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post #1 of 186 Old 01-28-2017, 07:17 AM - Thread Starter
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Amplifiers: Effects on sound if power does not double when impedance is halved?

Hi,

For many amplifiers the power output does not double when the loudspeaker impedance is halved (at least not down to 2 Ohms or so). Ideally an amplifier that has 100 W at 8 Ohm should have 200 W at 4 Ohm. But many do not, and only have e.g. 150 W at 4 Ohm.

Now what I wonder is whether that only comes into play at loud listening volume?

I'll clarify with an example:
* Amplifier: 100 W @ 8 Ohm, 150 W @ 4 Ohm.
* Loudspeaker: 8-4 Ohms, depending on frequency.

Now if one is listening at a volume requiring 50 W at 8 Ohm, that would require 100 W at 4 Ohm to sustain the same power delivery to the speaker. According the the spec, the amplifier in the example is capable of 150 W at 4 Ohms (i.e. less that the required 100 W). So does this mean that it is sufficient, and that even if the amplifier would have had 200 W at 4 Ohms instead that would not improve the performance anyway?
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post #2 of 186 Old 01-28-2017, 07:27 AM
 
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Amps do double their power output with halving of impedance load at small signal levels. They don't at full power, but that's of no significance, since you shouldn't be approaching full power anyway.
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post #3 of 186 Old 01-28-2017, 08:59 AM - Thread Starter
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Ok, thanks. So when does it become a problem in the example mentioned previously? Is it when 150W @ 4 Ohm is reached, corresponding to 150W/2=75 W at 8 Ohm?

Also, what's the point then of buying expensive heavy weight high-end amplifiers such as Krell and Mark Levinson that double the power output when halving the impedance all the way down to 1 Ohm (even at full load)? Is it basically waste for nearly all audio setups and normal listening conditions? (I know higher quality components in those expensive amps improve the sound too, but my question here is just regarding the power output aspect specifically.)

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post #4 of 186 Old 01-28-2017, 10:38 AM
 
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Originally Posted by Fjodor2000 View Post
So when does it become a problem in the example mentioned previously?
It's never a 'problem'. If your amp isn't powerful enough to drive your speakers to full output with adequate headroom, get a bigger amp.
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post #5 of 186 Old 01-28-2017, 11:08 AM
 
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Originally Posted by Fjodor2000 View Post
Hi,

For many amplifiers the power output does not double when the loudspeaker impedance is halved (at least not down to 2 Ohms or so). Ideally an amplifier that has 100 W at 8 Ohm should have 200 W at 4 Ohm. But many do not, and only have e.g. 150 W at 4 Ohm.

Now what I wonder is whether that only comes into play at loud listening volume?

I'll clarify with an example:
* Amplifier: 100 W @ 8 Ohm, 150 W @ 4 Ohm.
* Loudspeaker: 8-4 Ohms, depending on frequency.

Now if one is listening at a volume requiring 50 W at 8 Ohm, that would require 100 W at 4 Ohm to sustain the same power delivery to the speaker. According the the spec, the amplifier in the example is capable of 150 W at 4 Ohms (i.e. less that the required 100 W). So does this mean that it is sufficient, and that even if the amplifier would have had 200 W at 4 Ohms instead that would not improve the performance anyway?
Short answer; a loss in fidelity.

Signal compression, and spike in momentary or longterm THD & IMD, TIM, etc...

In essence all the short falls that the entrance into clip bring about.

Now if long term, your amplifiers protection will most probably real back the Voltage to protect the amplifier, in doing so THD, IMD, TIM etc, will not increase near as much but you will still incur signal compression.

Short term occurrences are very common for most of us that like to listen to our systems above 90dBc, say ranging between 90-100dBc.

Essentially all modern amplifiers are rated in dynamic terms, short term power bursts while connect to non-inductive loads - phase and load linear. Their actual usable, reliable power is always much less, than these dynamic testing standards depict. As a rule of thumb, I say take the rated, dynamic RMS and cut it in half, and one would have a fair idea of their amplifiers true, longterm, useable, and reliable power.

