[Wasnex]

Can you please clarify what I should be taking away from your calculations?

See this post: Yet another thread on heads, cabs, watts and amps

 

[billleeee]

Yes, one:
A 115 and a 210 are usually considered a good enough match, because the 210 and the 115 are in the same ballpark concerning speaker surface area.
However, you picked a 4 ohm 210 and an 8 ohm 115, so if you combine them, a lot more power from the amp will be directed towards the 210, creating an imbalance.

On top of that, there is always the issue of "the full range of sound from mixed drivers". Both cabinets will get a full range signal and do their best to replicate it all. They absolutely CAN help each other out - if one sounds a bit muffled but has huge bass while the other sounds crisp up top but lacks down low, they can help each other out. But there is a list of possible issues, like phase, comb filtering, cancellation and so forth that can make two mixed cabs sound worse than you planned. If cabs are constructed to be played with each other (by solid engineers), they obviously should work. If you mixed different brands, there's no guarantee that the result will be to your liking.

Thanks. I’m not necessarily set on matching the amp maker to the cabinet maker, but absolutely would match the cabinets to each other for all the reasons you note. It’s a bit odd to me that GK would only make full power available by using a pair of speakers with different impedance, and it’s only on the lower rated amp. The 800 watt version gives you the same power whether you use a 4 and an 8 or two 8s.

 

[billleeee]

I'll only add about your comment, "fairly large step up in my gear investment."

Unless you're opposed and want to go with new equipment for warranty reasons, have you considered going used?

Since you've done your research and have figured out what amp head and combination of cabinets will probably work for you, I'd suggest your watching the T.B. Classifieds, Facebook musical classifieds, Craigslist, ect. for the particular equipment your considering.

With patience and a watchful eye you can find just what your looking for, faster than you might imagine, just by watching the T.B. classifieds.

Your gear money will go much further by considering used but not abused.

I’ve considered it, and have purchased both used and new gear in the past. It’s your last part that is the tricky one - often tough to tell when something has been merely used vs abused. I’m also somewhat picky about the condition of my gear, and I’ve not come across too much used gear that hasn’t visibly lived its life to the fullest. Most of my gear looks generally the same as the day I got it.

 

[billleeee]

Those two cabinets you're talking about... Are they the same brand with the same type of drivers? As already mentioned, mix and matching different cabs can be hit and miss. Also, the whole idea that you get the "full range of sound" by mixing drivers of different sizes is rather outdated.

I haven’t finalized, but yes, the ones I’m looking at would be from the same manufacturer, and the same line from that manufacturer, so similar speaker construction, drivers, etc. It’s quite possible I’m old fashioned, but I do feel like I hear a difference between the 15” speaker I have at home and the 10 and 12” speakers in my band’s practice space. And even when I’m watching speaker shoot outs on YT, the 15 always seems to be the most pleasing to my ear. Subjective, sure, but it is what it is.

 

[Wasnex]

It’s a bit odd to me that GK would only make full power available by using a pair of speakers with different impedance, and it’s only on the lower rated amp.

This is unusual but not unheard of.

The limiting factor is current. In general, solid state amps try to produce the same voltage regardless of the load. As the impedance goes down, the current and power increases. More current = more heat. So at some point the current must be limited or the amp will let out the magic smoke.

This amp has an impedance switch. AFAIK it does not actually adjust the output impedance. What it probably does is reconfigure the power supply so the working voltage stays in a safe range for each impedance. Another possibility is it adjusts a limiter threshold so output voltage stays in a safe range for each impedance. It may do both.

I am guessing this is the amp

Here is our Formula Wheel for Ohm's law and Watt's law.

Let's figure the voltage and current for each power and impedance level.

180W at 8 ohms

• Voltage = sq rt(180 x 8) = 37.95V
• Current = sq rt(180/8) = 4.74A

300W at 4 ohms

• Voltage = sq rt(300 x 4) = 34.64V
• Current = sq rt(300/4) = 8.66A

450W at 2.7 ohms

• Voltage = sq rt(450 x 2.6666667) = 34.64V
• Current - sq rt(450/2.6666667) = 12.99A

300W at 2 ohms

• Voltage = sq rt(300 x 2) = 24.49V
• Current = sq rt(300/2) = 12.25A

It appears the limiting factor for 8 ohm power may be the voltage, which is probably determined by the power supply.

Notice the voltage is the same at either 4 ohms or 2.7 ohms. At 2.7 ohms the current is much higher at 12.99A

At 2 ohms the Voltage is only 24.49V. Notice the current is 12.25A.

This suggest the safe current capacity of the output section is a bit under 13A.

 

[el murdoque]

Thanks. I’m not necessarily set on matching the amp maker to the cabinet maker, but absolutely would match the cabinets to each other for all the reasons you note. It’s a bit odd to me that GK would only make full power available by using a pair of speakers with different impedance, and it’s only on the lower rated amp. The 800 watt version gives you the same power whether you use a 4 and an 8 or two 8s.

It is a very common thing that a solid state amp offers the maximal power when you have the minimum speaker load attached.
Would you have preferred that the amp has a minimum load of 4 ohms instead of 2,67, thus eliminating the possibility to connect a 8 and 4 ohm cab altogether?

There are combinations of speakers where this split makes a lot of sense. A 212 and 112, where the 212 runs at 4 ohms and the 112 at 8 ohms, or a 410 and 210 - or a 215 and 115.

 

[WRM]

When both cabs are used, you're essentially running three 8 Ohm drivers (with two sharing an enclosure), and each driver will receive equal power. Be careful with that 210 If you get a more powerful head.