~ QUAD 405 Modification Examples ~

~ M12368 iss.9 or 10 PCBs to DCD Mod–3 (2008 layout) ~

Example M12368 DCD Mod-3.pdf

~ M12368 iss.9 or 10 PCBs to DCD Mod–4 (2005 layout) ~

QUAD 405 M12368 iss.9 PCB
On the left: My DCD Mod–4 implemented on an M12368 iss. 9 or 10 PCB using MJ11015 and MJ11016 darlington devices for the "dumpers" and an improved class A amplifier with a 50mA current sink (Tr108 – Tr11) load for Tr7

The original 2003 layout (see S Mannas DCD–Mod4) kept the +Ve supply to Tr7 and Tr1a via the red link wire fitted across the board by QUAD but this with too high open loop gain for the class A stage and high bandwidth op–amps like AD843 or OPA627 proved unstable at VHF causing the amplifier to compress or even block

NOTE ~ On M12368 PCBs the "Zobel Network" R39 in series with C12 is connected to the input ground which is WRONG ~ These PCBs are only stable if both grounds either side of R2 are connected to 0V ~ preferably chassis or if the ground end of R39 is moved to the output ground point between C15 and C16 on the PCB and the 0V speaker connection is also taken from this point
Although this Mod may look complicated it actually simplifies the design as can be seen from the amount of components missing

There are 8 wire links fitted on the component side of the PCB above ~ 6 un–insulated links marked in blue and 2 yellow insulated links by D6 and D13

On the back of the PCB some tracks are cut and heavy wires link the ±50V to the output transistor collectors and the +ve supply for the class A amplifier

The leads of R3 are used for the op–amp non inverting mod and 3 x 1µF poly caps are fitted across C2 and the op–amp supply (not to prevent oscillation but to provide a good a.c. signal path to ground)

QUAD 405 PCB solder side modification

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