~ QUAD 405 and 405-2 Dual Power Supply ~

For many years I have fitted Dual Power Supplies for customers from 'other suppliers' ~ Some require drilling the base plate but if you are going to drill holes in the base you may as well fit 4 new caps with their connections upward using capacitor clamps

When the PSU capacitors and rectifiers are PCB mounted in the 'traditional way' the connections to the board can become very messy ~ especially if the original wiring is to be used even when additional screw connections or spade terminals are used

Picture: QUAD405-2 Dual Mono PSU mod - start
Picture: QUAD 405-2 Dual Mono PSU mod - 10,000uF Caps fitted
For my proto–type dual PSU I decided that if one capacitor needed replacing in future then all should be replaced ~ Gluing the capacitors together on a base plate made from old PCB material enabled wiring from the top and they could be removed cleanly if it did not work

In the picture above the capacitor mounting plate is held in place by two M4 bolts one side and under the curved case of the transformer the other side ~ The capacitors were glued with "modified silicone" to the base and with silicone rubber forced down between the gaps

Each half of the dual PSU was fed from the separated secondaries of the mains transformer after checking that the windings were on a common core and not 2 separate 110V cores paralleled at the secondaries because this would not work with the primaries in series for 220–240V

The 2 Bridge rectifiers used were KBU6G with long thick leads which connect directly to the pins of BC 10,000uF capacitors

Picture QUAD amplifier with Dual Mono PSU fitted
With the capacitors mounted this way the original internal wiring need not be changed in any way ~ The connections to each amplifier PCB were made directly to the terminals of the relevant 10,000µF capacitors along with 4 polypropylene bypass capacitors which cannot be easily fitted on other PCB mounted Dual PSUs

Fitting separate "floating" PSUs to each half of the amplifier enables use of an active ground circuit as fitted to the QUAD306 and QUAD606 ~ This circuit provides active correction of the voltage at the "centre tap" of the PSU capacitors and ensures that the supply voltages track during switch on and off preventing pops and thumps

The active ground circuit components were fitted to each amplifier PCB after removing the speaker protection crowbar components ~ Trying several times to use the existing holes and tracks with the components on the "component side" of the the PCB I gave up and mounted the active ground circuit "open frame" on the solder side

QUAD405 Dual Mono PSU PCB under test
Building the active ground circuit on each amplifier board was a bit awkward and I wanted to fit fuses on the a.c. side of each PSU so the fuses on the amplifier PCBs could be bypassed

The only way to make a neat Dual Mono Block PSU would be to fit the active ground circuit and a.c. fuses on a PCB which would also mount the capacitors ~ Following on from the proto-type the PCB was made "upside down" mounted on the capacitors

The Dual Mono PSU block is easily fitted to all versions of the 405 but may require changes to the Chassis and Speaker 0V connection for optimum noise performance

The active ground circuit sets a 0V midpoint for the PSU capacitors by adjusting for their leakage current ~ The circuit prevents d.c. current from passing through the loudspeaker under fault condition and so acts as d.c. speaker protection

QUAD405 Dual Mono PSU loudspeaker protection can be removed
The separate active ground circuits each with a single a.c. input fuse allow the components outlined in yellow above to be removed and the PCB mounted fuses to be bypassed ~ The ± power connections can be wired directly to the points marked with Red and Black dots ~ The 100nF caps C15 and C16 can be left in circuit or replaced with larger polycaps in the space where the fuses were fitted

An active ground circuit ensures the ± supply voltages track as the PSU capacitors charge and discharge thus preventing thumps during switch on and off ~ If you own a QUAD 306 or 606 or other amp with active ground and hear a burst of low frequency or thump at switch on or switch off suspect the PSU caps unbalanced or a fault

QUAD405 Dual Mono PSU block fitted to 405-2 My QUAD 405 Dual PSU fitted in a 405-2 ~ The original wiring could have been kept in place as it fits neatly to the PSU PCB but separating the signal cables from the mains and from each other improved the noise measurement

Bypass polypropylene caps were fitted in the space where the fuses and crowbar circuit were but as you can see they could be easily fitted across each PSU capacitor leaving the amplifier PCBs untouched

In the past when fitting Dual PSUs from other internet audio suppliers I had noticed an increase in measured background noise and 100Hz side bands on the residual distortion output from my test gear after the secondaries were separated

The noise was reduced slightly by NOT connecting the centre of the PSU caps to the front panel and by re–arranging the 0V and chassis connections but it could not be completely eliminated

QUAD405 Dual Mono PSU transfomer under test
The spikes at 100Hz (10ms) intervals looked like rectifier diode recovery noise but closer examination showed they were the wrong shape ~ Various snubber Capacitors were fitted across the bridge rectifier and transformer connections and more re–arrangements of chassis and 0V connections were tried but none cured the problem

With the base plate loosely fitted I noticed a switch on thump would occur with either half of the Dual PSU connected but never when both halves were connected ? ~ This made me curious so when I noticed the thumps on the next QUAD 405 with my Dual PSU fitted I looked for a magnetic solution instead of moving 0V connections

As seen in the picture above the Transformer is held by 4 bolts to the front frame which holds the heatsinks ~ There are also 2 bolts that hold the Transformer to the base/back panel but with these bolt tightened and removed the 100Hz noise was still present ~ The heatsink contacts were cleaned but the noise was still there

Although tightening and loosening the Transformer bolts had no effect the problem did turn out to be with the Transformer

The Transformer is a C core construction so has distinct air gaps in the magnetic circuit unlike EI or UT laminations ~ The cores are held together with metal straps which often become loose and a classic problem with the 405 is acoustic(al) hum from the transformer

My guess is the Transformer has 2 x 110V bobbins and 2 x 35V+35V bobbins either side of the air gaps ~ Wiring the primaries for 110V or cross coupling the 4 secondary sections as show above greatly reduces the 100Hz noise

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