Who makes

So if we have a one ohm load, to do 45 watts the current will be the square root of 45, or 6.07 amps.

Thanks Atmasphere. I think you are talking about speaker impedance and how it requires a certain amount of power to drive it? But what about musical dynamic peaks? That's what I'm, mistakenly(?) trying to get at. If you play a song at 90 dB with 110dB musical peaks like rim shots, don't you need the current to give that dynamic range for the 10 milliseconds?

the power supply of the amplifier is shorted out for 10 milliseconds. FWIW, we make tube amps with greater amounts of current by *that* measure...

So tube amps have more current on tap than ss?

If you play a song at 90 dB with 110dB musical peaks like rim shots, don't you need the current to give that dynamic range for the 10 milliseconds?

yes, you do. this sudden burst of current comes from the power supply capacitors. like you wrote, the rim shots (your example) are extremely quick & fleeting events. By the time the bridge rectifier reacts to this quick event, the event itself has passed. The power supply itself cannot react fast enough to quick events & that is by design - it's supposed to be a DC power supply that remains steady no matter what (given the amp is being driven within its limits). SO, it's the capacitor bank of the power supply that reacts to these quick events. That's why many amp manuf boast about how much power supply cap they have + you'll find many other amp manuf to have bypass caps in parallel w/ the power supply main cap. These bypass caps are much smaller (10,000uF) with very low ESR such that they can react very quickly to rim shot events.

So tube amps have more current on tap than ss?

Not as a general rule; more often than not the opposite would be true. It goes without saying that generalizations are not likely to be meaningful if drawn based on a comparison between a $349 amplifier and amplifiers that are in a VASTLY different league in terms of performance, quality, and price.

... what about musical dynamic peaks? That's what I'm, mistakenly(?) trying to get at. If you play a song at 90 dB with 110dB musical peaks like rim shots, don't you need the current to give that dynamic range for the 10 milliseconds?

Good response by Bombaywalla, of course, to which I'll add some further specifics.

I took a look at the specs of the Zamp, and it appears that what Ralph (Atmasphere) surmised about the 12 amp current spec is correct -- it most likely represents how much current the amplifier can supply into a short circuit (zero ohms) for a miniscule amount of time. Also, I would infer that the reference to 12 amps "peak" probably means "peak" not only in the sense of maximum, but also in the sense of being distinguished from RMS, which is the form in which the voltages and currents corresponding to maximum continuous power ratings are defined. For the sinusoidal waveforms upon which these numbers are based, RMS current equals peak current divided by the square root of 2, so on an RMS basis the maximum current rating is only about 8.5 amps.

In any event, what is important to realize is that the specified peak current is unlikely to ever be available to a real world speaker load, because for a reasonable load impedance the amplifier will not be able to supply the voltage corresponding to that current times that impedance, and it will not be able to supply the power corresponding to that current squared times that impedance.

What I think you are really asking about in this question is what is referred to as dynamic headroom, meaning the ability of the amplifier to deliver greater amounts of power to a speaker for brief amounts of time than its specified continuous maximum rating.

Dynamic headroom is often unspecified, and when it is specified there is often no indication of the amount of time the power increase can be sustained for, so comparing that spec for different amplifiers is usually not very meaningful. Also, having more dynamic headroom is not necessarily a positive attribute. It can be looked at in two ways: The amplifier is ABLE to deliver more power for a short time than it can deliver continuously, or it is UNABLE to continuously deliver an amount of power that is close to what it can deliver for a short period of time. Some of the world's best amplifiers have essentially zero dynamic headroom.

My impression is that typical dynamic headroom numbers range from zero to a few db, and rarely if ever exceed or even reach perhaps 6 db. 6 db corresponds to a four-fold increase in power, and would raise the sound pressure level heard by the listener by no more than 6 db, and perhaps somewhat less due to "thermal compression" in the speaker.

A number that generally says more about the robustness of a solid state (but not tube) amplifier than all of the foregoing is how closely its 4 ohm continuous power rating approaches being double its 8 ohm rating. The two ratings for the Zamp are 45 and 60 watts, which may be a better ratio than most other amplifiers in its price class have (many of which do not even have a 4 ohm rating), but does not approach the factor of 2 that many multi-kilobuck solid state amplifiers can achieve.

Regards,
-- Al

"A number that generally says more about the robustness of a solid state (but not tube) amplifier than all of the foregoing is how closely its 4 ohm continuous power rating approaches being double its 8 ohm rating."

That's the most common spec out there to use to help gauge ability to handle a "difficult" load.

Not perfect or 100% reliable always (numbers are meaningful but no single number can tell the whole story) but is generally a useful benchmark to help weed out pretenders.

If a similar rating is even provided into 2 ohm, that is usually a good omen as well that shows the maker really cares about these things.

Even more bankable than the specs is when reviewers like Stereophile actually perform measurements with 8 4 and 2 ohms when bench testing a product as part of the review.

Mapman, I agree with your comments buy you might be missing something: how the use of negative feedback affects this.

As you probably know, most transistor amps (especially ones that can double power nicely even to 2 ohms) employ negative feedback. **All** inexpensive solid state amplifiers do as well.

Now if the speaker is only going down to 4 ohms, the fact that the amp can't double power into that impedance does not mean that it is not a voltage source. This is due to the fact that the feedback of the amplifier will make it act like a voltage source independently of the amp's ability to double power. If you have heard of the Wolcott tube amplifier, this amp employed enough negative feedback to also act as a voltage source, and it was completely unable to double its power into lower impedances.

Its right about here that I see where a lot of designers get into a little bit of trouble in understanding the effect of output impedance on how the amplifier responds to load. The thing that clears the air is something called Kirchoff's Law- the law of energy conservation.

Now its understood that adding negative feedback to an amplifier reduces its output impedance, right? But right here we see that this really is not the case at all. If a circuit really has a lower output impedance, it can therefore drive lower impedance loads without loss of performance. So if negative feedback really did reduce output impedance, you could make any amplifier drive 2 ohms without losses just by adding more feedback!

Obviously that does not happen- if you really want to drive lower impedances you need things like more power tubes/transistors, bigger output transformers/heatsinks, etc. IOW Kirchoff's Law stands in your way. IOW adding negative feedback to an amplifier does not affect its actual output impedance at all, only its voltage response.

(Kirchoff's Law BTW is a basic law that says that the energy in an electrical circuit cannot be more or less than the amount of energy put into it. Its one of the first things you learn in electrical engineering.)

From this we can see that the term 'output impedance' as used by the Voltage Paradigm does not in fact refer to the actual output impedance of the amplifier at all! Instead, it refers to the how the amplifier *reacts* to its load impedance with its voltage response. That is something quite different.

So in our example of the inexpensive solid state amp that cannot quite double its power into 4 ohms, it is still a voltage source as its feedback causes it to *limit* its output power into lower impedances, based on what it can linearly do into higher impedances. This can be a bit confusing! On the ground what this means is that the example amplifier probably will not ever put out 60 or 65 watts unless the loudspeaker has a very flat 4 ohm impedance curve.