DRN, I have some questions regarding your situation. Have you requested a postmortem from the manufacturer of the two subwoofers which quit working. Do you know if there is a timing circuit
present in the preamp which allows the preamplifier circuitry to stabilize before connecting to the
outside world through the preamp out jacks on the back of the amp.
The reason for these questions are that most tube preamp circuitry is single ended and only swings positive
above ground. They are usually cap coupled at their output to block DC from reaching downstream
amplifier stages. There are frequently bleeder resistors to ground from the caps so that they don't sit
with high voltage on them when the amplifier is turned off and not in use.
When the preamplifier stage is first energized the DC blocking caps see a substantial voltage surge
which persists until the cap is fully charged. At this time it can pass a potentially damaging voltage spike
to downstream circuitry. With a timing circuit that keeps the output of the preamp isolated until the caps
have charged and the circuit stabilized this occurrence can be eliminated.
I would hope that most subwoofers are protected from DC and a voltage spike emanating from the source components.
Here is where some rampant speculation comes in relating to my hypothesis about what you have experienced. If the integrated amp is passing a voltage spike to downstream components from its preamp
outputs upon turn on, repeated applications of this to the subwoofers inputs may overwhelmed its input stage and any protective measures it may have had. Both subs initially worked and then subsequently died.
I am not a fan of subs or any other equipment that relies on sensing a signal present and then turning it self on. There is no delay in their activation. If an upstream signal source sends them a high amplitude voltage spike instead of a legitimate signal they may go right ahead and amplify it unless some safety measures are in place. The safety factor in this regard is an unknown unless a subwoofer manufacturer makes a specific
claim regarding it. This goes to the well known practice among
audiophiles of always turning a preamp on first and then the power amp and upon shut down always
turning the power amp off first, and in the case of big ones, waiting until the power supply voltage
has dropped and then turning off the preamp.
These subs are always on and waiting
for a signal and are a parasitic drain on your wallet when it come time to pay the power bill.
Also in the case out of sudden power outage, depending on how your system powers down you can get thumps and other mayhem.
Steve can probably chime in and correct any of my misconceptions regarding my basic assumptions and the speculation which led to my hypothesis.
Hope your new subs are compatible with your system. Have you had time to judge results of the upgrade to
the CM2? Enquiring minds want to know.
Scotty
