Driving different pairs of speakers with different speaker wire lengths


Looking for some sage advice.  My GF’s stereo is comprised of components I’ve replaced over the years.  It consists of a Denon CD changer, an older Technics multi channel HT receiver, a pair of Sonus Faber bookshelf speakers and a pair of Klipsch bookshelf speakers.  The Sonus Fabers are a couple of feet from the receiver, and the Klipschs are around 30 ft from the receiver (connected with 14 gauge speaker wires that run under the floor).

The receiver has connections for A and B speakers along with rear/surround and subwoofer channels.  I have the speakers connected to the A and B speaker outputs.

I am a civil engineer by degree, and had to take intro to EE twice, and not because I enjoyed it so much the first time - the concepts of matching impedance, etc escape me.  The system sounds very well balanced, but after about an hour of listening the receiver will stop and display an overload condition.  I suspect the difference in the speaker loads, coupled with the vastly different lengths of speaker cable are too much for the receiver to handle.  This irritates the heck outta the GF, so I gotta come up with a solution.

I’m thinking of getting a solid state preamp with two line level out connections (single ended) and a pair of two channel Class D amps - one for each pair of speakers.  This would let each two channel  amp handle the load presented to it individually without the other speaker load adding to it.  Intuitively I think this will work.

The Wyred for Sound MPRe preamp has two sets of RCA preout connections, has a DAC (not needed now, but could come in handy later as I improve her system), and is reasonably priced.  I think this might be part of the solution.  I now need to find a pair of two channel Class D amps that are reasonably priced as well.

The space for all this is really limited, so Class D is my current direction.  I’m scouring the used market for a pair of two channel class D amps that are small, and reliable.  Chances are I’m not gonna find a matching pair so I solicit any recommendations on new ones that are reasonably priced, and thoughts on my overall approach.

Thanks,

Mitch
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Run it without the 30ft  cabes and speakers,  and see if it still dies after extended use. If so, its trash, likely not worth repair.  Time to move up to better anyway. :)

Possible solution; affordable active speakers for surrounds. Perhaps receiver has pre output you can use with such speakers.  Drawbacks k is they must be powered, ie plugged in. 
Douglas,

The speakers are each running on full range signal, and they will stay in place.  I thought about some powered speakers as an alternative, but the ones I have already are really nice...

I agree this just might be the opportunity for an upgrade anyhow...

I thought about an A/B speaker switch too.  Both pair of speakers run to it, then it presents a single load to the receiver...

it’ll all come together eventually...

cheers,

Mitch
I think the Sonus Fabers are giving it a hernia.  They are probably 4 ohms and dip lower.  That older receiver can't take it.  It's not the Klipsch or the wires.  14 gauge can't make that much of a difference.
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You can get an Andover Model One Songbird streamer for $129 and a Tidal subscription. No need for the Denon changer!
Instead of two stereo power amp, you also can look for some multi-channel power amp to save space. Using four of the channels and leave one channel unused on a 5channel power amp is perfectly ok.
Its unlikely that the different speaker cable lengths are doing anything bad, unless the cables have some sort of high capacitance which is known to mess with certain designs that have poor phase margins. If the cables simply have two conductors side by side this is probably not the case. If anything, the longer cable will cause the amp to see a slightly higher load impedance but IMO/IME it won't amount to a hill of beans.

If the amp is on a shelf and there isn't much room above it, it could be overheating due to poor circulation. The output transistors are mounted on heatsinks and the heatsinks have temperature monitors- if they get too hot the amp will shut down. Otherwise it could be that you simply need speakers of higher efficiency so you don't have to push the amp so hard.
Sorry I wasn't clear. Runs. It without 2nd set of speakers.  That'll tell you.
Cobble together add on components is a mess. Start over, better. :)
Those A+B switches on receivers were always deceiving, as with most speakers doing that kind of double duty was unrealistic.
Ralph nailed it, the amp is simply overheating and shutting down. Ventilation is a factor but since this seems to be happening pretty regularly after an hour its probably because the total impedance load is stressing and causing the amp to overheat even at moderate volume. 

Pull out your EE book, or do a quick on-line search for series vs parallel impedance. The simple fix, first run the amp with each set of speakers individually. Just to confirm the amp doesn't have something else wrong with it. Then re-wire the speakers in series https://youtu.be/tTk_Xf5S6ns?t=93

Watch this video. See the problem? If your four speakers are 8 ohms then 8/4=2ohm. But speaker impedance is nominal, that is to say averaged or normalized, so that a nominal 8 ohm speaker can easily dip well below 8 ohms. Which is usually fine if its just one or two. But with four you could be dipping down to below 2 ohms which is darn near a dead short. 

Re-wire in series. Zero cost. Maximum benefit. Girls only want boyfriends that have great skills. https://www.youtube.com/watch?v=XsiiIa6bs9I
Thanks everyone for the advice; I’m gonna try running just the sonus fabers alone for a couple hours alone and see if this is the problem...the GF is patient, and I’ll let ya know what happens.
The receiver has connections for A and B speakers along with rear/surround and subwoofer channels.  I have the speakers connected to the A and B speaker outputs.
This probably your problem.  Your AVR has separate amplifiers for the front left / front right / rear left / rear right / subwoofer.  However, you are connecting your two pairs of speakers to A and the B outputs, you are connecting them to the same amplifiers (series or parallel).
Connect one pair to the FR/FL and other to the RR/RL output amplifiers.