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Old 1st February 2011, 04:03 PM   #3951
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Has anybody tried paralleling LDRs in an attenuator? I am constructing a 100K shunt volume control for a tube preamp. The volume control will have a fixed series resistor with an LDR shunt. I'm a little concerned that an LDR failure will cause a bump in volume to zero attenuation that could damage a speaker. Two LDRs in parallel would moderate the bump and might even sound good, as has been sometimes observed when paralleling conventional resistors.
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Old 1st February 2011, 04:27 PM   #3952
udailey is offline udailey  United States
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David
It will also give you lower min volume as min NSL32SR2 resistance should be set at 40R.
To address your concern over LDR failure you would also wire their power parallel, not series.
Any particular reason you like using such high resistance pots to control tubes? I know a lot of people do it but that doesnt make it right I have used much lower values on tube pre's and if you are designing your own you might consider as low as possible which will allow for lower noise and less filtering of the high frequencies.
On your pre you may want to put a fixed resistor from signal to ground, which would set the input impedance and also perform part of the function you were concerned about losing if an LDR went south.
You wont get very reliable operation from ~22k-100k with the LDRs either. Best to design for max resistance on the LDR of 22k. Past 22k and on average the matched LDRs will begin to take their own path which will not match the other. This is not a hard/fast rule but I see it to be true on more LDRs than not.
Build your circuit so that the LDRs turn off AFTER the PreAmp or you will have dramatic increase in volume very very quickly. Perhaps putting a bleeder resistor on the PreAmps power supply caps and extra capacitance with no bleeder on the LDRs power supply will help you.
Uriah
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Old 1st February 2011, 04:49 PM   #3953
udailey is offline udailey  United States
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Thinking about it some more and if you parallel you will have to take the LDRs up to 200k each to get your 100k and that will be so crazy as far as trying to get anything similar at all in their resistance curves. Seriously, these LDRs that match at 22k might be as crazy as 200k and 500k with same min current through them.
Anyway, paralleling is still great idea for low volume and I have had it working in my house for quite some time. I removed it recently just to use different circuit and have turned up resistance on my series LDR to compensate for the 40R min of shunts. I am probably running 10k-12k as max which gives me respectable low volume. I would prefer 6k but then I am back to paralleling and it looks like I would rather peruse DIYaudio than parallel another LDR
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Old 1st February 2011, 05:12 PM   #3954
joemana is offline joemana  United States
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Try keeping the entire circuit away from transformers as they seem to inject a bit of hummmm.
Uriah[/QUOTE]

I'm planning to mount in between the two output transformer of the DAC at an inch away on both sides, is the output (not supply) transformer will going to cause a hum?

Joe
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Old 1st February 2011, 05:23 PM   #3955
udailey is offline udailey  United States
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Joe
That should not cause any problem. I was only talking about the 50/60Hz humm we get from supply transformers.
Uriah
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Last edited by udailey; 1st February 2011 at 05:46 PM.
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Old 1st February 2011, 06:16 PM   #3956
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Uriah, This preamp has an integral phono section with an output impedance of around 2K. Thus I probably need at least a 50K volume control. I'll use TX2575 for fixed series resistors, and control the shunt LDR for each channel with a separate pot-- or possibly one dual pot for master volume, with separate low-value trim pots on the front panel to trim any imbalances. With this approach tracking should not be a problem.

Also, this is a balanced circuit, so for each channel there will be a TX2575 series resistor on each signal phase(each with R= 1/2 the nominal value of the attenuator). The variable LDR shunt will connect between the +/- signal phases.

I will leave the LDRs constantly powered up to minimize temperature drift.

Perhaps when left powered up constantly at low resistance as needed for high attenuation , two LDRs in parallel in the shunt position might last longer than one.

Dave

Last edited by David Garretson; 1st February 2011 at 06:19 PM.
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Old 1st February 2011, 06:36 PM   #3957
udailey is offline udailey  United States
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David
Different than I had imagined. However I believe you will have issues with matching LDRs at the resistances you are talking about. Nice series resistors!
Can you put the LDRs on the input of the preamp? I have not acquainted myself with phono designs at all so I am unsure what you would have available to drive the LDRs on that side.
Uriah
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Old 1st February 2011, 06:46 PM   #3958
tinitus is offline tinitus  Europe
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Lightspeed combined with Pass B1 buffer

I wonder if there could be dc or other kinds of issues if using Lightspeed powerswitch as a mute function

apart from being a mute function, it would solve at least two other problems,
having LDR's on for too long set on low SPL, where its known to put more wear on the LDR's
and the mute function switch is not part of the signal chain

any issues doing so ?
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Old 1st February 2011, 06:59 PM   #3959
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Uriah, I am currently running LDRs 35R-150K as variable load resistors at phono input. It requires a 1meg pot on 5Vdc PS to obtain this wide a range-- with a 25K pot in series for accurate adjustments in the 35R-1K range. That application doesn't require close L/R tracking, as an RCA or XLR patch cable between the input jack to an ohmmeter can be employed to set resistance.

A 50K volume control with a fixed series resistor and a variable shunt resistor that attenuates -10db to -63db, requires LDR shunt resistance up to around 25K. This is inside the range of reasonably close tracking for matched LDRs.

Last edited by David Garretson; 1st February 2011 at 07:02 PM.
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Old 1st February 2011, 07:15 PM   #3960
udailey is offline udailey  United States
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Tinitus
Problem is only that the shunt LDRs will gain resistance very quickly and volume will go loud BEFORE mute. So give extra capacitance on Shunts power after the 100k pot but problem might be that 100k pot might burn if caps draw a large amount of power quickly so I would choose carefully the size of the caps for shunt LDRs. It will only allow a very small amount of time but the series LDRs will be hitting megohms before shunt gets close to catching up if you have a bit of capacitance feeding shunts. I dont know what value is best or what value would burn the pot, but its something to consider.


David
If you need only 25k then this is perfect and good application for LDRs. If paralleling two LDRs to get 25k then you will obviously have 50k on each and like I said will have matching problems. However, since resistance to ground will be averaged out I think this has a real good chance of working and I wonder if several in parallel would ever give better matching than just one since their averages would begin to produce a resistance that is closer to a 'norm' than individual curves. I dont believe this would work well with values up over 100k since they are so odd at those values that the lowest LDR would basically control the resistance but at values like 50k I think it has merit.
Uriah
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