It's pretty rare that you CAN bridge a 2 channel amp. Unless the amp has a mono/bridge switch or jumpers, you can't. Most do not.
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B_limo, Only single ended amplifier can be bridged. Single ended means that one of terminals is inactive (ground). Some amplifiers have differential outputs and cannot be bridged (being already bridged). Bridging them will work but won't change output voltage - therefore won't change output power. In order to bridge amplifier phase of one channel has to be inverted. Bridging doubles output voltage and therefore quadruples output power but has some disadvantages:
- might not be able to drive some speakers since it requires double current hence it is the same as loading it with half of impedance. My amplifier, for instance, wouldn't be able to drive half of my speaker 3.8ohm min. impedance since it is specified at 3ohm minimum.
- second problem is that each amp would have to deliver twice power being not designed for that (power supply, heatsinks) and will likely overheat.
- third problem is amplifier's output impedance that will double and your speaker electrical damping will be twice worse.
- It might damage amplifier and will most likely void the warranty.
Even if you limit loudness to use only nominal power of each amp (double vs. quadruple) it is not worth it IMHO since it will only make it 22% louder. Amplifier's that are designed for bridging have most likely phase inverting switch plus oversized power supplies and heatsinks.
Does this line of reasoning hold true for a dual-mono design with separate power supplies, transformers and heat sinks? Would impedance issues still be a factor?Yes it does still hold true, although an amplifier like that may tend to be better able to tolerate the increased current.
It's pretty much a moot question, though, because if the amplifier isn't designed to be bridged it won't provide the phase inversion that is necessary.
Regarding paralleling, as opposed to bridging, the near zero output impedance of a solid state amp would cause essentially unlimited current to flow between the outputs of the two channels if for any reason their gains were to ever become significantly unequal, or if the input signal to one channel is ever absent for any reason. The result would stand a good chance of being a cloud of smoke. Presumably and hopefully the Mac and Bryston models NGJockey referred to have specific provisions in their designs to address those possibilities.
Tube amps are more likely to be capable of being paralleled, because their higher output impedance would provide some degree of current limiting in those kinds of situations. But I still wouldn't parallel a tube amp without a specific ok from the manufacturer, and I'm not sure I would do it even with an ok.
And since paralleling would provide a significant increase only with respect to current capability, and not with respect to voltage swing capability, its potential benefit would be limited to situations where the impedance characteristics of the speaker require more current than the amp can comfortably provide.
As Al stated dual mono might be "better able to tolerate increased current" but impedance issues are still there. One Audiogoner drives, with great results, 1 ohm planar speakers (Apogee Acoustics Scintilla) with 2ohm minimum rated class D amp (H2O M250SE) but it is an exception. These speakers are only 76dB efficient and require a lot of power but amp somehow can manage that. He worked on this project with designer of the amp.
Thank you guys. My reason for asking is that I've just had 3 Acoustat TNT200 stereo amps rebuilt with mono capability via internal switch. The work was done by Roy Esposito who is one of the design engineers of the amp. He assures me they are still stable at 4ohms in mono. It's not that I don't trust Roy, but you can't have too much consensus.
For the most part any transistor amplifier can be bridged, you need to run the output of one channel to the negative side of the input differential pair. Just follow the feedback of the amplifier to the diff pair, you will have to reduce the voltage from the output of the leading channel you will have to play around a bit to match the gain. Start with a 15K ohm 1/2 watt resistor and measure the amplitude of each channel, adjust the resistor until you have equal gain. Be aware the amplifier should be stable enough to drive a 2 to 4 ohm load in stereo if you intend to drive a 8 or 4 ohm load. The input will be the leading channel the one you are driving the back side of the diff pair with. The plus lead will be the leading channel and the other output will be the minus. This will now be a floating ground and can not be used with speakers like the polk SDA series that interact with between the umbilical cable, this will destroy the amplifier