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.

 

[Super]

In my opinion, an enclosure is one thing you'd be better off making yourself. Personally, I'm not too fond of particle board for use as a sub enclosure, which is all too often the case with older cabinets. At my local home depot, 2 4x8 sheets of MDF can be purchased for 20 bucks, with plenty of scrap left over for internal bracing. A little wood glue, a table saw, some screws, and a port or piece of pvc pipe and voila. You have a much sturdier enclosure with less resonance. Its up to you how much money you want to save, but in my opinion, the enclosure for a subwoofer is too important to cut corners with.

(Oh yea, if you go with the Shiva, don't rule out the possibility of a sonotube enclosure. Mine is almost completed and is looking pretty good. But if you use the Titanic or Peerless drivers, dont try a downfiring design. They werent designed for this configuration, and gravity will cause the driver to sag.)

[Edited by Super on 09-18-2001 at 10:30 PM]

 

[GRollins]

Wear out the channel?
Man, I hate when that happens!
(Ahem...)
Even with the Titanic/Shiva/whatever, you're not going to get to 16 Hz without some electronic help (bumping up the bottom end by one method or another). Those drivers are good for -3 dB in the 25-28 Hz range. -10 dB is pretty much useless. It's something the marketing people came up with so companies could claim ludicrous frequency specs. -3 dB is already half as loud. -10 dB...fergitit.
Corner placement does not give flat response. You're trapped between 1/8th space radiation patterns and a crude horn. Ugh.
In my mind's eye, I'm looking at an old (large) Advent. I'm thinking it's about half the volume you'd need to get one of those drivers down to the upper 20 Hz range. With a smaller box, you're going to roll off early, with a horrendous bump right before rolloff. Those boxes weren't so well constructed that I'd go all rubbery in the knees wanting to gut one to use for a subwoofer. Bear in mind that 3/4" MDF goes for less than $20 a sheet. You could build your own enclosure, at the proper volume, probably with one sheet.
If you're thinking of a 10" driver, you might--maybe--be able to get a box that's within shouting distance of a properly sized enclose, but either way, you're going to wreck havoc on the Thiele-Small response.
(And, in passing, you hit the nail on the head when you said rotted drivers in those old speakers. Why? Foam surrounds! Like I said...beware.)
You're likely to have trouble mating a passive radiator to one of those drivers.

Grey

 

[Super]

I'm not sure if you noticed, but the Adire Audio website also gives examples of buyer's projects, and some reccommended designs of their own. They can be rather helpful when designing the enclosure, considering some of the designs have already been tested and tweaked themselves.

 

[kelticwizard]

Grey:

"Corner placement does not give flat response. You're trapped between 1/8th space radiation patterns and a crude horn. Ugh."

My understanding is that corner placement is no good for midrange and highs, but recommended for bass. How bad are these corner irregularities, and are they markedly improved by movng them to another spot along the wall, (I assume you don't put the subwoofer in the middle of the floor)?

 

[kelticwizard]

Super:

Did a search for sonotube. Do you mean the round cardboard tubes used for puring cement? I have a pair I was going to use for another purpose, but didn't.

I thought of them as loudpeaker enclosures, but it seems to me the that although the cylinder shape gives added strenth, they would need re-inforcing. It is easy enough to slice another tube lengthwise with a knife, and fit the second one around the first. But I woonder how many layers are necessary?

It also seems to me that you can cut a 45 degree angle near one end, turn it around and have a 90 degree angle at the bottom, where you can fit the speaker. This avoids up or down firing woofers. Then construct a tiny stand to hold the thing upright.

Just spitballing a few ideas here.

Where can I read more about your sonotube project? Do you have a link I can click on?

 

[kelticwizard]

Thoth:

Re-checked out the circuit you gave on that link, thanks.

Have you ever heard of anyone improving the performance of a mid-fi amp or reciever by adding larger capacitors to the power supply? Often wondered about that. It would seem easy enough to do.

