December 2005 I bought one of the last "incarnations" of the QUAD 405–2 power amplifier and also a number of last issue of the 405–2 PCBs ~ The separate PCBs were specially made by QUAD and intended to power an active dipole speaker "system" for a British loudspeaker manufacturer who since 1995 have also been part of the Chinese owned IAG along with QUAD

These separate PCBs were populated without current limiting networks or the "ESL57" voltage limiter ~ I wanted them to build and test several copies of my "DCD Mod–4" using darlington output devices rated at 30A ~ The first DCD Mod–4s made in 2003 used PCBs M12368 iss.9 which are much easier to modify than the later issue PCBs especially the 405–2 PCBs ~ See further down this page

Although the last QUAD405–2 had the later grey finish with phono connectors it still had a horrid looking red LED for the power indicator ~ The yellow Hewlett Packard LEDs I had used for the 405 and 34 mods did not look so good with the grey colour scheme ~ But a white LED run at very low current should look good against the grey with white text

Experiments with some Nichia and Hewlett Packard white LEDs showed that using such devices as panel indicators would not be straightforward ~ Most White LEDs are made for illumination and consume 10s of mA at about 4V ~ In order to reduce the light output to a point where the LED could be used as a panel indicator the current had to be reduced to less than 0.5mA and even then the on axis light was much brighter than required

The voltage across the white LEDs dropped to less than 2V at 0.5mA and placing 5 in series showed that the light output varied greatly and some even tended to change colour to green/blue ~ Even if they could be made to work as a power indicator for the QUAD405–2 white LEDs would not be suitable for a matching grey QUAD34 and QUAD FM4 due to the colour and intensity differences at the lower output

What was needed was a "grey" LED ! ~ But it appears that no manufacturers are interested in making low level LEDs for panel illumination ~ Like diode lasers the white and blue LED manufacturers simply want to make "bigger and brighter" not "subtle and suitable" for direct viewing

The QUAD 405–2 pictured above has its speaker connectors better located than the earlier 405 ~ It also has gold plated phono connectors for the signal inputs which are isolated from chassis and an IEC mains input/output loop through ~ Although more practical none of these features were added to make it sound any better than any other 405 with the additional D13 and good condition capacitors fitted which can be a bit disappointing but is to be expected

~ Phono input connectors ~

If you look through the schematic evolution of the QUAD 405 designs you will see all the PCBs have a 10Ω resistor R2 fitted which links the input stage and output stage ground connections ~ It is likely that the design was found susceptible to radio frequency pick–up and the original 4 pin DIN input connector with its common ground for left and right needed to have the ground point connected to chassis

R2 allows the individual PCBs to be bench tested outside the chassis then helps prevent a low impedance internal hum loop when the PCBs are installed in the chassis with the output stage ground connected to chassis via the heatsink bolt and the input grounds connected to chassis at the 4 pin DIN input

R2 also allows the current through Tr2 and the operation of the d.c. control circuit around the whole amplifier to be determined during bench test by measurement of the d.c. offset at the output terminal ~ Thus a quick check of the basic operation of the amplifier PCB can be made before applying a test load and test signals

If measurements of THD+Noise are made on the later 405–2 with isolated phonos they can appear better than 405s especially if the test generator ground is also isolated from mains/chassis ground ~ R2 still allows a small amount of internal hum loop current through the input ground when the input screen is connected to chassis but the THD is no worse and the induced 50Hz is inaudible

If the PCB input ground is not connected to the chassis or internal 0V ~ the output offset voltage across the speaker terminals will be about 20mV or more whereas connected to chassis 0V as with the 4 pin DIN input it is less than 1mV ~ Current flowing through Tr2 and hence R2 causes a potential difference across R2 which raises the 0V reference to IC1 pin 3 and so increases the output offset by a similar amount

Although 20mV is a low offset it could be less than 1mV with the phono ground connected to chassis ~ Because the input phonos are isolated if the ground of either input is connected to a piece of equipment with a different ground potential then this can seriously affect the output d.c. ~ If the source is floating the input leads my pick–up radio signals or similar interference which nowadays would most likely come form switched mode power supplies in other equipment

