Anyway, todays Quad mods.
The 405 was produced with one eye on driving their electrostatic speakers with their certain requirements, and has voltage and current limiters fitted in order to protect said speakers. Both of these can impact dynamics as they impose non-linearities (in a pretty non-linear way in this case!) on signals with extreme transients or extreme dynamics. Therefore, it makes sense to remove these limitations if you are using this amp with conventional speakers. You can see the space on the circuit boards where the current limiters were, and the sockets for the voltage limiter resistors now empty.
Haven't had a chance to listen yet, but whilst researching the V & I limiters found some pages by some chap called Keith Snook who is big into Quad stuff and has done a lot of work on them, and confirmed that the limiters have a negative impact. He has a lot of damn good ideas, so I'll take a look at his work and look at applying some of it to my amps.
1) The input op-amp (the voltage gain stage) by it's design is noisier than it could be due to it being of inverting design to maintain overall non-inversion. As this doesn't really matter a damn, it can be reconfigured to reduce noise.
2) The discrete class A stage current source can be simply improved easily.
3) The feedback circuit can be improved easily.
4) I am going to have the aluminium case parts anodised 3DV! (Just kidding, gunmetal grey like later Quads)
Anyway, more adventures tomorrow!
"Two very small men cutting steps in the roof of the world"