Hey germanium, you never did tell us the source of this mod. You mentioned to me a few times about it, but I never found out where it started. If you just Google "ZXR Mod" on my computer, the first hit is this thread and I don't see any other hits related so I am curious just where this did originate from.
My own idea.
Electricity usually takes the path of least resistance however in this case the load is shared through bass & lower midrange.as the impedance is still fairly close to each other however as you move up in frequency the metalized film takes more of the duty over as it's output impedance drops much faster than the electrolytic caps does.
Electrolytics are far from a perfect cap. A perfect cap output impedance would drop to the point that at some frequency it would be like a superconductor. No Cap is perfect however all have some inductance which restricts their high frequency potential. The point at which the capacitance meets inductance is what determines the resonant frequency of the cap. Virtually all caps resonate at some frequency & usually well above audio frequency. electrolytics also resonate but the have an odd property in that their resonance is highly damped. When something is damped it gives up energy I.E. it gets turned into heat. This tells me that something is getting lost & it may not be just heat It may just be signal that is lost as well, particularly soft sounds that form the background of the music.
When one talks of the damping factor of an amplifier we are talking of controlling the driver by providing a means of braking. That means of breaking is by shorting the back electromotive force through the amps power supply. Guess what this uses some amplifier power to do & this power is given up in the form of heat. Same thing happens with electrolytics. This does not happen with metalized films to even near the same degree & consequently they will resonate quite a lot at their resonant frequency. This lack of internal damping in the metalized films allows them to pass all signals better below their resonant frequency & since their resonant frequency is very far above the audio range they pass the signal very well without adding anything.
As you can see metalized films are pretty much the ideal cap for audio
When used in the power supply they help maintain a low output impedance across all frequencies of interest. With transistor amps it is as important if not more important to maintain a power supply that has a low output impedance than it is to maintain rock solid voltage which is why most really good sounding power amps are still unregulated in the power supply. The best ones are regulated though. I'm just saying it is possible to have a good sounding amp without rock solid voltage regulation.
The interesting thing is these mods work well in regulated amps as well as unregulated & can be placed either before or after the regulators & have the identical same effectiveness. Why these mods work is that the power supply is in the direct signal path & the sound effects are the same as replacing an electrolytic coupling cap with a metalized film one.
The reason tube amps sound so good or at least part of it is that the power supply is largely outside of the signal path & is only supplying a constant current to the output tubes through the output transformer especially in a pure class a1 push/pull transformer coupled tube amp.
These mods bring you much of this type sound the you get with tube amps to transistor amps without the tube problems of old like soggy sounding bass.
This taken from a PM I sent someone else here
Edited by germanium - 4/11/15 at 10:42pm