Power Output vs. Load Impedance - Nerdy Curiosity

[Dino Boreanaz]

This is just a question out of curiosity and a desire to better understand more so than any practical need to know the answers. Sorry for the length of this rambling post, it includes several examples in an attempt to explain and understand the relationship between power output and load impedance.

Manufacturers of solid state amps will often provide power output ratings at two (and sometimes) three different load impedances, but there is no one consistent “scale factor” which applies across all amps relating how much the power changes as the impedance changes.

In some cases, such as the Ampeg RB series (class D), the power output is halved when the impedance is doubled. Is it correct to say that this suggests the amplifier’s output voltage remains constant regardless of load impedance, so the power simply scales inversely with the impedance? Or perhaps are there many other factors, all of which vary, and the net effect coincidentally works out to this simple inverse ratio?

In other cases, such as the Fender Rumble series (also class D), the power output decreases by 1.4 times when the impedance is doubled. This seems to suggest that the output voltage does not remain constant for varying impedances. Is this the inherent nature of a particular type of power amp? Are there specific choices made by the designer which could alter the 1.4 scale factor to some other number like 1.2 or 1.7 and are there inherent limits?

Finally, there are some manufacturers who provide the power output at three different impedances and there doesn’t seem to be much consistency in how the power output varies with varying impedances here either. Some, like the Vox MV series of class D mini guitar heads, behave like the Ampeg RB series in that the power halves for each doubling of impedance (50, 25, and 12.5 W at 4, 8, and 16 ohms). Others like the Marshall 2195 guitar and bass head from the late 1970s reduce power not quite in half for each doubling of impedance (from 100, 65, 36 W at 4, 8, and 16 ohms). Yet others like the Peavey TKO 65 from the early 1980s output maximum power at 8 ohms with reductions at 4 ohms and 16 ohms (50, 70, and 40 W at 4, 8, and 16 ohms). Does this suggest some form of current limiting to maintain safe operation at the lowest impedance level?

I’d be curious to get input from those more knowledgeable than myself as I try to understand why there are so many differences in the relationship between output power and load impedance.

 

[agedhorse]

In some designs the (maximum) output voltage remains constant across all rated impedances, in some designs the output voltage sags under load, and in some designs there are different limits that apply to different output impedances, so the answer is "it depends".

 

[S-Bigbottom]

I’d be curious to get input from those more knowledgeable than myself as I try to understand why there are so many differences in the relationship between output power and load impedance.

They do what they were designed to do.
Do not over think it, read the label of whatever amp you are seeing/using and use it as is written.

When shopping, again read this information and get what best suits your playing situation and needs.

 

[lug]

Do not over think it

Your statement could end Talkbass as we know it!

 

[S-Bigbottom]

Your statement could end Talkbass as we know it!

Is this cable good?
Yup.
Ok, thanks.
End Thread

Speaker good for this amp?
No.
How about this one?
Yup.
Ok, thanks.
End Thread

Is this bass good for metal?
Yup.
Ok, thanks.
End Thread

 

[Dino Boreanaz]

In some designs the (maximum) output voltage remains constant across all rated impedances, in some designs the output voltage sags under load, and in some designs there are different limits that apply to different output impedances, so the answer is "it depends".

Thank you Andy! That's pretty much what I thought when I took note of how this relationship was not the same across all amps regardless of class or other published information.

If you don't mind, I have one follow-up question. When you say "some designs" does this refer to a particular power amp type (topology, amplification device type, etc.) or rather the specific circuit with the myriad of decisions embedded within it? In other words, when a designer chooses a particular amp type are their hands tied in terms of how the power varies with the impedance or are there still circuit-level decisions (component values, etc.) which can alter this relationship? Or perhaps I have the cart before the horse (no pun intended!) and the designer in fact chooses other parameters which then dictate, or lend themselves to, a particular amp type. I guess that was more than ONE follow-up question.

Thanks again for indulging my curiosity. Like I mentioned, this is not of any practical value, just looking for a little understanding. And I'm starting to understand that there will be many things which I will never fully understand!

 

[fdeck]

Having dabbled in DIY amps, and tested a lot of amps, I can confirm that it's the specific circuit. Having a perfectly constant output voltage limit generally involves adding more "stuff" to the design, such as higher performance power supply regulation, and doesn't relate to any urgently needed purpose of bass amps. So it's more likely to not have a perfect ratio.

 

[agedhorse]

It’s usually related to how the power supply integrates with the power amp.

Sag (dropping voltage with increased current) occurs in all types of power amp as well as in power supplies. In switch mode power supplies, there is a global feedback mechanism that can be used to prevent the voltage from falling as current increases (called regulation in a power supply). A tightly regulated supply prevents much of the sag, so the amplifier tends come closer to doubling maximum power with a halving of load impedance.

The other part of the equation is how much loss there is in the amp as the current increases. In general, class D amplifiers perform closest to an ideal voltage source with the least losses. Linear (class AB/B) with bipolar devices generally have the next lower losses and MOSFETs generally have the highest losses unless the output devices are driven from a boosted source.

Regulated SMPS integrated with class D currently offer the best performance in this regard.

 

[Wasnex]

I scanned through the posts and did not see this mentioned.

Some solid state amps list three impedances and associated power levels. For example, the specs may be: 400W at 8 ohms, 800W at 4 ohms, and 800W at 2 ohms.

Obviously, the power is doubling when the load impedance changes from 8 to 4 ohms, so the power supply voltage is holding constant. But the power at 4 ohms and 2 ohms is the same (800W).

An amp with this sort of specs often has a 2-position impedance switch labeled "4/8" ohms and "2 ohms". This labeling can be a bit ambiguous as the intent is to configure the amp in 2 ohm mode anytime the nominal load is under 4 ohm. For example if you run an 8 ohm cab and a 4 ohm cab, the load is 2.67 ohms and the switch should be set to 2 ohms.


My understanding is the impedance switch in most of these amps reconfigures the power supply (lowering the voltage) so the current passing through the output devices stays within a safe range. However, I have recently seen at least one schematic where the impedance switch appeared to change the settings in the amp's compressor/limiter module, without reconfiguring the power supply.

 

[Killing Floor]

Every speaker 10” and smaller I use inverse Siemens and every speaker 12” and larger I use Ohms. Don’t use Ohms on your 10s or tweeters or you’ll overheat the cab which can cause the Tolex to peel off. Says so in the owner manual.

 

[teemuk]

There's no universal rule how an amp will behave, other than Ohm's law.

Assuming that (peak) output voltage will not change the Ohm's law will tell that output power drops to half when load impedance doubles, vice versa and et cetera.

But the problem is that in many amps the peak output voltage can change.
- An amp can feature an impedance matching transformer, or autoformer, which scales output voltage to correct value at different impedances. This applies to all classes of amps whether tube or solid-state, though class-d amps seldom employ such transformers whereas tube amps mostly always have them.
- An amp can employ current feedback, which means it will have different voltage gain at different nominal impedances, and therefore peak voltage changes at different load impedances. All kinds of amps can feature current feedback, even tube amps and class-d solid-state amps.
- The amplifier can have a power supply that provides higher supply voltage to high impedance load setting and lower supply voltage to low impedance load setting. You may find such feature from all classes of solid state amps, tube amps with impedance matching output transformer seldom need this.