~ QUAD 405 Power Amplifier Modification ~

Since 1993 I had intended to keep the 'hobby' web site Valve Audio only but in 2002 I was offered another QUAD transistor set–up consisting of an FM4 Tuner a QUAD 34 pre–amplifier and a QUAD 405 power amplifier ~ All were 'brown' models with DIN plugs which attracted little interest and would be difficult to sell ~ I decided to keep the set together and use it in my study office but first I wanted to experiment with a few modifications to the 405 to see if the rumours were true

Many QUAD 405 PCBs had the input op–amps mounted in sockets which of course encouraged the substitution of every pin compatible OP–this and AD–that with the usual subjective analysis and discussions ~ Often with the most expensive or lowest distortion devices sounding best ~ even if still very noisy ~ Due to the input topology simple op–amp substitution will NOT fix the problem of high input stage noise as I shall try to explain below

After applying some of the web published mods or 'upgrades' I decided to tackle the problem of the input stage noise which further improved the amplifier both objectively and surprisingly subjectively ~ The QUAD 405 now sounded very impressive and worthy of further work ~ It was treated to new larger value smoothing capacitors ~ Better output connectors and gold phono inputs paralleled to the 4 pin DIN so that it could be readily connected into many systems allowing direct comparison with other amplifiers [I know but we all do it]

There are a number of 'modifications' to the QUAD 405 that can be found ~ Most are not modification or even upgrades but simply replace the capacitors and resistors ~ The best information that I found on the web in 2002 was from Bernd Ludwig [2010 update] and I suggest you read his words along with mine

The information below is based on Bernd's work except for the non inverting input stage change and repositioning of C8 and the input/output terminal changes ~ Other changes like removing the current limiters and providing a better current source in place of Tr1 have been subsequently applied and are mentioned on the 405–2 page

If you wish to repair or upgrade your QUAD 405 or just want to learn more about the 405 and 405–2 you can download the QUAD 405 Service Data that I have recreated in pdf format ~ Also see my QUAD 405 evolution which is a set of pdf schematics from the second [first production] issue of QUADs M12333 405 up to some of my mods ~ Also you must read this article by Peter Walker and Michael Albinson

The following changes were made to a QUAD 405 with M12565 iss.3 PCBs ~ other issues of the PCB are similar but different ~ see examples

When the 405 was switched on it was clear that something was wrong ! ~ QUAD had even placed a bright red LED on the front panel to indicate so and this was the first thing to be changed ~ It was replaced with a Yellow HP LED run at a current 7mA by changing the series resistor R40 to 6k8Ω ~ The old Green and Red LEDs on the 34 pre–amp and FM4 tuner were changed for Yellow and Red HP LEDs run at ⅓ of original current

With extensive listening this QUAD405 sounded . . . reasonable ~ Just like a QUAD606 or QUAD306 at domestic listening levels but with noticeably more noise at lower levels ~ There was also some hum on the left channel but other than that it appeared to work fine and was given a further listening tests with friends and several pairs of speakers before being transferred to the workbench

Testing with a Tektronix AA501 and HP3582A [I now use an AT35670A] showed the distortion at 30W to be 0.005% on the right channel and 0.029% on the left while using the AA501 400Hz high pass filter to eliminate any hum from the measurement ~ Without the 400Hz HP the left channel residual showed sidebands of 100Hz around the fundamental and harmonics which explained the audible hum and contributed to the worse distortion measurement

When attempting to test the distortion below 1W noise and hum affected the readings with each channel measuring about 0.03% [400Hz–22kHz] ~ The hum on the left channel was cured by replacing C5 ~ A better solution would be to fit a better current source in place of Tr1 but for now 100µF 16V tantalum capacitors that were to hand were fitted for C5 in both channels ~ The hum was greatly reduced but the remaining noise was clearly from the input stage op–amps and ~ due to the design topology ~ would not be improved by fitting 'better' op–amps

C10 was changed for 100µF 100V electrolytic ~ NOT a bipolar type ~ Some mods suggest bipolar capacitors all round but this is crazy where the capacitor is clearly biased with a d.c. polarising voltage and at a.c. will never be subject to a reverse voltage even on the lowest bass notes ~ Bipolar capacitors by design have higher ESR and hysteresis than equivalent rated standard electrolytics and when subjected to a permanent d.c. bias may be worse and may even suffer earlier failure

