Question for our experts - why aren't more amps designed for 2 ohm loads?

[bherman]

Greetings all, I have a question about the choices that amp designers/manufacturers make about design characteristics of amp heads. Most modern amps seem to be designed for a 4 ohm minimum impedance. There are a few companies that allow for a 2.67 minimum, and only a few that design all of their products to be 2 ohm capable, most notably Mesa and the (now defunct) Acoustic Image.

My question is why most manufacturers don't design for 2 ohm minimum loads. Is is strictly a cost consideration, or is there something about meeting that requirement that impacts the design process?

Particularly flagging this for @agedhorse and @Wasnex, two of our more technically qualified TB members.

To be clear - I am not asking for personal opinions on why you think its a good idea or not, I just simply want to understand the reasoning behind the decision. Unless it adds considerably to design and production costs, it seems to me that there's no downside to it (other than the fact that it relies on the user to flip a switch on the back of the amp when changing to 2 ohm, thereby opening the door to operator error ).

 

[Wasnex]

Greetings all, I have a question about the choices that amp designers/manufacturers make about design characteristics of amp heads. Most modern amps seem to be designed for a 4 ohm minimum impedance. There are a few companies that allow for a 2.67 minimum, and only a few that design all of their products to be 2 ohm capable, most notably Mesa and the (now defunct) Acoustic Image.

My question is why most manufacturers don't design for 2 ohm minimum loads. Is is strictly a cost consideration, or is there something about meeting that requirement that impacts the design process?

Particularly flagging this for @agedhorse and @Wasnex, two of our more technically qualified TB members.

To be clear - I am not asking for personal opinions on why you think its a good idea or not, I just simply want to understand the reasoning behind the decision. Unless it adds considerably to design and production costs, it seems to me that there's no downside to it (other than the fact that it relies on the user to flip a switch on the back of the amp when changing to 2 ohm, thereby opening the door to operator error ).

My perception is it relates to both production cost and product optimization.

In general, solid state amps try to hold the voltage constant regardless of the impedance of the load. As the impedance is reduced, current and power increase. Current results in heat, so too much current means the output section of the amp will be damaged. So if a constant voltage amp is optimized for 2 ohms it will make less power at 4 ohms and 8 ohms.

Some solid state amp have an impedance switch. The associated circuitry adds to cost and complexity. These switches can do different things. For example the switch my reduce the operating voltage of the power supply or it may limit how much voltage the output section can develop. The end result is the amp is limited in a way that keeps it in a safe operating range..

Normally when an amp can hold the voltage constant, the power doubles when the impedance is cut in half.

The Mesa 800W Subways are rated 400W at 8 ohms, 800W at 4 ohms, and 800W at 2 ohms. So these amps hold the voltage constant when the impedance drops from 8 ohms to 4 ohms. But they do not hold the voltage constant at 2 ohms.

The 800W Subway amps have an impedance switch.

If the load is under 4 ohms, the switch must be set to 2 ohms for safe operation. If the amp is operated into 2 ohms with the switch set to 4/8 ohms, the amp will either go into protect mode or incur damage.

You can also trick these amps into limiting themself to about 600W into 4 ohms by setting the switch to 2 ohms. This could be useful if your speaker can not handle the full power of the amp.

If you want to see some of the math read on...if not stop here:

This formula wheel shows common Ohm's Law and Watt's Law equations.

Let's calculate the voltage for 400W at 8 ohms, 800W at 4 ohms, 600W at 4 ohms, and 800W at 2 ohms using V =sq rt(PZ)

400W at 8 ohms

V = sq rt(400*8) = 56.57V​

800W at 4 ohms

V = sq rt(800*4) = 56.57V​

600W at 4 ohms

V = sq rt(600*4) = 48.99V​

800W at 2 ohms

V = sq rt(800*2) = 40V
​

Now let's calculate the current using I = sq rt(P/Z)

400W at 8 ohms

I = sq rt(400/8) = 7.07A​

800W at 4 ohms

I = sq rt(800/4) = 14.14A​

600W at 4 ohms

I = sq rt(600/4) = 12.25A​

800W at 2 ohms

I = sq rt(800/2) = 20A​

It may also be informative to consider (P = VI) so you can see the voltage and current side by side. In the following, we input the values of V and I that were calculated above.

