A brace of good quality OPA134 op-amps to replace the not so hot (though state of the 'fet input' op amp art of the time) TL071s in the pre and power amp - a starter for ten. Also some decent speaker connectors to replace the woeful original spring push ones. A mod also done by Quad to later 405's.
Got the pre-hooked up to my 'phones this eve. Bass is super tuneful, treble nice and sweet and easy - bass definitely easier to follow than the Arcam. Mid... jury out. Slightly 'thick' sounding maybe? Maybe lacking in detail? I know the op-amps will reduce the noise level, but detail? Who knows... one step at a time.
Pleased with the effort that's gone into the power supplies on the pre, will leave that alone aside from faster rectifiers. Still not convinced that I need a buffer on the CD input - may add a 'straight through' switch to the 'Aux' card, with a bit of attenuation - one less active stage must be a good thing. Input switching, uses CMOS 4066's from eons ago, which apparently add 0.01% distortion all by themselves. (I designed a pre back in about '85 and thought about using them, but gut feeling led me to abandon the idea and I used CMOS latches to drive telecoms relays instead) Yes, 0.01% is low, very low, but it can be bettered by later versions of the 4066 so I will probably look there. Torn though as ideally I would fit reed relays rather than relay on CMOS (spit!) to do my switching but I dont want to make it less 'Quad' so I'll try Maxim's versons of the 4066 which have huge bandwidth an should be almost an order of magnitude better THD-wise in a low signal level switching application, which generally means other aspects of performance will be better also.
It's never going to be the most transparent of pre's, but it is a lovely piece of design work. A joy to work on and to use. The power amp however, that has the potential to be truly phenomenal. A very clever design, and something I'm sure will respond to reworking with a few modern ideas and components applied sensitively. Arise FrankenQuad!
<edit> listening to some old Rage Against the Machine - bass guitar is about as 'real' as I've heard on anything. Yes, the low end certainly does have some serious dynamics!
"Two very small men cutting steps in the roof of the world"