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~ The QUAD 22 Pre–Amplifier or Control Unit ~


The following information is based around a late model QUAD 22 pre–amplifier ~ If yours differs in some respects it's probably an earlier model ~ Considering that QUAD Hi–Fi (Acoustical Manufacturing Company) like most good manufacturers only made changes to their products for reasons of improvement [unlike today] I suggest that you bring your amplifier up to the latest spec ~ Download pdf Schematic

I don't recommend that you change every component just for the sake of it but you will most likely find that unless your QUAD 22 has been previously correctly repaired or manufactured within the last few years! it will require many discrete components changing ~ I'll list the usual suspects in the order I remember they are likely to turn up and will try to give a description of the symptom from the fault


~ QUAD II Power Amp fault can damage QUAD 22 Control Unit ~


A common fault with the QUAD II power amplifier is that R12 (180Ω @ 3W) goes open circuit. ~ R12 actually dissipates about 3.8W under perfect conditions but with aged and or mismatched output valves the dissipation in R12 is often over 5W ! ~ When R12 fails the HT voltage rises and if it's the QUAD II connected to the yellow channel of your QUAD 22 you will most likely have to replace R32 and R25, possibly R30 and R31 but hopefully not C15

Capacitor C15 can get "dried out" if the system is run for some time with the above QUAD II fault especially if R32 has been replaced with a higher power modern resistor which no longer acts as a fuse! ~ If C15 does dry up and so becomes low in value this may show as the system "motor boating" ~ You're playing some music loudly or with the bass control set to maximum and the speakers start to pump furiously and sound like an old motor boat and continue to do so even after you turn down the volume to minimum ~ it's time to look at changing C15 along with C12 and C7 which are unfortunately in the same can

You could simply lift the connections from the C15 section and substitute a single capacitor but it's likely the other sections require replacing so it's best to change the whole thing if you can find a replacement ~ If you cannot find a direct replacement and decide to substitute all three sections with modern types ~ 22µF to 68µF at 350V to 450V will be fine ~ larger values will add nothing and may be hard to fit ~ You could try and fit three new 22µF capacitors inside the old can


~ Components Age ~

The original "carbon composition" resistors R32 R25 R20 R30 R31 R18 R19 always appear to go high in value due to age and temperature but due to the nature of the design this may not be noticed until they have increased excessively in value ~ The pink high stability carbon film resistors R16 R17 R21 R22 R23 and R24 very rarely give trouble and should be left untouched as the main cause of their failure is the end caps being loosened during rework

Once you have the lid off anyway you should check all the resistors with a meter ~ after the power has been off for some time but with the QUAD 22 still hot ~ and change those that are out of tolerance ~ Always suspect carbon composition resistors especially where they have a high voltage across them or they have high values say 220kΩ and above

The original solder joints will often have more solder than text books on the subject would recommend ~ they were also made with solder that had an acidic resin flux and a lead content ~ Unless you are using the same original solder try to remove as much of the old stuff as you can so you are not mixing your own solder alloy with unpredictable results !

Whatever you do ~ AVOID LEAD FREE SOLDER ~ It is not suitable for repairing vintage electrical equipment ~ The WEEE and RoHS directives requiring low or no lead simply do not apply – You are not manufacturing thousands of items with a view to disposal in a few years ~ You will be doing a more environmentally friendly repair which hopefully will give the amplifier many more years of active life

If your QUAD 22 has a QUAD repair sticker you will most likely find that ~ if nothing else has been done ~ the 1.5MΩ resistors R18 and R19 have been changed probably along with C8 and C9 which would originally have been 0.5µF Hunts AM104 paper types rated at 150Vdc or 250Vdc which were replaced by QUAD with 0.47µF 250Vdc polypropylene [PP] that should last for ever ~ R18 or R19 going high and or C8 or C9 going leaky will lower the gain of the respective EF86 stage which can show as a reduction in bass for the DISC input and possible distortion of both the DISC and TAPE inputs

