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Author Topic: 2 ohm loads on bottom end  (Read 31001 times)

John Roberts {JR}

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Re: 2 ohm loads on bottom end
« Reply #80 on: September 24, 2016, 10:48:08 AM »

Guys, please correct my understandings...

I've been thinking of 'headroom' as an amps ability to handle transients, above the amp's rated power.
"Headroom" is an actual term that describes an electrical paths maximum clean signal handling level above a nominal 0VU level, analogous to the physical headroom above your head when you walk through a doorway. 

Another related term or characterization is "crest factor" related to musical signals that describes the ratio of short term peaks to long term average. Crest factor is a little fuzzy without a sense of how long term the long term is, but I have designed simultaneous Peak/VU meters that showed crest factor in real time.   
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Rated power being FTC, EIA, or AES, with each of their own nuances.
Transients being defined as some arbitrary duration, short term waveform that exceeds voltage implied in rated power. Burst / peak specs.
There is fudge or wiggle room in rated power. Marketers typically want to squeeze every last watt out of a platform for ads, while rated power for FTC testing may be derated some just to pass. I generally advise to read the power number (and minimum load impedance) screened on the back of the chassis because that is the number UL and test agencies use.  8)
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Ivan, I get what you are saying about the maximum you can get is the power supply rail voltage.
I take it that is the unloaded rail voltage, and represents the highest burst / peak spec available.......but it becomes a question for how long.
This is why the old archaic IHF dynamic headroom spec specified an on time for the burst and an off time... IIRC the off time was actually -20dB. Even this -20dB average draw will cause any PS rail to sag. An amp hoping to realize a high dynamic headroom spec from a power supply with soft regulation (transformer winding impedance, etc) will often sag losing precious spec dB from that -20dB load, which is probably why they did it.   
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AFAIKT, rated power specs always imply a voltage lower than either either maximum voltage, or burst wattage, whatever info the manufacturer provides.
Even on high reputation amps ....I just ran the numbers on a Speakerpower amp  , whose max voltage is 2dB ) above rated power ..thxJohn :))
(particular worst case example chosen for illustration)
I recently bought some cheap amps to use in my home video playback system. The labelled as 120W amps I bought, after you distill down the BS power specs, only put out <10W clean at 8 ohm.

Rated power at what % distortion? A square wave puts out 2x the power vs a clean sine wave.
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It appears that all amps sag some under load, some alot more than others. 
And that the zone between the sagged voltage and the unloaded rail voltage represents potential burst / peak voltage for at least some minute duration.
As I already explained from my experience designing a "burst power" amp, the duration of burst power needs to be more than tens of mSec to be audibly significant.

What is the difference between a narrow transient clipped or not? (answer not much). I've had the misfortune to participate in single blind listening tests where the test subjects preferred the clipped sound.   >:( >:(
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I'm left thinking that voltage vs short term duration in that zone, is the key to understanding 'headroom' above 'rated'.
I used to argue with a colleague who would use the PS rail voltage to compute a theoretical transient headroom spec. I don't know that I ever convinced him otherwise, but he never spent time on a bench designing a power amp. 
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Or what I think of as headroom, and when the clip lights will come on if they are fast enough.
We're dredging up old stories with that one.. I recall one amp designer who would under report (slow down) clip indication and his customers believed the amps had more headroom (recall that slight clipping is not very audible). Another well known mixer maker didn't report clipping in their post fader gain stage and convinced another group of customers that their mixers had more headroom.  >:(
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Does the above make sense?

I've read that Meyer now uses 500ms for their "true burst" spec, and state they believe anything under 100ms is useless for music.
If they are right, is seems most modern burst specs are more than meaningless...and maybe just look to rated power like I'm sensing you guys are saying..

Thx,  mark
The difference between now and the good old days is that modern amps have gotten much more powerful. Back in the day it was possible to sell a 240W amp that would make 240W all day and all night. Now with amps making more than 10x that power not only are there no speakers that can handle that much power (for that long) or outlets that can deliver that much power (for that long).

