Don Pearce wrote:



This is worth a read. It describes an upgrade that reduces gain by
10dB - designed to match the higher output levels of more modern
source.

http://www.desmith.net/NMdS/Electron..._upgrades.html

Basically it is a change to the feedback circuit around the input op
amp, originally 330k/22k for a gain of 23dB, now 100k/22k which gives
a gain of 13dB.


** Quad built sub-sonic filters into the 405 & 405-2 for very good reasons - response was flat down to 20Hz, -6dB at 10Hz and -20dB at 5Hz.

The above mod unwisely removes this filter making the response extend to 1Hz.

Quad also built crow-bar circuits into both models which will place a short across the output if significant DC or sub-sonics appear. Perfectly harmless to do so if there is a sub-sonic filter inside the amp, since the crow bar will then only operate in case of a serious fault developing.



..... Phil