High Instantaneous Current Amp Spec

[Dan Banquer]

The High Current Thingy

"The first one, Power=Voltage times the current times power factor. Power factor=1 for d.c., and less than 1 for a.c. with a complex load such as a loudspeaker that is not purely resisitve."
Absolutely! I hope to cover more of this in some upcming articles, but since this thread started with amplifiers driving a staight resistive load I thought I would stick to the subject. Damn silly of me eh?
d.b.

 

[Votrax]

Maybe somebody could sum up how an amplifier actually works. I was under the impression that based on the input signal strength, the amp outputs a voltage of a certain value, say 12volts. Then, depending on the load (impeadence) the amp would have to output enough current to sustain that voltage. (water in a hose theory, volt=pressue, current=current) The lower the impeadence, the more current becomes necessary and thus volts*amps=watts. More watts are required. So in an extreme case, you could be talking about 75amps at a realistic volume into an extrememly low impedence. Am I right here or is this the wrong way of thinking?

An amplified signal is always limited by the rail voltage. A typical rail voltage in a receiver is +/-50VDC. So theoretically the greatest voltage swing the amplified signal could achieve is +/-50VDC. It's actually less due to the voltage drop across the transistor and any protection circuitry. Any further amplification of the signal would clip off the top and bottom of the signal. The rail voltage will also drop further as current consumption increases. Assuming a perfect power supply that doesn't drop below +/-50VDC, and an 8 ohm load, the peak current would be 50V/8ohms = 6.25A. To achieve 75A would require a speaker load of 0.6 ohms. There's no receiver that can drive that low of an impedance. Most of your flagship receivers have output transistors rated for only 15A anyway.

 

[thxgoon]

Sure, I can try. Power factor is defined as the Cosine of the phase angle between the voltage phasor (a.k.a. vector in the old days) and the current phasor.

Wow, very cool. Thanks Peng. Where can I go to learn more about this stuff? So would it be accurate to say that the smaller the phase angle the better the amplifier? Or is this totally independent of design and based on physics?

 

[thxgoon]

For an inductive load, such as a voice coil, the current will be lagging behind the voltage. For a capacitive load, the current will be leading. You can see the phase relationship between the voltage and current phasors if you display their waveforms on an oscilloscope. For a purely inductive load (i.e. with negligible resistance), the current is 90 deg lagging the voltage. Cosine 90 deg is 0, so you can only develop reactive power (unit is VAR, not Watt) into a pure inductive load such as an inductive coil. The real power dissipation (watt), is 0.

So, just trying to understand here. A voice coil is an inductive load because the current induces a magnetic field which translates to movement? Or does it mean something else, like induction in a wire? When it is 90 lagging, the current is to late and hence redered useless? In this case what happens to the votage and the current, are they just cancelled out or dissapated as heat somehow? Why does this lag happen? Inertia of electrons perhaps?? I have no idea. Thanks again for the technical explanation.

 

[thxgoon]

I mentioned the p.f. thing because like efficiency, it has to be in the equation for any a.c. power calculations, unless the load is purely resistive.

Wow, ok. Sorry if I have a million questions now instead of one. So if p.f. must be figured into power calculations, is this figured into amplifier output calculations or actual output power from the speaker calculations. In otherwords, how does this interact with the amplifier? Does it give it a 'break' so to speak? Thanks again.

 

[WmAx]

Wow, ok. Sorry if I have a million questions now instead of one. So if p.f. must be figured into power calculations, is this figured into amplifier output calculations or actual output power from the speaker calculations. In otherwords, how does this interact with the amplifier? Does it give it a 'break' so to speak? Thanks again.

I don't think you will find a better explanation than this link:

http://sound.westhost.com/patd.htm

-Chris

 

[thxgoon]

I don't think you will find a better explanation than this link:

http://sound.westhost.com/patd.htm

-Chris

Excellent! Thanks Chris. Unfortunately I don't have time to read through all of it tonight but I'll check it out the first chance I get.

 

[PENG]

I don't think you will find a better explanation than this link:

http://sound.westhost.com/patd.htm

-Chris

WmAx, I know there is someone out there who can do a better job than me trying to explain it technically. You did even better by finding this wonderful article. Thanks!

 

[PENG]

Wow, ok. Sorry if I have a million questions now instead of one. So if p.f. must be figured into power calculations, is this figured into amplifier output calculations or actual output power from the speaker calculations. In otherwords, how does this interact with the amplifier? Does it give it a 'break' so to speak? Thanks again.

