How To "Bridge" A Stereo Amp to Just One Channel

[kelticwizard]

Okay, let's just say I have an old stereo amp that I got from a tag sale and I want to drive a really excellent subwoofer-such as Parts Express' Titanic, Peerless XLS series, or the NHT.

All of those series are single voice coil jobs, so I cannot use both channels to drive the speaker.

I have heard of the technique of "bridging" an amplifier to achieve two channels in one, but I have not come across a hookup pattern.

When I "bridge" this stereo amp, I am hoping to be able to get full power from both channels. If series resistors are used, I suspect they will cut the power.

Does anybody know the technique? Thank you.

PS: I plan to use this amp to only drive the subwoofer. I will be putting my high cutoff network in the circuitry BEFORE the output transistors.

[GRollins]

Easy enough...
Invert the input to one channel.
Leave the other alone--the signal to that channel goes straight in.
Run the speaker off the red/hot/positive terminals only, thus 'floating' the ground.
Caveats:
Some output topologies aren't happy with this arrangement.
The impedance of the driver is divided between the two channels, meaning each channel will see half of, say, a 4 ohm load, meaning 2 ohms--be sure the amp can take that sort of load.
Implementation:
Basically, you can pass the signal straight in to one side. For the other, use a single stage of amplification (tube or solid state, your choice) to invert it, or an opamp using the inverting mode. Make certain that the signal level is equal to the straight-in signal (unity gain). If you use a single gain stage, you will need to use a voltage divider to bring the output back down. It will be easiest to filter before the inverter.

Grey

[kelticwizard]

Wow!! That was quick. Mucho thanks.

I had thought of inverting the input, but wanted to check.

The input can be inverted as well, I suppose, by cutting and reversing the wires in one of the interconnect cables to the subwoofer amplifier, (you can tell I am not into expensive interconnects, lol).

I have seen advice given where the negative leads are connected. Should I leave the negative leads unconnected in this configuration?

Just for the heck of it, what if I connected the two positve speaker cables together, and the two negatives, and connected the speaker between them? Notice I am asking first!

That advice about cutting the ohm rating in half was very important. Rats. On older amplifier/receivers likely to be picked up for cheap, it's a good idea to make sure they can handle 4 ohm loads, let alone 2 ohm!

A quick check of the subwoofers I mentioned shows that they are all 4 ohm! On the Peerless and NHT I could wire high power 2 ohm resistors in series, but I think I'll keep looking for a genuine 8 ohm. Any suggestions?

[Super]

Dont just twist the leads together. Its basically sending out two different electrical signals which can wreak havoc on the driver, and will more than likely destroy your amp, especially if the input is coming from a stereo source; i.e a left and a right channel. (This is basically the reason why its necessary to bridge an amp. If the leads could just be twisted together, then no one'd bother with an alternative method).

Theres a good chance that the amp may be able to drive a 4 ohm load though. Check the back of the amp near the speaker terminals for an indication. If not, Adire Audio's "Shiva" driver has dual 8 ohm voice coils, and is the driver I'm using in my current SonoSub project. The thing has a monster magnet, and appears to have very impressive build quality. I've also heard good price/performance comments on the Dayton DVC series drivers, again, with dual 8 ohm voice coils. The each channel of the amp could drive one of the voice coils without having to worry about impedance issues.

The Shiva can be bought from http://www.adireaudio.com, and the Daytons can be purchased from http://www.partsexpress.com. I haven't heard the DVC series, but from what I hear, they are better for home theater than music, whereas the Shiva was highly praised in all areas. The Shiva goes for $115 (often on backorder because of its popularity) and the DVC's go for about $120 each. (I'm assuming that you are looking for 12 inch drivers)

Good luck

By the way, I've found this to be the best audio forum I've ever been to. Almost all fast, polite, and courteous responses. Whats not to love?

