could a normal stereo amp power a passive sub

befroe i bought my av amp, i used quite a good technics lifstylesque system, i only now use it for radio tape and CD's, the ampis no longer in use . firstly, would this be suitable for powering a passive sub and secondly, it is only 30w per channel, so is there any way of using both channels to power a sub without shorting it and blowing up the amp/sub

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'This one's got a bad mmooooooooooooooooootivaaaaaaator (in a very camp voice)'

member formerly known as hank hill

The short answer is yes you can, but the effort, cost and final sound would prabably not warrant you going down that route.

If you already have a passive sub then fine, connect it up, you've got nothing to lose.

However, if you're going to go out and buy a passive sub to use with this amplifier, then, unless you're getting the sub at a bargain basement price you would be better off spending the money on an active sub. There are many reasons why but I'll highlight just a couple.

1. With a passive sub the amplifer will just amplify all the frequencies passed to it. If you don't filter the imput first then the sub will get a full range sound signal which will not integrate with your existing speakers, depending on the natural cut-off point of the sub it could be actually act as a 'point source' and sound very odd.

To cure this you may want to add a low pass filter to the sub to make sure it just outputs bass frequencies, this will be an additional cost unless the sub has one built in.

If you sound system has a dedicated LFE output then the above issue is less of a problem but may not entirely be solved, again it depends on the cut-off frequency of your main speakers.

2. Subs need a lot of power to move the cones to give you the 'punch' when you get the kick of the bass drum, or explosions on videos/dvd's. Your amplifer output of 30watts would mean that your sub probably won't have much 'ooomph'. Having said that I should qualify it that the 'punch' is determined by how good the power supply in the amp is. If it's got really beefy capacitors in it then for the first 'kick' of the drum or first 'explosion' it will be ok, we're then relying on how quickly the rest of the power supply can recharge the capacitors (recover) for the next 'kick'/'Explosion'.

It's would be possible to bridge the stero amps to get a higher output but this is not a straightforward option. To do this you need to supply the signal to one of the channels out of phase (inverted) with respect to the other channel. You can buy some electronics to do this, or depending on the type of preamp in the Technics you may be able to 'tap' in at a suitable point where the signal is inverted. This would be determined by the type of preamp it uses, whether it's discrete components or an IC (integrated circuit). Even if you were able to do this, ultimately the increase in power would be governed by how good the power supply was.

Depending on your requirements it would probably be better to sell your Technics and use the money towards an active sub. The ability to change the phase and cut-off frequency of the sub to integrate it into your system will ultimately be much more rewarding than finding a 'good use' for your previous amplifier. If it was good for you in the past then it will probably be good for somebody else in the future.

Regards
Gary