QUAD 22 valve pre amplifier RIAA replacement PCB

Most of my repairs and modifications to QUAD 22s have been made much easier and even possible since 1995 when I made the QUAD 22 RIAA PCB which is based on the original QUAD circuit (pdf with layout) and fits neatly in place of the circuitry and valve bases of the original EF86 stages V1 and V2 — Also see QUAD 22 information for an explanation of the QUAD22 RIAA circuit and required changes to the input switching Once the old V1 and V2 valve bases and the insulated posts for C2 — C3 — R17 and other EF86 components are removed the PCB can be fitted to provide the amplification and RIAA equalisation required for the DISC stage — leaving the input switch bank free to provide more stereo inputs as required by a pre-amp today — see example schematics QUAD 22 Switching "B" or "C"
The QUAD 22 RIAA PCB is based on the QUAD DISC schematic but provides a more accurate RIAA equalisation and much better channel phase balance while still being true to the original QUAD design Using the original circuit it requires a high (>100kΩ) load impedance but if a recording device is not used this will not be a problem		Assembled and tested PCB plus mounting kit is 60GBP inc. UK P&P
New ceramic valve bases are fitted on the solder side of the PCB which is mounted on M3 by 8mm stand off pillars using the holes left after drilling out 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 never 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 could also be used as a stand alone valve phono pre-amp or fitted inside other valve pre-amps provided the loading is high enough — which is often the case with valve pre-amps — If required different equalisations like various course groove 78rpm or tape head can be provided and even different equalisation for each channel with 2 mono channels each specially optimised for a particular playback EQ — Just ask I have also made a triode amplifier and buffer PCB in the same format which can be used as a flexible — flat response — gain stage with low output impedance in place of V1 and V2 or could be used as a stand-alone pre—amp section or maybe coupled with the RIAA PCB to make a DIY audio valve pre—amp
				On the left a QUAD 22 control unit modernised to the drawing QUAD 22 Switching C where the tape output [TO] phonos are used as another input giving 4 line inputs This is only possible because the switch bank is now free of the DISC EQ components by using the QUAD22 RIAA PCB
QUAD 22 tone control replacement capacitors — February 2012
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 & C21 — 3n6 for C22 C23 C28 & C29 — 1n5 for C30 & C31 ( C24 to C27 are usually already polystyrene but not extended foil )
A kit of 10 x 1% 160V polystyrene capacitors for the 22 Tone Control as shown above are 12GBP inc. UK P&P C24 C25 C26 C27 — 220pF 1% 630V polystyrene extended foil are also available for additional 1GBP each when ordered with the kit above or I will supply 10% parts for C24 to C27 free of charge if required Bare PCBs and Kits
			Bare PCBs and Full 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 are 8GBP each or supplied with 2 valve bases and mounting spacers are 14GBP each inc. UK P&P Complete kits with the quality components and mounting spacers as shown below — supplied in a "really useful" project box — are 45GBP each inc. UK P&P
If you buy the Kit you will find it very easy to assemble following the schematics and these instructions — Start by fitting the lowest profile parts first The MRS25 resistors have their leads 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
			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 or RF
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 "eased" to fit the tight space while allowing good clearance for the high voltage and signal tracks — The valve base footprint and mounting method is thus unique to this design
	I found a small footprint B9A ceramic valve base available in quantity and NOT with gold pins — The pitch circle diameter {PCD} of the pins had 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
			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
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 lenghts 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 fitted to the PCB before it is fitted to the chassis or PCB pins need to be used — Black wire is used for all ground/chassis — Out—Y and Out—B do not have corresponding ground wires as they use the chassis
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

— QUAD 22 Pre Amp RIAA replacement PCB —