400W at 8 ohms

P = 56.57*7.07 = ~400W​

800W at 4 ohms

P = 56.57*14.14 = ~800W​

600W at 4 ohms

P = 48.99*12.25 = ~600W​

800W at 2 ohms

P = 40*20 = 800W​

FYI, I use ~ to mean an approximate value resulting from rounding errors.

 

[Marko 1]

I'm guessing that the VAST majority of players run one or two 8-Ohm cabs, or one 4-Ohm cab.

 

[Clark Westfield]

I just picked up a Bergantino HP2 for this exact reason, that it does indeed go to 2 ohms.

I have a combo of cabs that are 8 and 4 ohms, and now have no limitation anymore.

Choices are few, but this is a high priced head, so the option can be incorporated into production easier offsetting costs.

 

[gtirard]

Jad Freer Coda (Class G power amp) and Sisma (Hybrid amp) are rated 700W @ 4Ohm / 350W @ 8Ohm / 950w @ 2Ohm

 

[bherman]

My perception is it relates to both production cost and product optimization.

In general, solid state amps try to hold the voltage constant regardless of the impedance of the load. As the impedance is reduced, current and power increase. Current results in heat, so too much current means the output section of the amp will be damaged. So if a constant voltage amp is optimized for 2 ohms it will make less power at 4 ohms and 8 ohms.

Some solid state amp have an impedance switch. The associated circuitry adds to cost and complexity. These switches can do different things. For example the switch my reduce the operating voltage of the power supply or it may limit how much voltage the output section can develop. The end result is the amp is limited in a way that keeps it in a safe operating range..

Normally when an amp can hold the voltage constant, the power doubles when the impedance is cut in half.

The Mesa 800W Subways are rated 400W at 8 ohms, 800W at 4 ohms, and 800W at 2 ohms. So these amps hold the voltage constant when the impedance drops from 8 ohms to 4 ohms. But they do not hold the voltage constant at 2 ohms.

The 800W Subway amps have an impedance switch.
View attachment 5345343
If the load is under 4 ohms, the switch must be set to 2 ohms for safe operation. If the amp is operated into 2 ohms with the switch set to 4/8 ohms, the amp will either go into protect mode or incur damage.

You can also trick these amps into limiting themself to about 600W into 4 ohms by setting the switch to 2 ohms. This could be useful if your speaker can not handle the full power of the amp.

If you want to see some of the math read on...if not stop here:

This formula wheel shows common Ohm's Law and Watt's Law equations.
View attachment 5345345
Let's calculate the voltage for 400W at 8 ohms, 800W at 4 ohms, 600W at 4 ohms, and 800W at 2 ohms using V =sq rt(PZ)

400W at 8 ohms

V = sq rt(400*8) = 56.57V​

800W at 4 ohms

V = sq rt(800*4) = 56.57V​

600W at 4 ohms

V = sq rt(600*4) = 48.99V​

800W at 2 ohms

V = sq rt(800*2) = 40V
​

Now let's calculate the current using I = sq rt(P/Z)

400W at 8 ohms

I = sq rt(400/8) = 7.07A​

800W at 4 ohms

I = sq rt(800/4) = 14.14A​

600W at 4 ohms

I = sq rt(600/4) = 12.25A​

800W at 2 ohms

I = sq rt(800/2) = 20A​

It may also be informative to consider (P = VI) so you can see the voltage and current side by side. In the following, we input the values of V and I that were calculated above.

400W at 8 ohms

P = 56.57*7.07 = ~400W​

800W at 4 ohms

P = 56.57*14.14 = ~800W​

600W at 4 ohms

P = 48.99*12.25 = ~600W​

800W at 2 ohms

P = 40*20 = 800W​

FYI, I use ~ to mean an approximate value resulting from rounding errors.

Thx this is all helpful. My takeaway from this is that it’s a bit more complicated (thus more costly) to design and build, and requires safeguards to ensure that users don’t inadvertently damage their amps.

