This post contains pictures that might shock sensitive Quad-perfectionists. If you read ahead it's on your own risk!
I never felt very happy with Quad's LS protection circuit.
In fact all it does is short-circuiting the outputs if there is a DC-component in the speaker-signal.
Word goes that in the 405 MK II the protection circuit has been improved. I'm sorry to say that's not a fact. The only thing that changed is that it was integrated on the circuit-boards. In the 405 MK I it was soldered to the LS-plugs.
2 Weeks ago I got an e-mail from Olivier, who bought a 405 on Ebay I revised and sold on Ebay a few months ago:
"Bonjour, je viens de faire l'acquisition d'un ampli Quad 405 modifié par vos soins à Chagoumy (pseudo ebay). L'essai chez lui s'était avéré positif. Je ramène l'ampli chez moi et clac: bruit énorme sur la voie droite..et pire. Le tweeter des 104/2 (voie droite) grille !! Je ne comprends pas. Je rééssaye sur un autre système. Idem."
One of the BDY77 endstage-transistors was grilled and the 405 has the bad attitude to put -50 Volt on the speaker-outputs when this happens. Unfortunately in Olivier's 405 someone had replaced the protection-circuit with 4A fuses. This will protect the speakers against overload but not against DC-voltage.
Olivier's 405 is up and running again but it was time to think about a better protection-circuit. A circuit that monitors the output and switches on the speakers after a few seconds when everything is OK and that switches them off immediately if there is a DC-component or when the power is cut.
I heard about the Velleman K4700 protection circuit (see www.velleman.be and surf to "projects"), so I built one and tried to put it in the 405.
There is not much space in a 405 so I had to screw it on the top-cover. Anybody who has a better idea will be rewarded with a box of Belgian Chocolates.
Anyway, it's ugly but it works fine ;-(