After reading Bernd Ludwig's notes about D13 which was an extra diode that QUAD added from PCBs M12565 issue 6 to increase the standing bias on the dumpers and with my 405 PCBs at issue 3 it looked like a good place to start some modification

Rather than cut the track between D5 and D6 I formed a tripod of two 1N4006 diodes and the 1W 10Ω resistor and only cut the track at the base of Tr9 ~ The 'tripod' is mounted on the rear of the PCB after removing D6 as shown on the right ~ D5 was also changed to a 1N4006

A slight improvement in THD was measured especially below 1W but the crossover spikes previously seen in the residual trace output from my AA501 analyser were only slightly reduced ~ The fundamental energy of the spikes is way above the audio frequency range but this is no excuse to ignore them

After reading all I could find about the QUAD 405 on the Web [2002] I turned to my supply of Wireless World magazines and re–read the original articles about current dumping and the QUAD 405 from P. J. Walker and M. P. Albinson WW December 1975 I had in mind something about the 'crossover' spikes being related to current dumping bridge balance and wanted to see if they could be further reduced or even eliminated

A better clue was found in follow up letters and the article from J. Vanderkooy and S. P. Lipshitz [WW June and July 1978] But note that their tests were done on a very early QUAD405 that had C9 and R19 fitted as were many other critiques like the reports from Gordon J. King and Hugh Ford ~ Bear this in mind when reading magazine articles about the QUAD 405

If you compare the 'Bridge' L2 ~ R38 ~ C8 and R20//R21 as implemented in the QUAD 405 and 405–2 with the QUAD 306 and QUAD606 . . . onward ~ It is obvious that compromises were made with the QUAD 405 ~ Probably due to instability problems when applying 'feedforward' to a transistor amplifier via a bridge in its 'feedback' loop ~ If this is taken into account much of the analysis [especially using the first production units] of the design can be ignored ~ When implemented correctly the current dumping Maths works and it works well

I say the Maths works but 120pF x 500Ω does not exactly equal 3uH / 47R as it should ~ The problem as with most problems solved by formulae the initial assumptions are wrong ~ Although it is reasonable to assume the value of L2 is 3µH because it is marked on the schematic it is more likely to be 2.8uH and have some loss resistance

If you follow the QUAD Change History for the 405 and compare the schematics with the QUAD 306 and QUAD 606 etc. you will see that C8 is not placed optimally in the 405 Bridge circuit ~ Ideally C8 needs to be returned to the emitter of Tr2 ~ The 75Ω resistor R44 placed across L2 in later versions of the QUAD405 was a change intended to redress the bridge 'balance' but after applying the C8 move below I could get no better results at several output levels and frequencies with R44 fitted so left it unfitted

A variable capacitor for C8 in its original position could not be used to reduce the crossover spikes and the best results were found to be with a 1% 120pF polystyrene capacitor for C8 placed either side of R17 ~ Low level crossover spikes are an indication of good Bridge balance but the spikes can also be reduced by increasing the bias on Tr9 and Tr10 ~ However the QUAD405 would then be more a class B amplifier which could suffer thermal runaway

Looking at the change history you can see QUAD Experimented with the position of 'bias' diodes D5 and D6 and later added D13 to reduce the spikes but long before that Walker and Albinson had chosen an inductor L2 and capacitor C8 instead of resistors in the bridge and these components also reduce the crossover spikes and allow negative feedback to be used around the class A stage

Even after adding the extra Diode D13 and 10Ω resistor to my M12565 iss.3 PCBs the spikes due to dumper switching were still present but after moving the polystyrene C8 to Tr2 emitter they were greatly reduced

Residual Distortion output from the AA501 showing spikes at crossover with 1kHz sine wave at 30W output

Same conditions as picture on the left except C8 is connected to Tr2 emitter ~ see below

My first modification ~ I now recommend removing R18 and Tr3 and shorting Tr3 base/emitter ~ Tr4 [now Tr3] is fitted with a ZTX753 ~ C11 [and C9 on early models] are shorted with a wire link and R23 [and R19 on early models] are removed

