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Thread: Rebuilt Quad 405 susceptible to mains noise - any suggestions?

  1. #1
    Join Date: Sep 2009

    Posts: 18

    Default Rebuilt Quad 405 susceptible to mains noise - any suggestions?

    Hi,

    Last year I bought new boards for my 405 off the online auction site and completely rebuilt them to the new Dada spec.

    I also recapped the PSU and used new bridge rectifiers, changing the supply to now be wired as dual mono. I have used lots of smoothing caps across both the secondary and primary windings of the transformer and also across the main smoothing caps, but to no avail!

    I'm really pleased with the sound from the new boards, however it is now plagued by pops through the speakers when any lights or anything is switched on around the house (fridge cutting in and out etc.).

    It did not used to do this and my Lecson AP1 and Meridian 103D do not suffer from this.

    Please can someone advise a remedy?

    Many thanks,
    Colin

  2. #2
    Join Date: Oct 2016

    Location: Bolton, England

    Posts: 612
    I'm Andrew.

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    What is "the new Dada spec"?
    I don't like the sounds of smoothing caps across the primary and secondary windings of the mains transformer - you have to be very careful about caps across the mains.
    The pops could be caused by a burst of RF being picked up by the amp or by a spike on the mains. Unfortunately tracking down the reasons behind faults like this can be very difficult. First off, could you post a circuit diagram of the changes you made?

  3. #3
    Join Date: Feb 2008

    Location: Middlesex, UK

    Posts: 2,788
    I'm Alex.

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    All the standard Quad amps that I have used, II's, 303, 405 and 405-2 have been pretty well immune to mains noise.

    Were your boards the Chinese ones, the Dada ones, or something else?
    Spendorman

  4. #4
    Join Date: Sep 2013

    Location: North Island New Zealand

    Posts: 826
    I'm Chris.

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    I would simply rewire it to return it to exactly the way Quad made it.

    A 405 schematic shows the secondaries to be a parallel arrangement in each half with 4 secondary windings
    about a grounding centre tap. I suspect you have ended up with unequal voltages or unequal halves
    and problems arising from the centre tap being displaced slightly. Quad were concerned with how earthing
    was achieved with the 405, hence R2 on the original schematic separating power supply earth from chassis earth.
    Back to back diodes probably would have been a better choice.

    You should have DC rails at 52 volt positive, and 52 volt negative.

    As other contributors have pointed out capacitors across mains primaries also involving a transformer is not a good idea.

    The biggest fault with the 405 was mediocre zener supplies to compound what was a fairly cheap op amp installed.
    Those who had owned 303's up to 1976 must have wondered what Quad was up to releasing the 405, as the 303 was
    dead quiet, and some would say still is, their stand out amplifier

    A standard 405 is always noisy, usually on its left channel. A LM6171 op amp is a nice device to replace the 301 or 071 device.
    A set of Net Audio Mk 3 boards should be on your shopping list, once the transformer is wired back to normal.

  5. #5
    Join Date: Feb 2008

    Location: Middlesex, UK

    Posts: 2,788
    I'm Alex.

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    "Those who had owned 303's up to 1976 must have wondered what Quad was up to releasing the 405, as the 303 was
    dead quiet, and some would say still is, their stand out amplifier
    "

    I certainly did, have had a 303 for years, then bought the 405 just after it was released, didn't find it noisy, but did not really like the sound. It sat on the shelf for years until the 405-2 came out. It used TL071's, so bought a couple for a few pence, put them in the 405, and it transformed it. Later bought the -2 boards from Quad, didn't really notice any difference.

    A few years ago replaced the 071's with some Burr Brown op-amps (forget the number), and modified the zenner circuit feeding the op-amps, not sure if I can detect any further improvement.

    Amp is still quiet though. I did fit a push button on-off switch on the back panel in place of one of the rubber grommets. Very useful.
    Last edited by spendorman; 09-01-2017 at 10:30.
    Spendorman

  6. #6
    Join Date: Oct 2016

    Location: Bolton, England

    Posts: 612
    I'm Andrew.

