Please explain "high Current" receivers.... (1 Viewer)

[Aslam Imran]

That statement is not correct.

Fredrik, in the context of our discussion about multichannel receivers putting out 42 A/Ch , let me rephrase my response: Even Krells cant deliver 42 A/Ch (RMS) and the wall outlet in your home couldn't withstand that kind of output IF A 5 CHANNEL RECEIVER WAS PUTTING OUT 42 A/CH DRIVING REAL WORLD SPEAKER LOADS.

Now lets put this into perspective. Suppose, like you said, the wall outlet was able to deliver the 42 A/ch into 5 channels (and it can at low voltages I am not disputing that). Now your 15 amp outlet would deliver 110x15=1650W. and if you translate that power to 42x5 amps of current the voltage the amp can deliver is 1650/(42x5)= 7.8 volts which translates to (7.8/5=)1.56 volts. Now you tell me is 1.56 volts enough to drive a speaker to listening levels? Maybe a 0.1 ohm speaker but never a 4-8 ohm nominal impedance speaker.

So once again I reiterate that a wall outlet cant support 42 A/ch (RMS) into 5 channels in a real world HT.

And Matthew you are still wrong. Please dont ever believe these marketing gimmicks by any mass market receiver maker.

The true indication if a high current design is if an amp can double its output when the impedance is halved, period. Any thing else is just marketing hype.

The specs for a true high current design would read something like this (taken from Mark Levinsons No 33. mono amp specs)

300 w/ch continuous rms power @ 8 ohms

600 w/ch continuous rms power @ 4 ohms

1200 w/ch continuous rms power @ 2 ohms

2400 w/ch continuous rms power @ 1 ohms

Now thats what I call high current amplifier.

 

[Aslam Imran]

do you wan't to guess which one costs more? that's right the smaller, lighter one...and companies being driven often by market research often use larger heat sinks and cheaper heavier power supplys just to get the weight up.....

LOL
Please dont ask me why, cos I can go on and on and on about amp power supply design of high current class A amps (very heavy and inefficient requiring big heat sinks) that sound like magic and cost a fortune and power supply design of so called switching digital supplies (very light and highly efficient) but sound like PA system (and are rightfully used in them) and cost like dirt compared the Class A design. Puhleassee don't ask me.:frowning:

 

[Sankar]

and if you translate that power to 42x5 amps of current the voltage the amp can deliver is 1650/(42x5)= 7.8 volts which translates to (7.8/5=)1.56 volts.

Aslam,
I am in agreement with the spirit of your argument. However, I think that you divided by 5 twice. 1650W = (1650/5)W per channel = 330W per channel. At 42A, this reduces to 330/42 = 7.8V. Furthermore, to support this amount of current at this voltage, the speaker impedance must be of the order of 0.18 ohms. If these levels were sustained for any reasonable length of time, I suspect that the output stages of most amps would be fried ..

 

[Aslam Imran]

Sankar, thanks for correcting me.

 

[chung]

Now lets put this into perspective. Suppose, like you said, the wall outlet was able to deliver the 42 A/ch into 5 channels (and it can at low voltages I am not disputing that). Now your 15 amp outlet would deliver 110x15=1650W. and if you translate that power to 42x5 amps of current the voltage the amp can deliver is 1650/(42x5)= 7.8 volts which translates to (7.8/5=)1.56 volts. Now you tell me is 1.56 volts enough to drive a speaker to listening levels? Maybe a 0.1 ohm speaker but never a 4-8 ohm nominal impedance speaker.

If you read the poster's message again, he was talking about a 2-channel receiver that's capable of 42 A/ch, for 2 channels only and not for 5 channels.That is for peak instantaneous power, and not steady-state rms power. Also you divide by 5 twice.

I believe that perhaps what HK meant was that the total peak current (for both channels) is 42A. Which means that for peaks, each channel by itself can deliver 42A (with the other channel not using significant current). Which is pretty impressive, since the output transitors have to carry 42A's without self-destruction. Of course, that also assumes that the speaker is presenting a low impedance, since the HK does not have the voltage headroom to drive, for instance, 42A into 8 ohms (which requires 336V peak).

 

[Aslam Imran]

I believe that perhaps what HK meant was that the total peak current (for both channels) is 42A. Which means that for peaks, each channel by itself can deliver 42A (with the other channel not using significant current). Which is pretty impressive, since the output transitors have to carry 42A's without self-destruction. Of course, that also assumes that the speaker is presenting a low impedance, since the HK does not have the voltage headroom to drive, for instance, 42A into 8 ohms (which requires 336V peak).

