My thoughts on the subject echo what Joseph and TheBoringPianist said, and especially what HeartKeys said about “satisfied” versus “same thing”.
From an action point of view, the top hybrids do a superb job of emulating the grand piano experience to build and maintain technique, because, for all intents and purposes, what you find in a hybrid is exactly what you will find in an acoustic. The only main difference is the hammers are replaced with weight-appropriate stoppers, and, in the case of Yamaha, the weight from the damper is missing - and the impact of those things is calculated for and of minimal influence overall.
Overall, though, it’s fairly identical in terms of action.
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Where the issues crop in is the sound. There isn’t a digital on the market that has a sound that matches an acoustic piano. Period. And that’s because all digitals use sounds that are a mere snapshot of an acoustic. Some are powered by a very simplistic sampling processes which just record 3 or 4 velocities per note (an absurd limitation of a piano’s capability and tonal/timbral variation which may not even capture the entire dynamic range), are not recorded in highest fidelity, and are haphazardly assisted by mediocre sound effects. Others use an extremely, extremely throttled version of “modeling” which may not even actually be modeling because true modeling requires a level of computing power that no digital has. Now, A.) for styles of music that stay in a very “present” and straight-forward tonal/timbral range, B.) for musicians who don’t need extreme nuance for their playing style/performing style/tastes, C.) for simpler styles of music and arrangements, and D.) for beginner/intermediate students of styles that do require nuance - for these use cases, the extremely nuanced and infinite-within-its-physical-range-of-capability sonic responsiveness of an acoustic isn’t necessary and/or may not even be desired. But for styles of music or pianists who require the near infinite nuance of an acoustic - there isn’t one digital that matches what an acoustic piano can produce.
That said, the use of VSTs help mitigate this because they solve that problem by contain a more complete and nuanced capturing of what an acoustic piano can do.
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Now there are still some issues.
RE THE EXPERIENCE OF PLAYING: None of them contain the action you’d find in a concert piano - both Yamaha and Kawai are using actions that would be from 5-foot/6-foot (Yamaha) or larger 6-foot (Kawai) pianos. Thus, the super fine responsiveness that comes from the very long keysticks and specialized regulation on a concert piano will never be present on the current hybrids. So, even if your VST sounds like a 9-footer, your hybrid will never truly play like one.
Also, not all hybrids are created alike. Some prefer Yamaha Avantgrands, some prefer Kawai Novus. Reasons and differences presented vary from person to person.
Also, in my experience, and in the testimony of at least two others, I’ve seen reports about hybrids having a bit of difficulty creating the very lowest dynamics - this may be due to how the optical sensors work or are calibrated.
RE LIFESPAN AND REGULATION: Typical regulation is pretty much impossible as they can’t be lubricated normally (lubricant would disturb the optical sensors) nor can the hammers be altered (unless you willing to shave/shape/drill-tiny-holes-through metal/plastic). Most technicians won’t touch them for that reason and those that will are still limited in what they can do. You’re pretty much stuck with factory regulation, which basically doesn’t exist. The shelf life is also a fraction of an acoustics: while the acoustic action inside of it could last for decades - the digital/electric aspects will not - eventually the optical sensors, motherboard, circuitry, computer inside will stop working, replacing the parts will become more and more expensive out of warranty, and then impossible when production stops. An acoustic has no such issues, and can last for centuries so long as you take care of it, maintain it, and refurbish it when necessary.
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I think HeartKeys made a very good point about “satisfy” versus “the same”.
For some uses cases, a hybrid without a VST may well satisfy, but it won’t be the same as an acoustic or equal what an acoustic can do - for these use cases, this may not matter or even be detectable.
For other use cases, ONLY a hybrid without a VST probably won’t satisfy. Partner them together, and you can almost fool yourself into believing you’re playing a $200,000 grand. And it will suffice as a practice instrument, and maybe even a performance/recording instrument. Almost.
But it will never full equal or replace the experience of playing on a finely regulated concert instrument. You may be SATISFIED. But it will never be a replacement, and when you sit down at the real thing, if you have the demands/ear/experience, you will notice.
For most, the hybrid is a compromise. They want the most realistic action, but can’t have a loud acoustic in their living space, so they get a hybrid as their main instrument or their practice instrument. Or they don’t have space for an acoustic. Or they can’t afford an acoustic or find a suitable one in their budget range.
For my own use case, I live in a shared space where the walls are made of paper, so an acoustic piano is completely undoable - my neighbors would be after me with pitchforks. Moreover, the type of piano I’d want is out of my budget - I’m an independent artist surviving off grants and fellowships - my budget is strict and I live a rather spartan life and budget months in advance to afford nice things for my arts ventures. So even if I had space for an acoustic, I couldn’t afford anything but a bad used one at best, unless I got really lucky.
Hence hybrids being a solution for me (although I’ve since moved back to a digital that is somewhere in between a hybrid and a regular - the Casio GP series).
But if I had the money and the living arrangement - psh, I’d have an acoustic. And quite frankly, I think most who have a hybrid would agree. It’s the compromise we make based on our circumstance.
And it’s a superb compromise one. And even a willing one. But, I think most of us were all aware that it was a compromise regardless, and that in exchange for having most of our needs met, we’d sacrifice the most demanding of them.