[Wind Chaser]

When an amplifier doubles its 8 ohm power output at 4 ohms and doubles it again at 2 ohms, will it cut in half at 16 ohms and cut in half again 32 ohms? And what happens if the impedance spikes at 95 ohms?

For example, if amplifier has a power output of:

                                      100 watts at 8 ohms
                                      200 watts at 4 ohms
                                      400 watts at 2 ohms


How much power will it avail at:

                                       16Ω
                                       32Ω
                                       95Ω

[dB Cooper]

If a speaker had a 95 Ω impedance spike, I'd avoid it. A spike that big suggests a wildly out-of-control mechanical resonance or completely bad tuning or both. I don't even know how you'd get that if you deliberately set out to. Maybe from a 16Ω starting point, but otherwise, nah.

[Wind Chaser]

[Wind Chaser]

Okay, the question of 95 ohms aside, what happens to the power output at 16 Ω and 32 Ω?

[poseidonsvoice]

Okay, the question of 95 ohms aside, what happens to the power output at 16 Ω and 32 Ω?

50 watts at 16 ohms and 25 watts at 32 ohms.

Best,
Anand.

[Speedskater]

To over-simplify. E = sq.rt. P*R
So if the amp is voltage limited.

 2 Ohm 400W = 28.3V
 4 Ohm 200W = 28.3V
 8 Ohm 100W = 28.3V
16 Ohm  50W = 28.2V
32 Ohm  25W = 28.3V
95 Ohm  8.4W = 28.3V

[poseidonsvoice]

To over-simplify. E = sq.rt. P*R
So if the amp is voltage limited.

 2 Ohm 400W = 28.3V
 4 Ohm 200W = 28.3V
 8 Ohm 100W = 28.3V
16 Ohm  50W = 28.3V
32 Ohm  25W = 28.3V
95 Ohm  8.4W = 28.3V

Exactly. Voltage is preserved. The other important factor to understand is how the impedance changes with frequency and also how the sensitivity changes with frequency. With that information you can predict the max spl available at 1m since you know what the power output is at that particular frequency.

Best,
Anand.

[Wind Chaser]

Voltage is preserved. The other important factor to understand is how the impedance changes with frequency and also how the sensitivity changes with frequency. With that information you can predict the max spl available at 1m since you know what the power output is at that particular frequency.

Voltage is preserved meaning?

So even though a given amplifier may be capable of 400 watts at a low impedance, it may put out as little as 8 watts at a higher impedance. If this is so, then there is a lot to matching an amp (in terms of power) to a given pair of speakers than most might be inclined to think?

[poseidonsvoice]

Voltage is preserved meaning?

So even though a given amplifier may be capable of 400 watts at a low impedance, it may put out as little as 8 watts at a higher impedance. If this is so, then there is a lot to matching an amp (in terms of power) to a given pair of speakers than most might be inclined to think?

The RMS voltage is always the same in this example. Notice how it is 28.3V RMS.

P = V(squared)/impedance. Plug the numbers and you will see.

You are correct. For more details, I urge you to read Chapter 16, of Floyd Toole's book "Sound Reproduction" titled 'Loudspeakers and Amplifiers' where he details in laymen terms how if you wanted to maintain a specific SPL (say 90dB), the output power you will need can change dramatically dependent on frequency only because the impedance of MOST loudspeakers isn't flat (and this is also assuming the efficiency stays flat, which most do not). In addition, the length of your speaker cable and hence it's impedance and the output impedance of your said amplifier plays a very big part as well. I mostly aim for amplifier designs where the output impedance (over 20Hz to 20khz) remains at least 10 times below the lowest impedance of my speaker, or else SPL fluctuations may occur. It explains why a lot of SET amplifiers (with higher output impedances, say 2-10 ohms) sound softer in the bass or more forward in the midrange. Once you hear neutrality however, it is difficult to go back, especially with better modern recordings.

Routledge has 20% discounts from time to time: https://www.routledge.com/Sound-Reproduction-The-Acoustics-and-Psychoacoustics-of-Loudspeakers-and/Toole/p/book/9781138921368

Best,
Anand.

[Speedskater]

An amplifier like this is optimized for very low impedance loudspeakers. A lot of it's costs are for pieces & parts that work with that kind of load. This amp is only cost effective with these low impedance loads.
While other amps designed for more reasonable impedance loads have a Safe Operating Area circuit.  It limits max current into low impedance loads and max voltage into higher impedance loads.

But the speaker curve in reply #2 is a different story. The impedance spike is at the bass-reflex tuned frequency, the loudspeaker's efficiency goes way up at that frequency so it needs hardly any power.

* * * * * * * * * *
We can view that speaker as about a nominal 7 Ohm load, so the amp is a 114 Watt amplifier.

[poseidonsvoice]

But the speaker curve in reply #2 is a different story. The impedance spike is at the bass-reflex tuned frequency, the loudspeaker's efficiency goes way up at that frequency so it needs hardly any power.

+1. Totally agree.

Best,
Anand.

[Wind Chaser]

Thanks Speed and Anand, that was very helpful.

[planet10]

It is worth noting that most of what has been discussed here applies to voltage amps. Current amps increase power as impedance increases.

A voltage amp will double power if you halve the impedance as long as it doesn’t run out of current.

A current amp will double power if you double the impedance as long as it doesn’t run out of voltage.

SETs with high output impedance are typically voltage amps that are approaching the point where an amp crosses to a current amp (amp output impedance = speaker impedance).

P = V(squared)/impedance.

Note that that the impedance is a comples value.

P = V x I x cos(phase angle) , as the phase angle gets larger cos(phase angle) approaches zero (and the power approaches 0. This is why amps that driver electrostats (impedan c elooks a lot like a capacitor) need to have serious current delivery capabilities.

dave

[FullRangeMan]

Okay, the question of 95 ohms aside, what happens to the power output at 16 Ω and 32 Ω?

Very few SS amps can work in 2Ω, even fewer can work in 16Ω I unknow any capable. Watts in SS amps at 16Ω are not informed why they cant operate in this hi resistance.

[Speedskater]

Very few SS amps can work in 2Ω

This is true because there a lot of trade-offs and added costs to work into a 2Ω load.

even fewer can work in 16Ω I unknow any capable. Watts in SS amps at 16Ω are not informed why they cant operate in this hi resistance.

Any well designed SS amp can work in to a 16Ω or even much higher load impedance. Including an open circuit.
Of course as in reply #5, power rapidly diminishes as impedance increases.

[Speedskater]

It is worth noting that most of what has been discussed here applies to voltage amps. Current amps increase power as impedance increases.
....................................... .....................
dave

Hi-fi current amplifiers are only slightly more common than unicorns.

[FullRangeMan]

I had a big Carver SS amp that has a cap at output so it was safe with no load.

[Wind Chaser]

Hi-fi current amplifiers are only slightly more common than unicorns.

I thought most SS amps were current whereas tube amps were voltage?

[FullRangeMan]

I thought most SS amps were current whereas tube amps were voltage?

+1 in the past many SS brands adverted his hi amperage numbers.

[Speedskater]

We are using different applications of the words  'current' and 'voltage'.

Almost all hi-fi amplifiers are designed as a voltage sources.  They all (SS & tube) have a very low output impedance. Within reasonable limits they will output about the same voltage regardless of the speaker load impedance.

Some DAC chips are current sources. They have a high output impedance. They output the same current regardless of load (within limits).

A hi-fi amp that is a current source is a very different beast than 99.999 % of hi-fi amps and requires different speakers.

* * * * * * * *
Yes tube amps are high voltage / low current amps, but then the output transformer lowers the voltage and increases the current.