Ohm Rating

gregure

Just curious:
Why do receiver manufacturers list power ratings into 4 Ohms when they aren't built to drive anything less than six? Is this most speakers produce resistance to 4 Ohms for at least some of their frequencies? Again, just curious. It seems like false advertising power ratings at Ohm levels that would kill the unit.

HBombToo

If your NOT a LOUD listener your fine with regards to the amp.

If you overdrive your lows you will have a meltdown with the bass speaker first. 2-3 Ohms is not uncommon.

I have ran 8Ohm rated amps at 4 Ohm +/- but paid very close attention to distortion. Distortion is Baaaaaaaaaddddddddd.



1/4twin

gregure

I don't plan to run 4 ohm speakers with an 8 ohm receiver. I was curious because I noticed the Onkyo 502 had dynamic power ratings for 4 ohms, and I know that receiver can't handle 4 ohms. So, I was curious as to why they list the 4 ohm power rating.

HBombToo

Originally posted by gregure
and I know that receiver can't handle 4 ohms. So, I was curious as to why they list the 4 ohm power rating.

the receiver can handle 4 ohms at a moderate volume... that was my only point.

Mike682

Here is a link to a good article regarding speaker impedance and how it constantly fluctuates:

page 14
http://www.polkaudio.com/downloads/newspaper/issue9.pdf

What Onkyo is stating is that during normal operation, that receiver can put xx amount of power into a 4ohm impedance load when the speaker drops to that level for a brief amount of time. It is these short bursts of power that allow for a more dynamic sound and hence the name dynamic power.

Dr. Spec

People talk about dynamic headroom as being a good thing. Actually the opposite is true. The peak power an amp can deliver can actually be more accurately described as a "dynamic collapse" of the power supply. IOW, the amp cannot maintain that
given level of power continuously. The BEST amps have the LEAST dynamic headroom and the HIGHEST continuous power ratings. Their power supplies are so robust, that their continuous power and their peak power ratings are almost the same.

Impedance and current flow capability must be considered. Ideally, if the impedance load is halved, the power from the amp must double. In the real world, that is often not the case. The ability of an amp to increase its power output as the impedance drops is a measure of the "stiffness" of the power supply. You won't see many amps actually doubling their power output when the impedance load is dropped from 8 ohms to 4 ohms; a 50% increase in power is generally considered exemplary from a consumer amp when the impedance is halved. If you find an amp that actually doubles its power when the impedance is halved, check the price
tag.

PolkWannabie

Regarding amps that double the continuous wpc as the impedance is halved, the Sunfire amps do this ( for example - http://www.sunfire.com/400SevenFrontPR.htm ) but as you state they ain't cheap either ...

However I have a followup question to one of your other statements ...

If two amps are capable of the same continuous output at a given impedance but one amp has more headroom than the other then are you stating that the amp with the additional headroom is for some reason inferior ?!

scottnbnj

Originally posted by Dr. Spec
People talk about dynamic headroom as being a good thing. Actually the opposite is true. The peak power an amp can deliver can actually be more accurately described as a "dynamic collapse" of the power supply.

this, for sure, is above my pay grade, how does nature of the 'dynamic collapse' in pw's scenario differ in the two amps?

)

Dr. Spec

If two amps are capable of the same continuous output at a given impedance but one amp has more headroom than the other then are you stating that the amp with the additional headroom is for some reason inferior ?!

No, I wasn't making that type of comparsion. What I meant was if you have an amp that is rated at 50 watts continuous, and 200 watts peak (typically 40 milliseconds but there is no standard), that's nothing to get all lathered up about.

Dr. Spec

Yep, the Carver is a nice piece - a linear power increase as the impedance drops. And he rates the amp with all channels driven AND in continuous power with bandwidth and THD - no **** specs here. Although his use of the term "rms" is superfluous.


