Is this Clipping?

I have recently upgraded my rear and side surrounds to Polk LSIFx and LCi in ceiling speakers. This means I now have 7 channels of 4ohm speakers. Before, I had 8 ohm Polks for my rear and side surrounds. I notice now that when I watch very dynamic movie scenes such as explosions or car chases etc. that there is considerable amount of distortion coming from my speakers at a volume of -15 to -25 db.(and louder) This is a new phenomenon and it had not even occurred to me that it could be clipping with an amp like the 770 but......?? I am not even sure what clipping sounds like but it has been suggested to me that this is it.

Silly question:

Did you re-calibrate the output levels after changing the speakers?

Oh yes. I have actually been running some tests since my post.

I tried setting the cross over higher to transfer more of the power requirement to the sub amps. With each step(60,80,100,120) there was less distortion at -25db.

Then I disconnected my rear surrounds (Polk Lci) and played my throw down demo scene ( Amidala's ship exploding in AOTC). This time the explosion was as I remembered it with no distortion. I am afraid to test it at anything higher than -20 though for fear of damaging my speakers.

Can I alleviate some power sucking on my 770 by putting a couple of monoblocks on the LSIfx's? I like to play my HT at -15 on average so I may just need some extra juice. That 770 runs wicked hot also with all those 4 ohm vampires.

-15??? I've gotten into the habit of watching movies at -6. 8-) I tried 0dB once with Episode II and after a few minutes I knuckled under and went back to -6dB.

Cheers,

Wow! Hardcore..ha. I have gone to -10 on a consistent basis but anything louder for me is just uncomfortable.

I've got a pretty big room so it's not piercingly loud at -6dB.

I doubt that monoblocks will help because unless your amp is defective, it's not your amp. It's either your room, pre/pro, or speakers. The 770 can supply 300WPC into 4 Ohms.

Suggestions:

1. Calibrate your speakers

2. Go to http://www.myhometheater.homestead.com/splcalculator.html and calculate the peak watts needed for each speaker to supply your desired SPL at the listening position. As an example, if you like to listen at -10 (this is 75 dB, with 95 dB peaks), you want to adjust the watts on the calculator until the speaker dB is 95.

3. Add up all the watts for all speakers calculated in step 3.

4. Post your watt results to us.

I am willing to bet that each channel of the 770 will supply less than 100W to each of your speakers... clearly waaaaay less than the 770 is capable of. You may want to "rent" a receiver from Circuit City or Best Buy to use as a pre/pro for some serious testing just to eliminate your current pre/pro as the source of your problem.

-T

There is no way the 770 can drive 300 watts
into 7 channels. That would be 2100 watts
assuming power supply could even hand it...

I bet four monoblocks for your surrounds
and bi-amping your front, center, and right,
speakers with the 770 would ROCK!

I have a 770 driving 7 x 6-ohm loads, which I have listened to briefly at extreme volumes in the 7-stereo mode of the 950. This demands more power over all and feeds much more power to the surrounds than almost any surround sound source. No problems with distortion onset as higher levels are reached with all sub crossovers set to 80Hz. Not a large space but got the ol’ RS SPL meter over 120dB for a little while. Admittedly, most of the time the needle got that high was during moments of heavy bass, but the other speakers were still giving off sound which gave me mildly ringing ears. (I certainly do not do that but once every other year or so – I’ve got to maintain decent hearing for work and continued audio enjoyment.) The 770 got warmer, but not overly hot to the touch. I have almost a foot of clearance over the 770 for warm air to float away.

I agree that there are some simple tests one can do to check other problems as others have suggested. Another test might be to feed each of your three speaker pairs one pair at a time with a well known, simple stereo signal, see how each pair performs on their own at various volume levels. You may have to power down and swap a few cables to perform each part of the test since you don’t want to use processed surround signals for this test. With this test you know you are not near to exceeding the total power supply capacity of the 770 as you slowly raise the volume to high levels on just two channels at a time.

Another simple test might be to use a plain meter to monitor the line voltage supplying the 770 – see if there is more than a volt or two drop during moments of seven-channel high output.

One more test, then I’ll quit: use the same surround sound set-up and source material that lends itself to providing the distortion you can easily hear. Using the same part of one scene again and again, listen to only the center channel, then only the front L & R, then only the side surrounds, then only the rear surrounds. See if any set of speakers will tend to produce distorted sound whether played moderately or loudly. If a set of speakers are “clean” during the “simple stereo” test but “distorting” during a surround test, it may have something to do with the source or the surround processing, not the amplifier or power situation.