About 500mV between phono ground and mains ground (chassis) will give about 300mV modulation of the output and 2 or 3V between equipment grounds is not uncommon in the UK ~ This was most likely never a problem for many QUAD owners because their equipment was all QUAD with short signal cables and the mains cables linked via the equipment IEC loop throughs

My personal opinion is that for all versions of the QUAD 405 it is best to have the input grounds and 0V from PSUs connected to chassis internally ~ Even if a dual supply is fitted the 0V at the PSU caps should connect directly to the corresponding 0V speaker terminal and also to chassis along ~ The input screens should connect to chassis so the 2 supplies must be separate and "floating" to prevent internal hum loops

QUAD 405 PCBs ~ M12368 iss. 9 and 10 ~ had a serious mistake where the output Zobel Network was connected to the input ground side of R2 and {unless modified with the ground end of R39 moved to the PSU ground between C15 and C16} these PCBs MUST have their input lead screens and PSU 0V connected to the same point ~ preferably connected to chassis

~ Output Voltage limiters ~

The 405–2 voltage limiter is the same as that introduced to PCBs M12565 iss.3 ~ A wire link is fitted to reduce the maximum output voltage of the QUAD 405–2 to about 20V rms or 50W into 8Ω as recommended when using QUAD ESL57 loudspeakers ~ Although it was provided to prevent overdriving the ESL57 speakers is not the "speaker protection" generally referred to ~ see Offset Voltage Clamp below

The current dumping stage has a fixed voltage gain of 3.78x and the output from the op–amp is clamped by zener diodes D8 and D9 so that the output after R10 clips at +8.8V and –6.2V ~ D8 and D9 are different values to allow for the fact that the base of Tr2 is at about +1.3V so the output after R10 is clipped at ±7.5V and the speaker output is clipped at ± 28.35V or 20V rms or 26dBV as it is shown on the graph in the handbook which can be confusing

As part of my modifications R10 gets replaced with a wire link and it is best to then remove D8 and D9 and the voltage limiter sockets to prevent the link from being inserted ~ For M12368 PCBs the link was a 1.8kΩ resistor R11 which formed a potential divider with R10 and with the ±12V op–amp supply this gave an output after R10 of ±7.5V when the op–amp limited on its supply rails

~ Current limiters ~

The 405 current limiters are not speaker protection but because the ESL57 speakers presented a low impedance above 10kHz once again the QUAD 405 and QUAD 405–2 were fitted with extra components to accommodate the ESL57 speakers (and other low loads) and to protect ~ what were at the time ~ the expensive output devices from short circuits across the speaker terminals

The QUAD 405–1 had a relatively crude current limit that acted individually on each of the output transistors or current dumpers ~ The components that provide the current limit function are marked on my M12368 to DCD~Mod3 schematic in grey with blue reference designators ~ There are various mods published that delay the effect of the limiters and/or ensure they only operate at currents close to d.c. but I do not subscribe to these

Modified or otherwise the discrete current limiters have an effect on the output ~ The limit circuits are the same for both current dumpers but as applied to Tr9 and the compound Tr8/Tr10 their effect is different and their removal can be detected ~ Beware that when removed you now have no output device protection and if you are modifying your PCBs and you do not have a current limited PSU and have limited test equipment it may be best to leave the current limit components in place until everything is working okay

Although not part of the signal chain the limiters will always have some effect on the signal even if not clearly measurable so you may decide to remove them after doing any mods and once everything is working okay – If you replace the output transistors for MJ15003 as I recommend for DCD Mod–3 then the SOA protection may be too conservative for these devices

Either way it is certainly easier to remove and replace as a separate board so the results with and without the limiters or SOA networks in circuit can be easily be determined if you wish

