This page is about the Quad22 preamp control units.

There is a lot of interest in the audio community about the range of Quad tube amplifiers and
I hope to provide some details of my work which will answer many questions I have been asked.
Occasionally my work involves improving old Quad tube amplifiers to meet modern
expectations of amplifiers.

If you are familiar with old Quad 22 preamps, my pictures of my work will show much of what
I have done.

Some owners wish to retain their pre-amplifiers exactly as they were made many years ago
and without modifications. Where no modifications are wanted and an owner is happy with
the original but has a malfunctioning unit, I can usually just repair it easily with a clean out,
replacement of a couple of resistors, capacitors or a tube or two.
But there is a likelihood of malfunction in future unless all resistors and capacitors are
replaced with more modern types especially if the unit has never been serviced before.
Thus it is possible to keep a Quad-22 preamp original in design and function but we could
very happily wave goodbye to all the carbon composition resistors and all capacitors
because these items change there value and may give noise or otherwise cause problems.
But Quad-22 control amps like many others are not such wonderful sounding amplifiers.
I believe old Quad amps are not precious in their original state and can be be much
improved and simplified for better sound and for compatibility with other modern electronic
items and cabling. The original Quad-22 was meant for use with a couple of Quad-II power
amps and it could not be used with anything else. This was and remains most inconvenient
for a lot of people.

I have one owner who had me repair his Quad 22 preamp and change nothing because his
record replay system depended upon the Quad original circuit because of the many
available eq settings for many disk recordings made before the RIAA eq curve was
accepted by all record makers. Quad made some of the most reliable audio equipment,
and one customer has been using his Quad-II and 22 control unit since 1960 with perhaps
only 3 serviceings. He has been lucky. He has had me service his Quad22 only twice in
the last 10 years.

Anything electronics made in around 1960 will have many parts which have degraded.
Along with R and C replacement, potentiometers can wear or become noisy. Quad-22
switches are very reliable. The only problem is that there are so many switch contacts
in the signal path at any given time. Sometimes switch plastic buttons will disintegrate
or fall off and get lost. I have sometimes replaced all with metal buttons which will never
degrade. The potentimeters often wear out and have values which lead to noise. 

But other Quad owners want to use their Quad-22 preamp control unit with other brands
of power amplifiers so they need to replace all the old terminals at the rear of the amp
and will need a separate power supply.
They do not need the complexity of the switching within the Quad 22, and also want it
to sound better. They want to be able to select the normal modern range of line level
inputs sensitive to 200mV for aux, AM/FM tuner, tape in from a cassette player perhaps,
TV input or set top box input, sound card, and they will also want a CD player input
sensitive for 1V input. They may or may not want to play disc records. Where they do
want records, it will have only RIAA eq for vinyl LP and 45, and the phono disc input
will be sensitive to 3mV from a MM cartridge. It is possible to make a Quad-22 able to
be used with an MC cart but I have never had to yet.

QUAD 22control

An example is a Quad 22 preamp in which I did some serious rewiring back in about 2001.

You can see that there are 7 tubes, all twin triodes, which replace the original two pentodes
and two twin triodes. I have fitted a new back plate with gold plated RCA sockets to make
the unit compatible with today's cables. It may be used any system where RCA cabling is used.
This sample was powered from new power supply. Quad-II power amplifiers may still be used
if they have an RCA input socket fitted and have their mains cabling taken to wall plugs
where they may be switched On-Off . But with Quad-II power amps the Right Thing To Do
is to instal IEC mains input sockets with earthing to allow safe earthing of the chassis.

There should be an On-Off switch on each monobloc amp.

The control knobs have a slightly different function to the original and are set up from left to
right as Volume, Balance, Bass, Treble, HF Filter.
Of the original 5 plastic buttons, 3 were broken so I made new aluminium buttons, hand
engraved them, and bolted them onto the metal switch levers with some epoxy glue, ( Araldite ).
The face plate of the amp itself was slightly altered since there is no concentric balance lever
on the volume control and the 'QUAD' illuminated badge has been replaced with an LED.