QUAD-Hi-Fi-QUAD-22-R18-R19
Compare the full underside view in the picture above with the one opposite which shows elements of a very common QUAD repair around V1 and V2 – Affecting DISC and TAPE inputs

The late production unit pictured above had been repaired after a few years with only R18 and R19 changed to carbon film types whereas the QUAD 22 shown left was repaired by QUAD in the 1980s and also had C8 and C9 changed as well as the electrolytic capacitors C5 C6 C10 and C11 ~ Note that C4 and the other moulded paper capacitors were not replaced and unlike the new C8 and C9 they will not last another 20 years

The original electrolytic capacitors were much larger than today's suitable replacements (compare the sizes of C6 in the inset above with those shown in the full view higher up the page) they were also the least reliable components 30–40 years ago but modern replacements should give excellent results for many years especially if they have a high temperature rating ~ 105°C or better ~ but not an excessive voltage rating ~ up to 16V is good enough and should ensure the capacitors remain formed

I know some people rave about using Tantalum capacitors where electrolytics have to be used ~ like C5 C6 C10 and C11 in the QUAD22 ~ If you feel the need to use Tantalum capacitors fit the tinned brass can military axial type ~ Not the small resin covered radial beads which look ugly and often have short leads that break off when mounted in an axial position


~ QUAD 22 Control Unit switches mains to QUAD II Power Amplifiers ~

If C7 C12 or C15 (see above) start to "arc" the noise through both power amplifiers can sound very worrying ~ A similar "arc" sound may be heard if S4A or S4B (on/off switch on volume control) becomes dirty ~ If left without attention the switch sections will eventually fail and the respective channel will not power on

Slightly less common ~ and only affecting the blue channel is the failure of S1A which is a mains rated switch linked to the STEREO button ~ If either S1A or a section of S4 are faulty swapping the power amplifiers will show the problem to be the QUAD 22 control unit and not a power amplifier

The switches S1 and S4 can be cleaned with a good contact cleaner such as Electrolube EML ~ Aim the tube provided where the solder tags meet the insulation and the oil will get drawn into the switch ~ EML is also excellent for cleaning the volume and tone control pots and other switches

Total failure of S4 often occurs and no amount of switch cleaner can fix it because the insulated arm inside becomes worn and cannot operate the switches ~ It is possible to remove the switch section from the volume control and repair it with epoxy or a new paxolin arm hand crafted with a small file or maybe stolen from another old switch

If S4 is beyond repair ~ or you just want to fit a new possibly "better" volume control that does not have a mains switch ~ it is possible to wire the STEREO/MON switches as an on/off control ~ S1A is normally used to switch off the blue channel for MONO listening on the yellow channel only but nowadays it is likely that this feature is not required and the STEREO switch can be used as an ON control, the MON switch used as OFF and 2MON becomes on in MONO if required


~ QUAD 22 Control Unit combined Volume and Balance controls ~

Often the original volume control will show bad channel balance ~ As volume is adjusted the stereo image shifts ~ This can be due to several factors ~ First check that the balance control is not moving as the volume is adjusted ~ This can often be cured by removing the volume knob and oiling between the volume and balance shafts ~ Also check that the volume knob is not rubbing on the balance disk ~ If the balance disk is set too far forward it may be easier to operate the small balance knob but the balance can also be moved by the volume control
Note above how the volume knob has rubbed the balance disk which is shown back in its correct position flush with the spindle

The balance control is wire wound and often goes "noisy" before the volume control shows problems ~ It is not easy to use contact cleaner on either of the controls because they are usually sealed types but if you remove the front panel as shown above you can rotate the balance fully to its extremes ~ The balance control is wire–wound in 3 sections and that part in the limited centre range is fine wire with a total resistance about 9kΩ ~ Either side of centre is coarse wire which if you rotate the balance back and forth to its extremes several times can actually clean the wiper and clear the problem