The 405-2 PCB pictured above is one of the worst builds of the product range ~ Although the circuit had been improved and hence the title "405–2" the use of cheap resistors and a small ceramic capacitor for C8 which often "broke down" and under rated electroytics which "dry out" was not a good move ~ but keeping the RCA transistors was

If you intend to modify your QUAD 405–2 amplifier modules it is possible that you could make a simple mistake changing components or with the cut and strap on the PCB ~ My advice is to leave the SOA networks in place until everything is done and tested and even then if you can't determine any difference with and without them fitted leave them in place to protect the output transistors should your speaker cables ever get shorted

It is not easy to test the current limit in circuit without a fair amount of nerve as you may well destroy the devices intended to be protected ~ In the past to establish the individual effect on the dumpers I replaced the sense resistors R35 and R36 with higher values which brings the limits into effect at lower currents

Increasing R35 and R36 is also a good idea after modifying the 405 PCBs if you do not have a ±30V to ±50V current limited power supply you could increase the sense resistors to say 1Ω to test the basic operation of the PCB without damaging the output devices if a fault is present

~ Output offset voltage clamp ~

The output voltage offset circuit is provided as protection for all speakers ~ It prevents a d.c. offset from damaging your loud speakers if the amplifier module develops a fault causing the output to swing to one of the PSU rails ~ Although the circuit is fairly crude it does work in conjunction with the fuses and it is the fuses and not so much the output crowbar circuit that appears to upset many Hi–Fi purist

Below: A section of the 405 schematic M12565 iss.3 onward ~ where the output offset protection is built onto the PCB ~ Point X indicates a solder pad link that is bridged to connect the circuit and remove it for testing the circuit

T2 is a 8A Triac with low gate current requirement and a gate trigger voltage about 1V to 2V so a d.c. voltage of about 9V across C17 will cause T1 to breakover and trigger Triac T2 which will short the output blowing a fuse but saving the speaker

C17 is a bipolar capacitor and the only bipolar capacitor that you should fit on a 405 PCB ~ The time constant C17 x R42 is such that even for large low frequency output the instantaneous d.c. voltage across C17 never gets to the 9V trigger threshold in either direction

When there is a d.c. offset fault at the output C17 performs another role ~ Without C17 the circuit would not trigger even with 50V at point X because there would be insufficient current through R42 ~ During an offset fault C17 gets charged via R42 and when the 9V threshold is reached it discharges via R43 and T1 and the short pulse of current which is now limited by R43 triggers T2 which blows a fuse or two

There are instructions how to test the offset circuit in the QUAD 405 service data but a much simpler way using a now readily available current limited variable PSU is to set the current limit to about 500mA and the voltage to less than 1V and connect the output between Amplifier 0V [ how depends if mounted in chassis or loose on bench ] and the R42 side of pad X after clearing it of solder

It does not matter which way the PSU is connected but when it is switched on the current should be zero and remain so as the voltage is increased [you may see a small current change as C17 charges ] until at about 8V to 10V when the current limit will kick in as T2 shorts and the voltage from the PSU should be less than 1V~ This is correct operation for the circuit and it should be repeated with the PSU output reversed

Note that due to the way the T2 operates the threshold may not be the same in both directions but should be in the range 8V to 10V ~ Try several runs and reversals to be sure it's working well ~ The circuit if fitted on the output terminals can be tested by connecting the PSU to the output terminals with the Amplifier PCB outputs disconnected and safely isolated

If the circuit draws high current before the threshold is reached it is likely that T2 is damaged ~ If the circuit fails to trigger in one or other direction it is likely that C17 is faulty or less likely R42 or R43 are high resistance ~ T1 rarely gives trouble but if either T1 or T2 are faulty these parts or better equivalents are still available today ~ or you might decide to replace the circuit with the QUAD 306 QUAD 606 type active ground floating Dual PSU ~ See Below

~ M12565 iss.7 PCB to DCD–Mod–3 ~

~ Non inverting input stage ~

Modification of the input op–amp stage to make it non inverting to the signal but remain inverting for d.c. control can be done independently of the other modification stages ~ This mod greatly increases the signal to noise S/N of the amplifier which was one of its criticisms