C8 can now be connected between R38 and the emitter of Tr2 where it ideally should be in the bridge ~ The result of this simple modification is amazing and if C8 is made a close tolerance high voltage polystyrene rather than a ceramic [albeit an NPO COG type] then…

With C8 in its new position in the bridge the distortion on both channels was about 0.0044% at 1kHz 30W and the nasty spikes were not seen even when varying the load ~ The noise from the front end op–amp still prevented accurate measurements at lower than 1W and I was convinced that the noise could be reduced by a change of design around the input op–amp

The op–amp ±15V supply is derived from the main ±50V using Zener shunt regulators which can be seen in bottom left of the picture above ~ Despite what you may have read elsewhere this works well provided you don't shunt the zeners with too large capacitors in a mistaken attempt to reduce the supply noise

The 3k3Ω resistors R7 and R8 pass about 10mA without an op–amp fitted ~ The original op-amps and TL071 have supply current less than 1.5mA leaving 8.5mA for zener regulation ~ Fitting an op-amp like OPA604 with 5mA supply currrent is getting close to starving the zeners so I change R7 and R8 to 2k7Ω >0.6W and fit 1µF across the zeners to supply peaks of output shunted by R12↴C6 ~ Making the –ve supply C a bit larger than the +ve can help reduce power–on thumps

~ The 405 input operational amplifier IC1 performs several roles ~


It provides low distortion gain ahead of the power stage ~

15x or 23.5dB gain from the amplifier input to the junction of R10 and R12 which is more than enough to drive but not overdrive the Current Dumping stage using mnay pre–amps including those from QUAD ~ This op–amp front end gain is not used in the QUAD 306 and QUAD 606 etc. which use an op–amp only for output d.c. correction

The 405 op-amp has 22kΩ R3 in series with the source and 22kΩ has a maximum S/N of 104dB ref. 0.5V which equates to a maximum theoretical S/N of 63dB for 1W output ~ it also makes the overall amplifier inverting whereas most other power amplifiers are non inverting

The QUAD 303 was also inverting and like the 405 was intended to be used with the QUAD 34 and 44 pre–amps which are inverting so overall phase is maintained ~ My input stage mod makes the 405 non–inverting and reduces the noise due to R3 R4 and R6

The QUAD 405 input op–amp forms part of a d.c. feedback loop ~

At d.c. the input op~amp is operating at maximum gain and its output is d.c. coupled through the current dumping stage to the output terminals ~ The output terminal voltage is fed back to the inverting input of the op–amp via R4+R5 = 26k7Ω to be compared with the ground reference at its non–inverting input ~ As there is no corresponding high resistance at the non–inverting input any input bias current will give a d.c. offset at the speaker terminals

If you feel you must change the op–amp consider the input bias current specification which is generally higher for high gain bandwidth [GBWP] products ~ FET input devices like TLE2071, OP134 and OPA604 are all good for audio and have the low input bias current required for a low output voltage offset in this design

The QUAD 405 op–amp output operates in Class A ~ There is little or no benefit fitting some modern 'distortion cancelling' op–amps that 'do voodoo' around the zero crossing point ~ IC1 output never crosses zero and the rest of the circuit is effectively Class A including the current dumpers as they are in the Bridge with the Class A stage

The National LME49710 may be a replacement option with only 7nA bias current but it has a high GBWP which could make it unstable and you may not get the published specification in the 405 circuit ~ Many application circuits for these 'new' op–amps show current sinks connected from output to ground

Beware of bipolar input devices like the NE5534 which is often substituted in the QUAD 405 ~ The NE5534 and the dual NE5532 etc. are very good 'audio' op–amps and were probably used to produce much of the music you listen too but they can give an output offset of 150mV when used in the 405

C2 removes the a.c. content of the feedback signal as explained next but may also introduce distrortion at low frequency .  .  .

