Bridging power amps

[stevekendal]

I have a 200+200watt stereo power amp, originally intended for disco use that I use to play my bass guitar thru. Each has its own power supply, but they are within the same case. The mega big bass guitar amps have a "bridge mode" where the two amps feed one output. Could I just parallel the outputs so as to feed one output, or are ther other technical considerations? Steve.

[pinyoro]

Unfortunately you can't just parallel the channels. When you bridge an amp you invert the input signal of one channel and connect the speaker accross the "hot" terminals of the 2 channels. So there are technical considerations Please see link below with more info:

Bridged and paralleled amplifiers - Wikipedia, the free encyclopedia

[AndrewT]

A bridged amplifier puts out double the power into double the load impedance.

Or put another way, a two channel 200+200W amplifier driving a pair of 4ohm speakers is exactly the same as a bridged pair of 200W amplifiers driving an 8ohm speaker, i.e. 400W total into 8ohm total.

If the load has two drivers/speakers then use one channel to drive one speaker and use the other channel to drive the other speaker.

You don't need to parallel or bridge to get the same output power.
You gain nothing by bridging.

[bob91343]

I did that recently. I used an old stereo amplifier, 8 W per channel, as a guitar amplifier. I bridged the output and now am getting around 25 W into 8 Ohms.

In order to accomplish this, I built an inverter using an opamp I had laying around. This way the second channel has an inverted waveform and I get about double the voltage. Into the same load impedance, I would get four times the power except for the limitations of the power supply. So I figure I am getting more like triple.

[lgreen]

You have spoken about "bridging" amplifiers but then made the suggestion of playing them in "parallel" which is different. So "bridge" and "parallel" are kind of opposite concepts, but related.

You want to run the amps in "parallel" and probably this will work (not sure of the specific amp). but you will most likely need to put a .2 to .4 ohm resistor in series with each + terminal do to so in order to prevent the 1 amp from blowing up the other amp.

"Bridged" is another, different, idea.

[ervinl]

What if we bridge 2 amps, but 1 of amp input source is just connected to ground, instead of get the inverted phase of the next amp. Will we still get more power?

[AndrewT]

Originally Posted by bob91343

I did that recently. I used an old stereo amplifier, 8 W per channel, as a guitar amplifier. I bridged the output and now am getting around 25 W into 8 Ohms.

this is not the correct way to look at the effect bridging has on the power and load specifications.

When a pair of bridged amps are compared to two single amps, the loading and voltages are quite different.

If your single amp is specified @ 8W into 8ohms, then bridging a pair of them gives you two times 8W into two times 8ohms, i.e. 16W into 16ohms.

If you now put an 8ohm load on the bridged pair you are taking the amplifier outside it's specified load range and as a result taking excess power and current that is not specified for the amplifier.

The reason we have to look at it the other way is that each half of a bridged amp sees only half the load impedance. Load bridged with 8ohms and each half thinks it is driving 4ohms and currents appropriate to 4ohm loading will flow.