~ QUAD 405–2 & 405 Modifications ~

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 for a British loudspeaker manufacturer who since 1995 have also been part of Chinese IAG along with QUAD?

The 7 separate PCBs were populated without current limiting networks or the ESL57 voltage limiter ever being fitted ~ 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 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 up to 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

Placing 5 white LEDs in series with a current of 0.5mA 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 on indicator for the QUAD405 white LEDs would not be suitable for a matching grey QUAD34 and QUAD FM4 due to the colour and intensity differences at lower current

What was needed was a 'grey LED' ! ~ But it appears that no manufacturers are interested in making low light output LEDs for panel illumination ~ Like diode lasers the white and blue LED manufacturers simply want to make bigger and brighter and whiter than white but 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 its ground point connected to chassis

R2 allows 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 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

At the change of PCB from M12368 to M12565 R14 was changed from 560Ω to 470Ω which lowered the current through Tr2 and thus R2 when not shorted ~ With R14 =470Ω the voltage across R2 will be about 20mV because the Tr2 current ≈2mA ~ With R14 =560Ω the current can be as high as 5mA so maybe change to 470Ω

If the PCB input ground is not connected to the chassis ~ the output offset voltage across the speaker terminals will be about 20mV or more whereas connected to chassis 0V at 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

If you bench test a standard 405 PCB using a typical dual 32V bench power supply the current through Tr2 can be µA not mA because the Tr1 current source relies on the voltage across R14 being 1V and with a 32V supply the voltage drop across the diver R13/R14 is barely enough to turn on Tr1

The THD+Noise of the later 405–2 with isolated phonos they can appear better than DIN input 405s especially if the test generator is floating ~ With input ground connected to chassis at the DIN there can be 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

Although the 405-2 output offset voltage is only 20mV 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. and could cause noise or hum

With isolated input connectors if the source is floating long input leads my pick–up radio signals or similar interference which nowadays would most likely come form switched mode power supplies in other equipment ~ In the past many QUAD 405s were used at BBC radio stations but not the isolated phono version

A 500mV signal 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 not likely seen to be 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 through

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 then to chassis ~ Both PCB input screens should connect to chassis so the 2 supplies must be separate and 'floating' to prevent internal hum loops

405 PCBs M12368 iss. 9 and 10 had a serious mistake where the output Zobel Network C12+R39 was connected to the input ground side of R2 and these PCBs MUST have their input lead screens and PSU 0V [bolt on the heatsink] connected to chassis ~ This serious problem still exists on several Chinese copies and some also have R14 =560Ω

 

~ 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 provided to prevent overdriving the ESL57 speakers it is not the speaker protection generally referred to ~ see output 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 ± 28.35Vpk or 20V_rms or 26dBV as it is shown on the graph in the handbook which can be confusing

As part of my 405-2 modifications R10 is replaced with a wire link and it is best to then remove D8 and D9 and the voltage limiter sockets to prevent a link from being inserted ~ For 405 M12368 PCBs the link was a 1.8kΩ resistor R11 which formed a potential divider [not a clamp] with R10 and with the ±12V op–amp supply this gave an output after R10 clipped at ±7.5V when the op–amp limited on its supply rails ~ M12368 iss. 9 & 10 had ±15V op-amp supply?

~ Current limiters ~

The 405 current limiters are also 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 closer 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 applied to Tr9 and the compound Tr8/Tr10 their effect is different ~ Beware that when removed you do not have 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

 

 

The 405–2 claimed "improved current limiting" or rather Safe Operating Area (SOA) protection for the dumper transistors but the identical circuits are still applied "unevenly" to Tr9 and Tr8/Tr10 ~ The QUAD 405–2 SOA networks N1 and N2 are no doubt an improvement on the original design and use external 47µF capacitors C18 and C19 to prevent them operating on signal peaks

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

 

 

Looking at the schematic for the SOA limiter network can be a bit confusing because when first introduced the 405–2 drawing M12333 iss. 7 was drawn wrong and the mistake was carried through to the last issue 10

The correct schematic for the QUAD 405-2 SOA current limit network is shown left ~ Many of these ceramic PCBs were dipped in resin but some were left naked and the circuit can be followed ~ Compare the marked up picture above with the schematic left