The QUAD 405 input op-amp forms a high pass filter ~

Because it controls the output offset voltage as stated above it naturally makes a high pass filter [HPF] ~ C1 C2 and C4 are required to ensure the input op–amp gain is maximum at d.c. and so has sufficient open loop gain to control small output offset errors ~ If the output offset control was done with say another op–amp then R5 and C2 would not be part of the input circuit ~ C1 would still be required to prevent d.c. from damaging the speakers ~ C2 and C4 could be shorted and R5 removed

But that's not how it is so let's put these components back one by one ~ C1 [680nF] and R3 [22kΩ] form a HPF at about 10.6Hz [a time constant of 15ms] which prevents d.c. on the input upsetting the amplifier ~ R6 ~ R3 and R4 set the stage gain at 15x but when C4 [47nF] is back in series with R6 [330kΩ] the frequency response of the input stage becomes in theory flat down to a few mHz because the C4+R6 time constant is also 15ms and negates the effect of C1+R3 for as low as the amplifier gain holds out

Putting C2 and R5 back into position will cause frequencies below their turnover point to be subject to feedback from the output stage ~ the closed loop gain from input to output is 56.67 and the turnover point of C2+R5 is 0.3386Hz which multiplied by the closed loop gain gives 19.19Hz ~ it appears that the aim was to provide a HPF response with a –3dB point at about 20Hz which fits with general audio power amp design philosophy but due to the interaction with the other time constants the effect is a HPF with a turnover at 13.95Hz and ~12dB/octave roll off

Note a 75Ω source impedance is assumed for the calculation and a gain of 15 for the op–amp stage allowing for the effect of R3 and R4 ~ The effect of input impedance is minimal but if the gain of the op–amp stage is changed by altering R3 R4 or R6 then C2 will have to be scaled proportionally to restore the original response ~ You will also have to adjust C1 when changing R3 and C4 when changing R6

If you look at the Bernd Ludwig modification that started me off on this pilgrimage you will see that he reduces the gain by making R6=100kΩ and restores the 15ms time constant by making C4=150nF ~ R3 and R4 are unchanged and so the gain is reduced by 3.333 and C2 is adjusted to 33µF in proportion to the gain reduction restoring the overall response of the original design

The QUAD 306 to 606 including the 500 series of professional amplifiers use an op–amp 'outside' of the signal path for d.c. output offset control only [The QUAD 909 and later are a 606 that IAG make with a buss lane and different cases]

Now [2019] online I see comments like 'Peter Walker didn't see much point in amplifiying subsonics so he included a subsonic filter' ~ He did this rather than just relying on coupling capacitor C1 because the 405 was to be used with electrostatic speakers ESL57 and ESL63 which have low impedance below 20Hz and when used with a record turntable footfall could cause high level low frequency signals [The past is easier to change than the future]

The QUAD 405 input op-amp is a buffer for a passive low pass filter ~

If any amplifier is subjected to signals that it cannot handle it will produce Transient Intermodulation Distortion [TID] ~ Many valve amplifier designs attempt to push more bass through the output transformer than it can handle with horrible results ~ While bass is generally not a problem with d.c. coupled transistor designs exceeding the high frequency capability can be

The current dumping technique requires that the unbiased transistors that do the dumping switch in and out to aid the low power class A stage ~ The effect of this switching can be seen in the oscillograms above where fast spikes are produced by a 1kHz sine wave ~ As Peter Walker wrote in his Wireless World article of December 1975:

'We have said that the dumpers have to be sufficiently fast to come to the rescue of the class A amplifier to prevent its overloading. Clearly they must be sufficiently fast to achieve this over the audio spectrum of the programme. There is, however, nothing whatever to say that they must do so at frequencies outside the audio range provided that steps are taken in the design of the whole amplifier to ensure that any such frequencies that may be present do not embarrass the amplifier performance within the audio range. If the system is properly designed it is possible to use relatively slow devices ~ inherently more rugged than fast devices ~ and to show in theory and practice that they will never fail to come to the rescue of the low powered amplifier to any programme. If, however, the criteria are thought to be response to step functions, square waves and other factors not relevant to programme, then of course faster dumpers must be used commensurate with the rise times involved.'