The new non inverting topology also makes changing the input impedance and gain easier should you wish to do this ~ because the signal input is to the high impedance non inverting input of IC1 R3 ~ which is the input impedance ~ can be made higher or lower within the limits of the op–amp ~ C1 needs also needs changing so that C1 x R3 still equals R6 x C4

Only the components C2~C4~R3~R4~R6 and R10 around IC1 are changed ~ some track cutting and a wire link as shown below is required ~ R4 is used to change the gain for different input sensitivities but this is no longer required to reduce the noise which is now determined by the signal source impedance {not R3} ~ C2 has to be scaled with the gain change to restore the input filter response ~ see Schematic

~ New current source for Tr1 ~

The current source Tr1 which is the load for Tr2 can be greatly improved by the addition of a second transistor Tr1a as shown on the DCD Mod–3 schematic ~ The original current source relies on capacitor C5 to prevent any hum or noise on the +ve supply from modulating the signal ~ Whether C5 appears in the signal path or not is debatable but failure of C5 or even the type of the capacitor fitted for C5 can measurably and audibly affect the output of the amplifier ~ With the Tr1a / Tr1b mod in place C5 should not be fitted

Again this modification can be done independently of the other mods ~ Tr1a emitter and collector occupy the position of R14 and a hole drilled close to the Tr1(b) end of R15 connects the base to R15 and Tr1b emitter ~ The source current is approximately 0.6V/R15Ω ~ about 2.7mA with R15 = 220Ω ~ If you have hum due to C5 this mod will give much lower hum and noise than any replacement C5 and could even be cheaper ~ This current source can be made with transistors other than the 2SA1085E I use ~ maybe BC214 as fitted by QUAD for Tr1

~ Replace Tr2 & Tr4 ~

Although I have always shown Tr2 changed to 2SC2547E I think it deserves a little more explanation ~ 2SC2547E is an excellent low noise high voltage transistor and when driven via resistor R12 it achieves very low noise and good linearity ~ The reason for C7 // R17 is not obvious and even with the original Tr2 ~ ZTX304 they are not required to reduce the voltage at Tr2 collector ~ They are more likely compensation for the feedback via C8 and or C11

With Tr2 changed to 2SC2547E and R23 removed and Tr4 replaced with ZTX753 ** and C11 replaced with a link there should be no need for C7 // R14 and C7 can be replaced with a wire link ~ I suggest these changes are done together although Tr2 could simply be replaced by 2SC2547E at any time

** do not fit Tr3 if you are fitting Tr4 position with ZTX753 and Tr2 is 2SC2547E as there will be too much open loop gain which could make the amplifier unstable ~ Less open loop appears to make a better sounding amp!

~ Reposition C8 ~

The ideal position for C8 is between the Class A amplifier output at R38 and the emitter of Tr2 Thus forming the feedback bridge correctly ~ Looking at the QUAD 405 Evolution you can see that the "Tr2 end" of C8 was originally placed above C7//R17 and later at the collector of Tr2 but not at the emitter of Tr2

It appears that the inclusion of R23 and C11 to accommodate transistors Tr3 and Tr4 ~ due to insufficient voltage rating at the time ~ placed some constraints on the feedback arrangement which could be unstable without C11 and C7 ~ However if you replace Tr2 and Tr4 with higher voltage transistors like 2SC2547E and ZTX753 then C8 can be connected to the emitter of Tr2 with amazing results

Measurements before and after moving C8 may not be so different but the stereo sound stage is significantly better ~ I try to avoid describing how a piece of audio equipment "sounds" but the C8 change of position and its replacement with a high voltage Polystyrene capacitor does make the QUAD 405 sound more natural with a significantly better stereo image

The improved stereo imaging is probably due to better phase matching with the tighter specified pole capacitor I use for C8 and the removal of the odd a.c. paths through C7 and C11 which were horrible ceramic types ~ Ideally the Class A amplifier from the base of TR2 to the collector of Tr7 should be a d.c. transconductance amplifier and with C7 and C11 shorted it becomes one