In this light, your 75-watt amp, would fall in the range of 35-40 watts, still quite usable. The caps will provide for momentary demand peaks, but such power is very short term and easily exhausted.

Your speakers efficiencies, the distance that you sit from them, and the average range of dB that you typically listen to them, is what determines the power demands / requirement from your amp. Your music selection of coarse, also has a weighted factor, in that some music is more dynamic than others and will let your amp breath, if you will, but most music today is so highly compressed that theirs virtually no dynamics left, its a b@ll's to the wall mix down.

If your speakers are say, 88dB efficient, on-axis, without obstruction or diffraction, at 1w/1m and you sit say 3 meters from them, in theory, with 1 watt of injection they would show and efficiency of 79dB. But in real measurement we discover that it tends to be a little less, due to the power envelops dispersing differently, than within the field of the first meter. So lets just round down to 78dB @ 1 watt.

Now lets say that you like to listen in the range of 85-95 dBc. We need to take the highest dB within the range 95dBc, to determine how much true power that you will need, outside of short term, dynamic power.

78dbc@3m = 1 watt
81dBc@3m = 2 watts
84dBc@3m = 4 watts
87dBc@3m = 8 watts
90dBc@3m = 16 watts
93dBc@3m = 32 watts
96dBc@3m = 64 watts

In this example, it is likely that power compression and increases in THD, IMD, etc, etc, will start to trifle with elements of the faithfulness of reproduction.

If someones speaker system is say 86dB efficient, these troubles would set in sooner. If there speakers are out past 3m, from their seated position, the same will be true. Lastly if they like to listen to their systems with peaks above 95dBc, they same will also hold true.

If you are someone in all three camps, then big power is absolutely required, to ensure the highest fidelity reproductions from your audio system, as a whole.

Now for an important caveat: For those of whom that listen to highly dynamic masterings, it is advisable to double your estimated RMS, long term power needs. At least if your desire higher potentials toward fidelity.
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post #6 of 186 Old 01-28-2017, 11:44 AM - Thread Starter
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Originally Posted by Bill Fitzmaurice View Post
It's never a 'problem'. If your amp isn't powerful enough to drive your speakers to full output with adequate headroom, get a bigger amp.
That seems kind of contradictory. If the amp is not powerful enough it is a problem, isn't it? The question is when that would occur in the example I mentioned. Would it be at 75 W @ 8 Ohms, or something else?

Also, any comment about the other question, regarding the usefulness of having high-end amps that double the power when halving the impedance, all the way down to 1 Ohm?
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post #7 of 186 Old 01-28-2017, 12:00 PM
 
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Originally Posted by Fjodor2000 View Post
That seems kind of contradictory. If the amp is not powerful enough it is a problem, isn't it? The question is when that would occur in the example I mentioned.
OK, this was your initial question:
Quote:
Now if one is listening at a volume requiring 50 W at 8 Ohm, that would require 100 W at 4 Ohm to sustain the same power delivery to the speaker.
Power delivery doesn't determine how loud a speaker goes. That's determined by the combination of the amp's voltage swing and the speaker's voltage sensitivity. You're comparing apples to walnuts.

Quote:
Also, any comment about the other question, regarding the usefulness of having high-end amps that double the power when halving the impedance, all the way down to 1 Ohm?
Unless you have 1 ohm speakers it really doesn't matter at all.
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post #8 of 186 Old 01-28-2017, 12:03 PM - Thread Starter
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Originally Posted by Jady Jenkins View Post
Short answer; a loss in fidelity.

Signal compression, and spike in momentary or longterm THD & IMD, TIM, etc...

In essence all the short falls that the entrance into clip bring about.

Now if long term, your amplifiers protection will most probably real back the Voltage to protect the amplifier, in doing so THD, IMD, TIM etc, will not increase near as much but you will still incur signal compression.

Short term occurrences are very common for most of us that like to listen to our systems above 90dBc, say ranging between 90-100dBc.