To prevent high frequency signals affecting the dumper stage the QUAD 405 has a simple low pass filter [LPF] between the input stage and the current dumping stage ~ LPF R12=3k3Ω and C6=1nF has a turnover frequency or point where the response is –3dB about 48kHz and at 20kHz it is about –0.7dB ~ I have changed R12=2k7Ω which gives a –3dB response to 58kHz and is –0.5dB at 20kHz ~ It hardly seams worth it and I was able to check the TID before and after the change but if you can't test the result then leave it as it is

The QUAD 405 input op-amp provides a source for the power clipper ~

The 'power clipper' or voltage limiter provided to protect ESL57s from overload in this version of PCB consisted of two zener diodes put into circuit with a link to ground ~ To function correctly the voltage limiter requires a point with sufficient voltage swing from a known impedance like R10

For my input stage modification the limiter was not going to be used so the link pins were removed and a hole left in the PCB was used to connect R3 to ground for the input modification as seen in the picture above and described next ~ This is an area of the schematic which was changed several times so refer to the build numbers when quoting

~ Non-Inverting input stage topology can improve QUAD 405 ~

This change of topology places the source impedance in parallel with the amplifiers non–inverting input and allows R4 and R6 to be lower which reduces the input noise at the inverting input

With the values shown the sensitivity is reduced to 1Vrms for 100W but the high pass filter [HPF] characteristics are maintained by scaling the value of C2 proportional to the gain reduction

R10 has been shorted and with this particular QUAD405 I slightly increased the low pass filter [HPF] response to 58kHz by lowering R12

With IC1 configured as a non–inverting stage the signal to noise ratio [S/N] referred to 30W in a 22kHz BW is now better than 100dB [112dB ref. 100W my limit of measurement] and 1kHz 100W distortion is better than 0.0032% indicating that a 1Vrms common mode signal is not causing a problem due to the op–amp input capacitance now changing with common mode voltage [varactor distortion effect as described by Walt Jung]

Measurements at 1W were now more stable and meaningful but still dominated by 50Hz and 100Hz from the mains supply ~ The most likely source of this would be the supply capacitors or the Tr1 current source which relies on C5 for its power supply rejection performance ~ Download the QUAD 405 input mod. schematic

There is an a.c. voltage from the output via R5 across C2 which peaks to about 1Vrms at the turnover frequency of the input input high pass filter at 14Hz ~ When checking the distortion at 20Hz after making the IC1 topology changes I measured about 0.5% at 10W and 1% at a 100W but above 80Hz and 10W better than 0.004%

The problem was traced to distortion from C2 which in this modification was now the green 47uF 3V bead tantalum ~ The 405 originally had a 100µF 3V tantalum bead for C2 but the low frequency distortion was not measured and the voltage across it would have only peaked at about 0.5Vrms at 14Hz

Putting everything back to original on one channel showed the distortion at 20Hz was 0.05% at 10W and 0.004% at 1kHz ~ There was no reason the distortion at 20Hz was higher other than the capacitor was still producing distortion albeit at a lower level and mainly 2nd harmonic which was proved by replacing C2 with a technically worse [ESR and dissipation] 100µF 63V standard electrolytic

The many reports that appeared in 1976 following the introduction of the QUAD 405 did not mention any significant difference between distortion at 20Hz and 1kHz which I thought was suspicious but then noticed these first production items had different values for R4 and R5 which make the a.c. voltage across C2 only 0.2Vrms at 14Hz 100W

The non-inverting topology appears to exacerbate the C2 distortion but here I found a definitive mechanism where a capacitor produced distortion which could be considered significant ~ The original circuit helps cancel the signal across C2 which arrives at the summing junction IC1 Pin 2 out of phase with the input signal at 100W

Further tests showed that the best capacitors that still produced distortion mainly produced 2nd harmonic and these included OS-Con and other audio wonders ~ A simple C↴R HPF placed between a low distortion a.c. source and audio analyser shows easily measurable distortion at low –3dB turnover frequencies with >1Vrms across C

Some higher voltage bead and wet tantalum capacitors may be okay for C2 if you are applying the non-inverting modification with gain reduction and can select them ~ QUAD used 10V 100µF bipolar capacitors for the 405-2 which apear to work well but I would use a good make standard electrolytic if unable to measure parts