Thanks for pointing out the math error AGAIN.

I know HK specifies a high current rating and I believe that they mean total current (peak to peak) for all channel added. So if you read a stereo HK spec that says 42 amps current output it means (42/2=) 21 amps peak per channel.

For example the HK8000 flagship specifies '+/- 85 amps, ultra high bandwidth amplifier design' when it actually implies (85/5=) 17 A peak to peak per channel. This is what I believe. I hope you can agree to that? If you dont and if this was the amp output per channel then the HK8000 would beat most multichannel beasts on the market from Parasound, Rotel, B&K etc. For example the Parasound 2205 only specifies:

Current Capacity: 60 amperes peak per channel

and the B&K 7250 specifies:

Current Capacity per channel : 75 amps peak to peak.

and I dont think the amps in the HK come close to these amps by a long long very long shot.

So all I am saying is dont go by the ratings of these high current amps they are trying to mislead the gullible consumer. All you need to look for is how much the output increases for a decrease in impedance. Thats all you need to worry about and forget about the high current specs on these products. The power ratings at various impedances will tell the whole story.

 

[chung]

As I said, that 42A is for total current, not per channel. The specs say +/- 42A, so the peak current (for all channels) is 42A in either direction. I don't think they mean 42A peak-to-peak; that would be intentionally misleading.

The other products specify amps/channel, but that's not how the HK 8000 is spec'd. So if each channel in the HK8000 can deliver a peak of 17A, then they meet the peak instantaneous spec of 85A. In playing music, it is not likely to have all 5 channels delivering the same current, though.

Is it real? Hard to say without measuring it. But it is not impossible to design an amp that delivers a peak current of 17A for very short instants into low impedances, and have a power supply that has the capacity to deliver 85A total for very short instants. These specs are very similar to the HK PA5800. so maybe they lifted the design from that 5-channel power amp.

 

[Aslam Imran]

Ok Chung I give it to you.

 

[Paul Clarke]

"Please dont ask me why, cos I can go on and on and on about amp power supply design of high current class A amps (very heavy and inefficient requiring big heat sinks) that sound like magic and cost a fortune and power supply design of so called switching digital supplies (very light and highly efficient) but sound like PA system (and are rightfully used in them) and cost like dirt compared the Class A design. Puhleassee don't ask me"

Aslam,

Puhleassee! I'm asking. Some need to be reminded.

 

[Peter Johnson]

The specs for a true high current design would read something like this

300 w/ch continuous rms power @ 8 ohms

600 w/ch continuous rms power @ 4 ohms

1200 w/ch continuous rms power @ 2 ohms

2400 w/ch continuous rms power @ 1 ohms

(taken from Mark Levinsons No 33. mono amp specs)

Also misleading. An amp can never "double down" into an impedance load that is halved. Internal resistances as well as losses in cables etc (which are an issue at 1200W) prevent this.

Take an 8ohm load (assume its just a constant 8 ohms to keep it simple). Also add say 0.2 ohms for the sum of a 0.1R output impendance and 0.1R for cables. Thats a 8.2R load.

8.2/8 = 1.025, meaning that 2.5% of the power is lost into these two series losses. At 1200W, thats 30W.

Current is SQRT(P/Z) = SQRT(1230/8.2) = 12.25A RMS

As you increase the current (which we all know happens as you increase the load->lower impedance), the losses increase according to P = R*I^2

You can get close, but never actually double it.

What these companies do is underspec on the lower ratings. Ie, it might actually do 350W into 8 ohms, 630 @4, 1215 @ 2ohms and 2400 @ 1ohm (or whatever actually occurs, but something like that).

That way when tested, they also outperform their specs!

 

[Aslam Imran]

You can get close, but never actually double it

I agree that an amp can't truly double its current delivery when impedance is halved considering the path impedances but true high end designs can get quite close with the use of higher grade parts/cables. For example an amp output impedance of 0.005 is quite achievable in these designs. And ofcourse all the conditions external to the amp are common to all amps so are not relevant to the specs.

