QUAD 33 restoration (Pt 1)

The Quad 33 preamplifier is 50 years old, now.

It was introduced by Quad Electroacoustics in 1967 (to partner the 303 power amp) and 120,000 units were reportedly manufactured over 15 years.

The Quad 33/303 was Quad’s first Solid State consumer preamp/power-amp set.

My Quad 33 is serial no.6184, which means probably late 60s-early 70s manufacture.


Cosmetic condition of this example is less than 100%.


Rear view, showing 2 hatches for removable Tape Adapter & the Disc (phono) Adapter PCBs.


Quad 33 preamp block diagram (amended).


Case removed, showing compact, modular PCB design. (Note also, early ‘ribbon cable’ in 1967.)


Case removed, underside view.


Easily accessible Tape Adapter & Disc Adapter boards.


The Quad 33 has 3 inputs + a Disc (phono) input – all DIN sockets.

Original input sensitivities are:

  • RADIO 1 – 100mV
  • RADIO 2 – 100mV
  • TAPE REPLAY – 1V or 100mV or 400mV (user-selected by screws on that Tape Adapter board (see photos below).
  • DISC (phono) – 2mV or 5.6mV or 100mV (ceramic pickup) sensitivities (user-selected using plug-in Disc Adapter board (see photo below).

However, modern input ‘line level’ inputs should around 300mV. And source devices now (cellphones, CD players) have quite high output of 1V-2V, or more.

This preamp will be used for the household sound system, so I want to accommodate also a FM Tuner and maybe run my old Nakamichi Cassette Deck.

So, we need to decrease the sensitivity of the Disc (phono) and Radio inputs and set the Tape Adapter board sensitivity down to say 400mV.

Also, a number of other modifications recommended by other Quad 33 restorers.


The new parts – total cost UK£45 (~NZ$95).

I decided to replace all semi-conductor parts – the transistors with modern equivalents (BC550C) together with the diodes.  I used some bits from my own parts collection (2 x Wima caps and some diodes).

Resistors will be Takman REY metal film.  Electrolytic caps, Elna Silmic II.  Film caps, Panasonic radial polypropylene.

Good audio quality parts used, but not extremely costly – this preamp is for regular household use.


All sub-PCBs removed.


Underside of Phono Adapter board showing 4-sided plug-in contacts design.


Close-up of main Preamp/tone-filter ‘motherboard’ showing the PCB sockets. (The 2 Balance control caps (100uF 6.3v) have been replaced with Elna Silmic II 100uF 16v.)


The Power Supply board has been removed after de-soldering the ribbon cable and the rear AC Mains socket wire connections.


PSU board with new caps (Panasonic FC) ready to install.


Finished PSU board.

PSU mods:

  • PSU output voltage increased from 12vdc to 16vdc for improved stability – pi-filter resistances adjusted and original filter caps (400uF & 640uF) were up-sized to 1500uF 25v.  Zener diode changed from 12v to 16v.
  • Rectifier diodes replaced.
  • The supplementary 5v supply originally designed for power-amp switching, is removed.

(All as recommended by other restorers, see references.)


Upgraded PSU re-installed and tested – 17.55vdc out with no boards connected, looking OK.

(References – Quad 33 Service Data, Dada Electronics, QuadRevisie.)

Part II – upgrades to Amplifier, Disc, Disc Adapter and Tape Adapter boards, replace volume control.


19 thoughts on “QUAD 33 restoration (Pt 1)

  1. Pingback: Quad 33 restoration (Pt II) | D a r k L a n t e r n

  2. Hi there
    Many thanks for your write up.
    I have no power on the 33 all of a sudden and having changed all fuses I can only imagine it’s the psu. I see you changed yours, was it fairly straight forward to do?

    • Hi Alex – thanks for visiting.
      Before removing the PSU, if you have a DMM meter, I would first check any volts showing at the top + lead of the 1st PSU cap (as in the last pic above) – there should be 12+vdc between this pin & the green Gnd wire. Additionally, there should be 12vdc at the red pin 4 nearby on this bd. This will tell you if the PSU is working or not. And whether to investigate further upstream (within the PSU bd) or downstream (within the preamp).
      Let me know how you get on, Owen

  3. Hi there ,

    Good job ,

    I would like to do the same job , where can I buy the “The new parts – total cost UK£45 ” ?



  4. Lovely write up.
    I have two 33s and a 303. I had one upgraded along with the 303 by a specialist who installed the lovely Net-Audio upgrades. Sadly Richard who owned the business passed away a year ago, and the kits and parts are no longer available, though his website is. Worth a peek. I also carried out the Dada upgrade on the remaining elements that were not covered, though I have subsequently discovered, I needn’t have as the mains input upgrade is not required with the Net-Audio as it’s all driven off the tape card somehow. The mains is not required – woops.

