[www.keith-snook.info]

~ QUAD 22 Pre Amplifier RIAA PCB ~

Most of my repairs and modifications to QUAD 22s have been made easier or ~ like the one shown above ~ even possible since I made the QUAD 22 RIAA PCB (pdf schematic & layout) which fits neatly in place of the old valve bases of the EF86s V1 and V2 ~ It provides for 47kΩ loading of 3mV–6mV output MM cartridges

The PCB also mounts the components that would be in the DISC adaptor plugin and the Switch bank to provide cartridge matching and DISC equalisation ~ See QUAD 22 information for an explanation of the QUAD22 RIAA circuit and Modernisation & Modification for the changes to the switch bank required when fitting the RIAA PCB

Once the old V1 and V2 valve bases and the insulated posts mounting C2–C3–R17 and other EF86 components are removed the PCB can be fitted to provide the amplification and RIAA equalisation required for a better DISC stage ~ The input switch bank is then free to provide more stereo inputs as required by a pre–amplifier today
The QUAD 22 RIAA PCB is based on the QUAD 22 schematic but provides more accurate RIAA equalisation and improved channel phase and level balance while still being true to the original design concept

The QUAD 22 DISC circuit requires a >500kΩ load impedance but if a recording device is not used this will not be a problem

I fit silver plated copper PTFE insulated wires to test the assembled PCBs ~ PTFE is not easy to strip without the correct expensive tool so I cut the lenghts of wire to fit a QUAD 22 as shown in the pictures above and below which makes fitting the board simpler as no wires should need stripping ~ The original brown heater wires should be carefully removed from V1 base and soldered to pins 4 and 5 of V1 on the PCB which will be tinned ready
The left and right input leads are the twisted white/black and red/black wires in the picture left ~ These are intended to connect to RCA Phono sockets that are insulated from the back panel to eliminate internal earth loops and Neutrik/Rean NYS367 connectors are ideal for this

Screened cables should not be required inside the QUAD22 but keeping the input wires separated will prevent cross talk from unselected inputs as will wiring the switchbank to my "QUAD 22 Switching B" schematic

I sell asembled and tested PCBs complete with mounting posts and PTFE wires for 75GBP each including UK or EU postage ~ Price at May 2017 ~ If you buy the ready assembled board there is still a lot of work you need to do ~ Removing and cleaning the switch bank to accomodate the outputs from the RIAA PCB and additional line level inputs and fitting better input connectors

Ceramic valve bases are fitted to the solder or track side of the PCB which is mounted on M3 by 8mm stand off pillars using the holes left after removing the old valve bases ~ The ceramic bases are thus held between the PCB and the chassis allowing valves to be changed many times without damaging the PCB tracks as often happens with PCB mounted valve bases

The circuit is assembled using 1% MRS25 metal film resistors and 1% extended foil polystyrene capacitors for the equalisation ensuring a tight adherence to the RIAA replay curve and excellent channel level and phase balance which gives a precise stereo image not normally experienced when using the QUAD22

Provision is made for separate Heater and HT supplies for each channel if required or you could use d.c. heater supplies ~ In the rather severe modification shown above the heaters are a.c. powered separately from each QUAD 11 power amplifier ~ The HT supplies are combined with 1N4006 diodes to share the load between the QUAD II power amps to reduce the likelihood of transformer melt–down

This PCB can also be used as a stand alone valve phono pre-amp or fitted inside other valve pre-amps ~ If required different equalisation like various course groove 78rpm or tape head can be applied and even different equalisation for each channel with 2 mono channels each specially optimised for a particular playback EQ

I have also made a triode amplifier and buffer PCB in the same format which can be used as a flat response 'line' stage with low output impedance in place of V1 and V2 in the QUAD22 or it could be used to make a stand–alone valve pre–amp when coupled with the RIAA PCB or a triode RIAA amplifier with passive equalisation

QUAD 22 RIAA PCB modification by Keith Snook
On the left a QUAD 22 control unit is modernised to the drawing QUAD 22 Switching C where the tape output [TO] phono connectors are used as another input giving 4 line level inputs

This is only possible because the switch bank is free of the yellow channel DISC EQ components which are now mounted on the RIAA PCB


~ QUAD 22 Tone Control Capacitors ~

Another significant improvement can be made to the QUAD 22 by changing the Tone Control section capacitors for polystyrene 1% 160V extended foil types ~ 5n6 for C18 C19 C20 and C21 ~ 3n6 for C22 C23 C28 and C29 ~ 1n5 for C30 and C31 ~ The original tone section capacitors are not the best and are not suitably mounted

A kit of 10 x 1% 160V polystyrene capacitors for the QUAD 22 Tone Control section are available for 12GBP inc. UK P&P or 10GBP if posted with the RIAA PCB or other parts from this site ~ C24 C25 C26 and C27 should already be 220pF polystyrene but I can supply 1% 630V polystyrene extended foil types for an additional 1GBP or I will supply 10% parts for C24 to C27 free of charge with the tone capacitor kit if requested

The old capacitors were originally soldered with leads only a few mm long in some places ~ When fitting any replacement capacitors especially the 1% 160V polystyrene I supply it is best to fit them as shown on the blue channel above ~ The old C18 C19 C20 & C21 for the Bass control can be cut out without unsoldering and the replacements soldered directly to the potentiometer tags

