Question about the 7700

So, I have been the happy user of the 7700 for going on about 6-8 months now. At first, due to room lay-out, I was only running the front 3 channels. I am runnig Polk LSi9s and an LSiC up front. These speakers are notoriously hard to drive. But, the 7700 seemed to do it with ease.

After moving to a new house, I was finally able to set-up a full 7.1 system. All speakers are the Polk LSi series w/ a LFM-1 EX and a Polk PSW125 for subs. My source is a Oppo BDP-83 running to a 970. I also have upgraded cross-overs courtesy of the Skiing Ninja in the front 3, and I did my own in my rear surround LSi7s.

For Music, this set-up sounds awesome. For movies, it also sounds incredible 99% of the time. Every so often though, at normal movie watching levels (not reference, but also not night time when the wife is asleep levels) I notice break-up of the tweeter during high dynamic sections. Like an explosion or parts of someone yelling. I never noticed it when I was running 3.1.

Is this because of the low-efficiency speakers combined with running all 7 channels of the 7700? Is it the upgraded clarity of the speakers is showing me the problems in the actual recordings on the discs? So far, I think this has only happened on Dolby Digital recordings, and happens whether I use the Digital Coax or the 7.1 out of the Oppo...

The Polks are very hard loads to drive for any amp, especially in larger rooms - the LSi15 is probably the toughest, but the 9's have proven to be challenging as well. What you are describing doesn't match real well with what I remember hearing from other Polk owners, though. It is possible that you are hearing flaws in the source recording. Do you recall listening at these same levels in the old room when you were running 3.1? You could do a test: find a passage that produces this break-up, set the 970 to stereo (it would then downmix the surround), and see if you get the same result.

Might some connection have been loosened, but not entirely disconnected, at/in the problematic speaker during the move? If so, it could be that some cabinet-wide vibration occurring at moderate volume levels is causing the poor connection to make and break very rapidly.

Another guess: something in a crossover is allowing a small bit of lower-than-intended frequencies to travel to the tweeter. At lower levels, where this does not cause distortion, you wouldn't normally notice because these same sounds are also coming from the drivers intended to reproduce them. Once the levels are high enough, the tweeter is mildly over-driven by the unintended frequencies, but not yet damaged.

Both could be possible...

Except the x-overs were modified after the move, and the actual values of the components with in the cross-over are exactly the same, so the x-over performsexactly the same tricks as before.

Also, I don't understand how this wouldn't happen on music when it is played at much higher volume...

I don't own Polk speakers, so I have no experience with the demands they place on the power amp. My speakers do have similar sensitivity to many of the Polk models and are 4-ohm nominal impedance. My 7700 has exhibited ample headroom and could drive my speakers to an uncomfortably loud listening level, but I do only use five channels with both music and movies at the present time. However, I still have a hard time imagining a 7700 having a problem driving any similar speakers at a bearable level in a typical room. If the 7700 were at fault possibly because it was being driven into clipping or because the power supply was sagging, that would be noticeable any time there was a loud passage at high volume. Your problem doesn't sound like that. I didn't think I had anything to add to this thread, until I read the part about the crossovers having been modified between the time you used 3.1 and 7.1 channel. I assume this modification required some soldering. Are you confident there are no intermittent "bad" solder joints in any of the crossovers? That might cause the problem you are experiencing. If that's the culprit, it should be intermittent and not necessarily repeatable over the same piece of source information, although it's possible some source material is better than another at setting up a vibration frequency that exposes a solder flaw. It's also possible that if the flaw is in only one channel, it might be masked by the other channels at higher volumes and be less noticeable.

Bill

It definitely sounds like a crossover problem. Swap the offending speaker to a different channel and see if the problem swaps to the new channel. If the problem swaps, the issue is likely with the speaker not the amp. It sure sounds like the tweeter is being overdriven with some low frequency stuff. Do the Polks allow you to bi-amp such that you could disconnect the high freq side and test it that way. It would give you a way to try to overdrive just that speaker and see if it breaks up on more than one channel of the amp.