Mixed Impepences in home theatre [Archive] - Audio & Video Forums

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20to20K
11-07-2004, 07:32 PM
All,

I just had the following problem today...I tried to connect my Denon 3803 to the following:

Polk LSi 15 - front (4 ohm)
Polk LSi C - center (4 ohm)
Polk Fxi 30 - rears (8 ohm)

After about 20 minutes playing in 5 channel mode the reciever cuts off. I'm not playing it
loud. I call Tweeter. They claim AVRs tend to have a problem playing speakers with mixed
impedences and may shutdown in this configuration. The Denon has no impedence selector. This sounds fishy to me and if it's true sounds lame on Denon's part.

If it is true (I will continue to research) it appears I may need to buy a seperate amp to
drive the 8 ohm rears. This is still cheaper than upgrading the rears to the $900 LSi FX's.

Does this make sense to anyone out there?

Larry

anamorphic96
11-07-2004, 08:45 PM
You would probably be better off getting an outboard amp to drive the 4ohm speakers not the 8ohm speakers. The 4ohm speakers are whats causing the problem. The Polk LSi speakers are a tough load to drive properly. If im not mistaken the 3803 is not really designed to drive 4ohm speakers. It will do it but you wont be hearing what the Polks are truly capable of. The speakers are basically drawing to much current and the amp cant keep up. Try and look into buying a used 5 channel amp from Rotel, Adcom, or Parasound. You would be truly amazed at what these would do for your Polks.

Cheers,
Glenn

markw
11-08-2004, 04:53 AM
My 2802 manual cautions against 4 ohm speakers. Yours probably does also. Denon is not alone in this. Many AVR's also share this limitation.

kfalls
11-08-2004, 05:08 AM
I have a Denon AVR-3300 (105W/ch, 6ohm rated) which I believe has a lesser amp section than the 3803. I drive all 4 ohm speakers (Legacy Focus, Silver Screen center and T&A P-30 rears), but they're pretty efficient 92db-94db. I have an outboard amp driving the Focus' woofers in bi-amp configuration and believe this makes all the difference. I'm not familiar with all Polks, but did have a pair of the compact SRS series and they were terribly inefficient, kept shutting down my 100w/ch amp in stereo. They went back the next day.

20to20K
11-08-2004, 10:17 AM
You would probably be better off getting an outboard amp to drive the 4ohm speakers not the 8ohm speakers. The 4ohm speakers are whats causing the problem. The Polk LSi speakers are a tough load to drive properly. If im not mistaken the 3803 is not really designed to drive 4ohm speakers. It will do it but you wont be hearing what the Polks are truly capable of. The speakers are basically drawing to much current and the amp cant keep up. Try and look into buying a used 5 channel amp from Rotel, Adcom, or Parasound. You would be truly amazed at what these would do for your Polks.

Cheers,
Glenn

Based on the advice I've received (both on this thread and from my dealer) it appears that I'm committing two sins: 1) Driving three 4 ohm loads on a reciever that doesn't recommend any speaker lower than 6 ohms and 2) mixing multiple impedences at the same time. I'm not sure which is the worst evil. I've run the 3803 with the 4 ohm
LSi15 in two channel mode for long periods without a problem. When I added the additional 4 ohm load (center) and the 8 ohm rears I hit it with two whammies at once.

True that about the manual. I got the 3803 as a floor model ($599) and they couldn't find the manual. Me being Mr. bright audio guy tried to hook things up before taking the time
to download it off the Denon website. That's my bad. I've since done so and saw the comment about "lower than 6 ohms".

I recently bought a B&K 200 RMS amp that I was intending on using for the LSi 15s.
I should receive that amp in a day or two. I'll first attempt that configuration and hopefully
it will solve my problem. If the impedence mismatch is still causing me problems (because it's still being combined the the 4 ohm center) then I'll have to get a seperate mono amp for the center or another two channel for the rears. I'd perfer do the former and not the latter, but single channel amps (or brigdable 2 channel amps) are harder to find on Ebay.

To state the obvious...this is an expensive hobby! Keep thinking of Pacino in GF3:

"Just when you think your OUT....they pull you back in!"

Thanks for your help guys...

nightflier
11-08-2004, 12:18 PM
I was going to recommend a solid two channel amp just for the LSi15's, rather than a more expansive surround amp. There is also a very good 3 channel amp out there, I think from Adcom, for just this type of situation (actually for adding to a 2 channel amp, but same difference).

The LSi15's are very hard to drive because of the bass that they produce (I'm kind of surprised Polk did not add a seperate power source). By putting the speakers on a seperate 2 channel receiver, your Denon should be able to drive the rest. If not, you can also use the receiver as a preamp with a solid amp from Adcom/Parasound/Outlaw/Rotel.

