Protection circuit cuts in while my amp is cold

[MarkR]

I took the plunge last year and updated my 2ch system into 5ch.
My two front speakers bi-amped using RSX-1055 for mid/high and an
RA-980BX doing bass duties. The problem is the protection circut
cuts in when the volume level is over 72 and there is a lot of bass
in what is being played. The manual states this circuit is to protect the
unit from overheating but this occurs when the unit is only just warm.

[soundhound]

Sounds like your'e speaker impedance is nearing the lower end of what the amp can drive. When you start pushing it (get the woofers moving) your'e amp may be seing less than 2 ohms, wich is getting quite close to a dead short across the amps output. The thermal protection you refer to is normally controled via different circuit. Check the "nominal impedance" of your'e speaks, maybe post on what speaks you have and I am sure someone can tell you what it takes to drive them.

[MarkR]

Front speakers are Kef Q90, impedance is rated at 8ohms.

[soundhound]

You may want to take a meter and confirm, make certain you don't have a crossover problem, failing driver, or cable issue. Also what is the power capability of the 980bx?

[MarkR]

The 980bx is rated at 100wpc so it should keep up with the 1055.
I tried swapping the amps around today and I may have solved the
problem but not answered my question.

With the 1055 driving the woofers(in 2ch), I have now discovered the max bass capabilities of my speaker. At around level 75 on a bass heavy
music track I can hear the rubber surrounds on the woofer snapping to attention as they stop it from flying out of the speaker box.
I suppose this has answered one of my dillemas, what to upgrade next.
These speakers are rated at 150w and are about 8yrs old, am I asking
to much running 200w through them and winding up the volume?

Back to my original problem, as the 1055 is rated the same in 2ch
as the 980bx and can drive the woofers harder, can one assume the 980bx
is the weak link?

Also, am I right in recalling the lower the THD rating on an amplifier the better?

[soundhound]

The long and short of it would be to replace the RB980 with a healthier amp, say an RB-1070, RB-1080. The 1080 is capable of 200wpc, but it would give you a squeeky clean 150 should you choose to listen that loud. @ 85-100 watts your'e 980 is starting to distort wich added to the loading of your'e speakers is causing your'e protection to kick in. Is that 150 rating on the Kef's peak or nominal? You may also want to consider a Sub (if you have'nt one allready) if you like prominant bass.

[basementjack]

One other thought in reading this thread is to make sure the jumpers are off the speakers - if you are bi amping.

Probably pretty obvious, unless of course it's been overlooked.

[soundhound]

Not removing the jumpers would send both amps into protection fairly quickly,(low db), hopefully not blow fuses or let the smoke out of them as that would parallel their outputs.

[MarkR]

The speaker itself is 3way floorstand model with an 8" woofer. The only spec's I have for the speaker gives an 'amplifier requirement'
of 10-150W, I would assume this to be nominal?
I purchased a Rel Strata 5 late last year but I have had trouble
integrating it on 2ch with my fronts set to small. Its a lovely sub
with music but I have had to place it behind the couch for now, and the Rel
people recommend sending a full range signal to the front speakers so as not to degrade sound quality.

As for the jumpers, I thought my speakers were bi-wired for 5 years until one of my bannana plugs broke and I decided to barewire them, the jumpers fell out as I unwound the binding posts ops:

Thanks for reminding me!