Bridged amps vs stereo vs monoblocks.

I don’t have any real issue, but I’m curious about how the bridging of an amp functions. I have a 80 W stereo amplifier, which can be bridged to provide 220 or so power into a load. This amp also has a hefty power supply which makes it work beautifully with my Magnepan’s. However, I used to have two of these amps bridged.  One of which I lost due to it breaking so i replaced with an unbridged stereo amp, which I’m only using one channel. So what a strange hodgepodge of connections, right? It sounds great and I am have no problems but I’m curious if I was doing some thing that is easily changed to provide benefit. Mixing two different amps for example.  Or I read when an amp is bridged it is sensing a four ohm load as a two ohm load and therefore what does that even mean? Also, using one part of a stereo amp is odd, but does that change anything, especially if there’s one power supply? Curious, if there any principles I could learn about this from those more familiar with those equations thanks!


Although some here would be aghast about using different amps for each channel, if it's sounding good to you enjoy. The difference could be smaller than the room sound for each speaker.

A bridged solid state amp will see half the speaker impedance and output twice the current into the speaker load. The penalty is hotter running conditions for the pair of amps. Tube amps cannot be bridged. Instead a stereo tube amp can drive one speaker by paralleling the outputs of both channels. This gives twice the voltage and current into a single speaker. A McIntosh 275 paralleled as a mono amp will output 150 watts and twice the current.

A bridged GAS Son (80wpc) will output 240 watts into 8ohms and run considerably hotter. Most solid state amps will triple their wpc rating when used as a mono bridged amp.

@dain : I recommend changing the bridged amp back to stereo and using only one channel for one speaker. As you are already  doing with the other amp and speaker. Because each two-channel amp has a single power transformer this will benefit both amps by not stressing the power supplies.

So we all know V = I * R.

Bridging a 2ch stereo amp to mono doubles the output Voltage, halves the load Resistance each amp channel "sees", and then because of this each amp ch also *tries* to supply double the current (I), but how much of that it can actually supply depends on the design of its PSU and output stage.

The voltage is doubled by having one channel amplify the +V, and the other ch amplifies its inverse -V. Thus the difference makes V - (-V) = 2 * V, though each channel by itself does not see more than 1 * V in magnitude. However, each channel does try to double the output current, because they are each driving half of the same load (one 8 ohm speaker), rather than each driving two wholly different loads (Left and Right 8 ohm speakers).

The inversion for the -V side necessitates some kind of circuitry to do the inversion. This itself is not always without distortion artifacts. And the fact that the amp (both channels) are now operating into half the impedance load will also affect its distortion products, and thus sound. So if you value stereo image symmetry, it’s not great to run one side with a bridged amp and the other with half a stereo amp.

As for the power output of a bridged amp -- if you have an amp which "doubles down" its power into halved impedances, then you will net 2 * V and 2 * I, and Stereo Power = V * I, so you get Bridged Power = 2 * V * 2 * I = 4 * Stereo Power. Fully 4 times the power! Of course, it’s no free lunch. You’re driving the amp much harder and hotter in the conditions. Almost all amps, even "high current designs" will show their current limitations to some degree, and thus will not rate a full for a 4 times power factor.

I’ve tried bridging with a pair of Solid State stereo amps I really like (Phison A2.120SE). I found that their sweetness which I enjoyed in stereo mode largely disappears in bridge mode. So I kept them in stereo mode. It was better to run them in either bi-amping or single amp stereo mode configurations.

I also have a pair of stereo tube amps with mono mode, but in this case the 2 channels are paralleled, not bridged (it goes this way because of the output transformers). It only doubles the power (at maximum), but unlike bridging it only improves the sound quality!

Thanks @jasonbourne71  so if I understand I just don’t bridge to avoid stressing the power. That’s interesting a as to what stressing does. Shorten the life of the components? Isn’ta power supply just a coil of wire? Uses more energy? @mulveling i think you’ve touched on something. However this works it sounds great. I’m sure if I fiddle with it I may lose something, but such is this hobby. A bit of science and a bit of voodoo. 

If you do as jasonbourne71 suggested, and switch the bridged amp back to stereo, and use one channel of each amp per speaker, you may find that it sounds better in an unbridged mono block configuration. You’ll retain the benefit of the separation of the mono blocks, and the power supply will have less responsibility. It may be subtle and is hard to predict how audible it will be, but I think it’s worth a try.


Your original setup, two identical STEREO amps bridged MONO makes sense if more power was needed. A great solution if you start out with one, change your speakers to inefficient ones.

Balancing volume L/R was an issue (as is any stereo amp with separate Gain for L/R like my McIntosh mc2250. Happily, full gain matched (as advised by McIntosh: use preamp for volume). That MC2250 tested 305 wpc within specs, dead silent. I changed speakers, back to very efficient horns, sold the SS, moved back to tubes.

Lack of remote volume is why I finally changed from my wonderful pair of MONO amps (fisher 80az) Steve at VAS checked them for me pre-sale and made me sell them to him!

Now Stereo Cayin Tube AT88T, with remote volume behind a vintage McIntosh tube tuner/preamp mx110z.

As I understand it when you bridge an amp you will double its output at 8 and 4 ohms, but not 2 ohms. If your speakers dip to 2 ohms, depending on your speakers design (and listening levels?) their output potential (sound) may be compromised. FWIW

As I understand it when you bridge an amp you will double its output at 8 and 4 ohms, but not 2 ohms. 

That is not always the case - it depends on the amp. If you have one with a sufficient power section it will 'double down'

I own a pair of amps that put out 40 W into 8 ohms stereo, and 230 W into 1 ohm. I run them in bridged mode and they put out 160 W into 8 ohms and 500 into 1 ohm loads.  I am not using them with speakers that require that but when I bought them they were happily driving one of the harder speakers, a pair od Apogee Scintillas in 1 ohm setting.  The poster would have to research the amps he is considering to know whether they will be happy with a particular load.

You are correct, though, in thinking that it isn't always a strict doubling of power. My bridged amps put out 160 - 280- 400 - 500 W into 8 - 4 - 2 - and 1 ohm loads.

Personally I like a really good stereo amp or monos.   Buy a more powerful amp on the first place.  

I used to do the horizontal bi-amp with an affordable, more powerful amp driving the bass and a quality Krell integrated driving the top with ok results, but I finally broke down and bought a pair of Mc611 monoblocks with a dedicated pre/dac and can appreciate the difference. I would do what I was comfortable spending which probably cost me more than had I just done it right the first time,  oh well.