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Effed up Quad 909 (UK made)

Discussion in 'audio' started by Zombie, Sep 4, 2014.

  1. Zombie

    Zombie pfm Member

    So, I gave in to audiophilia nervosa and believed the fantastic reports from Dada on how much better the 909 would be without the ampbus card.
    So I removed it and ended up with a 2 x 140 W buzzing amp.
    The buzz is regardless of level and input. It was much more quieter without input cables and dead quiet with shorted inputs.
    So I thought I'd put the ampbus card back and problem fixed. Now I have a 2 x 140 W amp buzzing regardless...
    Anyone have any pointer to solving this earthing problem? I have seen some articles on the net about buzzing 909s and earth but haven't figured out what to do...
  2. Bluedroog

    Bluedroog pfm Member

    Avoiding Earth Loops

    Earth loops, also known as ground loops and hum loops, are the bane of every studio installer's life. Due to the sheer number of signal sources and destinations, earth loops are easy to create and, without thought and planning, almost impossible to rectify. Ea

    Figure 6. A method for avoiding earth loops with multi-channel unbalanced equipment.

    rth loops are created when the ground leg of a signal travels, via other wiring and equipment, back to its source. This loop picks up mains hum, digital noise, clicks and pops from any available source. Noise is introduced when the unwanted interference sources induce a current in the loop. In the real world, the loop cannot have zero resistance and therefore (according to Ohm's law) a noise voltage is developed across the loop resistance. This noise voltage is added to the genuine audio signal. From this description it should be easy to understand that all sorts of noise can be introduced in this way, as long as the source is able to induce a current in the loop. Because of this, I prefer to refer to earth or ground loops, rather than 'hum' loops.

    Although balanced systems are more tolerant of interference, an input's Common Mode Rejection Ratio (CMRR) will dictate how well the induced noise is attenuated. Often, in balanced systems, earth loops are not immediately obvious, as hum may not be apparent. Because CMRR will worsen with increasing frequency, low-frequency noise, such as mains hum, may be effectively removed, whereas noise with a higher frequency content may still be evident. Timecode and PCs are particular offenders in this scenario, and may often be heard 'screaming' away at low level in the background. Unbalanced wiring can be hugely affected by ground loops. Any induced noise appears directly at the input with none of the balanced system's cancellation improvements.

    The only sure way to avoid earth loops is to avoid any connection scheme which allows the signal earth path to connect back to itself. A typical earth loop is illustrated in Figure 4, and the path will often take a route via the mains earth of the equipment concerned. With balanced equipment, breaking the loop is very straightforward. There are a number of possible earthing schemes in balanced systems and each scheme will dictate where the earth loop is broken. My preferred method is to connect cable shields at equipment outputs and not at equipment inputs. If ground loops are experienced when introducing hire equipment into a system, 'ground lift' switches may also help. However, it is vitally important that the loop is not broken by removing a mains earth connect

    If all else fails, a resistor connected in series with the signal ground in the cable can often reduce earth loop hum to acceptable levels.

    ion, because this may result in the equipment becoming dangerous. Some installers choose to connect grounds at inputs rather than outputs, but this will give rise to problems at patch panels — balanced outputs will not be unbalanced correctly when connected to an unbalanced input (the balanced signal pair and their ground will not both available on the output socket), and therefore the signal level may be compromised.

    For unbalanced systems or part-balanced, part-unbalanced systems, the problem is much worse. Fortunately, many units sporting unbalanced connectors, typically phonos, do not require mains power grounds and therefore only have two-conductor power cable. This means that, with a single signal connection, no ground loop can arise. However, if you make a stereo connection, two phono cables will create a loop around themselves, as shown in Figure 5. Although this is not normally a problem if the cables are short, the kinds of lengths found in typical studio installations may well induce noise. This is simply cured by removing one shield connection at the equipment input.

    Unbalanced loops can become a real headache with multi-channel equipment such as many soundcards. If each cable had its shield connected at both ends, a large number of loops would be created. A simple trick to avoid this is shown in Figure 6. Here, none of the shields are connected at the unbalanced inputs. Instead, a single thick wire is connected between one output ground and one input ground. In this way, each signal is shielded and the two units have the same ground reference, but no loop is created. I have also used this method when constructing patch panels for mixing consoles. In many situations, particularly where the unbalanced equipment uses a power ground, the only sure-fire cure is by using a signal transformer (as shown in Figure 7) or a balanced interface unit.

