Adding a subwoofer of 4 ohms

Frank Elliott New member Username: Flyboy Hendersonville, NC USA Post Number: 1 Registered: Oct-05	Posted on Wednesday, October 19, 2005 - 04:03 GMT I have a pair of vacuum tube , single end monoblocks ..(and a 4 ohm 12" subwoofer.) The two channels are currently fed to a pair of 8 ohm JBL's. (some young whippersnappers roll their eyes as such low "wattage"-but trust me; at two watts out, people want it turned down so conversation can take place! It's plenty of LARGE sound. ) I was thinking of adding a center subwoofer, just for experiment.(because I have one...like Mt.Everest..."because it's there") I can easily tie the center subwoofer to the 4 ohm connection on the output transformer...and employ a 12 db 800 Hz crossover for it. The question came up...what if both channels are connected to the subwoofer. This message board is not suited to drawing connections...so has someone tried this? I was informed that at low frequencies, the output tubes would see what amounts to a dead short. I think they are right. What's your opinion?
Frank Elliott New member Username: Flyboy Hendersonville, NC USA Post Number: 2 Registered: Oct-05	Posted on Thursday, October 20, 2005 - 00:14 GMT Sorry! I said I could connect to the 4 ohm output connection and there is none. Just the 8 ohm connection for each channel. Thus feeding to a center subwoofer has me puzzled. I don't think it can be done. Both channels put through a crossover to send 800Hz and below, to one 4 ohm speaker would create a load on each channel owing to the decreasing reactance at low frequencies. Besides...it isn't set up for 4 ohm speakers..just the 8 ohm connections. It has my friend and I stumped. Any opinions appreciated. Like "Go to such and such website"
Jan Vigne Gold Member Username: Jan_b_vigne Dallas, TX Post Number: 6234 Registered: May-04	Posted on Thursday, October 20, 2005 - 00:37 GMT Any time two amplifier channels have separate ground paths you will do damage to the amplifiers if you tie them together as a common ground. If you want a subwoofer in this system, you can try running from just one amplifier. Bass is more or less mono at the lowest frequencies. The degree of bass response on the disc that you loose will depend on the amount of bass on the disc that is not mixed to mono. However, eight watts of single ended power is seldom enough for an enclosed subwoofer. You've given no sensitivity specs for either your JBL's or the sub. Unless the sub is very efficient, eight watts into a typical sub is not likely to keep up with the JBL's. Hooking the four Ohm sub in parallel to the eight Ohm tap on the amplifier will not be the proper impedance loading for the two speakers working together. With a transformer coupled amplifier, you probably won't do damage to the amplifier, but electrically this is a poor choice and and the amplifier's performance will be compromised in several areas. I would suggest, if you insist on a sub in the system, you would be better off with a powered sub where the SET's are not going to have to deal with the problems of a subwoofer. Then I'd let your friend take this one out of harm and temptation's way.
Frank Elliott New member Username: Flyboy Hendersonville, NC USA Post Number: 4 Registered: Oct-05	Posted on Friday, October 21, 2005 - 01:08 GMT Thanks to the guys for some good advice. In the meantime, I found a good site with fabulous interactive graphics that show plainly speaker connections, and explains clearly how to set them up with the correct crossover values of inductor and capacitor, either series or parallel. I think I lot of folks in this speaker section would like this site: http://www.bcae1.com/passxovr.htm

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