 

[Super]

Yes, sonotube is the forms used for casting concrete pillars. Sorry, but I haven't published the sub online. Actually, no additional bracing is required, because of the tubes shape. You can push in on one of the sides, but when internal forces from the driver push out on all areas inside the tube evenly, the tube wont flex, making it a very ideal enclosure. What some people do is add a circular insert inside of the tube, although it isnt necessary. I would suggest doubling and tripling up the endcaps in order to provide extra weight to keep the sub/speaker from vibrating on the floor. You will need to line the inside of the tube with foam however. If you don't, it can resonate and give off a ring, similar to a tapping the side of a bell or wine glass.

Your idea would work, but it would be even easier to just glue a face or gasket onto the area where you made the 45 degree angle cut, and let the speaker stand on the 90 degree angle. This site has links to SEVERAL sonotube projects which I found extremely useful: http://terryctheater.tripod.com/shivaphotoalbum/page12.html

If you have any more questions, feel free to ask.

 

[GRollins]

Yes, sonotube is the paper tube stuff, available in a number of diameters and thicknesses. Obviously, for speaker use you want it as thick as possible. Another similar possibility is PVC pipe. Some people use "T" joints with PVC to mount the drivers. For a 10 or 12" driver, you'll need to go to the larger PVC they use for sewer and such. I've got the stub of a 24" pipe that has 1" thick walls. Pretty stern stuff.
Yes, out in the room is the preferred placement for subs. People frequently speak of room reinforcement and corner placement as though they are magic wands. They aren't. There ain't no such thing as a free lunch. If--and I'm assuming, here--your goal is something like high fidelity and flat response, you'll be better off placing all speakers away from walls. Yes, subs are largish cabinets and don't always fit into peoples' decor, etc. Sometimes compromises must be made. But corner placement is to be avoided for nearly all speakers (and the speakers that are designed to be placed in corners are generally best avoided)--two walls at a 90 degree angle do not a horn make.
The best rule of thumb I've seen for speaker placement is one third of the length of the room from the front wall, and one third from each side wall. That's for starters; fine tune a bit, as necessary. My current speakers are one third down the room, but are spaced somewhat further apart. They're able to sustain an image across that width. The setup I was using before could not.

Grey

 

[carltonmdb]

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

Apologies for resurrecting a 17 year old thread, but this response has 80% of the info I need for an experiment, possibly more with clarification:


I have a Fender stereo guitar solid state amp (M-80) that I want to use on a single driver (EVM 18B) in a sub enclosure.



Creating, and subsequently inverting one of, a stereo signal is reasonably easy with one of these in the signal path. Alternatively, the sub out on the signal processor can be balanced or not; I'm thinking that I could create a "y" cable splitting the 2 positives into separate signals (shared negative), resulting in out of phase signals.



Where I am stumbling is the speaker hook-up; As I read the original post quoted above, both positives go to positive terminal on driver, and negatives just float. Should they tie together, via wire nut for example? Should the negatives get tied to a chassis ground, or something? And is there a way to determine if the amp topology will disagree with this wiring, again as referenced in OP?


Thank you!

 

[weltersys]

1)Where I am stumbling is the speaker hook-up; As I read the original post quoted above, both positives go to positive terminal on driver, and negatives just float.
2)Should they tie together, via wire nut for example?
3)Should the negatives get tied to a chassis ground, or something?
4)And is there a way to determine if the amp topology will disagree with this wiring, again as referenced in OP?

As you already know, for bridge mono operation the mono L/R input polarity must be reversed.
1) The "+" from the right amp output (1/4" Tip) connects to the speaker "+", the "+" from the right amp output (1/4" Tip) connects to the speaker "-".
The amp "-" (Sleeve) connections are not used for bridged mono operation.
Each amp side will "see" half the speaker impedance, so the single 8 ohm speaker will be equivalent to a four ohm loads on each amp side.
2) No.
3) The amp "-" speaker outputs are tied to chassis ground if I'm reading the schematic correctly.
4) Check that the two "-" (Sleeve) connections do share a common connection, they should read very near 0 ohms resistance between them.

If the amp has a stereo chorus, it might sound like a tremolo, as in the volume of the signal will rise up and down.