~ D13 and Tr3 ~

The QUAD405–2 PCB I show below is version M12565 iss.7 ~ The first PCB fitted with the SOA protection networks N1 and N2 was M12565 iss.5 which had D6 repositioned but D13 was not fitted ~ it also had both Tr3 and Tr4 fitted so to apply my Mod–3 to iss.5 PCBs fit an additional diode 1N4003 in series with D5 ~ Remove R18 and Tr3 and short the Tr3 position base to emitter ~ For iss.6 PCBs D13 is fitted so only remove R18 and Tr3 short Tr3 position base to emitter ~ R44 also appeared at iss.6 and this is not required when applying the C8 move

~ Change Tr7 ~ Tr8 ~ Tr9 ~ Tr10 and C10 ~

I do not consider changing components like capacitors or transistors for functional replacements as a modification ~ My DCD Mod–3 schematic shows the output transistors Tr9 and Tr10 as MJ15003 and Tr7 and Tr8 as MJE15033 ~ It is NOT necessary to change these transistors if they are otherwise okay ~ Changing Tr9 and Tr10 to higher current higher dissipation devices will allow higher current drive into low loads if the current limiters are also removed

Changing C10 for a newer type with a higher value say 100µF may help improve the overall performance of the amplifier ~ Note C10 is close to R30 and R31 which generate heat so mounting the new C10 on the reverse side of the PCB is a good idea

~ A Quick Guide to modifying 405–2 PCBs to DCD Mod–3 ~

Assuming the amplifier is working okay ~ Remove R3 – R4 – R6 – R12 – R17 – R23 – R14 – R15 – R44 – C2 – C4 – C20 – C7 – C11 – C8 – C10 – C20 – Tr1 – Tr2 – Tr4 – clean the PCB with white spirit or flux cleaner with a stiff brush ~ dry the PCB cut tracks and drill holes as shown on the right

Click on image for larger view

If you wish ~ the other resistors may be changed but this is not necessary and will NOT make a difference to the sound

Tr1b fits in place of R14 with its base lead in the hole drilled previously ~ R7 and R8 tend to get "warm" and so here they are mounted off the PCB ~ R4 is shown as 3k3Ω for a sensitivity of 1V so C2 is 47µF

Tr7 ~ TR8 ~ Tr9 ~ Tr10 ~ N1 ~ N2 are not changed see notes above and below ~ C10 has been changed but is close to hot resistors ~

Click on image for lager view

C10 can be mounted on the reverse side of the PCB to keep it away from the heat of R30 and R31 ~ solder C10 in place and then bend the wires on the top of the PCB to prevent the track lifting if it gets moved

Finally and most importantly C8 the new high voltage 1% polystyrene capacitor is fitted from one side of R23 to the emitter of Tr2

Click on image for lager view

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

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

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 405–2 input attenuator { Volume control } ~

Here an ALPS dual 10k log pot with 3kΩ series resistor is used same as the QUAD 303 A2 mod and once again its covered with copper tape but this time to prevent pick–up from the nearby mains switch

I think all 405s should have their input screen leads connected to chassis and this is the optimum point to do this

A serrated washer between the pot and chassis ensures a good connection to the chassis and if you look carefully you can see the insulation on the phono sockets

Maximum sensitivity with the 3kΩ series resistor and DCD–Mod3 PCBs is now 2V for 100W which gives the pot a useful range with a CD player directly connected

The input pot is an ALPS 20kΩ dual log finished off with a large black 1970s BBC control panel knob which matches the rest of the rear panel decor

You can see in the pictuure above this 405–2 was also fitted with a Dual Mono PSU ~ This was my last modification to further improve the QUAD 405 and eliminate the speaker crowbar protection ~ But as always QUAD prior to 1995 had already developed a good solution in the form of the dual floating PSU with active ground circuit