Phil Allison wrote:



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.


** Despite the author's claim of " ... flat response down below 1Hz " this may not be true. The feedback cap value in the op-amp stage (C4) has been scaled along with the main DC servo loop value (C2) - so there ought to be no change in the roll off behaviour at low frequencies.


..... Phil