 

[bherman]

I'm guessing that the VAST majority of players run one or two 8-Ohm cabs, or one 4-Ohm cab.

I agree, but if you don’t your options are more limited. I have 2 x 8 ohm 112s and a 4 ohm 110, if for some reason I ever wanted to run a 12 and 10 I’d need a different amp.

 

[bherman]

Jad Freer Coda (Class G power amp) and Sisma (Hybrid amp) are rated 700W @ 4Ohm / 350W @ 8Ohm / 950w @ 2Ohm

Yup there’s some out there but not many.

 

[Ostie]

Sone of the older Peavey amps were 2ohm, like 400 Series amps from the late seventies.

 

[logdrum]

My perception is it relates to both production cost and product optimization.

In general, solid state amps try to hold the voltage constant regardless of the impedance of the load. As the impedance is reduced, current and power increase. Current results in heat, so too much current means the output section of the amp will be damaged. So if a constant voltage amp is optimized for 2 ohms it will make less power at 4 ohms and 8 ohms.

Some solid state amp have an impedance switch. The associated circuitry adds to cost and complexity. These switches can do different things. For example the switch my reduce the operating voltage of the power supply or it may limit how much voltage the output section can develop. The end result is the amp is limited in a way that keeps it in a safe operating range..

Normally when an amp can hold the voltage constant, the power doubles when the impedance is cut in half.

The Mesa 800W Subways are rated 400W at 8 ohms, 800W at 4 ohms, and 800W at 2 ohms. So these amps hold the voltage constant when the impedance drops from 8 ohms to 4 ohms. But they do not hold the voltage constant at 2 ohms.

The 800W Subway amps have an impedance switch.
View attachment 5345343
If the load is under 4 ohms, the switch must be set to 2 ohms for safe operation. If the amp is operated into 2 ohms with the switch set to 4/8 ohms, the amp will either go into protect mode or incur damage.

You can also trick these amps into limiting themself to about 600W into 4 ohms by setting the switch to 2 ohms. This could be useful if your speaker can not handle the full power of the amp.

If you want to see some of the math read on...if not stop here:

This formula wheel shows common Ohm's Law and Watt's Law equations.
View attachment 5345345
Let's calculate the voltage for 400W at 8 ohms, 800W at 4 ohms, 600W at 4 ohms, and 800W at 2 ohms using V =sq rt(PZ)

400W at 8 ohms

V = sq rt(400*8) = 56.57V​

800W at 4 ohms

V = sq rt(800*4) = 56.57V​

600W at 4 ohms

V = sq rt(600*4) = 48.99V​

800W at 2 ohms

V = sq rt(800*2) = 40V
​

Now let's calculate the current using I = sq rt(P/Z)

400W at 8 ohms

I = sq rt(400/8) = 7.07A​

800W at 4 ohms

I = sq rt(800/4) = 14.14A​

600W at 4 ohms

I = sq rt(600/4) = 12.25A​

800W at 2 ohms

I = sq rt(800/2) = 20A​

It may also be informative to consider (P = VI) so you can see the voltage and current side by side. In the following, we input the values of V and I that were calculated above.

400W at 8 ohms

P = 56.57*7.07 = ~400W​

800W at 4 ohms

P = 56.57*14.14 = ~800W​

600W at 4 ohms

P = 48.99*12.25 = ~600W​

800W at 2 ohms

P = 40*20 = 800W​

FYI, I use ~ to mean an approximate value resulting from rounding errors.

While this is all impressive - you really need to give a business answer to the question. It will involve more math unfortunately and more data points.

If I need more equipment for my lab and have to ask management and get approval from my stakeholders, I will probably have no more than 15 % of my slide for technical stuff and the rest will be about cost and benefits even though most if not all my stakeholders are engineers and scientist themselves. Design decisions for products that are meant to be sold are mostly driven by money and charts speak more than equations.

 

[bobdabilder]

Money

 

[factory presets]

I agree, but if you don’t your options are more limited. I have 2 x 8 ohm 112s and a 4 ohm 110, if for some reason I ever wanted to run a 12 and 10 I’d need a different amp.

I can't remember the numbers from last century. But when I had impedance issues, I just put a big old ceramic resistor on the back of the offending cab anwired it in appropriately.