All the original ganged press button switches have been retained, but with the simpler and
more effective circuit. 1/3 of the original contacts are used. I have found these old switches
to be more rugged and reliable than the tiny switches mounted on printed circuit boards
found in more modern equipment. Different value gain and balance controls are fitted.
The line stage gain amp and tone control amp is fully deletable.
The complex arrangement for multiple eq for 78 shellac records has been abolished.
There were about 12 different contours for equalization of records produced before 1955,
but few people wish to play them now, so the new preamp has only RIAA standard eq, with
hard wired passive RIAA filters within the amp, rather than having the eq circuit for records
in a metal can that plugged into the back of the original Quad. The plug-in cans tended
to fall out and get lost. The tape eq can has been entirely deleted because nobody ever
now uses the tape source used in the early days of tape replay.

However, for those wanting to replay ancient recordings Quad II preamps are best simply
restored to exactly original condition, rather than modified like I have in this example but
a restoration ideally involves replacing all the R and C components since the degradation
of these parts over the last 50 years can cause serious distortions. The original Hunts
capacitors and carbon composition resistors are notorious for going rotten with moisture
absorption and corrosion.

In the amp I modified,  I have retained the LC filters to give a steep 12dB/octave HF cut
starting at 5 kHz or 7 kHz, or no cut at all.  Having done that I found out how useless
such filters were for making old records sound less noisy. Such filters remove the wanted
treble content and dull the music.
I have never used the filter feature. Bad noise from a
78 may sound like a hailstorm on a tin roof, but with a noise filter the noise just changes
to a storm with big drops of water.
The right way to combat noise from vinyl is to clean the record properly with a record
cleaner device and cleaning fluid and then vacuum the muck from the grooves while
the record is still wet.

But anyway, I used the original well made Quad 22 filter inductors, but I abolished the
"variable filter slope function" since it seemed to be a really utterly useless function
when I tried to use it with a particularly noisy record in an unaltered amp.

The amp was repainted and aluminium knobs polished.


QUAD 22 plus seartae
          poer supply.

This image shows the highly modified Quad-22 control unit on the left and power
supply on the right. PSU was an old tubed power supply I bought at a ham sale for
$2, but I completely rewired it to suit the needs of the preamp, and I made the perforated
steel cover and painted it gold to match the preamp. The power supply would normally
be mounted well away from the preamp on a shelf below the control unit.
A new umbilical cable was hard wired to the control unit and octal plug fitted to
suit the octal power output socket on the rear of the supply.

phono stage

For each channel, Phono input 6DJ8, V1, both halves in parallel, feeding passive
RIAA filter, ( no negative feedback ).
The 318us and 3180us time constant filters are before the second gain stage and
the 75us time constant filter is after the second gain stage but before the cathode
follower output.
Phono gain stage, 12AX7, V2, with cathode follower buffered output, V3.
The phono amp gives 46dB of gain at 1 kHz, which is plenty for all MM cartridges.
For MC a step up transformer will have to be used for low output MC since I found the
noise of the 6DJ8 was still too high for MC with outputs below 0.5mV.
And not shown is the earth terminal for turntable grounding leads.
Cartridge loading can be changed from the defacto values shown by using additional
components mounted on an RCA plug and plugged into the RCA socket shown beside R1.

Reformed Quad 22 control unit with tone control, hi-cut filters, and output buffers, 2001.
Schematic reformed
        Quad 22, 2001.

The original four Quad input source press button switches are shown near the three
'IN 1,2,3' sources plus the fourth switch for the phono amp output.
S2 and S3 are the two remaining button switches which can be used as shown for gain
and tone amp deletion. Later made Quad 22 had one less switch section included that
was never used in early models. A later revamp of a Quad 22 in 2006 required whole
set up to be different; it will be dealt with further down this page. But in this amp when
tone and gain isn't used, the input from the pole input from the 4 input switches
becomes directly connected to the top of the volume control.
If the following power amp is a sensitive enough then no gain is needed and the
signal does not experience unnecessary bits and pieces in the signal path.

S4 is a NOS replacement rotary wafer switch because the original had brass
connections which suffered metal fatigue and which broke very easily.
The tone control amp is a 12AX7 which probably will rarely be used.
A 12AT7 gives better line level amp micro detail with slightly different RL and Rk.

The tone amp is a "unity gain" type with a a standard Baxandal feedback tone control
network which is the only way to build a blameless tone control that won't have any sonic
signature when in the flat position.
V2 12AU7 line gain amp is a simple SET stage with current FIB with R14 unbypassed.
V3 is another 12AU7 which has a very high resistance input with low capacitance to make
sure HF losses after the volume and balance control are minimized.
The output resistance from the 12AU7 cathode is about 1k, so long cables,
power amp input resistance and input capacitance will not affect the preamp output signal.