Another mechanical problem affecting the entire control including the switch is the insulated wiper arms wear on the shafts and become sloppy ~ The only cure is to disassemble and thoroughly clean the parts and then glue the wiper and switch arms to the shaft with epoxy resin ~ This is not easy as unsoldering the sections is difficult and you have to ensure that the sections of volume control are physically aligned as the glue sets but if done carefully you will have no more problems

If the volume control is unbalanced only at low level and mechanically good measure the residual resistance at minimum volume but not switched off ~ if there is a great difference in the two readings with one more than 3 times the other ~ you could use a resistor in the ground connection of the lowest residual section to make the 2 readings as measured from the wipers to ground the same ~ This will better balance the two sections throughout the whole range ~ Also check the ground wire soldering ~ Beware that heating the tags of the volume control may change the residual resistance

If you can find a volume control with a 1/8" spindle (and better still with a mains switch) you could try to implement a "slider" balance as fitted in the QUAD22 volume replacement KIT R22VOLM which unfortunately is no longer available from QUAD ~ Even if you only have a pot with a 1⁄4" spindle you can still make a slider balance control but will have to machine the volume control knob ~ Such changes along with the modifications below can make the 22 a well featured "modern" design

The R22VOLM kit uses a dual 500kΩ volume control with 1/8" spindle and replaces the balance section with a 10kΩ slider

Note Yellow and Blue are reversed from the QUAD document because the yellow channel is now the Left channel after fitting Red and White replacement phono connectors

A small contersunk hole is drilled in the balance disk and a screw fitted to mount the connecting link ~ A bush is fitted to reduce the 1⁄4" hole to fit the the smaller shaft

500kΩ pots with switches are now hard to find and with 1/8" shafts even harder ~ A 6mm shaft dual 500kΩ without a switch could be used with S1A used to switch the mains

When fully assembled it is clear how the slider now replaces the balance control

As an alternative to this you could fit a dual 500kΩ without a switch and fit a balance pot on the back again using S1A to switch the mains or maybe do not have mains going to the 22


~ QUAD 22 Control Unit Phono or DISC equalisation ~

QUAD-Hi-Fi-RIAA-DISC-EQUALISATION

Click on image for bigger picture
Click here for plug–ins for various cartridges

The QUAD 22 provides DISC equalisation for many mono disks ~ On the left is a 115mm by 60mm plastic plate which was supplied with early QUAD 22 control units at a time when there were more 78rpm mono recordings than stereo 45rpm and 33rpm

The intention was that you screwed the plate inside your Hi–Fi cabinet door for reference ~ The DISC button alone provides stereo RIAA equalisation but there is additional equalisation available in Mono on the Yellow channel or both channels when using the 2MON button and the Mono DISC input

The simplified extract from the QUAD 22 schematic below shows how the combination of MIC–DISC–TAPE buttons provides equalisation (EQ) for the different manufacturers mono disks ~ The different replay time constants are switched by the MIC and TAPE buttons in conjunction with the DISC button for the Yellow channel only

When the MON button is pressed the Blue and Yellow pick–up connections are joined for mono and connected to the Yellow EF86 amplifier V1 unless a 2 letter PU Adaptor is fitted and the single MON PU is used (see table below) ~ If in addition the 2MON button is pressed then the mono output however it is derived is sent to both speakers in a stereo setup

If you refer back to the picture of the QUAD 22 control unit underside you can compare the diagram below with the physical layout and note the large area of "switch real estate" used for the DISC input to accommodate the various mono disk equalisations on the Yellow channel

The EF86 amplifiers V1 and V2 are also used for the TAPE input EQ and although almost all the 7 pin TAPE adaptors supplied were TAPE N ~ line level x1 gain or 70mV sensitivity ~ allowance was made to connect the low level output from a stereo tape head directly to the QUAD 22 to amplify and equalise the signal via the TAPE inputs but this could only be done for one speed or replay characteristic unless adaptors were changed

During the 1950s many manufacturers produced open frame tape decks [as opposed to tape recorders] for people to build into their own cabinets ~ These were often playback only and many had replay amplifiers with equalisation linked to the speed control but for those that didn't pre–amplifiers like the QUAD QC22 could be used to play tapes