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    This page has some very good information and would be a good place to start
    http://www.keith-snook.info/quad-405-mods.html
    His mods are quite extensive and may be more than you want to take on, but the page is certainly worth a read.
    BTW, I still don't know what the Dada mods are.

  7. #7
    Join Date: Sep 2013

    Location: North Island New Zealand

    Posts: 826
    I'm Chris.

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    Quote Originally Posted by RothwellAudio View Post
    This page has some very good information and would be a good place to start
    http://www.keith-snook.info/quad-405-mods.html
    His mods are quite extensive and may be more than you want to take on, but the page is certainly worth a read.
    BTW, I still don't know what the Dada mods are.
    Yes makes an interesting read, but there only so much that can be done to original boards. Layout on boards
    as we both know can be critical, and whilst Keith suggests some good ideas, it always comes back to
    modifying a fairly well designed board that has worked reliably the way Quad intended.

    I would instead invest in the Net Audio Mk3 boards and be happy - as you won't ever get anything better
    for a 405.

  8. #8
    Join Date: May 2008

    Location: A Strangely Isolated Place in Suffolk with far away trains passing by...

    Posts: 14,328
    I'm David.

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    Net Audio basically re-design the boards I gather and I'm deeply suspicious of one or two of their re-designs... Dada keep the original, but just update caps and so on which would be pretty knackered by now anyway. Not sure the 405 op-amps need to be as way-out as the ones suggested as too wide a bandwidth can apparently cause oscillations the crude originals miss. The OPA134 seems to be a good more modern compromise and still goes out to a few MHz.

    Check earthing again - A 405 isn't designed to be earthed directly to the mains as originally conceived. The case is tied to the signal returns and the amp is usually 'earthed' to the preamp, as is the 303 I have.

    Dada have a great friendly forum for just these issues. Well worth asking.
    Tear down these walls; Cut the ties that held me; Crying out at the top of my voice; Tell me now if you can hear me.....................

  9. #9
    Join Date: Oct 2016

    Location: Bolton, England

    Posts: 612
    I'm Andrew.

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    Quote Originally Posted by DSJR View Post
    Check earthing again - A 405 isn't designed to be earthed directly to the mains as originally conceived. The case is tied to the signal returns and the amp is usually 'earthed' to the preamp, as is the 303 I have.
    Yes, I suspect the fault has something to do with "earth" or the signal 0V. It's a much more complex subject than is often appreciated and changing any of the wiring can have unintended consequences. The amp performed without susceptibility to pops from switched lights before the mods so there should be no need to change entire boards - just get the 0V wiring right.

  10. #10
    Join Date: Sep 2009

    Posts: 18

    Default

    Hi,

    Thanks for all the replies - I believe this all comes down to earthing and I need to study it more closely. From memory I have taken the earth from the iec socket to the chasis (but I'm sure this is at it was before?) I will have a look on the Dada forum as well, thanks Dave.

    I tried dropping the earth from the plug top anti surge and this resolves another earthing issue that occasionally happened, which is the left channel hisses and distorts, but goes away when the volume is turned up.

    When I swapped the boards over it stays on the left, so has to either be a PSU or earth issue.

    The boards are Chinese built with all new components, mainly from Farnell, or the obscure inductors from Dada.

    I think it is an exact copy of the Dada V2 board (photo attached) which are considerably different from both Quad 405 MKI and MKII original boards.

    I'm going to have to put the 103 back in the main system and get inside the Quad again.

    I have used it with a Meridian 101 before rebuilding it and not had any issues.

    It is unlikely I will be able to take a look until the weekend now. I am going to remove the caps across the transformer and see if this helps; I cannot remember now where I saw this recommendation (will have to search through my browsing history) but these have been on since I first did the rebuild, so hopefully may be the cause.........
    Attached Images Attached Images

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