    Finally, in some situations, noise can be reduced to workable levels by inserting a resistor (100(omega), 0.25W) between the cable shield and the signal ground. This causes the noise voltage to be developed mainly across the resistor instead of in the wiring, thus reducing the resulting noise signal. This is a last-ditch solution, but is sometimes helpful. To avoid ground loops in complex systems, some or all of the above may be necessary. Also some experimentation may be required. The only hard-and-fast rule is: do not cure loops by removing the power ground!

  3. Zombie

    Zombie pfm Member

    The 909 hasn't any three-pin power outlet. The is no power ground connection between the preamp and the 909, the 99 preamp has the trhee-pin ground, but thanks anyway, I need to know what to do on the 909. The hum goes away when I turn the power off and the amp uses what's left in the caps.
  4. John_73

    John_73 pfm Member

    Ask Dada?
  5. Zombie

    Zombie pfm Member

    Already asked but kind of lost trust...
  6. Barrymagrec

    Barrymagrec pfm Member

    I`m guessing a bit here but given that the earthing would normally be via the ampbus board which you removed and now you still get hum with it replaced I wonder if there is a low value resistor some where which ties the 0 Volts to chassis and because you removed the normal earth that has blown open. I have had that happen on some equipment where the supplier reversed neutral and earth. Caused no end of aggravation.
  7. JohnW

    JohnW Trade: Lakewest


    If you can ship the amp to me in Czech Rep. then I resolve the problem for you and optimise its performance - for free :) (You just cover the shipping costs).

    Dont loose faith, the Quad 909 is worth a little TLC
  8. Zombie

    Zombie pfm Member

    Wow, that's a fantastic offer, John! Details on PM?
  9. linnfomaniac83

    linnfomaniac83 I bet you can’t wheelie a unicycle!

    John, although I haven't had any personal dealings with you, you are one of the true gentlemen of this industry. I have major selling products designed by you in my collection and you are known as a legend in audio design but you still find time to help people out.

    You get a big thumbs up from me!

    DAVEDWACK pfm Member

    Yes and a massive thumbs up from me also, seems to be a proper gent.:D

  11. John_73

    John_73 pfm Member

    Fair enough. I'm having second thoughts about getting the 606 upgrade kit now I must admit after reading this.

    I'm sure your 909 will be better than it was when new after John has worked his magic on it :)
  12. Zombie

    Zombie pfm Member

    It isn't about the 606 upgrade, but about taking out the ampbus card. The 606 doesn't have that. My 909 is very early (made in the UK) and maybe there revisions since then...
    But to get a generous offer from the person involved in the construction was more than I hoped for!
  13. Wobblybob

    Wobblybob pfm Member

    Thumbs up from me as well.
  14. Disbeliever

    Disbeliever pfm Member

    I find Quad amps QSP & Platinum not having three -pin power input are a pain. Can not use them.
  15. Radfordman

    Radfordman pfm Member

    I have no experience of the Quad 909, I have II's 303's, a 405 and a 405-2 and I have not read all the posts here.

    But I have many years of fault finding experience in electrical/ electronic/ mechanical systems.

    I would probably first, reverse any changes made since amp worked.
  16. Harry1212

    Harry1212 pfm Member

    Robert (Audiosmile - Bakeraudio {whatever}) is a whizz with all things Quad.

    Check out his website.

    Cheers, H.
  17. JohnW

    JohnW Trade: Lakewest

    :) PM sent :)
  18. S-Man

    S-Man Kinkless Tetrode Admirer

    The 909 sounds worse without the ampbus card - thin and a bit bright.
    China made 909 sounds better than 606II to me.
    I am not at all convinced by Dada.

    To fix your buzzing....

    have a look at the resistor/transistor network that sets the 0V of the main smoothing caps.
  19. Zombie

    Zombie pfm Member

    JohnW: your pinkfish mailbox is full so I ask here...
    Do you need me to enclose a Quadlink/ampbus cable?
  20. JohnW

    JohnW Trade: Lakewest


    Your amps arrived today safety - I'll take a quick look at it tonight,


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