About dynamic headroom (controversial topic :-)

[jeannot]

WARNING: This may be a controversial thread, let's try to keep it fun.

Been wanting to write for a while about the "dynamic headroom" thing. I will start with the most difficult to deny first...

Exhibit #1: The Dynamic Headroom is the ratio, expressed in Decibels, between the power that an amplifier can deliver during a very short amount of time (I think the standard is 100ms but I'm not sure) divided by the amount of power it can deliver continuously.

So far no opinion has been expressed about how relevant either of these tests is to reality.

Exhibit #2: Electrically speaking, the perfect power supply is one whose supply voltage is unaffected by the current demand.

What makes the difference in power between bursts and continuous modes is the change of power supply Voltage to the output stages at clipping. Given the two exhibits above, one must conclude that an amplifier with a perfect power supply has 0.0db of dynamic headroom, because it will clip at the same voltage regardless of the duration of the signal.

If the best power supply gives a 0.0db Dynamic Headroom, then to be logical, one MUST conclude that the worst power supply gives the highest dynamic headroom. Conclusion: the Dynamic Headroom is a measure of the poor quality of a power supply.

Claiming that more dynamic headroom is better is akin to saying that the harder you hit your head on the wall, the better it feels when you stop. Hence, you should hit your head on the wall as hard as you can.

Probably the second or third post down below will say something like: "No one listens to continuous sine waves at clipping", or "musical content is made a lot more of bursts that continuous signals", or something like that. This is not what this post is about. I would recommend starting a new thread titled "How relevant Dynamic Headroom is to real life", thank you.

This post is about a measurement for which common belief is to aim for the highest number, when actually we should aim for lowest numbers.

Flame shield ON.

 

[PENG]

Flame shield ON.

Make sure it is UL, CSA, NFPA approved, it's coming..............

 

[GranteedEV]

Issue #1 is this:

Dynamic headroom defined only as "short-burst potential"

I think of dynamic headroom as the output at 1% THD - a clipped signal. For a short transient we accept it, but any longer and we too easily hear the distortion.

 

[jeannot]

Issue #1 is this:

Dynamic headroom defined only as "short-burst potential"

I think of dynamic headroom as the output at 1% THD - a clipped signal. For a short transient we accept it, but any longer and we too easily hear the distortion.

Yes, that could very well be the reason why higher distortion is better tolerated in short burst.
Also, the signal produced by a burst is very much helped by the filtering capacitors, but they help a lot less for a continuous signal.

 

[jeannot]

I forgot an important piece: All comparisons between two amps for their power, dynamic headroom, or even the same amp in ACD or stereo, must be done with all other factors being equal: Same load, same distortion limit, I would say same lab, we might even go further and say same session.

 

[3db]

There was a s discussion either on this forum or another abiyt dynamic headroom and the arguement was basicalluy waht you stated. Dynamic headroom is a way fo rmanufacturers to cheat from putting more money into the powersupply. I have to agree with this arguement..If a receiver can use its power supply to its limit for short or long burts, then this would be better than it covering the short bursts only. There will be some questions asked about the real life relevance of source material demanding sustained peaks. I think hearing damage will occur much sooner in an amp with the perfect power supply.

 

[PENG]

There was a s discussion either on this forum or another abiyt dynamic headroom and the arguement was basicalluy waht you stated. Dynamic headroom is a way fo rmanufacturers to cheat from putting more money into the powersupply. I have to agree with this arguement..If a receiver can use its power supply to its limit for short or long burts, then this would be better than it covering the short bursts only. There will be some questions asked about the real life relevance of source material demanding sustained peaks. I think hearing damage will occur much sooner in an amp with the perfect power supply.

I am not sure if I can agree to everything said in Jean's post, but I am not a fan of this good dynamic headroom yet poor continuous rating kind of deal either. Now if everything is equal, i.e. two amps having the same continuous rating, then I may prefer the one that has more headroom. I guess that's pretty much what you are saying too, no?

 

[jeannot]

Okay let's say you have two amps, with power supplies giving +-50V DC at full continuous power. So they're both rated approximately, let's say 150 WPC.

Now, let's say one of these amps, amp A has a power transformer with cheaper and smaller wiring, but more turns at the secondary so the power supplies +-65VDC at idle, while the other amp B has 50VDC at idle, same as full power.

As the music or the movie sound is output, there may be large power swings, hence large variations in power supply demand.

