Tubehunter's monoblock 405 amplifiers

[Barry]

...or the equally stunning sounding Quad 405s Duncan had modified into monoblocks (more on those later).

Hi Marco,

Are you able to divulge any details? Has Duncan bridged a standard Quad 405 (effectively turning it into a '410')? Does the bridging connect the amplifiers in series or in parallel? Has Duncan applied any other mods such as those suggested by the likes of Dada or NET or those of Keith Snook?

Cheers

[Marco]

Hi Barry,

I'll flag this thread up to Dunc, and he'll hopefully reply with all the details you need. All I know is that they sound STUNNING - unquestionably the best SS amps I've ever heard in my system!

Marco.

[tubehunter]

Hi Barry,

The Quad 405 Mk2's are a single board in each case with 22,000uf 63v F & T power supply capacitors.
Each board has been taken to Keith Snook Mod 3 level but keeping the inverted input with OPA604 Op-Amps.
All components, where possible are measured to within 1% tolerance with all resistors changed for PRP PR9372 Audio Resistors.
Polystyrene Film capacitors are used in the signal path.
Elna Cerafines where electrolytics are required.
All transistors changed for higher specification devices.
Power supply to the op-amps has been improved also.
K4700 Loudspeaker protection boards fitted which mean the Quad clamp circuits can be removed.
Also with the higher power output transistors the current limiters have been removed.
One of the most noticeable upgrades regarding sound quality is the move of C8 to the emitter of Tr2

Regards
Duncan

[DSJR]

Were these mk2 boards to start with? (starting with issue 7)

Quad made many layout changes over the life of this amp, but I don't know how many versions of the mk2 boards were done ('my' era was issue 7, but I believe they're up to issue 10 now if you buy virgin boards from them)

[tubehunter]

My Quad 405's originally had M12368 ISS 5 and ISS 9 boards.

I happened on some M12565 ISS 7 (405 Mk2) the latest boards, without Tr3 so modified these.
405 Mk2's started with M12565 ISS 3 and finished with ISS 7.

I think ISS 10 refers to the circuit diagram M12333

Duncan

[Barry]

Thanks Duncan,

That's interesting, but in what way are they monoblocks? Have you bridged the (normally) two channels in any way?

Regards
Barry

[tubehunter]

Hi Barry

There's a just single board in each case.