 

[bherman]

I can't remember the numbers from last century. But when I had impedance issues, I just put a big old ceramic resistor on the back of the offending cab anwired it in appropriately.

Makes sense, but in today's day and age the lawyers would do anything to preclude that from happening.

 

[beans-on-toast]

The early (1969) SVT was designed to handle 4 or 2 ohms. Same for the current SVT. The V4B had a switch for 2, 4, or 8 ohm taps. The current V4B also has 2, 4, and 8 ohm taps. There’s a lot of these amps out there.

New 2 ohm capable amps are readily available. Many companies don’t offer this because their target buyers don’t need the capability.

 

[factory presets]

Makes sense, but in today's day and age the lawyers would do anything to preclude that from happening.

Not saying you're wrong. But I wonder how they'd go about that. Rolls Royce Legal waved their lifetime guarantee at John Lennon to get him to change his psychedelic paintwork. I only wish Peavy had one of those to wave. (Not that it actually worked on the Walrus.) On reflection it was probably the RCFs in the home made cab that got the ohms anyway.

 

[SunByrne]

This formula wheel shows common Ohm's Law and Watt's Law equations.

Wow, flashback to the circuits course I took in college in... 1989. I'm pretty sure I got a B.

 

[agedhorse]

In reality, it’s much more difficult to design a class D amp that’s capable of 2 ohms and is also robust, reliable and safety-EMC compliant. This is why there aren’t many 2 ohm rated commercial bass amps.

 

[57pbass]

My perception is it relates to both production cost and product optimization.

In general, solid state amps try to hold the voltage constant regardless of the impedance of the load. As the impedance is reduced, current and power increase. Current results in heat, so too much current means the output section of the amp will be damaged. So if a constant voltage amp is optimized for 2 ohms it will make less power at 4 ohms and 8 ohms.

Some solid state amp have an impedance switch. The associated circuitry adds to cost and complexity. These switches can do different things. For example the switch my reduce the operating voltage of the power supply or it may limit how much voltage the output section can develop. The end result is the amp is limited in a way that keeps it in a safe operating range..

Normally when an amp can hold the voltage constant, the power doubles when the impedance is cut in half.

The Mesa 800W Subways are rated 400W at 8 ohms, 800W at 4 ohms, and 800W at 2 ohms. So these amps hold the voltage constant when the impedance drops from 8 ohms to 4 ohms. But they do not hold the voltage constant at 2 ohms.

The 800W Subway amps have an impedance switch.
View attachment 5345343
If the load is under 4 ohms, the switch must be set to 2 ohms for safe operation. If the amp is operated into 2 ohms with the switch set to 4/8 ohms, the amp will either go into protect mode or incur damage.

You can also trick these amps into limiting themself to about 600W into 4 ohms by setting the switch to 2 ohms. This could be useful if your speaker can not handle the full power of the amp.

If you want to see some of the math read on...if not stop here:

This formula wheel shows common Ohm's Law and Watt's Law equations.
View attachment 5345345
Let's calculate the voltage for 400W at 8 ohms, 800W at 4 ohms, 600W at 4 ohms, and 800W at 2 ohms using V =sq rt(PZ)

400W at 8 ohms

V = sq rt(400*8) = 56.57V​

800W at 4 ohms

V = sq rt(800*4) = 56.57V​

600W at 4 ohms

V = sq rt(600*4) = 48.99V​

800W at 2 ohms

V = sq rt(800*2) = 40V
​

Now let's calculate the current using I = sq rt(P/Z)

400W at 8 ohms

I = sq rt(400/8) = 7.07A​

800W at 4 ohms

I = sq rt(800/4) = 14.14A​

600W at 4 ohms

I = sq rt(600/4) = 12.25A​

800W at 2 ohms

I = sq rt(800/2) = 20A​

It may also be informative to consider (P = VI) so you can see the voltage and current side by side. In the following, we input the values of V and I that were calculated above.

400W at 8 ohms

P = 56.57*7.07 = ~400W​

800W at 4 ohms

P = 56.57*14.14 = ~800W​

600W at 4 ohms

P = 48.99*12.25 = ~600W​

800W at 2 ohms

P = 40*20 = 800W​

FYI, I use ~ to mean an approximate value resulting from rounding errors.