Essentially all modern amplifiers are rated in dynamic terms, short term power bursts while connect to non-inductive loads - phase and load linear. Their actual usable, reliable power is always much less, then these dynamic testing standards depict. As a rule of thumb, I say take the rated, dynamic RMS and cut it in half, and one would have a fair idea of their amplifiers true, longterm, useable, and reliable power.

In this light, your 75-watt amp, would fall in the range of 35-40 watts, still quite usable. The caps will provide for momentary demand peaks, but such power is very short term and easily exhausted.

Your speakers efficiencies, the distance that you sit from them, and the average range of dB that you typically listen to them, is what determines the power demands from your amp. Your music selection of coarse also has a weighted factor, in that some music is more dynamic than others and will let your amp breath, if you will, but most music today is so highly compressed that theirs virtually no dynamics left, its a b@ll's to the wall mix down.

If your speakers are say, 88dB efficient, on-axis, without obstruction or diffraction, at 1w/1m and you sit say 3 meters from them, in theory, with 1 watt of injection they would show and efficiency of 79dB. But in real measurement we discover that it tends to be a little less, due to the power envelops dispersing differently, than within the field of the first meter. So lets just round down to 78dB @ 1 watt.

Now lets say that you like to listen in the range of 85-95 dBc. We need to take the highest dB within the range 95dBc, to determine how much true, power that you will need outside of short term, dynamic power.

78dbc@3m = 1 watt
81dBc@3m = 2 watts
84dBc@3m = 4 watts
87dBc@3m = 8 watts
90dBc@3m = 16 watts
93dBc@3m = 32 watts
96dBc@3m = 64 watts

In this example, it is likely that power compression and increases in THD, IMD, etc, etc, will start to trifle with elements of the faithfulness of reproduction.

If someones speaker system is say 86dB efficient, these troubles would set in sooner. If there speakers are out past 3m, from their seated position, the same will be true. Lastly if they like to listen to their systems with peaks above 95dBc, they same will also hold true.

If you are someone in all three camps, then big power is absolutely required, to ensure the highest fidelity reproductions from your audio system, as a whole.

Now for an important caveat: For those of whom that listen to highly dynamic masterings, it is advisable to double your estimated RMS, long term power needs. At least if your desire higher potentials toward fidelity.
Thanks for elaborating on the topic.

To me it sounds like it's more important to have high power at the impedance actually used, than necessarily doubling it when the impedance is halved? I.e. better to have e.g. 150W@8 Ohm & 210W@4 Ohm, than 100W@8 Ohm & 200W@4Ohm?

But then why do so many high-end amps focus so much on the ability to doubling the power when the impedance is halved, all the way down to 2 or 1 Ohm (which nearly no speakers ever reach)? E.g. 100W@8Ohm & 400W@2Ohm.
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post #9 of 186 Old 01-28-2017, 12:15 PM - Thread Starter
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Originally Posted by Bill Fitzmaurice View Post
Power delivery doesn't determine how loud a speaker goes. That's determined by the combination of the amp's voltage swing and the speaker's voltage sensitivity. You're comparing apples to walnuts.
Of course, but I've never said anything that contradicts this, have I? So I'm not sure why you're saying I'm comparing apples to walnuts. As you're most likely aware, the speaker impedance varies with frequency. That's why I wrote "Loudspeaker: 8-4 Ohms, depending on frequency" in the example in the original post. To sustain the same voltage swing at 8 Ohms, the amplifier output power has to be double at 4 Ohms to sustain the same volume (when the same speaker and thus the same voltage sensitivity is used).
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Originally Posted by Fjodor2000 View Post
But then why do so many high-end amps focus so much on the ability to doubling the power when the impedance is halved, all the way down to 2 or 1 Ohm (which nearly no speakers ever reach)? E.g. 100W@8Ohm & 400W@2Ohm.
Marketing and bragging rights for people who are duped into thinking that this is important.

