is a Quad 22 preamp in which I did some serious rewiring back in
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.
should be an On-Off switch on each monobloc amp.
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.
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.
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.
I used the original well made Quad 22 filter inductors, but I
"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 PREAMP AND POWER SUPPLY, 2001.
shows the highly modified Quad-22 control unit on the left and
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.
channel, Phono input 6DJ8, V1, both halves in parallel, feeding
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
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.
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.
amp is a "unity gain" type with a a standard Baxandal feedback
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.
latest reformed Quad 22, V1 is an SET gain triode driving a V2
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
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
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.
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.
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.
POWER SUPPLY FEB 2006
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.
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 well.
To re-engineered amps
To Index Page