Amp A will see its power supply constantly swing between 65V and 50-55V. The amplifier circuitry will be constantly trying to compensate for these voltage swings while trying to have the amp output an exact multiple of the input. Amplifier topology like constant current sources and input/driver stages regulators can somewhat litigate that, it is definitely not ideal. It's like trying to eat soup while walking.

Amp B however, will see a constant voltage source, whatever the current demand. The circuitry can now concentrate on simply making the input signal larger, because it can count on a constant revenue.

I will always remember, one of the best amps of the 80s were made by Naim Audio. (They're still pretty good, but they have competition now...) These amps were mind boggling. They had the simplest topologies with few transistors, and the sacrilege of quasi complementary output stage, with nothing more than a pair of 2N3055. (They cooked their 3055s in ovens at high-temp, then manually matched them) They sounded great, solid, open, despite all their technical theoretical deficiencies. They could drive speakers that amps at twice the power couldn't even wake up. The weirdest thing, they had regulated power supplies. Their trivial amps could count on a perfectly stable voltage source. Anyone remember the Quad 303? Looked like a beige toaster, a grand classic that made history in the 70s. That mutant was even capacitor coupled at the output. Again, regulated power supply.

I'll stop typing now, what's wrong with me for Volt's sake.

 

[3db]

I am not sure if I can agree to everything said in Jean's post, but I am not a fan of this good dynamic headroom yet poor continuous rating kind of deal either. Now if everything is equal, i.e. two amps having the same continuous rating, then I may prefer the one that has more headroom. I guess that's pretty much what you are saying too, no?

I think thats what I'm saying ?? Out of those two scenarios, you just listed, I would take number 2 as well. What I'm trying to say is that headroom offers a manufacturer wiggle room in power spec dleivery and makes a lesser power amp seem more powerful than it really is. I would choose an amp that can deliver the same instantaneous power without clipping advertised with 0db headroom over an amp with almost equal power but with a 3db headroom. If your confused, don't feel bad. I am too.

 

[avnetguy]

So in simplest terms, you're testing the capacitive storage abilities of the receiver right?

Steve

 

[mtrycrafts]

There was a s discussion either on this forum or another abiyt dynamic headroom and the arguement was basicalluy waht you stated. Dynamic headroom is a way fo rmanufacturers to cheat from putting more money into the powersupply. I have to agree with this arguement..If a receiver can use its power supply to its limit for short or long burts, then this would be better than it covering the short bursts only. There will be some questions asked about the real life relevance of source material demanding sustained peaks. I think hearing damage will occur much sooner in an amp with the perfect power supply.

Well, reality has to be part of the discussions as that is what an amp is reproducing, reality of an audio soundtrack.

Music and HT sound comes in short burst when analyzed on a scope down to a very small fraction of a second, unlike a DC signal And, its power duration compared to a sine wave of 70.7% is a lot shorter, a lot shorter.

If a power amp can satisfy the peak requirements of a musical passage, perhaps all that has been recorded reasonably, with its continuous power rating, we don't need a dynamic headroom, right?

But, if it cannot, then, what do we need in dynamic headroom to satisfy the peaks? Perhaps, that depends on our individual listening practices and speaker sensitivity. Loud sound practices needs a different amp than something more on the normal side of listening.

There are no perfect amps out there, never will be to the specs stated of unlimited this and that, so, perhaps that should not be a yardstick even for discussion purposes.

Only the universe is unlimited, as far as I know.

 

[gmichael]

OK, let me see if I understand this in Average Joe speak. Tell if this is correct.
You are saying that an amp that can produce maybe 300 watts rms with 0 head room is better than an amp that can produce 200 watts rms with enough head room to peak at 300?
I know that this is technically wrong, but is that the general idea? If so, I agree completely.

 

[jeannot]

So in simplest terms, you're testing the capacitive storage abilities of the receiver right?

Not sure, mediocre capacitive storage may also give "good" headroom.
What you are testing, is the power of the amplifier if it HAD a perfect power supply. Continuous tests show how far the power supply is from perfect.

 

[jeannot]

OK, let me see if I understand this in Average Joe speak. Tell if this is correct.
You are saying that an amp that can produce maybe 300 watts rms with 0 head room is better than an amp that can produce 200 watts rms with enough head room to peak at 300?
I know that this is technically wrong, but is that the general idea? If so, I agree completely.

