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]

[QUOTE]

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.