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In Reply to: Can someone explain "dynamic headroom" and posted by dwpc on July 20, 2001 at 00:23:06:
I think Trevor is exactly right. An amplifier with a theoretically perfect power supply--one whose voltage remains constant regardless of power demands--will have a "dynamic headroom" of 0 dB. I think this would be the case for most true Class-A amplifiers as well as amplifiers that use regulated power supplies.Most non-regulated supplies in power amplifiers will sag when under constant power demand (the conditions under which amplifiers are rated). Under less stressful conditions--such as when an amplifier is asked to deliver only short bursts of power--the sag will be much less, thus increasing somewhat the available power output of the amplifier.
"Dynamic headroom" is basically a measurement of how much an amplifier's supply will vary between those conditions. In my opinion, everything else being equal, the higher the dynamic headroom, the poorer the supply and the less desirable the power amp.
Follow Ups:
If the power supply is running WAY below MOL in terms of current demands even when the amp is drawing as hard as it can, there will NEVER be voltage sag at the output devices other than that created by losses in the support circuitry. This negates the need for ANY form of regulation. This results in a "faster" supply, better rise times with less tilt, etc.... Products built like this follow "the theory of overkill".When making statements such as yours' and Trevors', you must think in what is possible and NOT what is commonly available on the shelves at K-Mart. Sean
>
> If the power supply is running WAY below MOL in terms of current
> demands even when the amp is drawing as hard as it can, there will
> NEVER be voltage sag at the output devicesIf an amp is "drawing as hard as it can," how can its power supply be running below its maximum current capability? Perhaps I don't understand your question?
> When making statements such as yours' and Trevors', you must think
> in what is possible and NOT what is commonly available on the
> shelves at K-Mart.I wasn't just thinking of mass-market designs. As I said, everything else being equal, I think an amplifier with a low dynamic headroom is preferable to one with a high dynamic headroom. Again, keep in mind that a theoretically perfect amplifier will have zero dynamic headroom. In reality, of course, everything isn't always equal, so your point that other factors may need to be considered is certainly valid.
"If an amp is "drawing as hard as it can," how can its power supply be running below its maximum current capability? Perhaps I don't understand your question?"Let me try to explain using "generic" figures.
If the output section of an amplifier can pass 50 amps continuously and 100 amps momentarily and the manufacturer was using a transformer / power supply that was capable of 100 amperes continuous / 200 amps peak, there would be NO sag. The only reason that voltage sag occurs is due to the fact that something in the circuit is "bottlenecked", "resistive" or "current limited". Since there would be no current limitation under any normal state of operation, there would be no voltage sag. There would therefore be NO reason for voltage regulation to the output devices themselves although temperature regulated bias circuitry might still be advantageous.
See where i'm coming from ??? Sean
>
> If the output section of an amplifier can pass 50 amps continuously
> and 100 amps momentarily and the manufacturer was using a
> transformer/power supply that was capable of 100 amperes
> continuous/200 amps peak, there would be NO sag.I don't think there's enough information here to say whether the power supply would sag or not--that is dependent on how well designed the suppy is, not just on current ratings. However, an amp with high DH is basically *guaranteed* to exhibit power supply voltage sag.
Of more concern here though, I think, would be the underrated output devices compared to the power supply. If the amp was "drawing as hard as it can," I would think the output devices would be destroyed before the power supply reached its maximum capability.
I do think I see where you're coming from, though, and I tried to give you my perspective in another reply to you elsewhere.
I have to agree with Sean here. There are amps that are overkills and there are amps that are well designed. One key term parameter is the continuous power at 8, 4, 2 and 1 Ohms. If the power doubles as the impedance is halved, that is an indication of a good power supply. These are very expensive.When playing music, your amp have to provide dynamic power. Pluking of strings in an acousitc guitar (Fleminco style) or sudden busrt of energy from a piano (Beethoven sontas) do require power (also depend on efficency of speakers). Theils for an example will drain your amp if you do not have sufficient power and dynamic headroom while Loewther drivers with obscene efficieny such as 105dB may not need that much. The point is, in music you come across these dynamic passeges all the time and if your amp fails to deliver that power on demand, your tweeters are sitting ducks! I have heard number of horror stories how tweeters/woofers were destroyed while playing these dynamic passges at high volume levels.
As Sean correctly pointed out, one must be able to balance both the continuous power as well as the DH depending on the speaker.
Good Luck!
PriyaW
> One key term parameter is the continuous power at 8, 4, 2 and 1 Ohms.
> If the power doubles as the impedance is halved, that is an
> indication of a good power supply.Maybe. Or it might be a case of an amplifier's specifications being derated to begin with so it can claim this for marketing reasons. Despite its misleading name, "dynamic headroom" is a better measure for determining power supply quality. (The lower the DH, the better.)
> The point is, in music you come across these dynamic passeges all
> the time and if your amp fails to deliver that power on demand,
> your tweeters are sitting ducks!I agree. So, if your system sometimes requires 100 W of peak power, why--other than cost--would you choose a high-dynamic-headroom 50-W amplifier that can output 100 W on peaks instead of a low-dynamic-headroom 100-W amplifier, when the latter is almost certainly likely to offer better performance?
But instead of going for an amp that may begin to saturate on peaks, why not go for an even larger amp with even more reserve ? So long as it is "fast enough" and "wideband enough" to cope with any dynamic signal that you may present it, there is nothing wrong with "overkill" or running a product with LOTS of reserve. Compare this to a product that clips or saturates at / near maximum rated power. Which one do you think will sound "smoother" or "less stressed" ??? Sean
>
> But instead of going for an amp that may begin to saturate on peaks,
> why not go for an even larger amp with even more reserve ?If you have determined that 100 W is the maximum power needs of your system, then really the question is which 100-W amp to choose? Since the amp with low DH is likely to offer better performance (within the 100 W range) than an "equivalent" amp with high DH, then I think the choice is obvious (everything else being equal).
> Which one do you think will sound "smoother" or "less stressed" ???
In my experience, amplifiers with the "stiffest" supplies easily sound the least stressed. In fact, they usually sound like much more powerful amplifiers. ("Smoothness" I associate more with the amplifier circuitry itself.)
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