external cross-overs by Derek Hughes (Graham Audio) for the Rogers 5/8 and your experiences with it

Discussion in 'Monitoring' started by vonrozen, Nov 27, 2018.

  1. vonrozen

    vonrozen Active Member

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    Hello !

    I have just bought a pair of external cross overs, developed by Derek Hughes and marketed by Graham audio especially for the Rogers 5/8 to enable the user to use any power amp and not be limited by the Quad 405 and its rather badly designed cross-over.

    Has anyone had some positive experiences with this ?

    Can it really improve the quality of the good old Rogers ?

    Thanks

    Alexander
     
  2. Boswell

    Boswell Moderator Distinguished Member

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    Hi Alexander!

    When you say you have just bought a pair of new crossovers, I'm assuming they line-level electronic type rather than speaker-level passive ones. If so, which power amps are you intending to use with them?

    I would be interested to know how you get on with them. I've been keeping an eye on your thread and the replies at vintage-radio.net.
     
  3. vonrozen

    vonrozen Active Member

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  4. vonrozen

    vonrozen Active Member

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    as i mentioned before, my fear is that the improvement in sound will be insignificant and the lack of tweeter protection is a concern too. Would be interesting to hear from someone, who has experience using the passive cross-overs.
     
  5. vonrozen

    vonrozen Active Member

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    Here is the photo of the circuit ! Do let me know what you think about the electronics of it, the circuit - is it likely to be better than the active ones inside the BBC modified Quads 405 ?
     

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  6. Boswell

    Boswell Moderator Distinguished Member

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    Those are very nice-looking passive crossovers. I can imagine in a product like that all the components are individually measured to give a response that matches the design response very accurately. However, no matter how good and accurate passive crossovers are, they have to interact with the electrical and reflected acoustic properties of each loudspeaker cabinet they drive, and, in my experience, these are far from consistent from one unit to the next supposedly matched one.

    The normal way round this problem is to power each transducer element (loudspeaker driver) from its own power amp, and to use crossovers at the input at each power amp. These input crossovers could be low-power passive, but it's a good opportunity to use active circuitry in order to get a wider range of available filter characteristics, as well as using esoteric components such as negative inductors.

    So much for the differences, but I can't really answer your question about which is better. The Derek Hughes passive design has the advantage of being recent, potentially taking advantage of developments in filter theory and component technology during the 40-odd years since the venerable BBC/Quad active crossovers in the 405 were designed. There remains your point about tweeter protection, but my feeling is that a very experienced designer like Derek Hughes would have taken that into consideration when designing these crossovers specifically for the Rogers 5/8.

    I would get them installed and enjoy them. Don't forget to let us know how they sound.
     
  7. vonrozen

    vonrozen Active Member

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    When you say :

    The normal way round this problem is to power each transducer element (loudspeaker driver) from its own power amp, and to use crossovers at the input at each power amp. These input crossovers could be low-power passive, but it's a good opportunity to use active circuitry in order to get a wider range of available filter characteristics, as well as using esoteric components such as negative inductors.

    what do you mean ? Do you mean that, in an ideal world, these ones would have to be 1. installed into each power amp and 2. be low power passive ?
    Ane when talking about power amps do you also mean that I should go for 2 amps (each per speaker) rather than one power amp ?

    Thank you very much and sorry to be so unknowledgable :-(
     
  8. Boswell

    Boswell Moderator Distinguished Member

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    A line-level crossover requires separate amplifiers for each output of the crossover. If you look at the many so-called bi-amped powered speakers available these days, they have internally an active crossover at the input and then separate power amps for the bass/mid and treble drivers. The amps are usually tailored for their respective duties, with a lower-power one for the high-frequency driver. This configuration has come about because medium-power class-D amplifiers can now be low cost, small enough and give adequate quality for average domestic loudspeakers and studio near-field monitors. At present levels of performance, I would not specify a class-D type of amplifier for powering the drive units in a top-quality monitor like the Rogers 5/8.

    The original design for the 5/8 used a BBC-modified Quad 405 stereo amplifier per cabinet, along with the active crossover ahead of the two amplifiers. The combined configuration had the designation of AM8/16. Since there was a separate amplifier for the tweeter, the limiter for that output could be made more agressive in order to protect the tweeter at higher output levels.

    In your case, as I understand it, you have the Derek Hughes/Graham Audio speaker-level passive crossover. The expected way of using these is with a single power amplifier driving the crossover input, and the two crossover outputs driving the bass/mid and the treble (tweeter) units. There is not a lot of opportunity to add extra tweeter protection, but, as I said previously, I would be surprised if there were not some form of protection in the HF output of the Hughes crossover. The Graham Audio website talks about tweeter level controls situated behind the grille of their 5/8 cabinets, but I don't know if that was part of the original Rogers design.

    Am I understanding correctly that you have a pair of the Rogers 5/8s loaded with the correct drivers, but that no power amps came with them? You now have a pair of the Hughes crossovers, but did they come with any specific ecommendations about how to connect to the drive units in the 5/8? The only relevant info I can find on the web is this brief descriptive sheet.

    It looks as though it should be possible to use a range of different stereo amplifiers to drive a pair of 5/8 cabinets, each cabinet fitted with the crossover you have. My guess is that you would need an amplifier that has a rated output power of at least 200W continuous per channel.
     
  9. vonrozen

    vonrozen Active Member

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    dear Boswell !

    thank you very much for your great and informative reply . Just to clarify the points :

    - i have Rogers 5/8 with the correspondig Quads. The idea behind the acquisition of the passive cross-overs was to widen the possibilities by not being limited to just the Quads 8/16. I prefer a more tube-like sound and with the cross overs this is possible because I could, in theory, use any power amp, as you explained very well.

    - each Roger is furnished with a small tweeter control ( I think) , next to the tweeter located at the inside (see photo)
     

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  10. Boswell

    Boswell Moderator Distinguished Member

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    By "corresponding Quads", did you mean you have two of the full AM8/16 spec units?

    If your plan was to use the speaker-level crossover on the output of the amplifier(s), were you going to by-pass the active input crossover? Was this just using one of the power amps per cabinet, or did you have a way of utilising both of them?
     
  11. vonrozen

    vonrozen Active Member

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    Boswell,
    Thank you. Yes i have the modified Quads ( i.e 8/16) , i bought 2 pairs of Rogers 5/8 during the bbc auction.

    My intention is to get a warmer sound and a more unified one ( as the Quads age badly) .
    So are you recommending one piwer amp per speaker ? I was thinking of Audio Research as they have a wide range of products although if you know of a less costly alternative i‘d be interested!

    Thank you v much
     
  12. vonrozen

    vonrozen Active Member

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    Here is the back plate 271991FC-EAF6-4E49-A8D1-36B9AC07365B.jpeg
     
  13. Boswell

    Boswell Moderator Distinguished Member

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    Well, yes, you can only drive the Hughes crossover from the output of one amplifier.

    It seems odd to me that Graham Audio sell these (relatively expensive) crossovers for the 5/8 cabinets and then don't tell you how to wire them up.

    If you can by-pass the active crossovers in the AM8/16 units, you could try driving the input of each crossover from one of the pair of Quad 405 amps in each AM8/16 set. This should work OK at low to medium power levels, and give you an idea of how they sound.
     
  14. vonrozen

    vonrozen Active Member

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    I know how to wire them . They are not for sale, very few were made, they are a technical rarity. I bought them from a private source ;-)
     
  15. Boswell

    Boswell Moderator Distinguished Member

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    Sounds as though you are good to go, then. Do check the phasing of the two transducers.
     

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