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filter problem on the Quads 405-2 BBC modified

Discussion in 'Monitoring / Headphones' started by vonrozen, Feb 10, 2016.

  1. vonrozen

    vonrozen Active Member

    Hello everyone !

    I have 2 Rogers Ls 5/8 driven in bi-amplification by quads 405 and 405-2. Porblem: one of the filter on the Quad 405-2 is faulty because the tweeter only reproduces very high range and leaves the mid-range out ( this is compared to the other tweeter of the other Quad. I know that the tweeters are both in perfect condition and I have swapped the Quads to confirm that it comes from the filter, which it does.

    Can someone tell me if those filters are difficult / costly to repair ? Here in Paris every intervention in HIFI is immediately 100 € , so the more info I have the better I can be prepared .

    Thank you all

    Alexander ( Paris )
  2. DonnyThompson

    DonnyThompson Distinguished Member

    If you've checked all your wiring and everything appears to be okay, my very uneducated hunch tells me that it might be a problem with a crossover somewhere? I have no idea how pricey or difficult those are to repair, or if that's even the problem... Sorry.

    Boswell is probably your man for this one.

  3. Boswell

    Boswell Moderator Distinguished Member

    Possibly. As we discussed in the previous thread on your loudspeaker problems, the best information I have is that the professional versions of the LS 5/8s did not have passive crossover networks in the cabinets; they were bi-amped with a low-level electronic crossover at the inputs to the two amplifiers.

    This probably means one of two things: either (a) the components you see inside the cabinet are not crossovers but protection filters plus attenuators to equalise the acoustic levels from the two transducers, or (b) your LS 5/8s have been wholly or partly converted into PM510s, the single-amplifier domestic version of the LS 5/8 with an internal passive crossover.

    Maybe the truth is somewhere between (a) and (b), with the two cabinets having been left in different states of modification. Alternatively, these may be very special and unique units, operating to a specification that is not available to us.

    Could I suggest that you take some further close-up photos of the networks and wiring in the two cabinets? Rather than clog up the RO server with many high-res pictures, you could send them on to me (and anyone else who dabbles in this sort of thing, such as @MrEase) via the Conversation mechanisms of this website.
  4. DonnyThompson

    DonnyThompson Distinguished Member

    Good idea, although for the benefit of others, could one of you please update here and let us know what the problem was? It would also serve as a good resource for anyone else on the net doing a similar search on the subject. :)
  5. vonrozen

    vonrozen Active Member


    my speakers are genuine - Ihad an ex BBC man verify this. I am speaking about the filter on one of the quads. Let me summerise again: would you or anyone else know how to repair a Quad 405-2 8/16 ( BBC modified ) filter that does not filter all the signals to the tweeter ?

    ideas are wilcome.

  6. vonrozen

    vonrozen Active Member

    PS : further to your email Boswel, I have indeed the Rogers Ls 5/8 with protection step-down filters to protect the tweeter against any forceful noise, such as that of a microphone - logical thing for monitors to have that have been often used for speech /drama / interviews monitoring.
  7. Boswell

    Boswell Moderator Distinguished Member

    OK, sorry, I misunderstood what you were saying when you wrote: "I have swapped the Quads to confirm that it comes from the filter". I took this to mean that you swapped the Quads and the fault remained with filters in one loudspeaker rather than being related to the crossover network in the amplifier.

    To repair an amplifier fault, it's necessary to find out whether it's the AM8/16 crossover network or one of the power amplifier channels that is giving the trouble. My inclination would be to say that it's likely to be the AM8/16, in which case it should be relatively easy to fix. It's something I would be happy to investigate, but I suspect that transport costs France -> UK and return could be large for an item of that packed size and weight.

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