Absolutely correct. However, what is not obvious is that I am also saying that a 200WPC amp with 0db headroom is better than a 200WPC amp with 2db headroom. Because it has a perfect 200W power supply rather than a mediocre 300W power supply.

The price the 2db headroom amp pays to be able to achieve the 300W peaks, is that the power supply constantly fluctuates, and likely collapses on low-impedance speakers.

 

[jeannot]

Well, reality has to be part of the discussions as that is what an amp is reproducing, reality of an audio soundtrack.

Music and HT sound comes in short burst when analyzed on a scope down to a very small fraction of a second, unlike a DC signal And, its power duration compared to a sine wave of 70.7% is a lot shorter, a lot shorter.

If a power amp can satisfy the peak requirements of a musical passage, perhaps all that has been recorded reasonably, with its continuous power rating, we don't need a dynamic headroom, right?

But, if it cannot, then, what do we need in dynamic headroom to satisfy the peaks? Perhaps, that depends on our individual listening practices and speaker sensitivity. Loud sound practices needs a different amp than something more on the normal side of listening.

There are no perfect amps out there, never will be to the specs stated of unlimited this and that, so, perhaps that should not be a yardstick even for discussion purposes.

Only the universe is unlimited, as far as I know.

Well despite my initial "This post is about a measurement ... ", I will address this new topic that you bring. You are correct that continuous tests do NOT represent reality, and it was never claimed that they do. (we could also discuss whether a 100ms 1KHz sound wave does.)

If I could summarize my initial post in one sentence, it would be nothing more than: "The poorer the power supply, the higher the Dynamic Headroom."

Now, if we talk musical content with a fair amount of dynamics, the fact that a power supply sags under stress means that in order to benefit from the extra 1.5db for the peaks (some peaks, not all), you have amplifier circuits that work with an unstable power supply for the 50db under that 1.5db, and your amp is likely to perform poorly on low impedance speakers.

 

[avnetguy]

Not sure, mediocre capacitive storage may also give "good" headroom.

If it were to give "good" headroom wouldn't that be "good" capacitive storage then? Not sure why you'd label that mediocre?

Steve

 

[jeannot]

If it were to give "good" headroom wouldn't that be "good" capacitive storage then? Not sure why you'd label that mediocre?

Yes. But capacitive storage is only part of the story, as it cannot cannot make up for a weak transformer. Especially since the transformer is what recharges the capacitors...

So after the capacitors have given, the transformer caves even more, because it has to try to sustain the high current demand without the caps helping, PLUS it has to recharge these caps, caps that still get discharged between every 120Hz rectified cycle...

 

[PENG]

Now, let's say one of these amps, amp A has a power transformer with cheaper and smaller wiring, but more turns at the secondary so the power supplies +-65VDC at idle, while the other amp B has 50VDC at idle, same as full power.

By using an numerical example you are making your point abundantly clear, and it sounds convincingly logical at first glance. However, you have to consider in practical term it does not work like this, at least not quite. You can wind more turns in the secondary winding to get a lower step down ratio hence a higher secondary voltage, but you can only achieve the same power rating by using it with a higher impedance load, as you did allude to later on in your post.

So as you previously stated, everything else being equal, say using an 8 ohm resistive load for both amps. In order to acheive the same continuous power rating as amp B, amp A has to use the same wire size. Again, to be clear, you can't rely on a higher voltage if you are keeping the impedance (resistance in this example) the same for both amps because P=I²R, or V²/R. As long as P and R are the same, V and I must be the same. Again, what you stated sound convincingly logical, but if you think deeper, it is kind of circular logic. Okay my turn to put the flame shield on.

 

[avnetguy]

Yes. But capacitive storage is only part of the story, as it cannot cannot make up for a weak transformer. Especially since the transformer is what recharges the capacitors...

That's right but I thought this was about dynamic headroom? So if the caps can sustain the draw during a short but high dynamic period, then there shouldn't be any real significant drop.

Steve

 

[PENG]

NAD amps are known for their well regarded dynamic power as well as power supplies, following is part of their specs for the C375.

Features
•2 x 150W Continuous Power into 4 ohms and 8 ohms
•250W, 410W, 600W IHF Dynamic power into 8, 4 and 2 ohms, respectively
•PowerDrive™ circuit
•Holmgren Toroidal Power transformer

That's just another example of a counter point. I really don't believe the C275BEE achieves their excellent dynamic power rating by putting in a lower grade power supply.