    The second 33 is still awaiting “surgery” for which I’ll use the Dada upgrade and a couple of other recommend items to increase the low end – remove the 30Hz cut off Quad put in to reduce rumble. I think its C100 or C400 off the top of my head….

    Always enjoy your work, and shall be popping back for tips in the 2nd 33 upgrade.

  5. Hi, I take out the phono adaptor board, which side should be the correct way to insert to the Pre-amp.

    • Ty
      As per Owen’s video, all have the components upwards and just slot in as per the card slot…. but if you mean by way of phono the turntable card, that depends on what type of cartridge you use. It is based on moving magnet (MM) or older, not Moving Coil (MC). For that, you’ll need a head amp as well and plug into the tape input.

  6. Hi Owen,

    Thanks for writing your walkthrough on the Quad restore – really appreciated.

    Just about to restore one myself and was wondering if you used standard wire wound 1/4w resistors or a higher w value? I checked out the online schematics and the values in the part lists threw me (as in R500 being 120 followed by a 5 – is the 5 a recommended wattage or am I over thinking lol)


  7. I have a couple of questions, Owen:
    One I put up before but wasn’t clear and two more.

    So the first was, that I wasn’t clear on, was about the 33’s low pass filter. It cuts in at 30Hz and is a pain. I had assumed it was C100 or C400, but I am not sure which or if it is even them. Can you, please advise.

    The next is, I think the same occurs at the top end. Doing a frequency range test my system is filtering at about 15kHz, and I am certain it is the 33. If so, how does one disconnect that too?

    LASTLY, honest. I need to get hold of a Disc adaptor board. Tried Dada, they don’t have any…. It is for my second 33 which has the Dada upgrade, which I plan to sell, would like to do so as a complete amp. In fact if I can’t sort the first 2 questions out I’ll probably sell the other one too… Sad really as the sound is lovely.

    PS: I know you were interested in my Mauhorn V mods and I did say i might have a go at writing something up, and very sorry i haven’t, however here are some pics and things FYI
    Construction pics of Mauhorn V mod:
    The principle was to round edges and corners for a smoother flow and at the mouth, which also extended the range and used the floor and walls better. I lengthened the horn too to get past the 1.75m and few other mods. I used a tool for measuring expected efficiency and got 135db @ 1m. I am not convinced but my 2 x 20W Leak Stereo 20 will split eardrums with the Quad 33 on 4, so had to mute it (the Leak) quite a bit and using a miniDSP 4×8 drop that by 27db in the input.

    Tweeter additions Raal D70 10D and Angel MkIIIs:

    I installed 4 x Faital Pro 12FH500 under the floor as Subs/bass extenders, using the whole sub floor as a speaker cabinet. The Lowthers are great full range, but I have found and read that if the strain of full range is reduced they perform even better and these certainly make it so.

    I am getting a very flat frequency range for the system 10Hz to 45kHz with, for some reason a -5db spike at 70Hz.

    • Hi Jonathan – thanks for looking in here & sorry about delay in replying…
      LF filter – I followed the Dada recommendations (see their Quad 33 modif. guide). They change the original C202/203 caps on the Tape Adapter board from 0.33uF (or 0.68uF) to 2.2uF, which will result in a low pass filter (with the 50k vol pot) at ~1.5Hz (-3dB) instead of ~10Hz originally – this means that this cct section will be flat down to ~10Hz, instead of ~70Hz originally.
      HF roll-off – I am not aware of any filtering in-built.
      It is not uncommon for IC cable capacitance to cause HF roll-off, in combination with the input impedance of the next stage. If your load impedance is greater than 100k ohms (eg. in vintage amps) then HF roll-off could be noticeable in modern systems. Hopefully your Leak Stereo 20 has had it’s input R changed from 1M to 100k or less.
      Thanks for the pics – feel free to post some up on our DarkLantern forum, where I’m sure there will be interest- https://darklantern.proboards.com
      PS – I’d be most interested to see details of your in-floor subwoofer 😉
      Cheers, Owen

      • Thanks Owen,
        I’ll take another look at those capacitors, though I thought they’d been changed before.

        HF, yes, the Quad 33 has top-end filters too. Both were introduced because at that time the concern was noise from equipment/source. The 33 has the 3 5khz, 7khz, 10khz buttons and the slop for said, but reading the instructions properly I am not sure it can be cancelled fully as pressing the cancel button removes the bass and treble too.

        The Leak: One of the issues of adjusting the sensitivity is loosing all the lovely qualities of the Leak. This almost keeps it all. To reduce the sensitivity of a Leak Stereo 20:

        1. Remove C2L+R. This may seem counterintuitive, or unnecessary, but it isn’t, as Leaks have marginal stability which the next step will upset. You will see other recommendations to address this, e.g. to put a step network into the anode circuit of V1, but removing C2 is the best solution. See Keith Snook’s Leak page for confirmation.