The term RMS - in and of itself - does not automatically translate into continuous. The three most common values you can derive from a voltage sine wave are: average, RMS, and peak. For a sine wave, the RMS value is the peak value x 0.707.
The phrase "watts RMS" is actually a misnomer. There is no such thing as "watts RMS" - watts are watts. As you know, power must be calculated using a given voltage value. Whether this value happens to be the average, RMS, or peak value of the sine wave really is a moot point. It's the OTHER qualifying parameters that define the actual performance of the amplifier, like time frame, bandwidth, impedance load, and distortion.

The word "peak" should not be confused with or automatically associated with the word "transient". There are actually two peak values - the voltage peak of a continuous sine wave and the voltage peak of a transient sine wave. Continuous (at least according to the FTC) is rated as 5 minutes. Transient is (typically) rated at 40 milliseconds. That's where all the hype about peak power became a problem back in the 70's - they were talking about transient peak voltage values, not continuous peak voltage values.

So when an amp is rated for "continuous watts" (actually the more accurate term to use), it should look something like this: "200 watts continuous into a 4 ohm impedance load from 20-20kHz with no more than 0.5% THD"

Now you can start to appreciate that it is really not necessary to further define the voltage value used to calculate the power in watts. It could have been continous average, continuous RMS, or continuous peak - we don't know, nor do we have to care. What we really care about is that the amp can deliver those watts continuously UNDER THE ABOVE PRESCRIBED CONDITIONS.

Now you get the FTC involved, and they specify that we must use the RMS voltage value of the sine wave to calculate the power. OK, fine, but it doesn't change anything since the other parameters will ultimately control the actual power rating of the amp. So now it becomes:

"200 watts continuous (as calculated using the RMS voltage value of the sine wave) into a 4 ohm impedance load from 20-20kHz with no more than 0.5% THD".

Now you have to consider how many channels are being driven. When they say "watts per channel", what they really mean is "watts with one channel driven". There are very few AVRs that can actually deliver their rated load into more than one channel, much less all channels at the same time. The only multi-channel amps that can do this are those specifically rated for "all channels driven simultaneously". Now you can see why true monster amps like the Carvers and Adcoms rated at "300 x 7 all channels driven" REQUIRES a dedicated 20 amp circuit, because it can actually draw that much current from the line (a true 2100 watts).

The typical AVR with all channels driven will drop to less than 1/2 of its rated power. For example, a 7.1 Denon 3803 rated at 110 watts will actually only put out about 35 watts continuous into all seven channels. The true power rating of the 3803 amp section is only about 250 watts, not 770 watts.

PolkWannabie

Originally posted by Dr. Spec
No, I wasn't making that type of comparsion. What I meant was if you have an amp that is rated at 50 watts continuous, and 200 watts peak (typically 40 milliseconds but there is no standard), that's nothing to get all lathered up about.

I understand and thought that was what you meant but wanted to verify as one MIGHT have read something else into what you had originally stated i.e. that head room in and of itself is a bad thing, which I think you would agree that all other things being equal is not true.

As for your statements about RMS and manufacturers claims about the amps in receivers I whole heartedly agree.

Dr. Spec

I understand and thought that was what you meant but wanted to verify as one MIGHT have read something else into what you had originally stated i.e. that head room in and of itself is a bad thing, which I think you would agree that all other things being equal is not true.

Agreed, all other things being equal, dynamic headroom - in and of itself - is not a bad thing. The kicker here is "all other things equal", which is often hard to verify.

While the FTC requires the continuous power rating to be calcuated using the RMS value, they don't prohibit or require qualification regarding additional claims about dynamic power.

It would be cool if the FTC required a standard for transient power, just like they do for continous power. Something that defines time frame, bandwidth, impedance load, and distortion. A possible example:


"200 watts continuous into a 4 ohm impedance load from 20-20kHz with no more than 0.5% THD"

and the same amp......

"450 watts up to 40 milliseconds into a 6 ohm impedance load from 200-3kHz with no more than 2.5% THD"

In most cases, just the transient power is listed; we don't know anything else about the transient performance (like time, bandwidth, impedance, and THD) to make an informed decision about the rating.