 

[jeff morris]

WOW!!
I am overwhelmed by the responces!!
Thanks guys for the lessons. I can see that most people on this forum take this whole home theatre thing much more seriously than I. And there's nothing wrong with that. We'er all into somrthing, one way or another.
It's just that I am a casual HT guy and didn't expect such an amazing amount of in depth responses.
Here is basically the crux of my original dilemma, and why I posted.
I am presently using a 12yr old Technics Dolby pro logic RX with Baby Advents (4 ohms) as my mains. Center and rears are 8 ohms. When I turn up the volume, my RX cuts out-thermal overload lights up. This never happened when I ran just the Advents, before I bought the cenetr and rear spkrs.
I plan on upgrading to the Onkyo 500 RX and was concerned that I might have the same problem with the RX cutting out during loud passages.
I plan on dumping the Advents and getting Polk RT25's so I think the impedance thing will be moot. Just hope the Onkyo will have enough stooch to drive 5 Polks.
Now I see why there aren't too many, if any, 4 ohm bookshelf spkrs being sold anymore to match with lowwer end RX's
Again - thak you all for you help and lessons!!

 

[Arthur S]

The 2 channel Crown Studio Reference 1 amp is rated at 1190 watts continuous per channel into 4 ohms with both channels driven simultaneously. This amp can be used in parallel mono and bridged mono. In bridged mono it is rated at 2,375 watts into 8 ohms.

Premier speaker tester Don Keele used one of these amps for many years to determine maximum power handling of speakers. "Peak input power was calculated by assuming the measured peak voltage was applied across the rated (4) ohm load."

In short term peak power tests he measured voltages in excess of 200V which is 10,000 watts, again assuming the peak voltage was applied across the rate 4 ohm speaker load.

Don was not handicapped by a 20 amp power circuit. Using both parallel and bridged mono modes he found several speakers over the years that could absorb the 10,000 watts. The Crown can be safely used into 2 ohms in paralled mono mode. According to the "power draw" chart for the Studio 1, it can draw 23.1 amps in 2 ohm parallel mode at 100 volts. They also note that

"Peak current draw with dynamic program material

may be significantly higher."

Perhaps one of you EEs can comment on this information.

Artie

 

[Peter Johnson]

I'll have a stab, but I'm not banking on it all being correct
I am pretty sure (like 99%) that an amps clipping voltage is set by the voltage rails. Peak voltage is the rail voltage. Anything higher is clipped.
Peak power = 200^2/4 = 10000W.
Average = (200/1.414)^2/4 ~= 5000W
They rate it at 2375W into 8R. Into 4R it should get to around 4500W. He tested 5000W..a 500W difference, but only a 10% difference.
Who needs this much power?
I would like to know what speakers will actually play music absorbing 10000W? In fact, make that 1000W?
I've never seen anything take that much power. Few will take 1000W with their thermal rating (probably only line arrays etc with A LOT of drivers). But excursion wise...even obscenely longthrow (like 15-20mm X-max)sub drivers cant even take 1000W at low frequencies..

 

[MatthewJ S]

Nasa w/ the US military needed amps that would reproduce the effect of a sonic boom out in the desert ,their specs called for some amps that could sustain over 1000 watts for the duration of the creation of the freq. wave test...they used B&k mono blocks .............6 of 'em , each capable of a 1400 watt burst?..........hey if you want to create a sonic boom, you gotta do what you gotta do!!!!

 

[Peter Johnson]

They also probably used 6 drivers, each which cost more than most people's house (with an enclosure about that size as well!), and had an efficiency of well over 100dB/W/1m.

 

[DavidRM]

Hi,

There is a simple answer here. Most people look at watts which is totally inaccurate. Pick up a high current receiver and one that is not. The one with high current will feel like it is glued down because it has huge capacitors, etc. Here is a quick example of why current is more important than how many watts: a flashlight can run off of 12 volts and so can a car, so one is getting a better distribution of power and more of it. That is why a non ES sony at 150 watts can't compete with a B&K at 150.

 

[Arthur S]

David, there are exceptions to your simple answer. Namely, switching amps and variations. For example, Carver makes a 200 watt X 5 amp that weighs about 42 pounds, and it can deliver! Carver amps run virtually stone cold and can make your ears bleed without even getting warm. They don't even have ventilation slots on top!
But as with everything, there is no free lunch. Switching amps generate high frequency noise, say around 80KHz. They build in stone wall filters to cut off the noise (and the frequency response) around 22 KHz.
What does this mean? You can get a hell of a lot of current from something that doesnot weigh a ton, but their is a trade off. But remember, there are always trade offs. If you have the money for cleaner high current amplification, by all means amps like Bryston, will do very well indeed. In fact many people will drive each of their speakers with a separatte 500 watt Bryston 7B. So if you have 15 grand to drop on amps go for the heavies.