Bill's right. As long as an amp has enough power without clipping for a given load, i.e. the load your speakers actually pose at any frequency that concerns you, then what it can achieve into some theoretical more difficult load that doesn't apply to you is immaterial for your given situation.
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In A/V reproduction accuracy, there IS no concept of "accounting for personal taste/preference". As art consumers we don't "pick" the level of bass, nor the tint/brightness of a scene's sky, any more than we pick the ending of a novel or Mona Lisa's type of smile. "High fidelity" means "high truthfulness", faithful to the original artist's intent: an unmodified, neutral, accurate copy of the original master, ideally being exact and with no discernable alterations, aka "transparency".

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Originally Posted by Fjodor2000 View Post
Thanks for elaborating on the topic.

To me it sounds like it's more important to have high power at the impedance actually used, than necessarily doubling it when the impedance is halved? I.e. better to have e.g. 150W@8 Ohm & 210W@4 Ohm, than 100W@8 Ohm & 200W@4Ohm?

But then why do so many high-end amps focus so much on the ability to doubling the power when the impedance is halved, all the way down to 2 or 1 Ohm (which nearly no speakers ever reach)? E.g. 100W@8Ohm & 400W@2Ohm.
There can be many reasons, but the primary one IMO, is amplifiers that can double down, if you will, clearly have superior power supplies to those that cannot. Ideally, an amplifier should be able to run any load (2-16ohms), without producing any changes in its performances, outside of Current production (this equals a potential for linear power production). The bad news, no such amp exists, but some come very close (Krell and Mark Levinson), and such amplifying devices, get a big fat check mark in the what is prime category, and what is prime coming from an amplifier...? - Linear current production throughout the impedance landscape, if you will (and delivery, but I wish not to digress).

There is more, as in many other reasons, but this is the prime reason, and it's a very expensive design choice, as the most expensive part of any amplifier, is it's power supply, from a manufactures perspective. Yes that case, heat sinks, and what not can be just as much or more, however, they truly don't relate to your questions, at hand.

Well, that's the still short but sweet answer, i hope it helps at least a little.
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Marketing and bragging rights for people who are duped into thinking that this is important.
Here is a good example,

Three years ago I finished building my line arrays in the garage and needed a fan cooled amplifier for summer use--and adjustable limiter to protect the speaker/my hearing and from random gunfire by blasting out the town. Being an array, PEQ would be very useful. So I bought a Crown XTi 1002 that spec'd 275 watts 8 ohms, 500 watts 4 ohms and 700 watts at 2 ohms and 1,400 watts bridged at 4 ohms.

My minimum impedance I measured was 5.5 ohms so I set the limiter at around 200 watts and pressed on. I didn't need 2 ohms but the PEQ, adjustable limiter and fan was included so no worries.

My son stops by for a visit and hauled some of his friends in to see what line arrays were--one of them looked up the Crown on his phone and was amazed it was a "1,400 watt amplifier". Monster speakers + Monster amp = Crazy dad award.

Did I explain to them it was "only" output 400 watts total? No...but if you can't dupe your kids, what kind of fun is that?
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^Feeling that the FTC's 1974 rules insisting the 5 conditions must be listed in advertisement power specs [# of simultaneous channels driven, imp. load, bandwidth, THD limit, continuous RMS] were being too restrictive, some manufacturers are moving to Japan's less stringent requirement for stating power outputs to inflate their figures for marketing using their newer ILS standard: if lightning strikes.
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In A/V reproduction accuracy, there IS no concept of "accounting for personal taste/preference". As art consumers we don't "pick" the level of bass, nor the tint/brightness of a scene's sky, any more than we pick the ending of a novel or Mona Lisa's type of smile. "High fidelity" means "high truthfulness", faithful to the original artist's intent: an unmodified, neutral, accurate copy of the original master, ideally being exact and with no discernable alterations, aka "transparency".
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Originally Posted by m. zillch View Post
^Feeling that the FTC's 1974 rules insisting the 5 conditions must be listed in advertisement power specs [# of simultaneous channels driven, imp. load, bandwidth, THD limit, continuous RMS] were being too restrictive, some manufacturers are moving to Japan's less stringent requirement for stating power outputs to inflate their figures for marketing using their newer ILS standard: if lightning strikes.
What Japanese standard are you referring to? I don't know of a single one.