Reformed Quad22,  front panel 2006.
Quad22, reformed,
        on bench.

Source selections are by pressing interactive switches 1,2,3,or 4.
For including the gain amp, press the gain switch, and to include tone control, press tone
control, and for both, press both in at once so they stay in; these are interactive switches,
but are easy to get used to.

The schematic of what is in the Quad box above......
Reformed Quad22
        feb22 2006 schematic.

In this latest reformed Quad 22, V1 is an SET gain triode driving a V2 bootstrapped
follower output in what is now called a mu-follower amp stage.
V3 is a normal SET stage used to drive the deletable tone control stage and had
"unity gain" because the tone network is a Baxandal feedback type with about +/- 9dB
maximum boost and cut to extreme frequencies.

V4 is a C output stage with CCS cathode current sink, so that the only signal load
is the load that is connected to the output of the amp.
This ensures the cathode follower has the lowest distortion possible, and maximum available
drive current.

S3 is to provide 3 HF shelved HF cut starting at about 1.5kHz so that there is a selection of
0, -2, -4, -6 dB cuts to HF. This is useful where harsh speakers, or "digital" over processed
recordings with excessive HF content above 1 kHz  would ruin the enjoyment.

Note that this particular Quad 22 didn't have enough switches on S1 to have more than
one switch used for gain deletion so when the gain stage is switched out
the input source is still connected to the R5 47k input feed to V1.
But this will have no effect because most source impedances these days are from
an op-amp follower or cathode follower and are low impedance below 1kohm.
A high impedance source is one rated at over 10k ohms, but it won't be affected by V1
input permanently connected.

Fixed bias is used to bias the grid of the cathode follower, the V2 follower after V1 and
the CCS MJE340 bases using divider R25, 26, 27 and 28.

Preamp power
          supply feb2006 schematic.

This simple linear power supply is exactly what was used for the Reformed Quad 22 preamp
above but the same schematic could be used for any preamp project.
I placed two octal power outlet sockets on the rear panel to allow for a simple phono stage
to be connected if desired.
I wound the power transformer myself with B = 0.85Tesla and with 32mm stack of 32mm
tongue width material which was amongst my stocks of pre-used GOSS transformer laminations.

The transistors for the emitter follower type of regulator can be almost any general purpose
transistors. I had a couple of T0220 flat pack power transistors normally used in low power
amps and so I used the two because they have an HFE which tends to be higher than one
large flat pack bjt.
As it has been drawn, a short circuit of the heater supply will blow the 4 amp fuse, F3,
but other refinements like a slower ramp up of the heater voltage could be incorporated.

Reformed Quad22, rear panel, feb 2006, with hand made new power supply.
Quad22 rear view
        with psu.

The umbilical cable is nicely flexible and will curl up easily.

Reformed Quad22 with cover off, feb 2006.
Quad22 cover off.

The 4 x 12AU7 can be seen standing in the original McMurdo tube sockets, and the 150uF caps
also can be seen mounted above chassis in the line of tubes. The Alps Black volume pot can be
seen to the right.

The existing Quad board for components for the tone controls was retained but fitted with new
yellow polyester capacitors and new light blue 3/4W Welwyn metal film resistors.
Two 4 mm screws hold the cover onto the amp chassis.

Reformed Quad22 under chassis wiring.
Reformed Quad22
        under chassis wiring, feb 2006.  It is very crammed inside this amplifier!

 You can see that 4 rows of switches are not used because
 there is a second side to the switch bank hidden
 under what is seen.

 All the original turret connectors used in Quad22 were
 removed and replaced with fibre strips with brass plated
 screws to give a better range of connection points.

 All the capacitors are polypropylene and red colored ones
 are Wima 0.47uF, 630V rated.
 The thick solid wire top to bottom above tube sockets
  beneath the 3 dimensional wiring with very short leads
  is the 0V buss rail, separate from the case and chassis. 

  Wiring for B+ rail voltages and heater wires is with
  well insulated stranded wiring but wires carrying signal
  is 0.6mm solid hook up wire taken from a multi pair
  telephone cable which had about 50 wires of different
  color coded wire. This makes it easy to trace wires
  in service work and it sounds

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