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~ QUAD 405 Modification Examples ~


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 when both grounds either side of R2 are connected to 0V as they are when fitted in early 405s chassis with a 4 pin DIN connector or if the ground end of R39 is moved to the 'output ground' point between C15 and C16 on the PCB

This is a QUAD 405 PCB with the 'Zobel Network' problem ~ The 10Ω Resistor R39 has now been placed on the reverse side of the board and connects C12 to the 0V point at the junction of C15 and C16 (middle of the picture on the right)

If this modification is not done the input cable screen must be connected to the chassis at some point as QUAD did at the DIN connector but in practice it is better to correct the problem now you know about it and make sure the chassis screws are tight so all parts are connected but in all cases the heatsink or the junction of C15 and C16 must connect to the power supply 0V and the chassis

These early PCBs also had the speaker protection mounted on the output terminals and not on the PCB itself ~ Many people change the capacitors on the amplifier boards but not the speaker protection 10µF capacitors which if and when they 'dry out' can blow the the output fuses and possibly output transistors Tr9 and Tr10

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

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

The 2003 problem was resolved around Tr2 and it confirms that PCB layout or modification is an intrgral part of design not seen on the schematic


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)

The screw connecting the output 0V to the heatsink has been removed and the output 0V from each half of a dual supply was soldered to the junction of C1 and C16

QUAD 405 PCB solder side modification

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