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roberte
Joined: 20 Sep 2006
Posts: 50
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 Posted: Sun Feb 07, 2010 8:25 pm Post subject: JR149 improved x-over type 24 (8R 7W res in particular) |
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Recently upgraded to a pair of Graham Hartle's x-overs in my recently acquired JR149s and in general the results are very nice.
I can't helping feeling though that the treble detail is now just lacking something, a little too smooth.
I was slightly surprised to see that the resistors in the treble are now ceramic, I understand that previous supplies are now exhausted but I did just wonder whether swapping out this component would be beneficial.
I have already removed the extra link (ie no 2.5 dB treble cut) so that just leaves the 8R 7W. From a quick look around though, this isn't a value where there is masses of choice.
So, any suggestions? (and is it likely to make much difference anyway?) |
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ColinR
Joined: 31 Jul 2004
Posts: 999
Location: Staffordshire
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| Posted: Tue Feb 09, 2010 11:02 am Post subject: |
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As the 8R in combination with the 22R to achieves 2.5dB attenuation of the T27's output, by itself it's just ornamental.
If you're into gold plated audiofool fuses or the thought of a bit of nichrome wire in the signal path offends your hearing, remove it and replace it with a bit of "snake oil" cable, although a paper clip would be more appropriate  .
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roberte
Joined: 20 Sep 2006
Posts: 50
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| Posted: Tue Feb 09, 2010 2:35 pm Post subject: |
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Sorry, not sure I understand there Colin.
I have already removed the extra link which, I assume, takes out the 22R and means there is no extra attenuation of the T27.
You seem to be saying that the 8R isn't doing anything at all now - is ornamental.
Have I understood that correctly? |
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ColinR
Joined: 31 Jul 2004
Posts: 999
Location: Staffordshire
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| Posted: Tue Feb 09, 2010 4:04 pm Post subject: |
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| Quote: | You seem to be saying that the 8R isn't doing anything at all now - is ornamental.
Have I understood that correctly? |
Yup  .
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roberte
Joined: 20 Sep 2006
Posts: 50
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| Posted: Tue Feb 09, 2010 4:29 pm Post subject: |
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schematic from Graham's site, hope he doesn't mind - will remove if anyone would rather it weren't here
My knowledge of circuits isn't that good, but you are saying that R2 here could be replaced with a length of wire and there be no difference? (assuming link1 has already been removed) |
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ColinR
Joined: 31 Jul 2004
Posts: 999
Location: Staffordshire
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| Posted: Tue Feb 09, 2010 4:56 pm Post subject: |
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| Quote: | | My knowledge of circuits isn't that good, but you are saying that R2 here could be replaced with a length of wire and there be no difference? (assuming link1 has already been removed |
A fixed attenuator requires two elements, a resistor in series and one in parallel (to dump current), removing the parallel component makes the series one superfluous  .
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roberte
Joined: 20 Sep 2006
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| Posted: Tue Feb 09, 2010 5:08 pm Post subject: |
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| R2 will still be using power though won't it, it will be turning current into heat? |
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ColinR
Joined: 31 Jul 2004
Posts: 999
Location: Staffordshire
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| Posted: Tue Feb 09, 2010 9:02 pm Post subject: |
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| Quote: | | R2 will still be using power though won't it, it will be turning current into heat? |
Yes P= Irms * Vrms, but it will not be of such a magnitude that it will be audible.
i.e. having connected a 1W warble tone audio source via zero, one and two 5R1 10W to a T27 SP1032, the reduction in audio output (if any) was <<<<<2dB, the accepted noticeable differential spl for the human ear.
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speakerguru
Joined: 18 Nov 2005
Posts: 349
Location: Green Hut, Tovil
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| Posted: Wed Feb 10, 2010 9:48 am Post subject: |
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| ColinR wrote: | | A fixed attenuator requires two elements, a resistor in series and one in parallel (to dump current), removing the parallel component makes the series one superfluous . |
Not entirely superfluous. Don't forget the drive unit itself is a shunt component (load) in parallel with the resistor going to ground, so a series resistor will still attenuate, just not as much as with the shunt resistor in place. |
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roberte
Joined: 20 Sep 2006
Posts: 50
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| Posted: Tue Feb 16, 2010 11:15 am Post subject: nothing's ever simple is it? |
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Have had to do a bit of reading on what an L-pad is and does, am also slowly coming to grips with what effect it's impedance has on the filter before it - slowly mind.
Anyway, having started out with the links removed and found the top end a little restrained I held off putting the links back for a while thinking the additional attenuation would take them further down that same path.
However, in the interests of open investigation I put them back in at the weekend, and my impression is actually the opposite of what I was expecting - they seem to have perked up a bit, crisper and clearer at the top.
Obviously there is more to this than I'd thought - pleased with the way they are sounding now though. |
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