The Quad 405

Quad 405

Why would I think about upgrading or revising my 405?

There are several reasons:

  • Most electrolytic capacitors tend to dry out after 15 years. Their value changes and in the worst case they give a short-circuit. This may cause a "rumble", a "hiss", more distortion and even a complete disfunctioning of the amp. Modern capacitors are smaller and perform a lot better. High-end capacitors have a life-time above 150.000 hours and less internal resistance.
  • The Opamp technology in the seventies and the eighties was not at all as advanced as it is today. The LM301 that was used in most 405's can not match the performance of modern Opamps like the Burr Brown OPA604. CD's did not exist at the time the 405 was designed and CD's give a lot more dynamics in the sound that vintage Opamps can't match.

Instead of revising my 405, why don't I sell it on eBay and buy a new amp?

That's an option, indeed, if you have a budget of above $2,000.00 to match the same transparency and dynamics.

What useful modifications can be done to my 405?

  • The capacitors have to be replaced by new ones. After more than 15 years capacitors dry out and change their specifications. Modern capacitors, by the way, are not only a lot smaller but also a lot better. For some caps it may be usefull to have "audiophile" caps inserted like Wima, Wonder Caps or MusiCaps if you have the perfect ear and a lot of money ;-) Good quality capacitors of a usual (and cheaper) brand will do as well. For 99% that is.
  • The Opamp (usually a LM301 or a TL071) has to be replaced by a modern high-end audio Opamp. At the time the 405 was produced Opamp audio technology was new. Those veteran Opamps are absolutely no match for modern high-ends like the Burr Browns. CD's were not invented yet when the 405 was designed. CD's have much higher dynamics than LP's, dynamics those veteran Opamps can't match.
  • Reducing the current-limiting may be an option. Quad designed the 405 for professional- and home-use at the same time. And for the ESL speakers. So they limited the current to 3 ... 4 Amps to stay on the (very) safe side.
  • The output- and driver-transistors could be replaced by faster types available today like the Motorola MJE15031 for the drivers and the 2N3773 or MJ15003 for the output-transistors.

For the rest the design of the current-dumping circuit is not critical about component tolerance. There is no use - for most components - to spend money on 1% tolerances when even 20% will do fine.

Will the 405 still have the typical Quad-sound after the upgrade?

Sure it will.

Even more than ever before.

The "behind the curtain" sound, due to the bad performance of worn caps will completely disappear and the transparency of the sound which is so typical for Quad will be outstanding. Modern Opamps will dramatically reduce distortion and improve dynamics.

But the current-dumping schematic remains the same of course. There is no amp below $2,000.00 that can even match an upgraded 405 in a "blind test".

What are the technical considerations when the Opamp is replaced?

  • Most modern High-end Opamps like the OPA604 have a working voltage of + and - 15 Volt instead of + and - 12 Volt. The power-supply for the Opamp should be increased from 12 to 15 Volt by changing the Zener-diodes D1 and D2 with 16 Volt zeners. Otherwise there will be clipping at higher input-voltages and the undistorted power will be limited to about 70 Watt per channel.

  • quad 405 boards

  • Modern Opamps eat more power. This may cause unwanted switch-off behavior ("Plops" in the speaker) when switching off the amp. See the other posts on my blog for solving this matter.
  • In order to stabilize a "modern" Opamp one should add 2x 100 nF capacitors across D1 anode and cathode as well as D2 anode and cathode to avoid the input-stage oscillating and C3 has to be removed.

    quad 405 board

    Can you revise / upgrade my 405 for me?
  • Yes I can.No matter how old or how broken your 405 is, it can always be repaired and made sound better than a new 405-2. Just send an e-mail.
  • Quad 405 DIY illustrated guidelines - Step 1 to 3

    I thought it was time to make an online step-by-step guide for upgrading a 405.

    The revision/upgrade will be a basic one without fancy or expensive things.

    Click on the pictures to enlarge.

    So here we go...

    Step 1 - Getting the components together

    This may be the hardest part... Especially if you are looking for audio-grade components at an affordable price. The components I will use in this revision are:

    High quality audio-grade10.000 MF 63 Volt for the power-supply.

    • 100MF and 47MF 63V Audiograde capacitors for C5 & C10. In the 405-2 we will need 3 47MF per circuit-board.
    • Bipolar capacitors of 10MF (for the clamp-circuit) and 47MF (for the DC-feedback) x2
    • Burr Brown OPA604AP-chips for the input-channel. We'll need 100NF caps to stabilise the power-supply of the chips and 16V zener-diodes to make the chip feel happy.
    • We'll also need IC-sockets, thermal compound.


    • What equipment will we need?

      • The minimum is screwdrivers, pliers, soldering equipment and a multimeter
      • Better would be a 2x50V lab power supply, a LF tone-generator and a 2 channel scope (but it is possible to do without them)
      Step 2 - Taking the 405 apart

    • The first step is to remove the top- bottom- and side panels. Sometimes, after 30 years, the screws are hard to remove but giving the screw a "tap" with a mallet with screwdriver in place aids in removal.

      We now understand why Quad made 4 hole's on the left- and the rightside of the backpanel- they were intended to facilitate manipulation of the screws of the circuit-boards. (remember to "tap or hit the screws with a mallet with screwdriver in place or these probably will not come out)

      I don't need to mention it's usefull to keep the screws apart. They can be replaced (they are M4 - which is a standard-size) but of course there is not much use in replacing the screws.

      The best way to proceed is to remove the top-panel, then the bottom-panel and then the 2 side-panels.


