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Tannoy Chatsworth 12" MG rebuild advice.

Discussion in 'classic' started by awkwardbydesign, Feb 26, 2021.

  1. cooky1257

    cooky1257 pfm Member

    I use V2, It's also great/ handy for accurately measuring caps, inductors and resistors.
     
    Fatmarley and Hoopsontoast like this.
  2. awkwardbydesign

    awkwardbydesign Officially Awesome

    Scrabbling around in WinISD, I managed to input (I think!) the T/S parameters for the MG12, and came up with a 160L box, 2 ports @ 10cm dia. 30cm long, tuned to 28Hz, which gave slightly down shelved bass response with a -3dB point at 30Hz (relative to 0dB at 500Hz). A starting point, perhaps, as the T/S parameters are a bit iffy.
    [​IMG]
     
  3. Hoopsontoast

    Hoopsontoast pfm Member

    Blimey that's going back some!
     
  4. awkwardbydesign

    awkwardbydesign Officially Awesome

    And I just discovered that the programme I used on Win 7 laptop is XSim not boxSim! :rolleyes: At least with that I can change crossover components on the fly, and see what effect they might have. For example, Mark (divedeepdog) measured the response of these drivers and found a dip at 1500Hz, so I can look and see if a crossover component change might ameliorate that, despite it probably being a suspension artifact.
    Never got round to using them, for PC speakers, and I never will now.
     
  5. Fatmarley

    Fatmarley Agnostic

    The big problem is not knowing what the crossover is doing to the frequency response. You don't know where your output at 500hz is (driver output) compared to 100hz and below (port output). There's also room gain to consider, -3db at 30hz in my room would cause a huge peak in centred around 30hz. You also have to consider port resonances with ports that are too long.
     
    cooky1257 likes this.
  6. awkwardbydesign

    awkwardbydesign Officially Awesome

    As I said, a starting point. If the cabinet size is realistic, the tuning can be optimised in-room with REW. Once I settle on a port diameter, the length can be tuned to suit. Or even lengths. My inability to handle computer simulations makes the job harder, of course. And there is no-one locally who can sit down with me and guide me through the initial set-up of programmes. Even it was allowed!
     
  7. Fatmarley

    Fatmarley Agnostic

    In software an fs of 20hz changes the cone weight to 120 grams (nearly double). Obviously the cone weight won't change, so I adjusted VAS from 206ltr to 400 and the cone weight went back to 62 grams. I'd imagine the spider has loosened over the years and caused VAS to rise and fs to go down.
     
    cooky1257 likes this.
  8. Fatmarley

    Fatmarley Agnostic

    What's the internal volume of your Chatsworths? Also, are they sealed?

    I just modelled them and assumed they are 75ltr sealed. If you want a bit more depth, and you're happy with the upper midbass, I'd go for a cabinet of 110ltr tuned to 23hz (3" diameter x 7" port). If you want tight bass make sure you use plenty of stuffing but keep it well away from the port. At such a low port tuning frequency, the driver is producing most of the midbass, so should still sound fast.

    If you feel they are lacking in thump, just shorten the port by a couple of inches or so.
     
  9. awkwardbydesign

    awkwardbydesign Officially Awesome

    Thanks for the help. I did assume the suspension had loosened, but didn't know how to model it. At the moment I can't really tell how they sound, except they do sound rather strangled. I am in the process of replacing the horrible spring loaded terminals with small binding posts, replacing the very hit and miss 4 pin connectors, and changing the resistors and caps in the crossovers.
    The Chatsworths are definitely sealed, but I'm not sure of the volume, and being basically 1/2" thick are somewhat lively.
     
  10. awkwardbydesign

    awkwardbydesign Officially Awesome

    BTW, I wonder if the dynamic mass is really 62gm. That is the figure for the HPDs with their ribs, while all the other monitor 12s are around 40gm. Does the rubber surround make that much difference?
     
  11. Fatmarley

    Fatmarley Agnostic

    Some frequency response measurements could help give us an idea of what's missing at the low end. A nearfield with the microphone right up close to the cone, around 5mm to 30mm, and one about 1mtr or so away, preferably with the speaker lifted off the ground and as far away from boundaries as possible.
     
  12. Fatmarley

    Fatmarley Agnostic

    I do know that rubber surrounds are heavier than foam, but I don't know how much. You can work out the Mms of a cone by the thiele small parameters (Vas, fs and Sd), so assuming they are correct, then the surrounds will be 62g.

    Edit: What the hell am I on about "surrounds" - I should have said Mms.
     