The old filter section caps C30 and C31 and the wires from the PCB tags to the filter potentiometer should be removed after lifting the ends of C26 and C27 ~ The long leads of the new 1n5 capacitors can be used to replace the wire removed as shown

There are 2 options shown above for the treble control caps ~ note the tag where used is only for support and not soldered ~ When fitting new resistors carefully remove the insulated wires before removing the old resistors and fit the capacitors and insulated wires last preferably with a single soldering operation

A good solder sucker is a must have tool for cleaning the tags of solder ~ Lead/tin solder should be used to fit the new parts ~ Lead free solder is not suitable for repair work on vintage equipment and I even use vintage flux solder for many repairs because it sits on the tags and does not run down them so readily


~ RIAA Amplifier Kits and bare PCBs ~

QUAD 22 Bare RIAA PCB by Keith Snook Bare PCBs or complete kits for this project are available ~ If the circuit is built to the schematic and following the silk screen on the PCB it should work first time and last for a very long time

Bare PCBs with green solder resist are 7GBP each or supplied with 2 valve bases and mounting kit are 22GBP each including UK or EU postage

Complete kits with mounting spacers as shown below are supplied in a "really useful" project box for 50GBP each including UK or EU postage ~ Prices at May 2017

If you buy the Kit you should find it very easy to assemble following the schematic and these instructions

Start by fitting the lowest profile parts first ~ The MRS25 resistor leads are bent close to the body and with them fitted either a few at a time or all together the board is turned over on a paper pad or card and the leads soldered and cut flush ~ Resistor colour codes can be found here

Part populated QUAD 22 PCB by Keith Snook
Picture QUAD 22 RIAA PCB Next fit C4 and C14 and then the 1% extended foil polystyrene capacitors C1 and C11 then C2~C6~C12~C16

Although the ends of extended foil polystyrene capacitors often look rough it is the fact that the foil ends are "spattered flat" and soldered over that makes these parts very low loss and good for audio and even RF ~ Take care when fitting as the leads need to be bent close the the ends of the caps

Next fit C3~C5~C13 and C15 and that's all the parts on the component side fitted

When I design PCB layouts I often make custom footprints and silk screen outlines to get a consistent style ~ In this case some footprints had to be reduced to fit the tight space while allowing good clearance for the high voltage and signal tracks ~ The valve base footprint requires some pin bending . . .

picture QUAD 22 RIAA PCB alnost built
Picture B9A valve base I found a small footprint B9A ceramic valve base available in quantity and without gold pins ~ The pitch circle diameter [PCD] of the pins has to be reduced slightly to fit the space available ~ By bending the pins out at about 45˚ and then bending them back close to the base they fit the custom footprint with its reduced PCD ~ Before soldering check that the shoulder of the pins is tight to the PCB pads and bend the pins inward on the component side to lock them in place as seen in the last picture below picture B9A ceramic valve base
picture QUAD 22 PCB valve base fitting The valve bases are mounted on the solder side of the PCB with M3 spacers to position the ceramic body in the chassis holes ~ The M3 machine screws are fitted from the component side then a lock washer and the spacer are fitted on the solder side and tightened so that they stay in place when fitted to the chassis with the M3 nyloc nuts ~ Imagine the chassis between the spacer tops and the nuts shown in the picture
Before fitting the PCB to the chassis prepare and fit the wires for Ground ~ HT ~ Signals (supplied with built and tested units but not the kit) ~ Input signal wires do not need to be screened ~ Twisted wires for ground and signal to each DISC phono are easier to work with and do not pick-up hum or cause crosstalk if short and routed directly

For the PCB shown brown wires link the heater connections and the original brown heater wires to V2 will be soldered directly to pins 4 and 5 of the V1 base on the component side

Picture QUAD 22 PCB complete
Built and tested PCBs are posted with PTFE insulated wires attached unless you do not them ~ I have not gone to the extent of making a test jig for the assembled PCBs plus the test pins would have to be on the same side as the valves ~ which is awkward ~ so PTFE wires are soldered to the boards for test connections

Because PTFE wire is difficult to strip without the correct tools I fit stripped lengths such that the board can be wired directly into the QUAD22 chassis with HT taken from the R18–R19 junction as shown above where I have also provided extra supply decoupling in place of C8–C9 which are removed as part of the mod

If you are building the kit the table below may help you prepare the leads which should be 0.5mm diameter solid wire or 7/0.2mm stranded to best fit the PCB holes ~ The wires need to be soldered to the PCB before it is fitted to the chassis ~ Black wire is used for all ground/chassis ~ Out–Y and Out–B do not have a corresponding ground wire

Wire from
Wire to
Colour
Approximate insulated length
In–B Gnd–B Left channel input phono White/Black ~ Twisted 150mm ~ Twisted length
In–Y Gnd–Y Right channel input phono Red/black ~ Twisted 150mm ~ Twisted length
Out–B Left output to switch bank White 100mm
Out–Y Right output to switch bank Red 100mm
HT–B Junction of R18 and R19 Orange 80mm
HT–Y Junction of R18 and R19 Orange 110mm
H1 & H2 H3 & H4 Brown ~ Twisted 70mm ~ Twisted length
Gnd x 2 Chassis posts near V1 V2

Black

50mm

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