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

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.

Keith,
At some point I briefly tried the CAD 200 on the woofers of my Acapellas and had no issues at all. My pre is Supratek Dual Cabernet. The CAD 200 is a nice amp for the money, with midrange being an antithesis of typical solid state sound, but its bass is unfortunately slow and underdamped, the exact opposite of what your Violons need.

Hi Keith,

I would expect that the short-circuit protective shutdown, which as I quoted from the manual kicks in when it senses a 1.6 ohm load, would result in no sound whatsoever (aside perhaps for a fraction of a second or whatever amount of time it requires to activate). Also, fyi, the manual indicates that to recover from that condition the amp has to be shut off for at least 10 seconds, and then restarted. It also cautions that repeated protective shutdowns can degrade the output transistors.

I wouldn't think that there is much significance to the fact that you used long cables. The slightly increased resistance would work in the direction of reducing the likelihood of a protective shutdown, by increasing the value of the ohmic load (since the cables are in series with the woofer). What might hurt slightly is the increase in inductance (for the reasons I have previously stated), but the cable inductance is likely insignificant relative to the woofer and crossover inductance. Cable capacitance is likely a non-factor here.

In itself, the unusual configuration of the two woofers is not particularly significant. What is significant is that the overall woofer arrangement is highly inductive, the voicecoil of one woofer being directly in series with the amp output, and the voicecoil of the other woofer being in series with the amp output through (undoubtedly) an inductive crossover element.

The reason that the valve amp's ability to drive the woofers is counterintuitive is that from a SOUND QUALITY standpoint a tube amp, with its higher output impedance and more limited output current capability, is typically a poor match for a very low impedance speaker or driver. But here we are not dealing with a sound quality issue. We are dealing with an amplifier self-protection issue, and this particular amplifier has a protection mechanism that considers 1.6 ohms or less to be a short circuit. With the woofer impedance being around 2 ohms, and undoubtedly lower than that at some frequencies, sad to say that is simply an incompatible matchup.

The tube amp's higher output impedance, while degrading the sound quality it can provide into a low impedance load, would in fact help it to deal with a low impedance load, from a protection standpoint. The higher output impedance would in itself somewhat limit the current that would be drawn by a short or very low impedance, and provide some measure of protection that a solid state amp, with its much lower output impedance, would not have.

And for many other conceivable reasons the protection mechanisms of the tube amp, assuming it even has any, would figure to have little similarity to those of the solid state amp.

In any event, despite Kotjac's successful brief use of a CAD200 with the (apparently) same woofer configuration, the matchup of the CAD200's protection mechanism with the 2 ohm or so woofers is obviously extremely marginal at best. Any factor which would even slightly exacerbate the situation, such as the increase in net inductance which would result from driving the woofers in the absence of the capacitive loading presented by the mid/hi crossover elements, clearly stands a very good chance of putting the situation over the edge and triggering the self-protection.

The bottom line is I feel virtually certain that if you want to biamp, you need a different amp for the woofers. There is simply an inadequate margin between the woofer impedance and the short-circuit protection threshold.

Regards,
-- Al

Hey guys, I'm wanting to do the very same thing as I have 2 sets of monoblocks I'd like to use with my Cary SLP-05. Two CAD-211 FE's which I'd like to use for the mid/hi and Spectron Musician III Mk II fully balanced mono's. The Spectrons have no problem running down through 1 ohm, but the conceptual issue I have before even getting this set up (waiting for a rack for the Cary amp's so they're somewhat protected from the kids running around) is that the impedence matching (input impedance) is so different for both amps and that fully bridged the Spectrons will need to be attenuated (add to that, and this I would think would be relevant for everyone doing this, isn't XLR running about 6db louder than unbalanced RCA connections?). So my LF would be far too loud relative to the mids/highs unless I introduce a passive, fixed attenuator (say in a XLR tube at the amp input) spec'd to the right level so as to minimize quality degradation. Anyway, this is my chief concern, level matching.

Silverlight99,

I took a look at the following documents:

MusicianIII Specs
211FE Manual
SLP05 Manual
Stereophile's SLP-05 measurements, particularly output impedance

The 211FE input impedance appears to be specified as 150K unbalanced, 300 ohm balanced. If that is correct (and I would suggest verifying the 300 ohm number with Cary), you don't want to use the balanced input, because it's input impedance is vastly too low in relation to the preamp's output impedance.

Otherwise, I don't see any problems in relation to impedances. The power amp's 20K input impedance is not quite ideal in relation to the preamp's output impedance at 20 Hz of 3400 ohms (1500 ohms at higher frequencies), but that will result in a low frequency rolloff of less than 1db at 20Hz, which is insignificant.

You WILL need passive attenuation on the inputs to the Musician III's. They are spec'd for a voltage gain of 26db, apparently in stereo mode. Bridged mode will increase that to 32db. Using the balanced inputs may or may not increase that to 38db, depending on the internal design (you should ask Spectron that question).

The gain of the 211FE's is a little unclear. It says that 2 volts input results in full power, but full power is specified with 3 different numbers for Class A, Class AB2, and Class B operation. My guess, and it's just a guess, is that the 2 volts corresponds to the 110W Class AB2 number. For an 8 ohm speaker load (connected to the 8 ohm taps) that would be a voltage gain of about 24db (which would be 25db if the 2 volt sensitivity rating is based on the 150W Class B number). For a 4 ohm speaker load, those numbers would be about 20db and 22db, respectively.

So once you've pinned down the uncertainties I've described that enter into the gain calculations for each amp, the amount of attenuation that is required can be calculated by subtracting the resulting gain number for the 211FE from the gain number of the Musician III's.

All of this assumes that speaker sensitivity (spl out vs. voltage in) is the same for the low frequency drivers as for the mid/hi frequency drivers, which is probably the case if all drivers are part of the same speaker.

Hope that helps,
-- Al