Tape recorders provided an equalised output for each speed anyway and as the recording process also required such a machine the idea of equalisation at the pre–amplifier never caught on and the flat response TAPE N adaptor became prevalent

The 7 pin adaptor was also offered in a MIC N version with a level response and 1.5mV sensitivity for home recording or PA use but again if you had a tape recorder it would most likely already have a microphone input

QUAD Hi Fi QUAD22 RIAA schematic
The 9 pin DISC adaptor of the QUAD22 was provided to accommodate different types of pick–up cartridge and also mono and stereo cartridges connected simultaneously ~ See pdf list ~ In the 1950s the two main cartridge types were Crystal or Ceramic and Moving Magnet or Magnetic ~ Moving coils were very rare and would have been used with a 'matching' step-up transformer using the DISC A adaptor

Magnetic cartridges produce a voltage output that doubles for every octave increase in frequency +6dB/octave or 20dB/decade ~ Ceramic cartridges with the correct high impedance loading produce a constant output with frequency ~ During the mastering of a record disk the magnetic cutter head excursions naturally reduce at 6dB/octave so as frequency increases lateral excursions become smaller and recorded signal to noise S/N would become worse

To correct for the cutter head output falling with frequency a +6dB/octave record equalisation or pre–emphasis is applied ~ Below a low frequency ~ commonly 50Hz ~ the drive to the cutter is not 'emphasized' to prevent these signals causing large cutter excursions ~ A 50Hz or 3180µs pre–emphasis alone would make the input drive to the cutter at 1KHz about +26dB and at 20kHz about +52dB which is more than enough to compensate for the reduced cutter excursion with rising frequency but would cause excess distortion and damage to the head so a step reduction in drive is made and levelled out either side of 1kHz as described herein for the RIAA or BS1928 EQ ~ Other disk record EQs follow a similar pattern but with different time constants to describe their EQ curves

When using a magnetic pick–up its natural +6dB/octave response has to be corrected with an opposite RIAA 3180µs replay time constant which in the QUAD 22 is provided by R1 and C1+C5 in the DISC adaptor ~ It may not be obvious at first but the value of C1 is multiplied by the gain of the EF86 by miller effect so it appears as 46nF from grid to ground and C1+C5 = 47nF * R1 = 68KΩ gives the 3180µs replay time constant

For the 4mV adaptor DISC A the gain is all that a single EF86 can deliver and is about 255 x 180pF = 46nF with a good valve ~ For the 10mV adaptor DISC B C1 is higher at 560pF and the gain is reduced by R2 = 10MΩ to about 82x ~ If there is a fault around the EF86 stage or the valve is simply low gain then the bass response will be reduced ~ It is also possible to "overload" the TAPE OUT and also reduce the bass

With a crystal or ceramic pick–up a constant lateral amplitude above 50Hz would give a constant amplitude output which has led some to suggest that RIAA or other replay equalisation is not required but this is not true as you will see below ~ crystal or ceramic cartridges require a high impedance load and capacitive coupling which is –3dB at 50Hz to correctly equalise at low frequency ~ The QUAD adaptors DISC E and F have high impedance by virtue of R1 = 1.5MΩ and 2.2MΩ respectively and gain lowered by R2

Most record disks cut from 1954 onward have a 3180µs time constant so the signal sent to the cutter head increases at +6dB/octave above 50Hz but this increase of amplitude with frequency cannot be allowed to continue because the lateral excursion would be difficult to cut at high frequency ~ Therefore 2 other time constants are used during the cutting process such that the relative record amplitude is made to fall by –6dB at 1kHz and then level out again at –12.4dB beyond 1kHz

The 2 additional time constants defined by RIAA or BS1928 for stereo 45rpm and 33rpm records are 318µs and 75µs which relate to nominal 3dB points either side of 1kHz at 500.5Hz (+2.64dB) and 2122Hz (-2.867dB) ~ The amplitude step between 500.5Hz and 2122Hz does not look significant when viewed on the graph below but if the 6dB/octave due to 3180µs is 'removed' the step is seen to be 12.5dB

