18-04-2023, 09:15 AM
I don't see a problem with the thermal cutout mechanism. The supplies are only +/-10V, so bringing them down to zero does no harm beyond increasing power dissipation in R21 and R23 (from 1.125 to 1.68 watts). Given that the thermal switch is only a single NO pole, how else would you have done it?
Yes, it would have been better to have included an output relay that was released by the thermal cutout - and that would allow the addition of a DC offset protection mechanism to protect speakers from a faulty amp. But such things were almost never seen in UK designed hi-fi amps throughout the 70s and 80s. A shame, but the designers I know cite penny-pinching accountants - all too common in the UK in my experience. In fairness, it is awkward to find a suitable relay with the requisite DC current breaking rating and a high voltage coil - as such, you end up needing an additional winding on the transformer, or wasting heat in dropper resistors, so you can understand the temptation to drop it. Remember, woofers were much, much cheaper back then in real terms. There is an output fuse, which is far from perfect, but much better than nothing.
As audio amps go, this is actually a relatively complicated design - not sure why people are so quick to say it's basic. Meridian did some innovative designs over the years - both technically as well as aesthetically.
As for re-capping costs, this amp only appears to have 5 electrolytic caps (assuming a chunky pair in the PSU). I agree that service engineers are generally undervalued, but this is hi-fi, where many enthusiasts think nothing of paying hundreds for "upgraded" mains cables! Of course, if this chap is popular, the price might be set to manage demand and perhaps exclude a certain type of customer - can't blame him for that!
Yes, it would have been better to have included an output relay that was released by the thermal cutout - and that would allow the addition of a DC offset protection mechanism to protect speakers from a faulty amp. But such things were almost never seen in UK designed hi-fi amps throughout the 70s and 80s. A shame, but the designers I know cite penny-pinching accountants - all too common in the UK in my experience. In fairness, it is awkward to find a suitable relay with the requisite DC current breaking rating and a high voltage coil - as such, you end up needing an additional winding on the transformer, or wasting heat in dropper resistors, so you can understand the temptation to drop it. Remember, woofers were much, much cheaper back then in real terms. There is an output fuse, which is far from perfect, but much better than nothing.
As audio amps go, this is actually a relatively complicated design - not sure why people are so quick to say it's basic. Meridian did some innovative designs over the years - both technically as well as aesthetically.
As for re-capping costs, this amp only appears to have 5 electrolytic caps (assuming a chunky pair in the PSU). I agree that service engineers are generally undervalued, but this is hi-fi, where many enthusiasts think nothing of paying hundreds for "upgraded" mains cables! Of course, if this chap is popular, the price might be set to manage demand and perhaps exclude a certain type of customer - can't blame him for that!