Also, all known North American standards measure and list the spec's that you have denoted. Their difference rest in other areas, such as signal agitation type, signal acquisition type, duration, minimum number of channels driven, and measurement conditions.

A spec, that doesn't list the conditions is in fact invalid.

As an example, we often see THD specs written: THD 1%. A valid spec would read more like the following: THD, 3rd Harmonic .013%, +4 dBu, 20Hz to 20 KHz, unity gain; this is a complete statement, well almost, the load, and load type is required, as well as the testing metric (standard); because anyone of these variable can have a profound effect on the score, let alone two or more...

In that most products are rated dynamically, using a CEA, IHF, EIA, etc. These are relaxed tests. No need to lookup and implement an easier one.

FTC guidelines apply only to US markets and has been unenforced since the first day it was released. Albeit a much more honest measurement standard, but far from ideal. Its a balancing act between consumers and manufactures. The best written to date, of all consumer standards, all others are inferior to FTC, in terms of estimating USEABLE device Wattage, etc.

Please share which Japanese standard that you are referring to, as i know of none. There are simply, spec's listed, with out third party validation.

The big one theses days is CEA-2006 - short term, dynamic power only...

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You didn't get that it was a joke?

In A/V reproduction accuracy, there IS no concept of "accounting for personal taste/preference". As art consumers we don't "pick" the level of bass, nor the tint/brightness of a scene's sky, any more than we pick the ending of a novel or Mona Lisa's type of smile. "High fidelity" means "high truthfulness", faithful to the original artist's intent: an unmodified, neutral, accurate copy of the original master, ideally being exact and with no discernable alterations, aka "transparency".
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You didn't get that it was a joke?
You are correct - how dare you make a joke, and not, make it clear that you're joking for dumb asses like me.

Thanks for setting me straight.
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(the) newer ILS standard: if lightning strikes.
Do you have a citatation for that standard!?? Are the specific joules of the strike specified?

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To sustain the same voltage swing at 8 Ohms, the amplifier output power has to be double at 4 Ohms to sustain the same volume (when the same speaker and thus the same voltage sensitivity is used).
I think I might understand what you're trying to say, that since the impedance of a speaker varies with frequency that the power delivery would have to be constant into any impedance load for linear response. That's not the case. The voltage output of an amp is constant into any load impedance.
If that's not what you're trying to say then you'll have to further clarify it.
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I think I might understand what you're trying to say, that since the impedance of a speaker varies with frequency that the power delivery would have to be constant into any impedance load for linear response. That's not the case. The voltage output of an amp is constant into any load impedance.
If that's not what you're trying to say then you'll have to further clarify it.
Say what now???

The Voltage is absolutely not constant, nor does it retain the same Voltage potential into all loads.

Bill, am I misunderstanding what you have posted?

Or have you made a clumsy posting error?
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Originally Posted by Bill Fitzmaurice View Post
I think I might understand what you're trying to say, that since the impedance of a speaker varies with frequency that the power delivery would have to be constant into any impedance load for linear response. That's not the case. The voltage output of an amp is constant into any load impedance.
If that's not what you're trying to say then you'll have to further clarify it.
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Say what now???

The Voltage is absolutely not constant, nor does it retain the same Voltage potential into all loads.

Bill, am I misunderstanding what you have posted?

Or have you made a clumsy posting error?
My guess is your first option, a lack of understanding.

Power amplifiers are voltages sources. They are designed to maintain a voltage (with gain) that is based the input voltage. As a voltage source the amplifier supplies whatever current is required to maintain the desired voltage into the load, whatever the impedance of the load may be at that instant.