If the 318µs and 75µs replay time constants are not used for replay as often occurred with some crystal or ceramic or strain gauge phono stages because they do not require the 3180µs correction of a magnetic transducer the replay response between 20Hz and 20kHz could fall 12dB or ± 6dB either side of 1kHz ~ Many Ceramic cartridges had a poor high frequency response and relied on mechanical resonances and damping and less than optimum output loading to provide a flat(ish) frequency response

Some modern strain gauge cartridge makers appear to use different rules

"Note that this is not a phono stage, the cartridge having a natural 6dB/octave roll-off. Pedants for accuracy will complain that the resultant output only approximates the RIAA curve, designer Peter Ledermann maintaining that our ears are considerably less sensitive to frequency response errors than to the distortions and phase shift aberrations that would result from employing correction filters" ~ John Bamford & Paul Miller ~ hi-fi news

"In reality, there are subtle changes in equalization in the RIAA curve that need to be compensated for, and Soundsmith's preamplifier does that—but it's still much simpler than a full-blown RIAA circuit, and the resulting frequency range is claimed to be an astonishing DC–70kHz" ~ Michael Fremer ~ Stereophile

Luckily the QUAD did not offer support for a strain gauge or have a plug-in for such ~ so sensible people like us need not worry about how flat the frequency response would be with 318µs and 75µs time constants as used by the 22 for 'non magnetic' pick-ups ~ If we did wish to use a strain gauge we would have to use the custom phono stage to power it and give a 'flatish' response into one of the line inputs

QUAD Hi-Fi RIAA curves

When using a Crystal or Ceramic cartridge the QUAD 22 applies all 3 time constants required for a correct disk replay response ~ Referring back to the schematic extract above the RIAA time constants at 318µs and 75µs are cleverly implemented by reducing the feedback and thus the miller effect of C1 which increases the gain as required between 500Hz and 2122Hz giving the 12.5dB step

For the yellow channel  (R7+R10) * C2 = 75µs and (R7+R10+(R8//R9)) * C2 = 318µs

Note that R8//R9 ≈ 47kΩ and R7+R10 ≈ 15kΩ and for frequencies below 100Hz C2 can be considered open circuit and beyond 10kHz C2 can be considered short circuit so the amount of feedback is reduced by the potential division ~ 15kΩ/(15kΩ+47kΩ) = 0.24 ≈ 12dB ~ The 318µs EQ is better balanced with blue channel if R8 is made 110kΩ

For 78 rpm disks the other 2 time constants vary between manufacturers but from 1954 onward the common standard became known as the "coarse groove characteristic" with time constants of 450µs giving a response –3dB at 354Hz and 50µs such that the response levels out beyond 3183Hz ~ The QUAD 22 also provides 450µs with 25µs and 318µs with 100µs to equalise a range of 45rpm and 78rpm disks as indicated in the "Record Equalisation Guide" card shown above and in terms of their time constants as shown in the table below

MIC
DISC
TAPE
Yellow Ch EQ with C2
RIAA


3180µs
318µs
75µs
R10+R7 & R9//R8

3180µs
318µs
100µs
R10+R7+R4 & R9//R8
Coarse Groove

3180µs
450µs
50µs
R10+R4 & R9
430µs
25µs
R10 & R9


Taking the coarse groove equalisation as an example ~ This requires the DISC and TAPE buttons pressed simultaneously and only works for the yellow channel or with the mono pickup input with MON and STEREO pressed ~ (R4+R10) * C2 = 47µs and ( R4+R10+R9) * C2 = 457µs

Although the 450µs and 50µs characteristic became known as coarse groove for commercial 78rpm disks it was also used for fine groove 33rpm recordings for archive use and transporting radio programmes between broadcasters from about 1951 to 1954 before the RIAA curve was ratified ~ These disks being for professional use would often have the speed and playback time constants printed on the labels so there was no confusion