[Edited by Super on 09-18-2001 at 12:00 AM]

[GRollins]

In order:
--Uh, well...no, you can't just reverse the wires in an interconnect. It doesn't work that way. One's ground and the other is signal. Running signal to ground and ground to signal results in the sound of silence.
--I'm not clear which negative leads you're talking about, interconnect or speaker. The negatives for the interconnects are probably already connected to a common ground in the preamp/crossover. The speaker will only have the one negative lead, and it should be connected to one of the positive speaker connections at the back of the amp. Connect nothing to the negative terminals. (Yes, I know this sounds odd, but that's the way bridging works.)
--Paralleling solid state amp outputs is not generally a good idea. (You can do so with tube amps, though. Chalk up another plus for tubes.) The usual result is smoke rising from the vents.
--You want an 8 ohm Titanic? Series two 4 ohm ones... I'm running mine in series-parallel at the moment, since I haven't had time to get twelve amps built yet.

Grey

[GRollins]

A quick addendum, as Bryan's post appeared as if by magic when I hit the enter key...
Bryan,
Yes, you could use a dual voice coil driver to blend channels, but it would be wasteful of power. Sometimes the signals would be telling the driver to go in opposite directions, so the net result would be for the cone to remain stationary, but still generating heat in the voice coil. Better to blend the signals ahead of time so that the driver doesn't receive conflicting impulses.

Grey

P.S.: Incidentally, wiring resistors in series with the driver to get the impedance up will work, but will negate the power gains from bridging the amp. You'll be burning wattage for no reason in the resistors. If it comes to that, just use one channel of the amp and let the other sit idle.

[GRollins]

This is like telephone tag...I hit enter and see that Bryan has done the edit thing...
Time for me to put on my Cassandra hat and utter my oft-repeated warning:
Be wary of foam surrounds. They tend to rot out in 5-10 years. Note Parts Express and others selling replacement foam kits--there's no need for replacement rubber surround kits, as they're stable.

Grey

[Thoth]

Elliot Sound Products has a project on their web page for bridging two amplifier channels together. It only works if the '-' output from the amp is ground, and if the amp can safely drive half the speaker impedance. It can be found at 'http://sound.westhost.com/project14.htm'. There are lots of other projects on his web pages, including a 300W sub amp.

Special note for subwoofer usage: Many amps (especially less expensive amps) seem to sound weak when driving a sub hard. I think that this is due to several things, including low power transformer, inadequate cap bank, and a barely adequate output stage.

Good luck.

[kelticwizard]

Thank you all for your replies.

I recommended a friend use one channel of an amp to drive a subwoofer and a technician claimed that it wore out the amplifier. As I know something of amplifier design, I could not understand that, but my friend decided to go with that advice. It is good to know that I can do that, after all.

I somehow managed to go to the Shiva Home Page and missed that it had dual 8 ohm voice coils. This is very useful.

Older amps really liked 8 ohm loads. A friend bought a Phase Linear back in the seventies-the Super Amp that had everyone talking-and it wouldn't carry 4 ohm loads!! For that reason, I generally prefer to put 8 ohms on older amps.

GRollins: when I say interconnect I mean the cables with RCA plugs that would go from the TAPE OUT of the main amp to the AUX IN or TAPE IN of the subwoofer amp. When I reverse the wires of one cable, would that not reverse the polarity of that channel? Anyway, it is just as easy to do it the way you describe, as I will add the dividing network before the output stage.

[kelticwizard]

Adding another Titanic or Peerless would not be a bad idea, but there are space limitations I was hoping to observe.

The basic idea is to see how close to big output at 16 Hz I can get with how little money and a little cleverness. Older amps and receivers in the 100 watt per channel range are available cheap at tag sales since everyone switched to remote control electronics. Maybe I might have to replace some output transistors, big deal.

The old Advent and AR boxes are also available for a pittance, often with speakers in them with rotted cones. Many other brands also made the traditional 2 or 2.2 cu. ft sealed box in the seventies, and the boxes were well made. I plan to cover the 12" woofer cutout with a 3/4" piece of plywood, and cut a hole for a 10" subwoofer. Also to put similar patches over any midrange or tweeter cutouts.

Finally, when I tune the box to 16 Hz, I should be about 10 db down at 16 Hz. I plan to put the subwoofer in the corner, which should bring that up 6 db or so.

If that does not "cut it", I plan to incrementally raise the tuning until I sense the right amount of "guts".

If I feel really adventurous, I might take a sabre saw, cut off the back of the box, and construct a "Passive Radiator Augmented" enclosure which is claimed to extend output lower.

What ever I get out of this, I will at least have the satisfaction of knowing that most of the money went to the subwoofer, not to ancillary parts like extra amplifier, crossover, etc.