About dynamic headroom (controversial topic :-)

[jeannot]

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.

At the clipping point, two amps have to have sensibly the same supply voltage, and current delivery. I do not see how this prevents amp A from having a smaller wire size and higher idle voltage.

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.

The higher voltage is the sum of the voltage loss in the secondary AND voltage supplied to the amp. These two voltages added give the idle voltage.
As you start drawing current, you start having voltage drop within the primary/secondary due to their internal resistance. The two V that are the same that you mention are the V delivered to the amp, of course they are, a watt is a watt. But at clipping, the weaker transformer wastes power in its secondary and its primary in the form of heat, because for the same I and higher internal resistances, then:
Power loss = VI = (Vidle - Vclipping) * I
The more difference between Vidle and Vclipping, the more power loss in the transformer for the same power delivery to the speakers.
I hope this logic looks a bit less circular now.

 

[jeannot]

NAD amps are known for their well regarded dynamic power...

Indeed, NAD practically invented Dynamic Headroom.

as well as power supplies,

That is completely different from good dynamic headroom. Could you point me to actual measurements of the NAD 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

First I want to say that I am leery of numbers that are not the results of real measurements, and distortions are not specified. But since you provide them... These numbers, if actually measured are actually atrocious, because they claim that the amplifier can deliver 410W into 4 ohms if you need it for 100ms, but if you need the power for more than a few seconds, it falls flat on its face and will only get 150W. However I do agree that it is definitely a High Current amplifier as far as the output stage goes, as the 600W in 2 ohms show.

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.

The $3300 Krell 250a delivers 207WPC continuous at 8 ohms and 407WPC continuous at 4. That's what a near-perfect power supply allows.

And you know what? Krell do not publish a specification for Dynamic Headroom. Because their numbers are probably among the lowest on the market, in the order of 0.1 to 0.2db if that...

 

[3db]

At the clipping point, two amps have to have sensibly the same supply voltage, and current delivery. I do not see how this prevents amp A from having a smaller wire size and higher idle voltage.

The higher voltage is the sum of the voltage loss in the secondary AND voltage supplied to the amp. These two voltages added give the idle voltage.
As you start drawing current, you start having voltage drop within the primary/secondary due to their internal resistance. The two V that are the same that you mention are the V delivered to the amp, of course they are, a watt is a watt. But at clipping, the weaker transformer wastes power in its secondary and its primary in the form of heat, because for the same I and higher internal resistances, then:
Power loss = VI = (Vidle - Vclipping) * I
The more difference between Vidle and Vclipping, the more power loss in the transformer for the same power delivery to the speakers.
I hope this logic looks a bit less circular now.

There's more than just winding resistance at play here. There is also saturation of the core itself which plays a much bigger effect than winding resistance. As you up the current draw on the secondary, there comes a point where the core becomes saturated and the transformer failes to operate as a tarsnsformer.

 

[jeannot]

There's more than just winding resistance at play here. There is also saturation of the core itself which plays a much bigger effect than winding resistance. As you up the current draw on the secondary, there comes a point where the core becomes saturated and the transformer failes to operate as a tarsnsformer.

Thanks for the correction, that was a big omission. I should add the core as a contributor to the transformer internal losses. That also explains why the transformer sags in a non-linear way as it reaches its limit.

 

[PENG]

At the clipping point, two amps have to have sensibly the same supply voltage, and current delivery. I do not see how this prevents amp A from having a smaller wire size and higher idle voltage.

Because the designer/engineer knows they have to size the wire they use to meet the current requirement. For example, AWG#14 for 15A, for a particular temperature range. The designer will not use smaller size than he should, whatever the voltage level is. Regarless of the transformer secondary voltage, the current magnitude is still dependent on the rail voltage and the impedance (resistance in your example).

The higher voltage is the sum of the voltage loss in the secondary AND voltage supplied to the amp. These two voltages added give the idle voltage.
As you start drawing current, you start having voltage drop within the primary/secondary due to their internal resistance. The two V that are the same that you mention are the V delivered to the amp, of course they are, a watt is a watt. But at clipping, the weaker transformer wastes power in its secondary and its primary in the form of heat, because for the same I and higher internal resistances, then:
Power loss = VI = (Vidle - Vclipping) * I
The more difference between Vidle and Vclipping, the more power loss in the transformer for the same power delivery to the speakers.

Again, it seems right but not really. It is only right if in fact amp A transformer has smaller AWG but as I explained before it is not the case. In other words, if amp A transformer use smaller AWG on the secondary, then it will not be able to develop the same power as amp B, given the same load resistance. To extrapolate your argument, you can get to a point the idling voltage (as you put it) could be very high and as you stated, under full load it will drop to the same voltage level of amp B, but the wire size would be so small that the copper will melt, that is, game over.