On my re-drawn QUAD 22 schematic I have marked C1 and C2 as 5n1 (5.1nF) capacitors because this is the nearest standard value available ~ But 4n7 1% capacitors that fit in place of the original 5nF Dubilier 400 capacitors (which were optimistically rated at 10% tolerance and may have been okay for a few years after fitting) are more readily available and are a good second choice

A more accurate EQ can be provided with 4n7 1% capacitors fitted for C1 and C2 if for the blue channel R6 is made 16k 1% and R5 51k 1% ~ For the yellow channel R4 and R10 should be made 5k1 1% resistors and R7 made 11kΩ 1% ~ R9 needs to 85k5 and the parallel R9//R8 should be 51k5Ω which can be achieved if R9 is made from 300kΩ in parallel with 120kΩ and R8 is 130kΩ ~ All resistors from my preferred E24 series


~ QUAD 22 Pre–Amplifier DISC Adaptors ~


QUAD Hi Fi pickup adaptors

PU Adaptor A ~ 4mV Magnetic ~ 68kΩ load
PU Adaptor B ~ 10mv Magnetic ~68kΩ load
PU Adaptor E ~ 300mV Ceramic ~ 1.5MΩ load
PU Adaptor F ~150mV Ceramic ~ 2.2MΩ load

TYPE
R1
R2
R3
R4
C1
C2
C3
C4
C5
C6
A
68kΩ
180pF
1nF
AA
68kΩ
68kΩ
180pF
180pF
1nF
1nF
AB
68kΩ
10MΩ
68kΩ
180pF
560pF
1nF
AE
68kΩ
330kΩ
1M5Ω
180pF
1nF
47pF
1nF
B
68kΩ
10MΩ
560pF
BA
68kΩ
10MΩ
68kΩ
560pF
180pF
1nF
BB
68kΩ
10MΩ
10MΩ
68kΩ
560pF
560pF
BE
68kΩ
10MΩ
330kΩ
1M5Ω
560pF
1nF
47pF
E
1M5Ω
330kΩ
1nF
47pF
EA
1M5Ω
330kΩ
68kΩ
1nF
180pF
47pF
1nF
EB
1M5Ω
330kΩ
10MΩ
68kΩ
1nF
560pF
47pF
EE
1M5Ω
330kΩ
330kΩ
1M5Ω
1nF
1nF
47pF
47pF
F
2M2Ω
1MΩ
330pF
33pF

The earlier Mono Quality Control Unit II ~ QCII ~ used a similar style 9 pin B9A based plug-in for Pick–up and Microphone matching but these were Marked R1 to R13 or M1 to M13 ~ The QC II Phono/Microphone amplifier stage used an EF86 but the equalisation feedback was applied in a more "traditional" manner similar to the LEAK and many other 1950s pre–amplifiers ~ The QUAD QC22 phono circuit was not one of these "me too" designs


~ QUAD 22 Pre–Amplifier TAPE Adaptors ~

MIC N ~ 1.5mV ~ 100kΩ load ~ Level Response for Stereo Microphone
TAPE N ~ 70mV ~ 100kΩ load ~ Level Response same sensitivity as RADIO
TAPE H1 ~ 6mV ~ 1.5MΩ load ~ 100µs CCIR 7.5"/s tape head replay
TAPE H2 ~ 7mV ~ 1.5MΩ load ~ 50µs NAB 7.5"/s & 15"/s tape head replay
TAPE H3 ~ 7mV ~ 1.5MΩ load ~ 33µs CCIR 15"/s tape head replay

TAPE N1 ~ 300mV ~ 430kΩ load ~ Level response for CD
TAPE N2 ~ 300mV ~ 100kΩ load ~ Level response for CD


TYPE
R1
R2
MIC N
100kΩ
100kΩ
TAPE N
100kΩ
100kΩ
TAPE H1
6.8kΩ
TAPE H2
3.3kΩ
TAPE H3
2.2kΩ
TAPE N1
430kΩ
100kΩ
TAPE N2
100kΩ
22kΩ