You asked some pointed questions and you may get all of them answered if you read the article WmAx posted the link for. Any remaining questions, such as the one on the 90 degree lagging one, I can try to get you an answer when I have more time this weekend. To answer some of your questiions in a technical sense, (based on electrical theory) is relatively easy. You can refer to a textbook if you have studied physics. To explain it in a physical sense, such as using easily understood analogy, is more challenging. Again, the article Chris posted may be the ticket, I'll read it myself this weekend.

 

[PENG]

"The first one, Power=Voltage times the current times power factor. Power factor=1 for d.c., and less than 1 for a.c. with a complex load such as a loudspeaker that is not purely resisitve."
Absolutely! I hope to cover more of this in some upcming articles, but since this thread started with amplifiers driving a staight resistive load I thought I would stick to the subject. Damn silly of me eh?
d.b.

At times we do seem to focus on using a straight resistive load. Thanks.

 

[gene]

Likewise, the Yammie's 500W number is a power consumption number.

That rating is based on a UL/IEC Standard (not sure which one) where 2CH driven at max power while all other channels driven at 1/8th power. Its NOT a max power consumption rating for the receiver.

 

[aarond]

so it will drive 7x120w all ch max like there literature states ie 840w!

 

[gene]

so it will drive 7x120w all ch max like there literature states ie 840w!

Good joke

 

[mtrycrafts]

so it will drive 7x120w all ch max like there literature states ie 840w!

Yes, great joke but the manual doesn't say all channels driven. Each channel will do that though and probably a bit more.

 

[3db]

Gene. These are pretty strong accusations

What people fail to realize is many of these publications are regulating the line voltage and sometimes even swapping out the rail fuses on the amps to even hit these ridiculous figures. What's even more bogus is some are using a sweep tone in their Audio Precision analyzers to plot power vs distortion which is an instantaneous NOT a continuous test!
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Umm Can you back this statement up?

 

[aarond]

we all know that it won't. the point is all of yamaha's advertising claims that its an 840 watt recvr when that is just an out and out lie!

 

[3db]

Outlaw and NAD

for sure and maybe Rotel are one of the few manufacturers that can back up their claims of all channel drivens. And its funny, of all the shops that I've been to that sell both Yamaha and NAD, the shop owners always said that the NADs sounded better than the Yamahas. Maybe the all channels driven test may not be the best measure of what an amplifier will sound like. I can't help but see a correlation here.

 

[Pyrrho]

fact or fiction?

1) an HK AVR 335 with 55wpc will beat a yam rx-v1500 with 120wpc because it has high amperage.

2) the yam 120wpc is BS


my definition of beat = it will sound better ; it will have more power in running a particular speaker

1 is fiction. HK has claimed "high amperage" for many products, and their ads, like Bose ads, have had an effect on people's beliefs. There is no standardization for the claim "high amperage", so any manufacturer can claim this about anything they make. If you are interested in power output at different impedances, look for the specific ratings for those impedances. A claim of "high amperage" means absolutely nothing.

2 is fiction. Yamaha has rated it for one channel driven; the fact that someone wants them to list an all channels driven rating does not make Yamaha's claim false.

 

[3db]

1 is fiction. HK has claimed "high amperage" for many products, and their ads, like Bose ads, have had an effect on people's beliefs. There is no standardization for the claim "high amperage", so any manufacturer can claim this about anything they make. If you are interested in power output at different impedances, look for the specific ratings for those impedances. A claim of "high amperage" means absolutely nothing.

2 is fiction. Yamaha has rated it for one channel driven; the fact that someone wants them to list an all channels driven rating does not make Yamaha's claim false.

Specs are like legal documents ..neeed to figure out the slant in which its coming from. Once this is mastered, the Audio world is yours

 

[gene]

Umm Can you back this statement up?

If you have any technical sense than I don't need to back up my statement about the Audio Precision Test sweep. Its a fact. Companies like HT Magazine use that test. Its a useful test, but NOT representative of a continuous power test.

As for the rail fuse and regulation, read the editorial notes in S&V magazine. If I recall correctly they bypassed the fuse in a denon receiver review. Since I don't subscribe to magazines I can only recite from memory what I read when I perused them at the bookstore.

As for regulation, again its a fact that they have to regulate since the #'s they are publishing simply aren't possible without doing so. S&V Magazine also stated the discrepancy with their Yamaha/Denon debacle when they tested the Yamaha RX-V1. If you don't believe it, simply find the following issue to that review and you can read it for yourself in their feedback section.

Or better yet, you can read the editorial from the owner of S&V Magazine who allowed me to print this several years ago.

http://www.audioholics.com/FAQs/yamahaRX-V1receivertest.php

Did I sufficiently back it up for you?