Consumers like to distrust amp makers as somehow trying to deceive them but unfortunately like low information voters the customers want to be told massive numbers. At the end of the day the amps making maximum power for only medium term time duration are a good match for speakers that can handle more medium term power than continuous. So these modern amps are a better solution for the real world applications than continuous power.

Don't over think this. Listen to reports from end users using these modern amps in real world applications (of course this being the internet you shouldn't listen to what everybody says... maybe not even me.  8)

JR 
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John Roberts {JR}

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Re: 2 ohm loads on bottom end
« Reply #81 on: September 24, 2016, 10:51:11 AM »

Maybe you can answer this in terms us normal people get. How big of a difference does 240v make in an amp compared to 120? I understand it is only on the primary of the transformer, the secondary side puts out the same on either voltage? Is there any notable sound difference. I think i know the answer but would like to here it from someone that actualy knows the truth. Thanks.
In theory it should make zero difference.

In practice the 240VAC line cord is drawing one half the current so external mains wiring will experience one half the IxR voltage sag.

JR
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Mark Wilkinson

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Re: 2 ohm loads on bottom end
« Reply #82 on: September 24, 2016, 01:22:24 PM »

John, thank you for your generous reply.
If I may I ask one more tech question.....and I'll try to stop  :)

When I think about voltage sag (believe you called it 'regulation' in another post),
I picture the cause of the voltage sag to be the amp's inability to provide sufficient current against the given load, to maintain pressure (voltage).
IOW, that it's a lack of current that makes voltage sag...

Is this picture on the right track?
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Ivan Beaver

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Re: 2 ohm loads on bottom end
« Reply #83 on: September 24, 2016, 01:57:30 PM »

John, thank you for your generous reply.
If I may I ask one more tech question.....and I'll try to stop  :)

When I think about voltage sag (believe you called it 'regulation' in another post),
I picture the cause of the voltage sag to be the amp's inability to provide sufficient current against the given load, to maintain pressure (voltage).
IOW, that it's a lack of current that makes voltage sag...

Is this picture on the right track?
Think of it this way.

The power supply is often the limiting factor in many amps.

It is a reservoir.  If it is small, then it drains quickly.  If it is large-it takes longer to drain.

In both cases it is being "refilled"by the incoming voltage from "the wall".

The first approach was to use switching power supplies.  This way the freq was a lot higher, so they could get by smaller caps (because they are being recharged faster)

If you pretend it is a standard water tower, the faster you can fill it up, the more likely it will not run low on water.

But when it runs out, all you have is the "refill" to deliver.

So you want enough in "storage" so that it can deliver a lot of water (power) during a hard event, but when it lightens up, it can refill quickly to have enough for the next "hard event".
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Kevin Conlon

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Re: 2 ohm loads on bottom end
« Reply #84 on: September 24, 2016, 03:01:35 PM »

Think of it this way.

The power supply is often the limiting factor in many amps.

It is a reservoir.  If it is small, then it drains quickly.  If it is large-it takes longer to drain.

In both cases it is being "refilled"by the incoming voltage from "the wall".

The first approach was to use switching power supplies.  This way the freq was a lot higher, so they could get by smaller caps (because they are being recharged faster)

If you pretend it is a standard water tower, the faster you can fill it up, the more likely it will not run low on water.

But when it runs out, all you have is the "refill" to deliver.

So you want enough in "storage" so that it can deliver a lot of water (power) during a hard event, but when it lightens up, it can refill quickly to have enough for the next "hard event".
This refill time, isn't that what used to be called "duty cycle", a spec not used in audio. My small welder has a duty cycle of [i don't remember} on that i think of it as a cool down time before it starts to fade in output current. Or am i thinking of slew rate maybe? the time it takes for the amp to recover. A number that isn't used anymore.
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Mark Wilkinson

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Re: 2 ohm loads on bottom end
« Reply #85 on: September 24, 2016, 03:14:14 PM »

Think of it this way.

The power supply is often the limiting factor in many amps.

It is a reservoir.  If it is small, then it drains quickly.  If it is large-it takes longer to drain.

In both cases it is being "refilled"by the incoming voltage from "the wall".