This picture shows one of the PCBs with some of the mods mentioned above implemented ~ Top left of the picture capacitor C8 is in its new position wired directly between R38 and R20//R21 and now in the bridge ~ The white wire from R3 to IC1 is part of the input stage modification

The two Yellow tantalum capacitors top left are supply decoupling for the op–amp which may not be doing much as the impedance of the zener diodes is low by comparison to their ESR ~ later these were changed for 1µF 63V polyester caps and a new current source in place of Tr1 was added

The QUAD 405 with its old power supply capacitors and output connectors removed ~ Looking at the holes for accessing the heatsink screws it was obvious that they would do a better job mounting new speaker terminals connected directly to each PCB with short leads

New 4mm binding posts fit neatly in the holes after filling slots for the locating pips similar to those in the original holes ~ RCA phono sockets were fitted in the holes of the original output connectors ~ They were the isolated type but were connected to chassis at the 4pin DIN where the signal inputs are also paralleled

New EPCOS [Siemens] 22,000µF 63V Capacitors work well and are just short enough to clear the lid by a few mm ~ The wiring between the transformer, rectifier and supply capacitors is now as direct as possible and all ground connections including speaker 0V are made to the bus between the two capacitors

The spade terminals on the PCBs were removed and these connections directly wired using 19/0.2mm silver plated PTFE insulated wire

First switch on after replacing the supply capacitors was a bit of a non event even the 'bumps' in the output as seen on an oscilloscope and heard through speakers was much less than before and the previously low level hum was no longer audible

The QUAD 405 with modified PCBs and new high quality supply caps ready for the side panels lid and base plate to be refitted

The only other mod not shown was to solder the TLE2071 op–amps directly to the PCB to prevent the urge to swap op–amps at a later date and a change of C2

The marked up picture of my first QUAD 405 Modification ~ Left ~ shows changes or additions in Green and removed items in Red

This can be used in conjunction with the schematic diagram which details more information regarding the improved current source for Tr1 and earthing arrangement in place of the heatsink bolt connection ~ Click on the image for a larger view

As with all tweaks you have to consider at what level to stop ~ The final outcome will depend on the initial condition of the equipment and the design changes and components used and of course your skill and patients

Try not to forget that the QUAD 405 is an excellent design and that some of the changes shown here and published elsewhere could not have been reasonably be applied by a manufacturer at the time they were made ~ At least not if they wanted to make a consistently good and reliable Quality Unit Amplifier Domestic at a profit

If the DIN socket is removed the grounds of the phonos should still be connected directly to chassis to keep the output voltage offset to minimum ~ See QUAD 405–2 modifications ~ With PCBs M12368 iss.10 and earlier the output Zobel network C12 and R39 was connected to the input ground side of R2 and connecting the input lead ground to chassis or 0V is required to prevent speaker and or transistor damaging oscillation

Some components that I didn't already have to hand were obtained from Farnell as some people have asked for them here are the part numbers ~ I also have 1% polystyrene extended foil capacitors for sale

120pF 1% 630V
C13 & C14
Tr3 & Tr4


White Phono
4mm Output Connectors
Since 2002 experimenting with the QUAD 405 and 405–2 has continued along side my other audio experiments and developments ~ In addition to the changes outlined above lowering the open loop gain by bypassing Tr3 and fitting a 'ring of two' current source for Tr1 gives my DCD–Mod3 which is a sensible level of modification for the QUAD405 or 405-2 ~ You can then fit higher rated output transistors and remove the current limiters

In 2004 I modified some M12368 PCBs replacing the Class–A load resistors R30/R31 with a 50mA current sink and the output transistors with complimentary Darlington devices to make the DCD–Mod4 which is very similar to the NET audio Mk3 which followed some time after publication of my QUAD 405 Evolution document

The QUAD 405 is a very good design in a useful format and so was used ~ like other QUAD amplifiers between 1950 and 1995 ~ by the BBC for quality monitoring and for custom amplifiers like the AM8/16 ~ See QUAD 405–2 modifications and QUAD 405 Evolution and Example Mods for more information about the domestic 405 and 405–2

If you have a 405 that has blown fuses and damaged the output transistors or you want to test your modifications safely but have limited equipment then check the QUAD 405–2 modifications link above and see testing without Tr9 and Tr10

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