Has anyone bi-amped with a Cary SLP-05?

Amfibius,

I have tried all the combinations you list with my Cary SLP-05, Cary MB 500 and Cary 120s tube amp and they all work for me.

I notice this statement in the manual for the CAD200:

Short circuit protection activates if load impedance is about 1.6 ohms or less.

I'm not sure how the protection circuit would sense what the load impedance is, but presumably it would do it by some sort of measurement of the relation between output voltage and output current. If the load is highly inductive, the current drawn by the speaker will lag the voltage by a substantial fraction of 90 degrees (exactly 90 degrees for a pure inductor, that has no resistance).

Therefore at the instant that current reaches its peak, the voltage will be at a considerably smaller value than the instantaneous voltage which caused that peak, which occurred somewhat earlier in the sine wave cycle. The protection circuit may interpret, however, that the peak current was caused by the instantaneous voltage occurring at the exact time of the current peak, in which case it would "think" that a lower impedance is present than is really the case (recall Ohm's Law, resistance = voltage/current; therefore a lower voltage producing the same current implies lower resistance).

When you run the speakers full range, as I said earlier, the capacitance which is likely to be a significant part of the impedance of the mid/hi drivers is placed in parallel with the woofer and its crossover elements, which would partially negate the inductive component of the loading presented by the woofer (the effects of inductance and capacitance on the phase relationships between voltage and current work in opposite directions).

So I think the statement in the manual about the protection mechanism reinforces the theory I presented earlier. It could be that the design of the amp's protection mechanisms is incompatible with your woofers, when they are driven alone. While you are waiting for the measurements you may want to ask Cary if a heavily inductive load, such as the woofer section of a speaker connected by itself, can trigger protective shutdown of the amp.

Regards,
-- Al

P.S: I just noticed in your system description that the woofer impedance "gets down to 2 ohms." I think that strongly reinforces the theory I have offered.

-- Al

Al's observation is probably right on the money, the problem looks like load impedance related than anything else. There aren't many amps that well behaves at 2 Ohms and below. You might want to audition First Watt F5, as it's supposed to handle down to 1 Ohm. It sounds awesome as well.

Thanks for your contributions so far, Al and others. Just one question - what would the symptoms of protective shutdown be? Would you expect the amp to make some sound, or would it refuse to power up altogether?

There are a couple of things which I did not mention in my initial post. The first is that I had to use 10m speaker cable, because I don't have a spare pair of anything shorter. When that failed to work, I hacked up some rather thin gauge lamp wire with the same results. I would imagine that both cables would present the amp with a large resistive load (and large capacitative load in the case of the 10m cable). Would this exacerbate the problem?

Also, I know something about the internal configuration of my speakers. It is rather weird - one woofer runs off the crossover, and the other woofer runs straight off the binding post and is effectively full range. What would this do to an SS amplifier?

Also, why does the SS amp refuse to drive the woofer, whilst the valve amp is perfectly happy to do so? It seems counter-intuitive.