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Old 13th December 2010, 03:17 PM   #3831
udailey is offline udailey  United States
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Kerux,
Nice job. Got pics?
Was at a friends house yesterday. He had been a DJ for a jazz station in Louisiana for several years back in the 80's. I've not met anyone with that many records/cds. "3000 more records out in the garage" he says. Jeeeeez!
Anyway with a LS in his system for the first time his words were "are those MY speakers?" pretty much what his wife said. The difference was huge. Fun times.
He had a Melos pre in there when I arrived which also sounded incredible, not equally incredible but I am predjudiced towards LDRs of course, but it was quite a different sound. I wouldnt mind trying that pre in my own system from time to time.
Uriah
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Old 13th December 2010, 05:22 PM   #3832
UnixMan is offline UnixMan  Europe
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Quote:
Originally Posted by georgehifi View Post
Wish you luck on the matching, and have some Valium handy.
George, for a balanced setup I would rather use a simple shunt setup (two fixed R in series + single shunt LDR between the two "sides").

Using series/shunt LDR (i.e., mkII) IMHO is NOT a good idea, any attenuation unbalance among the two "sides" will screw up the balanced environment.
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Old 13th December 2010, 05:30 PM   #3833
udailey is offline udailey  United States
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That solution would certainly be easier. I dont think George was recommending the solution he posted but simply giving the guy what he asked for. Its really hard to perfectly match up as many LDRs as are necessary for balanced, the reason for the Valium. Your solution might be worthwhile of course impedance is all over the place that way, but with a powerful CDP output and a real high input impedance on the amp it might not matter much. Why not use LDRs as the set resistance resistors?
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Old 13th December 2010, 07:35 PM   #3834
ondesx is offline ondesx  France
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Quote:
Originally Posted by TheShaman View Post
How hard would it be to make a balanced version of an LDR attenuator?
A much simpler way to build a balanced attenuator is to use "floating balanced" (see figure)
Using this method, you'll save one LDR per channel, less headaches for their matching and their driving !
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Old 14th December 2010, 01:02 AM   #3835
soongsc is offline soongsc  Taiwan
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How would the load impedance look?
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Old 14th December 2010, 09:49 AM   #3836
AndrewT is offline AndrewT  Scotland
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The series LDR must match very accurately.
This could be done with a switched resistor stack. A two pole 6 position switch each feeding a fixed resistance into the respective LEDs would achieve the separation that could allow for very accurate trimming of the LED resistors.
The shunt LDR is the continuous Volume controller and does not need to be matched.

However, I proposed something very similar in another thread and was referred to D.Self for why this shunt version of a balanced volume controller is not as good as it can be. I can't recall the details but the argument against seemed valid to me.
If you can accept or overcome Self's argument, then this would be very simple to implement.
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Old 14th December 2010, 05:29 PM   #3837
ondesx is offline ondesx  France
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Quote:
Originally Posted by AndrewT View Post
The shunt LDR is the continuous Volume controller and does not need to be matched.
Well, there are two possibilities here :
- if no match at all is performed, a separated left and right control could deliver the correct balance,
- if a stereo control is used, a balance correction is mandatory.

As for all LDR, the major drawback is to ALWAYS deliver the CORRECT current to LED to be sure to get the correct value for the resistor. This remains true for all series and shunts components. Using resistors to control them may be one solution, but it is a "fixed" method (excepted if the resistors could be replaced by real adapted values). A best approach is to be able to correct the currents for any divergence of these resistor values, from time to time, either manually or automatically, ...
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Old 15th December 2010, 02:07 AM   #3838
soongsc is offline soongsc  Taiwan
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I think correct current to LDR is only half of the story.
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Old 15th December 2010, 10:44 AM   #3839
ondesx is offline ondesx  France
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Quote:
Originally Posted by soongsc View Post
I think correct current to LDR is only half of the story.
And the other half is ?...

Last edited by ondesx; 15th December 2010 at 10:46 AM.
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Old 15th December 2010, 11:42 AM   #3840
soongsc is offline soongsc  Taiwan
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I was wrong, it's only 1/3 of the story. Even for the correct current, the resistance value may not be the same from device to device. If you get the resistance values to match at one point, they may not match at other points.
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