VR3

Receivers are listed in 4 ohms, because it is more watts than 8 ohms.

When people see the larger watts, they think it is better.

gregure

Thanks again, Dr. Spec, for sharing your wealth of knowledge with us. You mentioned that most receivers drop their output by 50% when all channles are driven, despite the fact that they say 100 WPC (which most people misinterpret). Brands that assert their ratings are conintuous with all channels driven include ARCAM, NAD, and B&K to name a few. I'm looking at buying a B&K 507. To they fulfill their promise? I would hope so for $2500. Thanks

PolkWannabie

For those kind of dollars you ought to give separates some real consideration ...

gregure

A decent pream and amp would easily be another $1000, if not more. I've heard the B&K 507, and I'm very pleased with its performance. It's still a couple months off, but I think I've settled on keeping it down to one unit for now. Plus, the equivalent amp by B&K is 200wpc, as opposed to the 507's 150wpc. I don't want to blow out the surrounds.

PolkWannabie

Originally posted by gregure
Plus, the equivalent amp by B&K is 200wpc, as opposed to the 507's 150wpc. I don't want to blow out the surrounds.

You won't ... mine are getting 425 wpc at the moment ...

gidrah

Originally posted by Vr3MxStyler2k3
Receivers are listed in 4 ohms, because it is more watts than 8 ohms.

When people see the larger watts, they think it is better.

You've gotta give the kid a hand here fellas. He's right!
I'd even venture to say that some companies that know damn well their amps can's handle a impedemce curve that usually coincides with a 4 ohm speaker, are more than happy to publish the fact that their amps can handle 4 ohms, but decidedly ignore the fact they they are talking about impedence dips/spikes at a given frequency, and that they could no better handle a 4 ohm average load than I could handle a stick of butter on a hot day. Sure I can touch it and even grasp it mometarily, but when I try to get a firm grip it just slips through my fingers and leaves a mess.

kingtut

Doc, with your vast knowledge in amplifiers and subwoofer, are you an engineer or same line of work by trade?

Dr. Spec

Doc, with your vast knowledge in amplifiers and subwoofer, are you an engineer or same line of work by trade?

I'm an engineer. Audio/HT is a hobby for me. Thanks for the props, but there are people who have forgotten more about audio than I'll ever know. I read a LOT more than I post, especially at advanced forums frequented by designers in the field.

Dr. Spec

I'm looking at buying a B&K 507. To they fulfill their promise? I would hope so for $2500. Thanks

The B&K is a very solid piece. Talk to Dan (Mantis) for more info on B&K; he's been on a factory tour.

For those kind of dollars you ought to give separates some real consideration ...

I agree (the goodness of the 507 not withstanding).

gregure

Supposing I do go with separates, what's a good brand name to go with in the neighborhood of $3000? I was looking at the B&K 50 coupled with the 200.7 amp. Not sure how much they sell for, though. Also, on the B&K pre/amp, the speaker preouts are big three-pronged plugs. I've not seen that before. What kind of plugs are these, and where can I find good ones? I think reg. RCA connectors are present as well. Which connection is better? So many questions. Are there other brands worth considering in this price range? I've looked at some ADCOM stuff, but that is expensive, and I've heard mixed reviews about ADCOM, whereas I've read nothing but positive reviews for B&K.

PolkWannabie

Originally posted by gregure
Supposing I do go with separates, what's a good brand name to go with in the neighborhood of $3000?

Since I think you said that you are a couple of months away from a purchase I would start looking through the preamp & amp sections on Audiogon & eBay on a regular basis ...

Your dollars will go much farther this way ... This may be a little out of your price range but in the last 6 months I picked up a Sunfire TG IV and dual Sunfire CG II Signature amps ...

Total Price (New) - $11,500

I got the preamp new for more than $1,000 off list on eBay from an authorized dealer and the 2 amps each at half their original $3,995 price each from Audiogon both of which were in perfect shape ... total around $6,500 and of course no sales tax.