I hope you can track down and solve the problem whatever its source.

Oh, if you are hearing “hard clipping,” the sound will be distinctly crackling during the highest moments of the signal. Of course you may be hearing a type of distortion that is not clipping.

One thing to keep in mind is that unless ALL your speakers are set to LARGE, they're not getting anywhere close to full output of the amp, even at extreme volumes since a lot of the low frequency signal is being filtered and fed to the sub.

Has Outlaw ever commented on this? I also find it strange that the amp's total output at 4ohms exceeds its rated maximum power consumption. So even assuming a 100% efficient amp, there just isn't enough power to go around.

Cheers,

Uh, the monoblocks put out 300W into a 4 ohm load (same as the 770)... 300 * 4 is 1200W... but the amps are about 50% efficient, so 4 monoblocks will require 2400W from the outlet. So I guess there is no way 4 monoblocks will drive his 4 surrounds either, using your logic.

So I am willing to bet that 4 monoblocks will not provide 1 iota of benefit.

-T

Interesting analysis. However, you are assuming that all 4 monoblocks are in the same outlet. In that case I would agree with you. Obviously, if they were on a different circuit then some benefit might be achieved.

Although, I must admit my curiousity is stirred. How could the 770 put out 2100W if it is only drawing 1800W maximum? Now bear with me, but if the total capacitance is sufficient then for some short time [t] with t<120ms [ milliseconds ] then it is possible for the 770 to do that. However, I believe if [t] is some long time in comparison say 10 seconds, then something would have to give. Of course, 10 seconds of a very high signal level in all 7 channels at the same time....not that much chance of happening.

Just my 2 cents worth, and I am probably missing something as well. Perhaps Gonk or Soundhound could help me out here?

Bob045,

I was using the same logic as tonydeluce, so I believe there was no mention of separate outlets.

-T

You are assuming you have to plug everything
into the same circuit :-)

Even separate outlets are often on the same circuit in the same room (unless you've specifically set things up differently).

Clearly, the 770 can't provide full output to all channels @ 4ohms simultaneously and continuously. There just isn't enough juice to go around. I suspect that it probably can't do it at 8ohms either when you consider that the amp isn't anywhere near 100% efficient so you're losing probably 30-40% of the power as heat.

Anyone from Outlaw care to comment? Is my math faulty?

Cheers,

That is why I said separate circuits. I had
two completely isolated 15 A circuits installed
and I use three separate circuits for powering
my A/V components.

I doubt it could do even 1400 watts ( 7 channels
into 8 ohms ) for more than a very brief period.

Unless you are listening at close to reference
levels driving all channels you should be o.k.

But if you are using six or more 4 ohm speakers,
forget about it, as they say :-)

This can happen because the outlet and more specifically the circuit breaker are NOT current limiting devices (except for a short circuit). The breaker works on a thermal trip function which means it will take many, many times it's current rating and still not trip as long as the time at the increased current is not to long. Most breakers will operate at 120% of rated current indefinitely and 500% of rated current for many seconds. Some seem to feel that the breaker somehow begins to inhibit the flow of electricity as soon as the amperage rating is met and abruptly cuts it off right above the 1800-watt rating of a 15 amp breaker, this is just false. Look at it this way the same 15 amp breaker used in Europe would have a "wattage" rating of 3600 due to the increased voltage. Another fallacy seems to be that a 20A breaker somehow flows electricity easier than a 15A breaker .... it won't. Breakers are also not prone to wear and have a lifetime in the thousands of thermal trips. They work on the same principal as the turn signal blinker in your car to the extent they both use bimetal to open a circuit.
I'm not inventing this or even claiming some superior expertise, anyone can read the same articles from Square D or Cutler-Hammer that explain the operation of a circuit breaker.

Capacitors in the power supply are also there to help with the momentary demands.

I don't think it can put out the power specified because the home's outlet is limited. This thing really needed 2 power cords, one to each power supply.

This review (http://ultimateavmag.com/amplifiers/147/index4.html) had 6 channels driven at the same time, for a total of 221W x 6 into 8 Ohms (1 KHz), and 347W x 6 into 4 Ohms (1 KHz). The test bench could only handle 6 channels. The variac saturated, and then blew a fuse (on the variac).

For 4 Ohms x 6, it put out 2082W... so it drew probably about 4000W from the source.

-T

If it put out 2082 it was for a very very brief
period of time...

2082 watts ... not only respectable but makes me grin