I hope this logic looks a bit less circular now

Sorry, it still seems circular because the whole argument hinges on using thin gauge wires but more turns will yield the same continuous power given that the resistance is fixed. Then if that was true, amp B that uses larger wire size without using more turns to get a higher idling voltage will have lower voltage drop between idling and at rated load. Well, the former is not true so the latter cannot be true.

By the way, transformer power rating does not depend on just the windings, it also depends a lot on the core. If the core is subpar, you can wind as many turns as you want, you can still get into trouble, including saturation.

 

[highfigh]

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.

Dynamic headroom was defined quite a while ago as the peak power available when the amp is already running at full power with a specified amount of distortion. Some manufacturers listed this spec because their products did well and the ones who didn't list it, basically couldn't pass the test. I remember seeing more than a few receivers rated as .1dB dynamic headroom and some had absolutely none.

 

[jeannot]

... the whole argument hinges on using thin gauge wires but more turns will yield the same continuous power given that the resistance is fixed.

No. The main thread is about manufacturers using a cheaper transformer that meet the same output power rating on an 8 ohms resistive load. And that these transformers result in a higher dynamic headroom.

We disagree on the specific of the how, and on the benefit of continuing this argument in a downward spiral.

 

[Nestor]

IIRC, companies like Proton implemented a second voltage rail that kicked in when power exceeded the normal output. Current was supplied for those brief peaks with capacitors.

 

[jeannot]

IIRC, companies like Proton implemented a second voltage rail that kicked in when power exceeded the normal output. Current was supplied for those brief peaks with capacitors.

That is a notable exception, unfortunately very few of these still get produced for cost reasons. (can anyone cite a few?) Yamaha also made them at one point, and I think Hitachi's named that dual-level power supply "class G".

 

[PENG]

No. The main thread is about manufacturers using a cheaper transformer that meet the same output power rating on an 8 ohms resistive load. And that these transformers result in a higher dynamic headroom.

I thought by cheaper, and based on your example your meant using smaller wire size and wire more turns in the secondary winding to get a higher voltage, implying that it could deliver the same power when it's voltage drop to the same lower voltage of the one that you think has a better transformer. This is the point I disagree with, and that if in fact the manufacturer uses such transformer, it will not result in a higher dynamic headroom and I have been trying to explain why it is not possible.

If amp B has a better quality and more capable transformer than amp A such as higher current rating, then it will have equal or better headroom than amp A, assuming(based on criteria set by you) everything else are equal. A transformer rated for say 120V/90V 1000VA at rated load may get you 2 to 5% (approx/typical) higher secondary voltage under no load condition. If you use smaller size wire and more turns it will have a higher secondary voltage, sure, but it won't get you a higher power rating unless you increase the load impedance.

 

[PENG]

No. The main thread is about manufacturers using a cheaper transformer that meet the same output power rating on an 8 ohms resistive load. And that these transformers result in a higher dynamic headroom.

We disagree on the specific of the how, and on the benefit of continuing this argument in a downward spiral.

I should mention that we do agree on certain points. For example, I am pretty sure we agree on two transformers can be rated the same given a specific load characteristics (say impedance) but the better build one, heavier gauge wire, better core, will do better with lower impedance loads.

The disagreement is in whether the "cheaper" one could give better headroom. I say the better quality transformer is the one that should have better headroom. The devil may be in the definition of "dynamic headroom"?

 

[Pyrrho]

NAD amps are known for their well regarded dynamic power ...

Indeed, NAD practically invented Dynamic Headroom.

I think it would be more accurate to say that they invented advertising claims of dynamic power.

Having much headroom above the continuous rating will tend to matter only if the continuous output is rather low. If one never needs the continuous power for the peaks, there is no need for any additional dynamic power at all.

And a high continuous power capability is much better than having dynamic power to that point, because the continuous power will be available no matter how long the peaks last, whereas the dynamic or peak power is only available for a very short time.


Also, if one wants to have a real idea of how "high current" an amplifier is, one should look for the lowest rated impedance that is supposed to be hooked up to it and see what the rated continuous power is for that impedance. 2 ohm dynamic power ratings do not mean that one can safely hook up a 2 ohm speaker to it and actually use it that way.

 

[Pyrrho]

IIRC, companies like Proton implemented a second voltage rail that kicked in when power exceeded the normal output. Current was supplied for those brief peaks with capacitors.

Proton amplifiers are interesting, but having an extended peak power is no substitute for having continuous power. One of my brothers repairs audio gear for a living, and more than one Proton amplifier owner has paid for not appreciating that fact, as they tried to get their amplifiers to put out power above the continuous rating for extended periods of time. Doing that tends to land the amplifier in a repair shop to be fixed, because they cannot handle doing that for long. In other words, there is a good reason why the continuous rating is much less than the peak rating on those amplifiers.

