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Using 'Lightspeed' passive LDR attenuator w ZKIT1 (Read 6702 times)
Evo
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #15 - 05/17/10 at 23:38:55
 
Hey guys. Forgot to mention that I have recently upgraded my DAC with some mods (USB Monica with 'Mojo', 'Audioslave', 'CCS2' and 'AC/DC' mods), so will wait for the caps to burn in before I try this mod, otherwise any listening impressions aren't likely to be that accurate Grin
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dank
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pair of dual 18
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #16 - 05/18/10 at 01:04:26
 
Matt

If you are going to replace the pot with fixed resistors you really must remove the pot from the circuit.  Whether you physically remove it, or remove it by removing J4, J5, and the two 1k resistors connected to the input tube control grid's is up to you.

I just set mine to "8" on a 0-10 scale and measured the pot.  I got wiper to ground = 34k and wiper to RCA input = 16k (remember I'm using 50k pots, not the 100k you are using).  If we assume yours are double this, it would be close to 68k for R2 and 32k for R1.  I would use 68k and 33k.  I would remove J4, J5, and the two 1k resistors, solder one end of both 68k and 33k resistors in the 1k resistor hole that connects to the tube grid.  So now you have two sets of 2 resistors sticking up vertically out of the pcb.  Connect the two 33k's to the J4/J5 hole that connects to the RCA connectors with 2 short wires, and run another short wire from the both 68k's to ground.

Dan
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Evo
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #17 - 05/18/10 at 02:25:31
 
Dan,

Sounds like a good plan. My concern re: not wanting to remove the pot only related to physically desoldering it due to the way my amp is made. I definitely want to 'remove' the pot though, so your suggestions re: changed circuitry fit perfectly and your suggestions on how to practically arrange the resistors sound good too.

Currently I am playing my amp through some old omnidirectional speakers as I am painting one of my DIY speakers. I notice that the hum isn't evident on my omnidirectional speakers at all at any level, so I might just wait until I get my good speakers back to see if when the hum occurs as I have changed DACs since I first looked at it. I will check the resistances at the desired volume level to see if they match your estimates Dan and then let you know. Thanks for measuring that for me. I appreciate the effort.

Just another quick question. Earlier you said that there was nothing special about 100k and that you use 50K instead as it seems less sensitive to noise at that level. Might I do better to have the R1 + R2=50K? So by your reckoning R1=16K, R2=34K.

Let me know what you think,

Matt
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dank
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #18 - 05/18/10 at 13:18:42
 
Matt

If it were mine, that's what I'd do.   R1=16K, R2=34K.  But I'd round up/down to standard resistor values so I'd use  R1=15K, R2=33K.  Then, if you want to adjust slightly, you can change just one of the resistors - say R1 = 10k to make it slightly louder, R1 = 20k to make it slightly quieter, etc  


Dan
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chris_m
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #19 - 05/20/10 at 01:11:10
 
Matt, you sound well on your way with Matt's advice.  I thought I would chime in with just a couple of comments though.

1) Assuming you did your build with the supplied printed circuit board, using some solder wick will make removing the pot a fairly easy task.  The PCB seems to be of good quality and takes heat well as far as I have seen.

2) I think the main different between a 50K pot vs. a 100K is really the load that is presented to the source.  With a 50K pot the source is loaded more than with a 100K pot.  That said, I would be surprised if most sources even notice the difference between driving a 50K load vs. a 100K in practice.  It could be a bigger deal perhaps if the source were driving through a long interconnect.  But your situation doesn't sound like that.

3) Finally, I don't know how your amp is physically built and what your constraints are.  Mine is still on a piece of wood (though I worry about electrocuting our cats).  If you inserted a switch in the signal path you could swithch between two inputs, one which goes though a pot and one input connected directly.  For the input connected directly I would still connect a resistor from that input to ground on the order of the 34K that Dan recommended so that the input does not float and cause a noice or hum issue.  That may be part of the reason why Dan reports less noise with a 50K pot.

Chris
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Torii MkII, AN DAC, home built MLTL with Jordan JX92S
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