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


I have this rather weird problem I am hoping the collective wisdom of Audiogon will be able to help me sort out.

The SLP-05 preamp has a pair of RCA outputs, and a pair of XLR outputs. I recently tried bi-amping using both outputs, and the result is the SS power amp always fails to work, no matter what output it is connected to. Here are the configurations which I tried:

SLP05-> (RCA)-> Cary CAD2000-> speaker
(Full range). Result: success

SLP05-> (XLR)-> Cary CAD200-> speaker
(Full range). Result: success

SLP05-> (XLR)-> Cary CAD211AE-> mid/tweet
SLP05-> (RCA)-> Cary CAD200-> woofer
(Bi-amped). Result: CAD211AE works, CAD200 produces no sound from the woofer.

SLP05-> (RCA)-> Cary CAD211AE-> mid/tweet
SLP05-> (XLR)-> Cary CAD200-> woofer
(Bi-amped). Result: CAD211AE works, CAD200 produces no sound from woofer.

SLP05-> (XLR)-> Cary CAD200-> mid/tweet
SLP05-> (RCA)-> Cary CAD211AE-> woofer
(Bi-amped with valve amp on bottom). Result: Now here is the interesting thing. Put the SS amp on the mid/tweet and the valve amp on the woofer, and it works! I am succesfully bi-amping!

Now obviously this is not what I want, because I would rather have the SS amp on the woofer. I checked and rechecked the connections and there was no problem. I swapped RCA and XLR cables to my spares and there was no problem. I swapped speaker cables and there was no problem.

I even borrowed another two SS power amps and the result was the same - each time, the SS power amp refused to power the woofer in bi-amp configuration.

I am wondering whether there is something about the higher input impedance of the valve amp that makes the SLP-05 preferentially drive it.

This problem has me beat. I can't figure it out. Can anyone help?
amfibius
Also, if at all possible try to arrange for the impedance measurements to provide the phase angle of the impedance (vs. frequency), not just the magnitude of the impedance (vs. frequency). That will allow the inductive, capacitive, and resistive components of the impedance to be distinguished from one another.

Regards,
-- Al
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.