I was going to say the same thing..

 

[Panek]

Greetings all, I have a question about the choices that amp designers/manufacturers make about design characteristics of amp heads. Most modern amps seem to be designed for a 4 ohm minimum impedance. There are a few companies that allow for a 2.67 minimum, and only a few that design all of their products to be 2 ohm capable, most notably Mesa and the (now defunct) Acoustic Image.

My question is why most manufacturers don't design for 2 ohm minimum loads. Is is strictly a cost consideration, or is there something about meeting that requirement that impacts the design process?

Particularly flagging this for @agedhorse and @Wasnex, two of our more technically qualified TB members.

To be clear - I am not asking for personal opinions on why you think its a good idea or not, I just simply want to understand the reasoning behind the decision. Unless it adds considerably to design and production costs, it seems to me that there's no downside to it (other than the fact that it relies on the user to flip a switch on the back of the amp when changing to 2 ohm, thereby opening the door to operator error ).

For example, when have you seen anyone schlepping around a 810 on the regular, let alone two of them that would go down to 2 Ohms? I have one and I like carting it around, but everybody's rocking stuff like 4x10's, 2x12's, 2x15's, two 8 Ohm cabs, etc etc. or just go cabless at all, resorting to pre-amp pedals and DI boxes, and everybody sounds nice.

I have two solid state amps from the '70's, a Traynor and a Acoustic. Both are built like tanks and perfectly capable of pushing 2 Ohms. In fact, I think the Acoustic is even capable of getting down to something like 1.8 Ohms, because the Cervin-Vega speaker in one if the cab models had some weird impedance and you could run two at a time. But don't quote me on that, it's just something I read once.

I used the 370 for a time with an old PA cabinet repurposed with two 15" 4 Ohm drivers just to get the full 2 Ohm experience. Why? Well the amp can do it and everybody hauled those bigass rigs everywhere they went, so I thought it's a good idea. Unfortunately, I was absolutely too loud at any gig or rehearsal I played, even by my punk standards. So I quickly sold it, but ended up getting the 810 which I find more versatile, believe it or not, even though it's also a beast by it's own.

Anyway, I think amps have changed because we have changed. There's always room for dudes with huge rigs who swear by their full tube SVTs, but most of the players go for smaller amps and cabs and the manufacturers just don't see any need to make an amp that can push 2 Ohms. If you want to use two 4 Ohm cabs, just get a second amp I guess...?

 

[gtirard]

For example, when have you seen anyone schlepping around a 810 on the regular, let alone two of them that would go down to 2 Ohms? I have one and I like carting it around, but everybody's rocking stuff like 4x10's, 2x12's, 2x15's, two 8 Ohm cabs, etc etc. or just go cabless at all, resorting to pre-amp pedals and DI boxes, and everybody sounds nice.

I have two solid state amps from the '70's, a Traynor and a Acoustic. Both are built like tanks and perfectly capable of pushing 2 Ohms. In fact, I think the Acoustic is even capable of getting down to something like 1.8 Ohms, because the Cervin-Vega speaker in one if the cab models had some weird impedance and you could run two at a time. But don't quote me on that, it's just something I read once.

I used the 370 for a time with an old PA cabinet repurposed with two 15" 4 Ohm drivers just to get the full 2 Ohm experience. Why? Well the amp can do it and everybody hauled those bigass rigs everywhere they went, so I thought it's a good idea. Unfortunately, I was absolutely too loud at any gig or rehearsal I played, even by my punk standards. So I quickly sold it, but ended up getting the 810 which I find more versatile, believe it or not, even though it's also a beast by it's own.

Anyway, I think amps have changed because we have changed. There's always room for dudes with huge rigs who swear by their full tube SVTs, but most of the players go for smaller amps and cabs and the manufacturers just don't see any need to make an amp that can push 2 Ohms. If you want to use two 4 Ohm cabs, just get a second amp I guess...?

When you see how loud you get be with a modern 2x12 @ 4Ohm and a 800W amp, who would need more?