The first approach was to use switching power supplies.  This way the freq was a lot higher, so they could get by smaller caps (because they are being recharged faster)

If you pretend it is a standard water tower, the faster you can fill it up, the more likely it will not run low on water.

But when it runs out, all you have is the "refill" to deliver.

So you want enough in "storage" so that it can deliver a lot of water (power) during a hard event, but when it lightens up, it can refill quickly to have enough for the next "hard event".

Yes, thx.
 
It's exactly that kind of analogy that makes me think pressure sags because available current (water) is less than needed, to be able to maintain pressure to the given impedance..
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Jeff Bankston

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Re: 2 ohm loads on bottom end
« Reply #86 on: September 24, 2016, 03:49:11 PM »

I'm wondering how the old QSC Series 3 3500 and 3800 amps do as for as headroom goes.
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John Roberts {JR}

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Re: 2 ohm loads on bottom end
« Reply #87 on: September 24, 2016, 04:11:43 PM »

I'm wondering how the old QSC Series 3 3500 and 3800 amps do as for as headroom goes.
IMO headroom is not a useful term for characterizing amplifiers.

Better for line level signal paths like inside consoles or for storage mediums (give me more bits baybee). 

JR
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John Roberts {JR}

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Re: 2 ohm loads on bottom end
« Reply #88 on: September 24, 2016, 04:41:37 PM »

John, thank you for your generous reply.
If I may I ask one more tech question.....and I'll try to stop  :)
No problem good questions are good thing, I just get a little tired of answering the same ones over the years.
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When I think about voltage sag (believe you called it 'regulation' in another post),
I picture the cause of the voltage sag to be the amp's inability to provide sufficient current against the given load, to maintain pressure (voltage).
IOW, that it's a lack of current that makes voltage sag...

Is this picture on the right track?
Perhaps for some of the amplifier current limiting mechanisms (there are several in the output stages) but "transformer regulation" is a simple IxR winding wire loss, and in the limit magnetic flux saturation so either not enough copper (winding resistance) or not enough magnetic flux (iron and turns of windings). This is why bigger (old school) amps have heavier transformers.

Sorry I am not trying to blind you with science but transformer regulation is just one of the limits. Power transistors have a finite on resistance so will lose voltage based on another IxR loss, not to mention most old school amps used emitter degeneration resistors in series with power devices to force sharing.

A crude way to quantify or estimate how lossy the amp is wrt current is to compare rated power between 8, 4, and 2 ohm loads. In an ideal world (that we don't live in and only politicians talk about) a 200W @ 8 ohm power amp, would deliver 400W at 4 ohm and 800W a 2 ohms. Looking at any real power amp specification reveals significantly less than theoretical output at 2 ohms. This shortfall is partially from transformer regulation, losses in power devices, and in the extreme max current output. Old school amps follow a fairly predictable power vs load relationship but some modern class D amps get funny with with voltage limited or current limited extremes.

There are other wild cards in here, like reservoir capacitor ripple. For audio frequencies lower than 2x the mains frequency charging rate, the capacitors are tasked with supplying more current than at higher signal frequencies. While this is getting a little esoteric, at high frequencies the sine wave alternates between the positive and negative swing drawing half from the plus supply and half from the negative. At very low frequencies the signal draw remains always negative or always positive for a full charging cycle (only a few mSec) but long for the caps. You can parse this out if a amp specs more power at 1khz than 20-20kHz (but make sure THD is same for both too).

Again this is TMI for most users. Modern amps are so good that we can all relax and buy something light that won't break our back or the bank.  8)

JR

PS: I didn't tell you everything there is about amps yet,,, but I am trying to calm you all down not scare you.       
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Jeff Bankston

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Re: 2 ohm loads on bottom end
« Reply #89 on: September 24, 2016, 04:54:52 PM »

IMO headroom is not a useful term for characterizing amplifiers.

Better for line level signal paths like inside consoles or for storage mediums (give me more bits baybee). 

JR
My storage unit has 8 feet of headroom.
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Re: 2 ohm loads on bottom end
« Reply #89 on: September 24, 2016, 04:54:52 PM »


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