In the schematic extract above I have also included the Radio input switching to show how it goes directly to the volume control ~ unlike the TAPE and MIC and DISC inputs which use the EF86 amplifier stages and are responsible for much of the wiring between the back plate and the switch–bank and the bad cross–talk between inputs and channels

It is possible to modify the tape input adaptor to have say 300mV sensitivity for a CD input ~ I did this on my QUAD 22 in 1981 not long after getting a Philips CD 104 and as you can see above there are several options but you should not make the input impedance too low
QUAD Hi-Fi rear panel adaptors
If you decide that you will never use a ceramic cartridge and you only require RIAA equalisation ~ because you only have vinyl disks made after 1954 ~ then you can dispense with the often troublesome TAPE and DISC plug-ins and the poor DISC equalisation and channel frequency response imbalance that gives a blurred stereo image

The TAPE input today would to be a nominal 100mV or 300mV sensitivity and would be switched directly to the volume control like the RADIO inputs are ~ The MIC+RADIO 2 input sockets can be used for a 300mV CD input which is also switched directly to the volume control bypassing the EF86 amplifier stage

For "modern" use only the DISC stage requires the extra gain and equalisation provided by the EF86 valves V1 and V2 ~ The components needed for RIAA (or other) equalisation can be replaced or even removed from the switch–bank and new components can be fitted around the EF86 valve bases to make a more reliable QUAD QC22 based on the schematic below

QUAD Hi-Fi QUAD 22 modification

There are some changes to component values on the above schematic to make the RIAA EQ more accurate by using readily available 1% standard value components and also to provide the cartridge with a 47kΩ load ~ This plan removes the EQ components from the switch bank so the EF86 valves are only used for DISC input

The tape output is provided as in the original circuit but modern recording devices may load the DISC stage too much and possibly the RADIO input if using a QUAD FM1 or AM1 tuner ~ There will be a reduction in overall level and bass response as the load on the tape output TO is reduced

Rewiring the DISC and TAPE plug–in circuitry around the bases of V1 and V2 is time consuming and the other EQ components are awkward to change without removing the switch bank so I made a replacement PCB to accommodate this circuit which makes modernising & modifying the QUAD22 much easier and neater if you only intend to use RIAA (or another fixed EQ) for DISC input and you make the TAPE input a direct connection to the volume control similar to the RADIO inputs
When I use the RIAA PCB or build them for others to fit they are tested on the bench for gain and frequency response and signal to noise ratio S/N with surprising results ~ The S/N is often about 60dB referenced to 3mV from an RIAA network and input terminated with a pick–up cartridge which is either a Goldring 1000 series or Sumiko Blue Point

The QUAD 22 handbook states Total hum and noise: Better than –70dB which we must assume is referenced to its 1.4Vrms output ~ Noise alone is defined as –80dB or where applicable, the equivalent noise of the pick–up load impedance at the input ~ which suggests the Noise Figure NF of the QUAD 22 DISC input is very good and my measured S/N near 60dB would suggest it is

The response of the RIAA characteristic improves the S/N by about 4dB compared to a flat resonse when measured in a 23434Hz bandwidth which is the noise bandwidth of my AA501 audio analyser ~ The S/N limit referenced to 3mV at 1kHz due to the 47kΩ input resistor of the RIAA PCB alone is calculated to be 57dB so it looks like the QUAD 22 EF86 pentode RIAA stage may have a NF less than 1dB which is very respectable

The S/N I measure will not be realised in the QUAD 22 unless hum from the heaters is addressed ~ While testing the RIAA PCBs on the bench with d.c. heater current I noticed that Mullard EF86s with mesh screens were about 10–15dB more noisy than Telefunken with solid screens ~ With the workshop fluorescent lights turned off all EF86s tested simlar low noise ~ EF86s with mesh screens appear to be modulated by light which does not happen when they are in an enclosure like in the QUAD 22 with its cover fitted

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