        2. Change R18L+R (12k) as follows:

        – for 500mV sensitivity, divide it by 4 (i.e. 500/125) and use the nearest standard value you can procure. – for ~775 sensitivity, divide it by 6 (i.e. ~775/125) and ditto.

        2.3 Whatever factor you divided R18 by above, multiply C9L+R (200pF) by the same value and ditto.

        In other words, the product of R18 x C9 should remain roughly constant.

        Example: for 625mV: R18=2k2, C9=1000pF.

        Regarding posting: I know I said I’d do something ages ago and have not got around to it. I shall, just keep prodding me! I had a bit of an accident Saturday. I wanted to just do a couple of very small tweaks to the system and used my wife’s laptop to tune the miniDSP. Disaster. She’d been having issues with it which I helped resolve, but it seemed to play with the software I use to test and tune.. Somehow I also managed to blow a channel on my power amp I used for the sub 600Hz. Not worth repairing so just replaced it. So a bit of focusing on that at the moment and also on my 3D printer which is in kit form.

        Not a lot to say about the subs/bass underfloor units. I have always felt that placing the units in an open space was best for bass and doing a lot of research on the subject that is the case, they are best in fee air. I can across a couple of individuals who build units in their roof which is something that didn’t cut it for me as regardless what people say there is some directional sound and I don’t want to necessarily fill the house. These guys were very interested in ultra-low and sub frequencies with a keen interest in organ music. The biggest two challenges are:
        1. Getting the listening area (typically the room) and the underfloor area as airtight as possible between each other.
        2. How to allow as much sound through the floor without risking the kids and SWMBO stepping on the units or just falling through the floor.
        All else are quite simple if uncomfortable building and fitting units under a floor in a tight crawl space.

        I measured the room and as a compromise decided to build dead centre as roooms get kmoved about, otherwise it would be between the units, slightly more right channel as some inconsiderate builder 115 years ago had made the joists slightly offset. I simply decided on a very high-density MDF board cut to fit across the floorboards, with holes for the units between the joists. This did limit the size of the units, but with, if I wanted, a whole floor area to play with not a real problem as I can have as many units as I wanted. Then made airtight between the joists using loads of screws and glue, adding mount points and plugs for the various cables, also allowing a switch between 4 or 8ohm (series/parallel combos) to test. Units: You need as much volume/area as possible to get as low as possible. Large units give easy volume, but loose accuracy and add harmonics and have a slow reaction and recovery times, so many smaller units are better. Ideally, bass and sub-bass units should have a Low Qts, a Flux Density >2, and, with a magnet made of Neodymium or Alnico. So I initially used 4 x Keff B139s, mostly because I know them and like them and also because they fitted nicely. They, however, turned out to be not up to it so replaced with 4 x Faital Pro 12FH500 – 12″ 500W Bass. Wired in 2 pairs at 8ohms, one pair each channel. They have their own power amp, just gone from a Rotel RB-03 to a QSC RMX 1450, rolling off at 600Hz which is a comfortable level for the Lowther 5As, and allows them to chill out and focus on the mid.

        Have to say it sound incredible and even my buddy with his >£200k system is very impressed. I get an almost flat 10Hz to 18kHz frequency curve without any filtering and it sounds warm, lively, detailed and real…

  8. A couple of bits n bobs to the above, sorry the SWMBO keeps calling me rebooking flights!

    Where I refer to putting the bass/subs centred between the units, I refer to the Mauhorn V speakers.

    To get to 10Hz requires about 11m2, and using underfloor if money and complaints were no object you could fill the whole sub-floor area with units.

    I sealed the two areas, not airtight but tight enough by using a floating underlay with one side sticky made especially for solid flooring and laid a 22mm solid oak (not veneered and block underneath, but solid, solid oak) floor. I worked around a few shortish boards that I cut the tongue and groove off of and laid above the speakers. Once completed I removed and stuck together these loose boards that because they are random lengths are not visible, but they come out so I can access the units for maintenance and cleaning whenever. I then found a pattern of holes and laid it on the boards, and drilled them out. Mine is a series of progressively enlarging holes and looks very psychedelic. The largest holes are about 50mm D and smallest 3mm.

    I forgot to mention, that not only do you hear the music, but you feel it. If you do this you will convert.

    Early days build with Keff B139s and some spec sheets. Incidentally, Lowther do a new unit and that would have been perfect, but it was too expensive.
    Something to try and show you the floor. As you can see it is all compromise. Dining room, my wife’s teaching room, and music room, and when I can get in my stereo room. Even being so crowded and small (5m x 5m) it sounds much bigger.

    Hope this helps and is of some interest.

    I’ll use this and do the same for the speakers, Quad builds, the Leak, Garrard, and system on your forum as requested.

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