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Replacement tweeters for R107: Matching

 
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Notthatclever
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Joined: 05 Apr 2018
Posts: 6

PostPosted: Fri Apr 06, 2018 10:32 pm    Post subject: Replacement tweeters for R107: Matching Reply with quote

Hi All
I've got some 107/1 with a /2 Kube. I replaced the tweeters with some Vifa Xt25gb60 trying to match the T33 parameters as close as possible eg 4 ohm, 93db but the resonant freq on them is much lower @ 600hz vs 1300, plus other parameters are different.

Is it possible to redo the conjugate hf circuit to move the 'nulling' of the reactive parts to match the new Vifa? Is this a worthwhile goal? What would it achieve?

I have a spare set of /2 hf boards Sp2166, one faulty, not sure if the fitted ones are the same /2 boards or the original /1 Sp2070 boards. I will check.

The /2 boards have 4.2uF cap in parallel with the tweeter and an inductor in series on the bridge of a pi network with a .68uf/1.5 ohm RC on the other leg. This I guess produces a tuned circuit at some central freq.

How would I go about rematching this to the new Vifa? Grateful for any help.

Marcus
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Notthatclever
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Joined: 05 Apr 2018
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PostPosted: Sat Apr 07, 2018 7:11 pm    Post subject: Reply with quote

I’ve checked and the heads were updated to SP2166 spec to match the /2 Kube. I think the LF Crossover was retained, and the hf part still requires the high pass conditioning that is done on the LF board....probably.

Looking at an impedance plot of the /2 in the Stereophile review 1991, it goes a bit knarly above 2khz with a big peak, which suggests the pi filter may not be the complex conjugate, but there to compensate for some other non linearity of the T33 SP1265 metal dome tweeter. However the 1987 Stereophile measurements say impedance is flat 20-20khz within 5%. Confused....Rolling Eyes

I do not have the SP1265 so wonder what these components will do to the response of the components I have, should they be bypassed?
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SaSi
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Joined: 24 Aug 2008
Posts: 234

PostPosted: Sun Apr 08, 2018 7:36 am    Post subject: Reply with quote

What happened to the original tweeters? If they only have hardened / dried ferrofluid, that's a relatively easy fix that's totally feasible. And you keep the original "character" of the loudspeakers.

Just a thought.
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Notthatclever
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PostPosted: Sun Apr 08, 2018 5:11 pm    Post subject: Reply with quote

Yes I have just acquired a pair of sp1210 and ‘decoked’ them, not a slight difference. So I will get that opportunity......

The hf crossover is still optimised for the SP1265.

They came with some Fostex which was okay, but a vigorous workout with Stevie Ray Vaughan popped one of them Very Happy
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Notthatclever
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PostPosted: Thu Apr 19, 2018 11:34 pm    Post subject: Reply with quote

Code:
“HF Crossover Circuit diagram in 107/2 head unit
                     
     —————-||——————-   
    |     1.22uF      |         L2
————-||———————-——-||——————————-&&&&————————
     3.5 uF | 10uF           |       |       |
            |                |       |       |
            &.         0.68uF== 4.2uF==   Tweeter
            &.  L1           |       |       |
            &.              1R5      |       |
            |                |       |       |
——————————-———————————————————————————-——

Left hand appears quite standard i.e. 3rd Order Acoustic Butterworth filter.
Right hand side has a strange pi filter which appears to be a low pass, but no doubt it’s conjugating the SP1265 tweeter.  Not shown the L-pad which matches the tweeter output to form the reference level output.”
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