A power amplifier is not a perfect voltage source. In a conventional amplifier, depending on a number of factors, such as the design of its power supply, the voltage of the supply rails will begin to drop as the voltage source is required to deliver more and more current to maintain an instantaneous desired voltage. When the rail voltage drops too low the amplifier will no longer be able to maintain the desired voltage and will clip the voltage peaks of the output signal.

Low impedance loads require more current from the voltage source to maintain the desired voltage. Various design factors, typically in the power supply, but also in other locations in the amplifier, restrict the current output and don't allow the amplifier to supply enough current (2x) into half the load impedance, which would be required for the amplifier to double measured power output (8 ohms reduced to 4 ohms for example).

Notice that the above doesn't mention power in the discussing how the amplifier operates. The amplifier maintains a desired voltage at any one instant by supplying current. The power output from all this is a calculation, but not fundamentally part of the how the amplifier operates. The amplifier doesn't "know" how much power it is supplying, it only deals with voltage and current.
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The amplifier doesn't "know" how much power it is supplying, it only deals with voltage and current.
Correct. To address the OP's concern as I understand it, if voltage swing varied with load impedance then music reproduction as we know it would be impossible, as the volume of the speaker would vary with frequency. Current does vary with frequency, and by dint of that so does power delivery, but speakers respond to voltage, not current.
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This is beyond what I've bothered to learn before but the current (no pun intended) topic has me curious. I know that current source/transconductance amplification is possible, and there are some who think it is the superior approach. Whether true or not... would that require a different transducer motor design, crossover approach, both or neither?

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post #23 of 186 Old 01-29-2017, 09:02 PM
 
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Originally Posted by Bill Fitzmaurice View Post
I think I might understand what you're trying to say, that since the impedance of a speaker varies with frequency that the power delivery would have to be constant into any impedance load for linear response. That's not the case. The voltage output of an amp is constant into any load impedance.
If that's not what you're trying to say then you'll have to further clarify it.
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Originally Posted by Jady Jenkins View Post
Say what now???

The Voltage is absolutely not constant, nor does it retain the same Voltage potential into all loads.

Bill, am I misunderstanding what you have posted?

Or have you made a clumsy posting error?
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My guess is your first option, a lack of understanding.

Power amplifiers are voltages sources. They are designed to maintain a voltage (with gain) that is based the input voltage. As a voltage source the amplifier supplies whatever current is required to maintain the desired voltage into the load, whatever the impedance of the load may be at that instant.

A power amplifier is not a perfect voltage source. In a conventional amplifier, depending on a number of factors, such as the design of its power supply, the voltage of the supply rails will begin to drop as the voltage source is required to deliver more and more current to maintain an instantaneous desired voltage. When the rail voltage drops too low the amplifier will no longer be able to maintain the desired voltage and will clip the voltage peaks of the output signal.

Low impedance loads require more current from the voltage source to maintain the desired voltage. Various design factors, typically in the power supply, but also in other locations in the amplifier, restrict the current output and don't allow the amplifier to supply enough current (2x) into half the load impedance, which would be required for the amplifier to double measured power output (8 ohms reduced to 4 ohms for example).

Notice that the above doesn't mention power in the discussing how the amplifier operates. The amplifier maintains a desired voltage at any one instant by supplying current. The power output from all this is a calculation, but not fundamentally part of the how the amplifier operates. The amplifier doesn't "know" how much power it is supplying, it only deals with voltage and current.
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Originally Posted by Bill Fitzmaurice View Post
Correct. To address the OP's concern as I understand it, if voltage swing varied with load impedance then music reproduction as we know it would be impossible, as the volume of the speaker would vary with frequency. Current does vary with frequency, and by dint of that so does power delivery, but speakers respond to voltage, not current.

Staying within the OP context:

Voltage will absolutely sag in most amps, when loads are half nominal, and will do more or less so, at different frequencies. This is a fact. Lower frequencies and load impedances have been known to more than sag Voltage, but to cause it to oscillate , and real back, sometimes to zero, as in enters protection.

