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Lesser known Quad power amplifiers

I did not like my 405 (bought new in 1976), sat on a shelf for years, changed the op-amp for TL071 and amp was pretty good.

Later I upgraded the boards to 405-2 (bought from Quad) and to be honest could not tell the difference to the 405 with TL071.
 
I replaced the LM301 with TL071 for the sake of it and honestly couldn’t hear a difference. Mmm.
 
The 405-2 sounds much crisper and more dynamic than the 303 to my ears but shares the bass shy and lacking slam nature. In both cases this is deliberate by Quad! Maybe to try and limit the amount of low frequencies fed to ESL's and to prevent saturation of the step up transformers in the ESL's. I consider the 405-2 a very good amp above 100Hz.

Most (all I've seen in fact) people modifying these amps seem to do nothing about this, which is IMHO the single biggest problem with them and responsible for most/all of the criticism they get for "lacking balls" etc.

The last two PCB issues of the 405-2 are the best as, not surprisingly, they have the most fixes for earlier issues plus the extra diode to help traverse the class C region by bringing it more to class B. This can be done to even the first 405's.
 
Arkless Electronics said:
The 405-2 sounds much crisper and more dynamic than the 303 to my ears but shares the bass shy and lacking slam nature. In both cases this is deliberate by Quad! Maybe to try and limit the amount of low frequencies fed to ESL's and to prevent saturation of the step up transformers in the ESL's. I consider the 405-2 a very good amp above 100Hz.

Most (all I've seen in fact) people modifying these amps seem to do nothing about this, which is IMHO the single biggest problem with them and responsible for most/all of the criticism they get for "lacking balls" etc.
Click to expand...
Exactly; limit saturating the original ESL57 transformer; and this - 1975-76 - when vinyl as The source, and warp and rumble could provide rally large ELF transients.

But - sort-out the LF time constants, wind-back the current-limiting , and the 405/2 is a damn good amp.
The later 606 then sorted-out excess input noise; the 909 a cosmetic makeover of 606, with cheaper parts; then the 306 - the final iteration of current dumping design, and I think - the best of the lot/ the sweet-spot/ an utterly-superb, compact 50w amp.

(the added diode that is all that essentially separates the 405 from the 405/2, is a big step forward, as Jez notes)
 
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