      The speaker connectors do not accomodate new large speaker wire or banana plugs and you "may" want to replace them. It is not "absolutely" necessary.

    • The cable-beam of the 120V-side of the transformer should stay in place, there is no improvement in replacing it as there is only +/1 Amp max going through it. On the other side of .

      Step 3 - Fitting in the Power Supply capacitors and the connectors

      We'll do this step first and we'll fit in the capacitors with circular Velcro dots on the flat side of the capacitors.

      As modern capacitors are a lot smaller (and better ;-) than the ones used 30 years ago we can either buy new fixing-rings to fit them in or use circular Velcro dots to fix them in the original rings. Another alternative is to wind some flat Velcro tape around the caps to increase the diameter. This will work if the caps are not much smaller than the rings.If you want to change the speaker connections and add RCA outputs now is the best time to do it with the capacitors removed.
    • This makes working on the back-panel easier.

      The speaker ground outputs do not have to be isolated from the chassis but the "hot" speaker outputs have to be isolated. New RCA-inputs have to be isolated as well.

    • The best order to mount everything is:

      1. The negative speaker connectors to a central ground point on the chassis (this wire will be connected to the central ground-point later). We put this wire in because it is easier to solder it when the capacitors are not in place. The ground speaker connectors don't need to be isolated from the chassis.
      2. The hot speaker connectors need to be isolated from the chassis.
      3. The RCA-inputs. They are soldered (later) to the L/R connectors of the inputs on the driver boards-plug with special wire.
      4. The capacitors. The wiring of the power-supply will be done later, after the Silicone has dried. Make sure to fit the first capacitor with the + on the left and the second one with the - on the left. The left-side will be our central 0Volt mass of the power supply.

    Quad 405 DIY illustrated guidelines - Step 4 to 6

    Step 4 - Upgrading the circuit-boards.

    OK, we'll remove all components that have to be replaced. We won't touch the other board for the time being. This way we can always compare.


    We'll replace D1 & D2 with 15V 1,3W.

    We'll replace the electrolyts C4/C5/C18/C19 with new audio-grade caps.

    We'll replace C2 and C10 with bipolar caps.

    Of course we'll replace the opamp with BB604AP or BB627BP.

    We'll add 100 nF capsacross the anode and cathode side of diodes D1 and D2.

    Before adapting the second circuit-board we have to test the upgraded board to make sure it's OK. This way, if there is a problem, we can compare the voltages on both boards to locate the error.

    Step 5 - Testing the circuit-boards

    When both boards are upgraded we'll connect them to a sinus-generator and to the scope. We'll connect + and - 50 Volt with our lab power-supply and connect a true-RMS multimeter to check the input- and output voltages.

    Following measurements are OK:

    - 0.01 Volt DC on the outputs
    - 30 .. 32 Volt AC on the outputs before clipping. This corresponds with 110 .. 125 Watt into 8 Ohm.
    - 1.7 Volt AC on the input before clipping. This corresponds with line-voltage for full power.

    If you don't have all this lab-equipment, skip this step. We will make sure later that there is O VDC on the output and that the power-consumption in the power-lines is 120 .. 130 mA. In this case we'll do Step 6 first.

    Step 6 - Cabling

    We start by cabling the power-supply.


    • We will do the cabling of the Power Supply lines and test the voltages first. They should be around + and - 51 .. 54 Volt DC. Before connecting the power re-check the polarity of the caps and the rectifier-bridge.

      Don't forget to connect the mass-lines of the capacitors (one + to one - side of each cap) and the ground-line of the transformer to the central mass-point on the chassis.

      It is better to twist all cables that have to be soldered to the same point together before tinning them, it makes soldering them a lot easier..

      Warning If the polarity of the caps is wrong they will explode!

      Once the power-supply is is checked, we attach both boards adding thermal paste to ensure maximum heat transfer with the chassis.

      We can now connect the + and - power-lines to the boards (don't switch them or you'll kill the output-transistors!) as well as
    • thespeaker output cables to the red speaker output-plugs.

      It is best to use Heat Shrink Tubing for insulating the +, - and output connections, as there are large voltages between them.

      Before connecting mains-voltage we should check all cabling again with the Ohm-meter. We should alse re-check that each channel uses 120 .. 130 mA in the + and the - power-line and that there is less than 1 Volt DC on the outputs (typically 0,01 Volt).

      If it passes these final tests we can now connect the speakers and the preamp and... enjoy the music ;-)

    Quad 405 basic DIY upgrade kit

    Price $115.00
    Includes all the capacitors resistors, opamps and diodes to upgrade the Quad 405. Installation manual included

    Quad 405 Deluxe DIY upgrade kit

    Price $185.00
    Includes all the capacitors resistors, diodes, Burr Brown OPA627BP opamps RCA connectors and gold plated 5-way binding posts to upgrade the Quad 405. Installation manual included

    Quad 405 Deluxe (with upgrade transistors) DIY upgrade kit

    Price $210.00
    Includes all the capacitors resistors, diodes, Burr Brown OPA627BP opamps RCA connectors and gold plated 5-way binding posts to upgrade the Quad 405. Installation manual included


    Quad 405 Basic kit with installation

    Price $259.00
    The basic Quad 405 kit (includes all parts) installed by Quad Mods USA.


    Quad 405 Deluxe kit with installation

    Price $309.00
    Includes gold plated RCA connectors and 5-way binding posts and opa627 opamps installed by Quad Mods USA.

    Quad 405 Deluxe kit (including upgrade transistors) with installation

    Price $335.00
    Includes gold plated RCA connectors and 5-way binding posts and opa627 opamps installed by Quad Mods USA.


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