I am asking what will be the drawback if the L/R line-out of a pre-amp is bridged (or Y-jointed together) and then connected to the LFE line-in of an active subwoofer?
The pre-amp is a spare one and will not be connecting to other power amp than the subwoofer.
I did connect this way and it worked fine. However, I wonder if this connection is good for permanent setup, and if not, will my pre-amp or the sub will get damaged someday by this connection? That's why I concerned the most. Will it?
The left and right channel of your preamp are in effect 2 separate and independent amplifiers, driven from 2 somewhat independent signal sources.
If you connect the outputs of 2 amps directly to each other, the risk is that the two amps may compete, i.e. may attempt to pull the output line high, while amp B attempts to pull it low.
You might think that wouldn't be a problem, since preamps work with fairly low power. That's true, but some preamps use correspondingly low-power output chips, which can still overload if they 'wrestle' with each other's outputs.
Second possibility: If the output stages pull in opposite directions, and one output transistor loses the contest, the drop across that transistor may exceed its absolute maximum rating and it may be ruined.
Third possibility: Under some input conditions, competition may result in clipping, where the tops of what should be sine waves get chopped off. The sharp corner on the wave is then faithfully reproduced by your power amp -- and when your speaker's voice coil can't change velocity that quickly, your power amp's feedback circuit cuts the drive current to zero because the speaker is overshooting. The speaker's voice coil then throws a fairly large inductive kick of over 100 volts back through the amp on every half-cycle, which is a leading cause of fried output stages.
In a really worst-case scenario, the left and right channels of your pre-amp don't just compete -- the feedback circuit connected to the output goes into oscillation for certain combinations of left and right signals. The oscillation is again faithfully reproduced by your power amp, but this being a worst case, you don't hear it because it's ultrasonic, because modern power amps can often handle 75 to 100+ kilohertz. You do however notice a faint rattling sound in the speaker, and as you listen close, a piercing pain goes through your head, which is hearing damage.
Since you didn't know that the answer to most questions depends on type-of-equipment, you're obviously a novice, which means you should use equipment only as it's designed to be used. If you were advanced, you'd also stick to using equipment only as the designer intended, because a lot of unintended things can happen even if you're a circuit designer, and even if you're THE circuit designer for that amp.
The right way is to use a mixer, or a mixer-preamp. See also Wikipedia.org article: for bridged amplifier, which assumes left and right channel are the same ( you didn't specify that either ).
You should generally not connect the 2 inputs of a preamp together, if they come from different souces. If you want to connect a single monaural source to both L and R preamp inputs through a Y connector, that's OK. You can also get a Y-cord with resistors built in to merge left and right channels, but they may not be a good impedance match for your equipment, which may increase distortion above rated specs ( particularly important for devices like microphones and tape drives ).
Hi I am new to ecoustics- can somebody help me with my subwoofer connections? I have a Sony SA-WX700 subwoofer and a HSU STF-2 and wish to link the two together, the sony has line -in- as well as line -out-, STF-2 has only line in, when I ran the sub'wire from the sony to the STF-2, only the sony responded, no sound from the STF-2, what could be the reason for this? both the subwoofers work well on their own, perhaps I could use a 'Y' adapter from the preamp and hook up both the sub's to the 'Y'adapter? Please help.