There are just oodles of good components for sale on these sites and if you know what you are looking for you can save a lot of money by buying used. A lot of the audiophile sellers at Audiogon turn over equipment as fast as the rest of us change our clothes in their never ending quest for the best and they very regularly have excellent used equipment for sale for relatively cheap that you wouldn't know was used to look at it or use it.

I'm not pushing Sunfire here although I think they make great toys ... What I'm suggesting is good used equipment.

gregure

Read up on the B&K separates. General consensus is that the pre-Ref 50 is great for HT, but a little dry for music. The 200.7 amp is very highly rated. Although, for the two you're approaching $4500, more than I can afford at present. Then I looked up Outlaw Audio, as the name has come up before. I could go with the 950 pre and 770 amp all for the same price as the B&K 507. Audio Review has a plethora of stellar reviews for these units (although everyone gripes about how ugly they are--not important to me). A few other reviews I looked over online were very favorable. I'm now leaning towards Outlaw, unless someone has had considerable trouble they can share with me. Sounds like these units are the best in their price field, with excellent sound irrespective of price. Any feedback would be appreciated.

gregure

PS. Thanks all for pushing me a little into investigated separates rather than walking blindly into the B&K. That's what the forum is for, I suppose. Re: my "bugs me" thread, this is what I'm talking about. When someone actually takes the advice to do some investigating and step up to the plate for quality equipment, rather than settling. Again, thanks for the urging.

PolkWannabie

You can lead horses to water but the horses have to decide whether they want the water or not ...

So again here's another push to the used trough if you like B&K ...

http://cls.audiogon.com/cgi-bin/cls.pl?homeproc&1093295885

... or ...

http://cls.audiogon.com/cgi-bin/cls.pl?homeproc&1092891058

... or ...

http://cgi.videogon.com/cgi-bin/cl.pl?prea71ch&1092308944

... and possibly ...

http://cls.audiogon.com/cgi-bin/cls.pl?ampsmult&1094262490

I don't see any 200.7's at the moment, but with any of these preamps for half price of new that should allow you some wiggle room for an amp ...

reeltrouble1

Excellent discussion but have to chucke that going for the B&K would be "settling". They are considered one of the better lines. If you are set on new then the Outlaw should serve you well, I have not owned one but have read enough from others that I would buy one.

Personally, I bought a terrific B&K 7270 that looks and performs flawlessly on Audiogon, their seems to be less ripping off there than other places. I paid less than half of the original 3000 price tag, although you have to watch for this type of gear.

If I remember correctly you are a classic buff, well versed in the complexity and detail of this type of musics signature keys. I could not say if the Outlaw will perform in this Genre, but the good news is the 30 day trial if you do not like the piece.

Since you have done the research glad you decided to go separates company aside. The three pronged connector is for what is known as a balanced xlr connection, has to do with better ground and is recommended for long runs for interconnects. Some believe they are the only way to go regardless.

The last thing I would consider is how long you would expect to keep your new gear will want to upgrade again in the near future. Both the Outlaw, B&K, and Sunfire gear mentioned sell quickly in the used market, holding some value but like I said I paid 1300 for a 3000 amp so somebody took a hit here.

Good Luck in your search.

gregure

Thanks again all. The used prices on the B&K are still very high. I don't think going with B&K is settling, but obviously enough people on the forum are confident that separates will yield better results. So, why go with a B&K receiver when I can go with highly rated Outlaw separates for the same price? B&K separates, however, are too expensive; even used the preamp was still $1500 in most cases. Plus, I'm trying to find a happy medium between a system that's good for HT use and music. I tend to use the better sounding DSP choices (like Neo:6), which may help with music playback. B&K ref 50 has a rep for being a bit dry with music. Also heard a demo of a 507 with classical music, and it seemed a little harsh and boxed in in the upper range. Could have been the speakers, but he used both Def Tech and Vienna acoustics and the results were similar. Outlaw, from what I've read, may be better with music, certainly in DSP mode. One thing I certainly do not have money to invest in at this time is two separate systems of separates for music and HT. That is a much farther down the line.