Quad 405 Mk. 1

[prestonchipfryer]

I have a Quad 405 Mk. 1 which I would like to refurbish with RCA inputs, proper speaker binding posts, etc. Any suggestions who could carry out this work would be welcome.

John

[DSJR]

The first thing to do would be to get a pair of new amp boards from the likes of Dada Electronics or Net-Audio, as this circuit is incredibly sensistive to component layout (it's measurable as well as audible, so not bunkum) and the latest boards have better output transistors and significantly less current limiting. Quad themselves went through several issues of amp boards, all laid out differently, before settling on the mk2, which was "issue 7" from memory. There's a site showing shots of crossover distortion on issue "3" boards until a resistor leg was moved directly to the transistor feeding it.

http://www.keith-snook.info/QUAD-405-mods.html

http://www.keith-snook.info/QUAD-405-2-Mods.html

Note the comment from the above site -

"Try not to forget that the QUAD 405 is basically an excellent design and that some of the changes shown here and published elsewhere by others could not have been reasonably be applied by a manufacturer at the time they were made — At least not if they wanted to make a consistent QUALITY UNIT AMPLIFIER DOMESTIC at a profit"

The main issue with almost all "original" 405's is the supply caps bulging and leaking. Modern tune-ups fit larger value caps and dada at least, do a twin reservoir supply as modern caps are smaller yet higher in value. I've used Dada for my 33 preamp update and the advice is good as well as the prices reasonable and not greedy IMO. They also have a blog-spot which goes into various issues with all Quad amps... Their "audiophile" boards feature most if not all of the tweaks to date.

http://www.dadaelectronics.eu/shop/dada-high-end-boards

If you use basic phono sockets, there will be hum-loops if you use the mains earth on the 405. A specialist engineer will correct me I hope, but I'd be inclined to use insulated phono sockets, keeping the signal return away from the chassis earthing if possible.

The horrid old push-button speaker connectors can then be ditched, along with the little circuit attached to them (it's duplicated on the new boards so unnecessary). I'd just fit some simple 4mm sockets as Quad did on later ones,but you may prefer summat posher ;)

I don't know who here would be willing and able to do these mods, but I's seriously do the supply and amp-boards since changing sockets etc will offer no improvement at all otherwise.

lastly, is it worth spending a few hundred on a 405? My answer is a definite YES!!!!! It's a great old design and, although I agree that modern transistors, caps etc would enable a simpler non current-dumping circuit to be used with superior performance on the test bench, a "sorted" 405 does have a lovely sonic charm to it, while staying pretty faithful to the "straight wire with gain" ideal :)

[prestonchipfryer]

:) Thanks very much, very informative. Will look further into your suggestions.

Regards

John

[spendorman]

I have done budget referbs to several 405 Mk 1's. This only costs peanuts and the reward sonic reward is large. Replace the op amps with TL072 or better (they are usually in sockets, so this is easy). Replace the power supply capacitors. I used capacitors from Cricklewood electronics.

Total cost would be about £10.