 

The 405–2 current limit may have been difficult to fit on the main PCB using discrete components similar to the rest of the amplifier ~ But as you can see in the picture it appears that it was made on a small PCB daughter board

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

 

In the mid 1980s QUAD made 405–2 amplifier PCBs for a once famous and respected British loudspeaker manufacturer who were going to produce a large active dipole loudspeaker and did not require the 405–2 current limit

It is likely that QUAD found it easier to supply the early 405–2 PCBs from the production batches before fitting the SOA limiting

It is also likely that the SOA network was made on thick film ceramic board due to mergers or deals with such a company

 

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 'breaks down' plus under rated electroytics which 'dry out' was not a good move ~ but keeping the RCA transistors I think was ~ The mid 1980s was a bad time for UK Hi-Fi production

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 or test without Tr9 and Tr10 ~ see below

~ Output offset voltage clamp ~

The output voltage offset circuit is the speaker protection ~ It prevents d.c. offsets greater than 9V 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 isolate it for testing

 

The circuit operation is simple but does rely on T1 which is a silicon bilateral switch [SBS] type 2N4992 or BS08D ~ The SBS is an avalanche device [similar to a diac if the trigger is not used] which breaksdown ~ or rather breaksover in diac terms ~ at about 8V

T2 is a 8A Triac with low gate current requirement and a gate trigger voltage of 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 and often output transistors but saving the loud speaker

 

C17 is a bipolar capacitor and the only bipolar capacitor that you should fit on a 405 PCB [maybe also C2] ~ 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 an output d.c. offset fault 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 ~ C17 gets charged via R42 and when the 9V threshold is reached it discharges via R43 and T1 and the short pulse of current 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 simpler way [with no ±50V leads connected] using a current limited variable PSU is to set the current limit to about 200mA and the voltage to less than 1V and connect the output between amplifier PCB ground 0V and the tab of T2 after clearing solder from link X

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 to 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 and current limit at 500mA

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 and from myself if you need

On earlier 405 amplifiers this circuit x2 was mounted on the output terminals and the circuit operation can be tested as above with the current limited PSU connected across the output terminals and the amplifier PCBs isolated by unplugging their output connections

 

~ 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 1.8mA with R15 = 330Ω ~ 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

If you bench test an unmodified 405 PCB using a typical dual 32V bench power supply the current through Tr2 can be µA not mA because the original Tr1 current source relies on the voltage across R14 being 1V and with a 32V supply or lower than 50V this is not achieved ~ with a modified current source you can use supplies as low as ±15V

~ 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 low noise and good linearity but it is also a high frequency device and allows C7 // R17 to be removed

The reason for C7 // R17 is not obvious even with the original Tr2 they are not required to reduce the voltage at Tr2 collector but may reduce Tr2 power ~ They are more likely compensation for the feedback via C8 and or C11 or to correct for group delay at the top end of the frequency range where ESL63 loudspeaker impedance is low

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 // R17 which can also 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 with 2SC2547E as there may be too much open loop gain which could make the amplifier unstable ~ Less open loop also appears to make a better sounding amp! probably due to less overall distortion correction

~ Change & reposition feedback capacitor 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 C8 was originally placed above C7//R17 and later at the collector of Tr2 but never at the emitter of Tr2 connecting to R20//R21

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 as was done by QUAD in the 306 and 606 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 and its not only my opinion

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 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 ~

I do not consider changing components like capacitors or transistors 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 current drive into low loads if the current limiters are also removed or bypassed

If you have blown fuses on a 405 or 405-2 and its not obvious why then removing Tr9 and Tr10 will allow the PCB to be safely powered and tested even if you do not have a current limited variable PSU ~ It is most likely that one or more of the output transistors is damaged anyway and both should be replaced

If you need to or want to replace Tr9 and Tr10 a 32V bench PSU with 2A limit allows you to safely check the output offset is near zero without any load before applying ±50V in the chassis but see Tr1 current source comments above

It is also possible the test any repairs or modifications including new Tr7 and TR8 before refitting Tr9 and Tr10 by simply powering the amplifier PCB without the output transistors as shown

This can be done in the 405 chassis if you do not have a bench PSU and once the output voltage offset is checked you can apply an input signal and check it operates as an amplifier but limit the output to <1W into 8Ω

