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Ldr for 250k input

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Posted on June 18, 2015 at 19:28:02
ChasLieb
Audiophile

Posts: 284
Joined: April 7, 2002
I have a pre with a phono stage that wants to see 250k from the attenuator which follows it. The stepped series pot I have is good but a 46 position ladder or ldr would give me more gradual control. I found a kit LDR with self calibration from Tortuga audio that goes up to 99k and was thinking I could slap a 150k resistor on the ground leg to get me up to 250k seeing how the output hits the grid of the buffer tube that follows. Will the blocking resistor after the LDR damage it or harm its performance?

Anyone have an idea if this sounds workable?

Chas.

 

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RE: Ldr for 250k input, posted on June 18, 2015 at 19:46:46
Abacus Music
Audiophile

Posts: 97
Joined: February 13, 2012
The autocalibration would try to re-calibrate your 150k resistor.

You should speak to Morten Sissener at Tortuga about creating super-high impedance curves for his Tortuga boards, they are all digitally driven and variety of impedance curve "maps" could be created.

One thing though, LDR's do not operate well in impedance ranges higher than 30k as they become very non-linear and calibrating them becomes a real challenge. Another thing is their high 3rd order distortion that makes them useful in mostly "passive" attenuators where distortion is not amplifier any further.

 

RE: Ldr for 250k input, posted on June 19, 2015 at 10:12:11
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
You should consider getting a different phono preamp (is it a phono preamp, or a phono preamp/linestage).

Only being able to drive a 250K load indicates a pretty crappy design with very high output impedance (maybe a bunch of 12AX7's). I wouldn't that it will drive cable capacitance very well either.

A 150K resistor between the LDR and ground will eliminate most of the positions you probably would use for normal listening.

 

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