06-08-2020, 09:04 PM
Perhaps surprisingly, they're much the same power; the RP25 might well be 18V, but it uses a 15Ω speaker, whereas the Roberts is 5Ω. But where the Hacker should score, of course, is the much bigger 5" by 8" speaker, which should be more efficient than the Roberts one, so more SPL for your electrical input.
The RP18 uses a 30Ω speaker. That amplifier doesn't produce half the power of the RP25 because, of course, the lower current means a higher voltage swing. The spec says 1.2W, whereas the RP25 claims 1.5W. Obviously, that difference is pretty much inaudible in practice. And back in those days, you'd see similar or bigger variations in loudspeaker sensitivity from nominally identical units - consistency wasn't brilliant back then in the hi-fi market, let alone in the cheap transistor radio space. Remember how BBC loudspeakers had tweeter level adjustments in half-dB steps? And they're all set differently. But talk to people who make BBC-licensed speakers today, and they'll tell you that they rarely if ever need to move them away from the 0dB position - the new version of the Audax HD 13D34H, for example, is impressively more consistent that the original.
I'll try to dig out my two R600s to how they compare to the units used by Hacker (there were 4 different types used in the RP25 - the A models usually got a silver Celestion) - I'll stick them in front of my loudspeaker measurement setup. It might take a while, as my Roberts sets are in a part of the attic that isn't very easy to access, but I'd quite like to find my R600s as a result of this thread - not least to see if the Lockfit installation is as bad as you reported earlier...
The RP18 uses a 30Ω speaker. That amplifier doesn't produce half the power of the RP25 because, of course, the lower current means a higher voltage swing. The spec says 1.2W, whereas the RP25 claims 1.5W. Obviously, that difference is pretty much inaudible in practice. And back in those days, you'd see similar or bigger variations in loudspeaker sensitivity from nominally identical units - consistency wasn't brilliant back then in the hi-fi market, let alone in the cheap transistor radio space. Remember how BBC loudspeakers had tweeter level adjustments in half-dB steps? And they're all set differently. But talk to people who make BBC-licensed speakers today, and they'll tell you that they rarely if ever need to move them away from the 0dB position - the new version of the Audax HD 13D34H, for example, is impressively more consistent that the original.
I'll try to dig out my two R600s to how they compare to the units used by Hacker (there were 4 different types used in the RP25 - the A models usually got a silver Celestion) - I'll stick them in front of my loudspeaker measurement setup. It might take a while, as my Roberts sets are in a part of the attic that isn't very easy to access, but I'd quite like to find my R600s as a result of this thread - not least to see if the Lockfit installation is as bad as you reported earlier...
