Simple Amplifier Bridging
If you've ever wanted to bridge a stereo amp into a monoblock, it's not that hard to do. You just have to feed the two channels out of phase and then connect the speakers across the two red binding posts on the back. The speaker grounds will not be used.
This is called a Bridge Tied Load, and provided the amplifiers are up to it, it can give you a power increase up to 4 times.
This is the spice schematic...
I doesn't get much simpler than this one!
You connect the Left and Right outputs to the inputs on your stereo amplifier. And feed it from one channel of your pre-amp's line out. Of course you need two of these, and two amplifiers, for stereo.
The circuit is very stable and quiet, but it needs to be in a shielded enclosure. An Altoids tin works a treat. It can be run on 9 volts either from a battery or a wall wart and uses almost no power. I'm partial to stealing power from the host amplifier and even hiding this small circuit inside.
Here are the outputs...
The little inverter is unity gain, clips at about 2.5 volts, so there's plenty of headroom. The frequency response is down .5db at 4 hz and flat from 10hz to over 100khz.
This basic circuit has been the focus of several projects I have been toying with for a while.
One caveat that the neophyte must know is that bridging will only work for single ended style amplifiers, where the black, or negative teminal is referenced to system ground. If the output is already bridged, you won't be doing anything that isn't already done. Check the literature for your amp, Google it, or call the manufacturer for verification. Alternately, you can use a multimeter in low ohm/continuity range and check between the speaker negative (black) and input ground (outer ring of RCA or the sleeve of the standard TRS 1/4" or 1/8" - the long barrel at the back of the plug) to see if it is very low to no resistance. If that is the case, it is most likely to be a good candidate. If the resistance is very high to reading open, it is most likely already bridged. Please note this is a generalization and not all amps are built the same. Always 100% verify first.
Excellent advice... thanks for jumping in.
I agree, this is really only for single ended amplifiers that do not have built in bridging capability. On amplifiers that are already BTL outputs it is very possible to damage the amplifier.
A very quick check is to do a resistance check across the Black binding posts with the amplifier turned off ... if they're connected together it's almost certain the amp is single ended. If you get a high resistance it may already be in BTL mode.