I was being polite when I suggested that I might be misunderstanding Bill's comment. Taking it literally, Bill states:"The voltage output of an amp is constant into any load impedance."

This is not so, perhaps maybe in potential, in a multi-stage, tightly Voltage regulated, SS, Class A like Krell, but even then there are limits. Output Voltages are constantly changing, in reference to the input signal. Only by injecting a test tone, can we make an amplifier output a steady Voltage. If we do so, and switch the load downward to half the rated nominal load, virtually all amplifiers will reveal some Voltage sag, with just one frequency passing through them! The more complex and robust the frequency bandwidth, the greater the load, the greater the potential for sag! Tube amps are the worst. Outside of monsters like some Krell's, all suffer sag, again, especially tubes, as they incur low impedances 4-1 Ohms.

I will be testing some Behringer amps soon, I will be able to evidnece this fact, as I perform sweeps from 8 down to 1 ohm. The Voltage will sag, always does, even into non-inductive loads. Reactive loads from speaker brings about some very interesting performance, which are even more non-linear.

I understand what you gents have posted, but an audio amplifier isn't a perfect Voltage source. Some of the most expensive Class A SS amps come close but outside of them, it's a sea of sag, if you will, as one approaches 1-Ohm (lower loads). I have benched 1000's of amplifiers, and have witnessed such first hand; over and over.

With regards to an amplifier knowing things - it knows nothing, it doesn't think, it responds to energy passing through its circuits, as designed. This includes the protection circuits which are designed to real back unsafe Voltages/Current combinations - Wattages. As energy passes through each circuit, there are losses within every stage, which are intrinsic, and can mostly be measured at its output, while under various loads tests. Voltage sag is amongst these losses.

In short as energy is transferred from source production to load absorption, the sag is proportional to the current demands of the load, in ratio to the source production capabilities and stabilities.

Running within the nominal load rating, and below clip, most SS amps don't suffer much if any sag, but some still do.

Less expensive amps, seem to suffer more from this than more expensive ones, IME.

We will take this one to the bench, and observe half nominal load Voltage performances vs nominal, etc...
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post #24 of 186 Old 01-29-2017, 09:22 PM
 
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Originally Posted by Bigus View Post
This is beyond what I've bothered to learn before but the current (no pun intended) topic has me curious. I know that current source/transconductance amplification is possible, and there are some who think it is the superior approach. Whether true or not... would that require a different transducer motor design, crossover approach, both or neither?
Not my wheel house, but I don't see why we couldn't use the same transducer technology. We would most likely have to use higher impedance driver, how high depends on the amps design.

I suppose if its to high, the transducers may produce more nonlinear defects, but I'm frankly guessing.

I've never contemplated designing such an amp.

IMO fidelity wise - a lessor topology, but again, that is an assumption, based on the little I know about the topology (high series impedances erode Drive Qes - electrical damping - this would in turn effect the passive crossover network - math would have to be adjusted, or damage would occur).

That's my 2-cents

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Quote:
Originally Posted by Bill Fitzmaurice View Post
Correct. To address the OP's concern as I understand it, if voltage swing varied with load impedance then music reproduction as we know it would be impossible, as the volume of the speaker would vary with frequency. Current does vary with frequency, and by dint of that so does power delivery, but speakers respond to voltage, not current.
Current is prime in the production and utilization of energy.

Without the presence of coulombs, their can be no electrical energy produced.

Voltage is a product of one factor of Current/ Coulombs (I) and one factor of resistance/impedance (R): E=IXR

Voltage-Electricity cannot exist without the presence of Current in a circuit.

A speaker cannot work without energy being injected into it and through it. This energy we call Power, when its able to perform work. Watts = useable power.

A Watt, like Voltage is a product. It is two factors of Current (IxI) and one factor of Resistance (R): P=IXIXR

Therefore a speaker in fact not only responds to Current, it does so twice as much, if you will. Firstly within the production of Voltage(s), and secondly within the production of usable power - Wattage(s).

