I think dual mono, for the most part, is a marketing term. I may be wrong, but to me, a dual mono amp is something like a BAT VK-500. But amps like that are pretty rare.
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The issue is power supply modulation, which can be very difficult to prevent! If the supply has noise (IOW, if the action of the amplifier has caused fluctuations in the power supply) the result can be intermodulations with the other channel running off the same power transformer.
This is why monoblocks usually sound better than dual mono, and why dual mono usually sounds better than a plain stereo amp; intermodulation distortion is pretty audible.
Thank you Atmasphere! If a single power transformer is used, are separate secondary windings for each channel used in all stereo amplifiers or is this configuration in the dual mono or partially dual mono camp? I'm just trying to understand why some manufacturers hype separate secondary windings as complete power supply separation for each channel, despite a solitary power transformer.
Atmasphere is right. My Vitus SIA-025 integrated uses a single Vitus-designed UI-core potted "floating" transformer with separate windings for the left & right channels. Isolating the transformer in a potted, "floating" magnetically sheilded enclosure certainly helps. Also being a true balanced design with well regulated power supplies helps improve s/n ratio. But yes, as a rule of thumb mono blocks usually sound better due to separate power supplies, reduced noise and lower intermodulation distortion as Atma said.
A friend whose ear I trust owns a Vitus SS-102 and Magico Q3's. He auditioned the Q3's with both the SM-102 monos and SS-102 stereo amp & preferred the SS-102 which sounds warmer and more tube-like, yet with similar resolution. Though the SS-102 runs separate transformers in a twin mono, true balanced design. Suffice to say, there are exceptions as mentioned.
If a single power transformer is used, are separate secondary windings for each channel used in all stereo amplifiers or is this configuration in the dual mono or partially dual mono camp? I'm just trying to understand why some manufacturers hype separate secondary windings as complete power supply separation for each channel, despite a solitary power transformer.
It is better to have dual windings than to have both channels share the same supply. The weakness of this approach is that the two supplies still can 'talk' to each other through the core and primary winding of the transformer, although certainly there will be less noise than if the two channels shared the same supply. IOW this is a step to better performance; better yet would be dual power transformers.
There are a lot of variables at play here so nothing can be considered case in concrete; I am merely stating how this works strictly from the point of view of power transformers. There can be other considerations in a design that may negate this issue- ultimately audition is the best course of action.
Thank you again, Atmasphere, for your complete and well explained answer. Given other considerations in design being well implemented, monoblock amplifiers and stereo units with dual power transformers should
lead to less IMD and possibly better sound. And yes, audition is a must, as "better" is quite variable amongst audiophiles.