Quad 405 DCD Mod 3 or 4?

[DarrenHW]

A quick re-cap on what I've done to the 405's so far:
Dada Service Kit,
New PS Caps,
Mono blocked.

A quick picture:

http://i1324.photobucket.com/albums/u606/drtwas/IMG_5182_zpsizsxkw41.jpg (http://s1324.photobucket.com/user/drtwas/media/IMG_5182_zpsizsxkw41.jpg.html)

Link to the original thread if interested: http://theartofsound.net/forum/showthread.php?34047-Quad-405-servicing-modding-advice

During an early spring clean over the weekend I unearthed the amp boards that came with the second 405, they are M12368 ISS9, one working, the other not. My intention is to rebuild these following Keith Snook's work with MJ11015/6 transistors and delete the current limiters which raises the question of whether to go Mod 3 or 4?

Mod-4 seems like a good option, less work and lower component count, however talk of instability and the lack of information on the net on Mod-4 makes me a little apprehensive.

Does anyone have any experience with these mods?

[Light Dependant Resistor]

Hi Darren
I would be happy with what you have already. The DCD Mod 3 and 4 well just take a look at the zener supplies
for the op amps. Why on earth chase every last bit of THD from this amp and put faith in the Power Supply Rejection Ratio
(PSRR) of the op amp somehow to deal with the noise of a zener diode ? I certainly would not.

If you look at the Quad 306 it was a step forward in design, but never the less can be improved upon. Once again
noisy zeners are in this amp too. But the 306 is easy to work on. I have posted some info about modding them in my DIY thread
"My day improving Audio equipment". I have a bit more to add to that still.

Generally Quad got everything correct with the current dumping principles and implementation
although there is improvement in finely balancing the bridge components. The real improvements are
in improving the front end components and their power supplies.

As for your present boards, I would find source / attenuator components that work well with them. Your wiring is tidy
they no doubt work well, ...project finished ... I would just enjoy them. :)

[DarrenHW]

Hi Chris,

Thanks for your reply.

I should have clarified that I'm happy with the 405's as they are, especially in conjunction with the Croft after adjusting the the input sensitivity to suit. So happy in fact, that I'd forgotten about the other boards and my intention to mod them until I unearthed them the other day.

Now I find myself with 2 boards, some of the components required and a case of the "whatifs". To be honest if it weren't for the fact that one of the boards needs repairing I would probably leave it there, but it does need repairing and DCD Mod's seem the easiest option.

Either way I'm grateful for your comments and take them on board :).