The picture above is a 405–2 PCB that had a blown output transistor Tr9 because the owner had fitted some uninsulated gold output posts ~ you know the ones ~ Although the PCB was to be modified [ I only mod damaged amplifiers ] I removed and changed both output devices and tested for any other problems like Tr7 or Tr8 damaged

 

~ Change and reposition C10 ~

Changing C10 for a newer type with a higher value say 100µF designed for high ripple 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 ~ C10 also carries some current and fitting a low ESR high ripple type is a good move

 

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

 

 

A one size fits all guide to modifying the 405 or 405–2 is difficult because of the PCB variations ~ However this next section is a "quick guide" to modifying a 405–2 with M12565 PCBs to my DCD Mod–3 ~ Refer to both the original and the DCD Mod–3 schematics and my notes above

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 images for the bigger picture

Drill a small hole next to R15 pad as shown for base lead of Tr1a ~ Cut a strip of track at R10 as shown and remove by heating with a solder iron and drill a hole in the track to pin 3 of IC1 ~ Cut between pads R3 and R4 and remove R3 pad with solder iron ~ Scrape solder resist away from the ground copper near "R" of R3 and tin with solder ~ Use only lead/tin solder do not use lead free ~ it will alloy with the original solder and could give dry joints

 

The Amplifier PCB should look similar to the one on the left and assuming C1 is the shorter 100V 680nF capacitor then there is a spare hole to run an insulated wire from C1 to pin 3 of IC1 as shown ~ If not you can run the wire on the track [ solder ] side of the PCB as shown later

R3 is fitted in the R3 position but the lead near the edge of the board is now bent over to connect to ground ~ Tr1a is fitted in place of R14 as shown with the base lead going through the new hole to the emitter of Tr1b

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

 

 

Fit the new resistors as shown using the DCD Mod–3 schematic as a guide and use the leads cut off to make the links for C11 ~ C7 and R10 as shown

Tr1b fits in place of R14 with its base lead in the hole drilled previously ~ R7 and R8 voltage droppers for the op-amp supply 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 although I no longer recommend tantanlum for C2

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 images for the bigger picture

 

On the reverse side of the PCB some 1µF 63V polyester caps are used to bypass D1 D2 and C2 ~ The op–amp IC1 should not require capacitors to prevent it oscillating if it does you are using the wrong type for this application

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

 

 

~ M12368 iss.9 or 10 PCBs to DCD Mod–4 {2005 layout} ~

 

 

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

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 uninsulated 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 } ~

 

 

The QUAD 405–2 has a blanked off hole next to the input sockets which is an ideal place to fit an input attenuator ~ Note the output sockets cleared ready for better binding posts

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

 

The phono sockets were kept isolated from the chassis as QUAD had done because the holes were large to take the insulators ~ The two grounds are now connected together and to chassis at the low side of the input pot along and to the copper tape

I think all 405s should have their input screen leads connected to chassis and this screened volume control is a good point to do this ~ QUAD 405 amplifiers used in the BBC either for AM8/16 speakers or as standard sterteo amplifers were fitted with RF pickup prevention but the best way on transmitter sites to prevent problems is have the input grounds connected to chassis

 

 

Although the mains wiring and switch are so close there was no sign of induced mains on the output at any setting due to the full screening of the ALPS pot

A serrated washer between the pot and chassis ensured a good connection to the chassis and if you look carefully you can see the insulation on the phono sockets which was left in place but bypassed internally

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

 

Having made several input volume control modifications to 405–2s and 303s for others it was about time to make one for myself and the mod below is essentially the same as that shown above with a few slight changes

The new input connectors are Nuetrik NYS367 phono sockets fitted without the plastic isolating washers they come with and which are grounded to the rear of the chassis with M8 serrated washers

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

The input impedance of each amplifier PCB was changed from 22kΩ to 220kΩ by removing R1 and making R3 220kΩ ~ In addition C1 was changed from 680nF to 68nF to maintain the input high pass filter response at 13Hz ~ Note these changes are only suitable with the non inverting input topology

You can see in the picture 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 their dual floating PSU with active ground circuit

" Got a feeling I've been here before ~ Won't you let me help you find the door "