    Last edited: Mar 4, 2021
  13. awkwardbydesign

    awkwardbydesign Officially Awesome

    Lead surrounds? :eek::D
    I can't take any measurements at the moment, as the speakers are in the loft. I can listen up there, and measure impedance, etc. but not the response. And at the moment they are in pieces being fiddled with; new binding posts, new 4 pin chassis connectors and crossover components. When all that is done I can drag them down again (loft ladder and stairs to cope with) and get the mic out. Then they will have to be dragged back up, as the QUADs are still here. So I want to minimise all that, as I am not as fit (or young!) as I was, and I have to do it alone.
    Changing VAS to 400 litres in WinISD, and playing around* with box, vent and tuning, I looked at a 150 litre box, 10cm vent and 23 or 26Hz tuning, to add thump, as you put it. My reasoning being that a larger box gives me more options, with cabinet wall shapes and driver placement height, it can be reduced in volume if necessary, the larger vent similarly (easy to sleeve down). Also Paul's dimensions are not far from that 150 litre size, so easier for me to scale my box, etc. At the moment I have domestic acceptance of those dimensions, and changing it too much means I have to go through all the explanations again! :D
    I do have a pair of Ripole subs I made for the QUADs, so there is another option for bass reinforcement, but I'm not really expecting to need them.
    * I say that as I am not sure I am getting it right. With a 150 litre box, one vent at 10cm , 26Hz tuning, I get -1dB@40Hz, -3@30Hz and -12@20Hz. At 23Hz, it's -2@40Hz, -4@30Hz and -10 or 11@20Hz. Vent lengths of 15.85 and 22.29cm. Does that match your numbers?
     
  14. Fatmarley

    Fatmarley Agnostic

    150ltr with the new 20hz fs and 400ltr VAS my sims pretty much match yours. I think there will be too much bass around 30hz once you add in room gain, but as you say, you can always make the enclosure smaller or add a shelf inside to reduce the volume. It's not so easy to make an enclosure bigger.

    Driver positioning on the baffle is also important. Assuming Tannoy got it right with the Chatsworth, you really should make the baffle dimensions the same and position the driver in the same place on the baffle. To see what I mean, download The Edge and have a play.

    http://www.tolvan.com/edge/help.htm
     
  15. awkwardbydesign

    awkwardbydesign Officially Awesome

    I think I have The Edge on the old laptop, but I'll download it to this one. But I'm not sure Tannoy "got it right", the Chatsworth is probably the smallest cabinet they could get away with.
    A relief that my sims match yours, at least now I can fiddle them. I plan to post a similar thread on Lenco Heaven, which is where Paul Coupe posts nowadays. He can then shout at me or help me, as he wishes. :D
     
  16. Fatmarley

    Fatmarley Agnostic

    When I said "got it right" I was talking about baffle diffraction, not enclosure volume. As you move the driver around the baffle or change the baffle size in The Edge, the frequency response will change.
     
  17. awkwardbydesign

    awkwardbydesign Officially Awesome

    Yes, I realised that. I just think the baffle diffraction is what it is, and the volume is what they worked to. They couldn't change the baffle size much due to the driver dimensions. And the cabinet is as shallow as they could make it; the back of the driver is pushing into the wadding. At least that is my suspicion.
    I want the acoustic centre of the driver just above seated ear height, when the plinth is taken into account. In the past, having the tweeter above ear height gave the greatest illusion of a live performance, but with a dual concentric driver that may not be the case. But having a plinth/base that I can adjust would allow some fine tuning there.
     
  18. Fatmarley

    Fatmarley Agnostic

    Read post #56 again.
     
  19. awkwardbydesign

    awkwardbydesign Officially Awesome

    You are misunderstanding me. I mean that I don't think Tannoy made the the Chatsworths to take account of baffle diffraction, just the minimum volume they could get away with. And therefore the baffle diffraction is what it is. On the Chatsworths.
    I understand it will change as you change shape and position; I have calculated it in the past, and incorporated compensation in my active crossover.
     
  20. Fatmarley

    Fatmarley Agnostic

    It's probably me not explaining it properly - When you design a loudspeaker, you build the enclosure, then take measurements, then design a crossover to give you a flat or at least pleasing frequency response. If you change the driver position or the baffle size, it changes the frequency response from what Tannoy intended, and there's a good chance it won't sound as good as it could. I'm pretty sure Tannoy would have done their best and taken a lot of time getting this aspect right.

    A well respected member of this forum has said that he's heard quite a few DIY Tannoy enclosures and thought they all had faults. I haven't heard any diy Tannoy enclosures, but I've built enough speakers from scratch to have a good idea of what the problems would most likely be. I'd argue the worst culprit would be a badly tuned port (the wrong enclosure size means no amount of messing with the port can fix it). Followed by a baffle or driver position change. A few decibels difference here and there can turn something special into something quite ordinary, or worse.

    I'd keep the baffle and driver position the same, just to be on the safe side. If Tannoy has built other models with the same driver and crossover, but with a different sized baffle or driver position, it could be worth looking at those, but if that's true, it's likely one model will have a more *pleasing* or linear frequency response than the others. Only extended listening combined with measurements would give you the answer there.

    *I use the word pleasing because sometime a bump or dip here and there can sound more accurate (yes, more accurate) than a flat line.
     

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