If the load is lower at a different frequency, the amplifier will strive to output a proportional increase in Current (I) at that frequency. It is as simple as that. If it didn't, load(s) would in fact not matter at all, which it obviously, does; and music reproduction as we know it, and enjoy it, remains quite possible!

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post #26 of 186 Old 01-30-2017, 04:51 AM
 
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I know that current source/transconductance amplification is possible.. would that require a different transducer motor design
It's sufficiently difficult to implement that no one bothers to.
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Stasis amps (I think, been a while), a few others, but realistically what is effectively a current-source output offers no advantage in practice as (a) speakers are designed assuming the conventional voltage-drive paradigm and (b) the feedback needed to control the speakers and keep the drive constant ends up making either approach look like a (mostly) low-Z output. At least the ones I have seen/heard/measured. The one or two that actually had high-Z outputs had severe frequency response aberrations into most speakers.

Note I have little experience with them, however, mainly because so few were ever designed.

IME/IMO - Don

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley

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post #28 of 186 Old 01-30-2017, 07:55 AM
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Quote:
Originally Posted by Bill Fitzmaurice View Post
It's sufficiently difficult to implement that no one bothers to.
Which is difficult to implement, the transconductance amp or appropriate transducer? I know a couple of current source amps are commercially available though I have no idea how close they come to actually being current source as I've had no interest in them. Does seem to be a rather obscure topic, for good reason I assume.

(Sorry, missed your post Don, was responding to bill)
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post #29 of 186 Old 01-30-2017, 09:27 AM
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Originally Posted by Bill Fitzmaurice View Post
It's never a 'problem'. If your amp isn't powerful enough to drive your speakers to full output with adequate headroom, get a bigger amp.
Not necessarily.

If your rig is running out of gas when you're cranking it, buying a more powerful amplifier will usually not be as effective as acquiring more efficient speakers.

It is often easier to increase your headroom by 6 dB with better speakers than with an amplifier four times as powerful (which may burn out your current rig anyway). And anything less than a 6 dB improvement probably isn't worth the trip.
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post #30 of 186 Old 01-30-2017, 10:36 AM
 
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To the OP:

By chance, I found examples of what various insufficiencies in audio amplifier power supplies, look like, in an amp of the same wattage rating as you have depicted yours to be - 70-Watts.

The best known means for testing power supply performance, comes from a company called Audio Graph, which has created a load device, which more accurately tests such, and graphs the results in 3-D. This permits quick visual summary of an amplifiers performances in this regard: Voltage, Phase, Load, Wattage. Using their PowerCube, as they have named it, in tandem with Audio Test Suits, From AP, Prism Sound or others, more accurate insights can be captured and document towards revealing the real world, outworkings of an amplifiers performances, when connected to actual speakers, at varying drive levels.


The above graph depicts a Non - Existent Amplifier - Very few come close - the mass majority are represented below.
Please take sometime and study this data.

Note how the power / Wattage remains linear into all phase angles, and loads. @ 8-Ohms 98 -Watts period, no ups or downs, then the same throughout.
Many (lay people) think our amplifiers are linear, they are not, they are just so-called such. They are not linear in Voltage production or waveform production, nor or they silent in the production of either - various forms of noise are intrinsic.

The power supply is the heart of any audio amplifier, it is also the most expensive aspect of an amplifiers material and R&D costs. As such, they are always designed from a budget driven perspective, and not a purest audio performance perspective. I know of very few exceptions, and none come out of China, or Japan; so that wipes out 99% of the consumer offering.


Good audio amplifier will produce graphs like this one. But many do not, as in most. Not the Voltage sag, is alive and well, in even so-called good amplifiers.


Here what a bad amplifier would look like. This is a very common occurrence, in AVR's.


This one shows evidence of bad current limiting scheme.



The one depicts Voltage Oscillation. Very Bad...


Here is a random sampling of well known brands.
Voltage sag is ever present, even in the Rotel!

In an ideal audio bandwidth Voltage source, Voltages would remain constant, irrespective of load/phase angle, but such a beast doesn't yet exist.
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