Of course, these are also expensive options and those LSi FX surrounds might not be such a bad idea, either...

20to20K
11-08-2004, 12:51 PM
I was going to recommend a solid two channel amp just for the LSi15's, rather than a more expansive surround amp. There is also a very good 3 channel amp out there, I think from Adcom, for just this type of situation (actually for adding to a 2 channel amp, but same difference).

The LSi15's are very hard to drive because of the bass that they produce (I'm kind of surprised Polk did not add a seperate power source). By putting the speakers on a seperate 2 channel receiver, your Denon should be able to drive the rest. If not, you can also use the receiver as a preamp with a solid amp from Adcom/Parasound/Outlaw/Rotel.

Of course, these are also expensive options and those LSi FX surrounds might not be such a bad idea, either...

The LSi25 are pretty much the same as the LSi15 with an additional power sorce to
drive the bass. I may not have had the problem as bad as with the 15's. Those speakers
do cost an additional thou and in my opinion, they didn't sound as good to me as the
15's when combined with at a $500 sub. Of course that's just my opinion.

As I mentioned, I have a B&K on the way...can't help but notice none of you mentioned
B&K in your suggested list of amps. You guys know something I dont?

anamorphic96
11-08-2004, 02:49 PM
Anotther thing you might want to try once you get the external amp for the 15's. Is to run the surrounds and center as small or cross them over around 80hz. This might take some strain off the amp. That B&K amp will really open those Polks up.

Another amp I forgot about is Outlaw. Kick Ass products and the mono blocks for 299 are a steal.

Glenn

20to20K
11-08-2004, 05:43 PM
Anotther thing you might want to try once you get the external amp for the 15's. Is to run the surrounds and center as small or cross them over around 80hz. This might take some strain off the amp. That B&K amp will really open those Polks up.

Another amp I forgot about is Outlaw. Kick Ass products and the mono blocks for 299 are a steal.

Glenn

I'm currently running the rears as small...but I've got the center a large...I assumed
(and I may be wrong here) that a pretty wide spectrum of sound may be required
of the center and the LSiC is rated down to 50 HZ or so. Wouldn't you lose some soundstaging
and imaging by having the center set to small? I mean is some helicopter or tank is sweeping across your screen from left to right wouldn't you notice that roll off as it comes
across the center channel?

Any other opinions?

Larry

kfalls
11-09-2004, 05:00 AM
A good sub-woofer would take care of that problem. When setup correctly there would be enough bass to fill the room and be non-directional.

Quagmire
11-09-2004, 01:40 PM
20to20k,

Something you should try, just to eliminate the possibility of problems with the receiver, is to put the receiver in stereo (2 channel) mode and play some music and movie sources. By stereo, I don't mean Pro Logic or any other mode which allows a stereo source to be spread out amoung all of the speakers -- only the front mains should be getting a signal. Even with 4 ohm speakers, the receiver should be able to power the front mains without shutting down. If it can't do that, you may have more problems than just inefficient 4 ohm speakers.

Q

anamorphic96
11-09-2004, 08:52 PM
Now that I think about that some more it may not be the best idea being your using the LSi 15's and they have built in subs. Probably the best thing to do is find an outboard amp to drive the center as well. The Outlaw mono blocks kick some serious butt for there price and size. Only 299.00 for a 200 watt mono block. As far as B&K goes they are a great company with great products. Just let the receiver dirve the surrounds and everything should be good.

Cheers,
Glenn

20to20K
11-11-2004, 05:07 PM
I recieved my B&K and the FedEx gorillas damaged it. I got reimbursed in full and the same day a 3 channel B&K popped up on Audiogon and I snatched it up. It's the 3220 and it pumps out 220x3 RMS. It only cost me $150 more than the 200x2. This should teach those 4 ohm Polks a lesson!

I'll keep you posted...

20to20K

Woochifer
11-11-2004, 05:19 PM
Most receivers out there are not rated to drive speakers under 6 ohms impedance, and the ones that I've seen that are rated to drive speakers down to 4 ohms recommend that you use no more than one pair of low impedance speakers at a time. People on this board who've used low impedance speakers with AV receivers have reported mixed results. Some people's systems work fine with a full 5.1 setup at 4 ohms, while others go into protection mode with just one pair of 4 ohm speakers.

In your case, I would try shutting down the center speaker or identify it as a Small speaker and see if that helps. Since you've already decided to go with an outboard amp, you should be fine. Just surprising to me that Polk has gone to a low impedance design, since they've traditionally been relatively easy speakers to drive. FYI, switching the center speaker to Small will not affect the imaging since the default crossover setting is 80 Hz, and the sounds at that frequency and below are nondirectional. Not a whole lot of low frequency information usually gets mixed into the center channel to begin with.