 

[audiofox]

Seems to me that "dynamic headroom" is just another term for transient response capability, which is a more well known terminology in electrical engineering circles (at least for SATCOM systems), especially for power amplifier design. I'm surprised that Dr. Mark has not chimed in about the Quad amps-their current dumping design would seem to be able to handle transients better than other more standard designs. Does NAD have a particular design feature that results in their dynamic headroom performance, or is it simply their way of characterizing the transient response capability of their amps?

 

[avnetguy]

Having a high continuous output vs a lower continuous output with high dynamic headroom would always be nice but for the average receiver user, especially for HT, I would think the difference is minimal. I'd think having larger capacitive storage would benefit more users than having a larger transformer, both in cost and the reality of the source material demands.

Add to the fact most home users will probably be running all their audio/video equipment off a single 15A line then the need for a larger transformer becomes kind of moot I'd think. My two cents ....

Steve

 

[mtrycrafts]

...Add to the fact most home users will probably be running all their audio/video equipment off a single 15A line then the need for a larger transformer becomes kind of moot I'd think. My two cents ....

Steve

I run everything off of one 15A circuit and don't have any problems
Check my link

 

[avnetguy]

I run everything off of one 15A circuit and don't have any problems
Check my link

lol, (wipes coffee spray off the monitor after seeing the boom boxes) I gather you mean your "other stuff".

While not quite as extensive as your AV equipment, I too run all my gear off one 15A line. Never had any dynamic headroom or problem maintaining continuous SPL level issues but if I did, I'd move the receiver to a separate 15A line before I'd worry about it's transformer.

Steve

 

[jeannot]

Seems to me that "dynamic headroom" is just another term for transient response capability, ...

I think they are different, because they are tested differently. Dynamic Headroom is tested with a sine wave lasting 50-200ms, and the measure (in decibels) is the ratio of where these signals clip divided by where a continuous identical sine wave clips.

I would say the transient response is more related to slew rate, and is measured in V/uS using square waves. The slew rate is a measure of the parasitic capacitances within the amplifier circuit, among other things.

 

[PENG]

Seems to me that "dynamic headroom" is just another term for transient response capability, which is a more well known terminology in electrical engineering circles (at least for SATCOM systems), especially for power amplifier design.

That's the thing I am having trouble with, the definition. However, Jean did define what dynamic headroom is for the purpose of this thread. I am not clear if 'dynamic headroom' is the same as 'dynamic power' that is often used by manufacturers such as NAD and Yamaha though. Based on that definition, I absolutely agree when shopping for amplifiers it may be better to consider amplifiers (mid range and up) that show lower dynamic power/headroom? because manufacturers who emphasize their higher dynamic numbers may in effect skimp on the transformer side but put in more gigantic capacitors or other clever technique/features to boost the extremely short term rating to impress the potential buyers. The entry level NAD C375 is a good example, relatively lower continuous power rating such as 2X150W but very good dynamic power rating, though they don't specify their definition of dynamic power. As such, we have no idea whether it is for a duration of less than or more than 100 ms. There is indication that it is for 20 or 200 ms but I am over simplifying it here. I can send you a link if you are interested.

Back to the OP's point, my only main disagreement (respectfully) with the him is that, if two mid range real power amplifiers have the same rated continuous power ratings, the one with higher dynamic rating is not necessarily indicative of having a weaker/cheaper power supply. It could simply be that it has more capacitance or other features in the power supply to boost that rating. If in fact an inferior PS transformer is used, the 'dynamic headroom' is not going to get higher.

Now if you look at it another way, say:

Amp A - 200W cont., dynamic headroom (as defined by OP) 400W or 3 dB
Amp B - 250W cont., dynamic headroom (as defined by OP) 315W or 1 dB

Then we have a choice to make, and I would likely choose Amp B as I perceive it has a more robust power supply transformer, but if Amp B's cont rating is also only 200W, than Amp A would be my choice for the better headroom. YMMV.

I should emphasize that the NAD example I used may not be appropriate, without knowing how they define their dynamic rating, but for argument sake I am assuming it is for 200 ms or less.

 

[Nestor]

Proton amplifiers are interesting, but having an extended peak power is no substitute for having continuous power. One of my brothers repairs audio gear for a living, and more than one Proton amplifier owner has paid for not appreciating that fact, as they tried to get their amplifiers to put out power above the continuous rating for extended periods of time. Doing that tends to land the amplifier in a repair shop to be fixed, because they cannot handle doing that for long. In other words, there is a good reason why the continuous rating is much less than the peak rating on those amplifiers.

The Protons were appealing to the budget-minded due to high power output. Had both car audio and home audio in the 80's and 90's. They made for compact car amps at the time, and their home amps had good bang for buck.

Their output transisters were a weak point IMHO, and I suspect the caps aged prematurely.

Great strides in Class D and H amps have rendered that cct obsolete. Is headroom even a concern anymore?