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This is something that can catch out the unexperienced keyboard player or buyer - I'm using "keyboard" to mean any generic keyboard instrument. Surely "Progressive Hammer Action" and "Balanced Weighted (Hammer) Action" keyboards are six and half a dozen. Unfortunately, you would be very much mistaken!
Progressive Hammer Action Keyboard
As you move through the various brands, some use something called a "Progressive Hammer Action Keyboard", often abbreviated to PHA. It is the more common type of keyboard action to be found on stage pianos and more so on furniture type digital pianos. It basically means that the weighted hammer action starts heavy for the lower notes and progressively gets lighter as you move towards the top of the keyboard, just like the action on a real piano. This is the more desirable option among pianists as a progressive hammer action is a salient feature of any piano.
Most popular brands sporting a PHA would be Yamaha in their 'P-series' and in their Clavinovas, Roland's digital pianos use a PHA throughout all their latest stage pianos and digital pianos, Korg with their SP series, Kawai, Casio, Classenti.... to name a few. Yamaha, Roland and Kawai have at least three variations to their graded progressive hammer action keyboards, obviously installing the better ones on their more expensive models where they also include their highest quality voices.
Weighted Hammer Action Keyboard
This part is harder to describe because there are terms used loosely by some brands to indicate a weighted keyboard action but not what I am mentioning here. I'll mention a few of these first of all:
“Weighted Action Keyboard”
Be careful with this one and get clarification as to what this means. Generally a weighted action is used on cheaper models within a product range, usually within the synthesiser bracket. It is not used very much now because it is too vague. It usually indicated some sort of weighted or even semi-weighted keyboard action. It is usually very impressive to play and for pianists who are looking for the quick and expressive piano-like response from this keyboard would be disappointed. They can feel as light as say the waterfall keys on a Hammond B3 or slightly heavier. These are sometimes called “semi-weighted” actions.
It’s also worth mentioning that Yamaha used a SFX keyboard action on their Motif XS6 and XS7 models which is lighter than a balanced hammer action but certainly heavier than what I have been describing here.
“Balanced Hammer Action"
Balanced Hammer Action (BHA) or what is sometimes referred to as “Weighted Balanced Hammer Action” is more common on synthesisers but not always. Most keyboard players like the feeling of some kind of resistance when they play, but not to the extent of the PHA keyboards. The main difference between a weighted and a hammer action is the presence of a hammer mechanism underneath the key, adding more weight, resistance and response from the keys. Some of these hammer actions also allow for an addition expressive feature called “after touch” (mentioned elsewhere in the FAQ section). BHA keyboards are not graded so as to allow for versatility as the musician moves between voices. Playing horns or a Hammond B3 emulation on a PHA would be very weird and unnatural. For pianists wanted to use piano voices on a BHA keyboard action, I’m afraid they will have to compromise on order that the vast majority of the other voices can be accommodated.
As mentioned earlier, in their infinite wisdom, a couple brands use a BHA on their stage pianos and not a PHA as you might have expected. Personally, I feel this is a strange idea considering that musicians want something as close to a piano as possible and a PHA would be one of those vital features – but there you have it! Make sure you pay attention to the use of these terms because they are not always used clearly or explained properly by those carrying out the demonstrations in music shops.