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
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