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Old 18th January 2013, 04:49 AM   #4921
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Both: first i suspected channel imbalance by ear, later this was confirmed on a scope.
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Old 18th January 2013, 09:08 AM   #4922
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I have purchased, but yet to build, Uriah's Lighter Note kit. I plan to use it with a DCB1 buffer in hot rod mode (which generates a lot of heat) in the same case. I never gave the heat issue much thought until now, but I see that I need to.

The LDR board is separate from the power supply and small. Would it be good enough to place the LDR board in a small box to keep temps consistent? If so, should it be plastic, metal, or wood? Do I need to fill it with wax, silicone, or epoxy?

If I understand correctly, it is not the actual temperature that is the issue, it is keeping the temperature consistent between the LDRs. A slow gradual rise in case temperature is OK as long as both LDR rise at the same rate.
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Old 18th January 2013, 09:23 AM   #4923
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@mike_mcf
That is what I am using (not the complete lighter note kit, just the lightspeed attenuator). No problem for me.. But I havent "hot rodded" the dcb1 yet. Using 10r resistors.
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Old 18th January 2013, 11:12 AM   #4924
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OK, that's a level 1 hot rod (200ma or less). Not too hot. I am looking to build a level 2 (600 ma minimum). How do like the LDR+DCB1 combo? Does the Lighter Note drive the DCB1 well?
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Old 18th January 2013, 04:02 PM   #4925
pwgtang is offline pwgtang  Armenia
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Quote:
Originally Posted by gootee View Post
You have touched-on some implementation details that are very important. (And it sounds like you did an excellent job implementing yours.) [The following is just general informaion and is not necessarily directed at only you.]

A short signal path IS important. It helps in not making an antenna. And it helps in minimizing the conductor's self-inductance. It's even more important to be absolutely sure that NO GAP is left, anywhere, between each signal conductor and its corresponding ground, on their way to and from the attenuator. Otherwise, you've made what's called "enclosed loop area". Another name for that is "antenna".

Any time-varying magnetic or electromagnetic field will induce a corresponding time-varying current in the loop (See Faraday's law.), in proprtion to the geometric area enclosed by the loop, and the strength of the field.

So twist the signal and ground wires tightly together, with at least four turns per inch, or, better yet, use shielded twisted pair, with the shield connected on one end only, to chassis ground (NOT to signal ground!). Hopefully, the AC Mains pair will be tightly twisted, as well as the secondary pairs, and the pairs connected to the inputs and outputs of rectifiers, and the output pairs, and the heater pair if it's a tube amp, and even the DC power and ground pairs, because all of those can and will act as transmitting (and receiving) antennas! (With PCBs, either use a ground plane everywhere possible, or put the pair's traces exactly overlapping on opposite sides of the PCB, or, if single-sided, keep them very close together, everywhere. RF (Radio Frequency) signals can also cause insidious effects, which are sometimes very subtle, which can make them very difficult to even identify, much less determine their source or eliminate them.

Your idea to have close thermal bonding of all four LDRs is a very good one. Encasing them then helped greatly, too. They could even be surrounded by a small metal box, perhaps with a resistor to act as a heater, so that the little oven's internal temperature would swamp-out any changes in the temperature outside the box. But you correctly understood that the main important factor is to not have changing differences between the temperatures of the LDRs, i.e. relative to each others' temps. Combining them all into one physical mass, with large contact areas between them, gets you most of the way there. Also then wrapping or potting or boxing them all inside of a single enclosure gets you almost all of the rest of the way there (and is probably more than good-enough). Doing that well-enough would enable you to place them almost anywhere, and there would be definite advantages to having them located as close as possible to the original signal path.

I wonder if anyone has ever played around with using "trimmer" LDRs, putting one in series with each of the main LDRs. Of course, you'd have to run them wide open (max current, to give lowest resistance) and then adjust a multi-turn trimpot by some hopefully-small amount, to reduce their LED current slightly to increase their resistance "offset" slightly. So the adjustment could only be upward in resistance. Assuming the first trimpot was fed from a separate fixed DC feed, they could optionally also receive some fraction of the main LDR's control current, which might enable better matching overall. But using them at all might also be a way to be able to use somewhat-unmatched main LDRs while still ending up with well-matched responses. I guess maybe it would be too complicated or too expensive.

Cheers,

Tom

I had built one used four trimmer for each LDR's. each LDR can independent
adjusted the resistance at four position of the pot. 7, 9, 12 and 5 O'clock.
For series LDR I adjusted at 7.5 O'clock= 20kr, 9=19kr, 12=15kr, 5= 50r, at
7 O'clock position the series LDR will open the resistor is greater than 20mr
the output is dead silent.
For shunt LDR at 7 O'clock=40r,9=1kr,12=5kr,5=20kr.
This attenuator had built in the channel balance pot and high/low impedance select switch also. The one I used now I adjusted high to 20kr and low for 10kr.
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Old 18th January 2013, 04:39 PM   #4926
udailey is offline udailey  United States
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Yes they are just that sensitive, especially when they are at higher resistances. At 250R or so you cant budge them but at 10k a 1 degree difference can mean hundreds of ohms. The motion of air from a door opening in another room can make a difference.
I have tested with two LDRs being monitored by one DMM each and have found that some LDRs will move more than others even though they get the same power and are initially set to the same resistance. So this is why I wait about an hour before I start testing and every time I change to a higher resistance I wait about 15 minutes before beginning the next test. I have a LDR attenuator running now that has been running for 2 years with the top off. My channel balance is pretty nice but its in a huge room and there are no hotspots around it. I have never built a LDR attenuator INTO an amp chassis. Not saying I wouldnt but I havent so I cant speak from experience. I can only say that table top testing of individual LDRs makes me think that, yes, temp changes will get them moving and the fact that they might move at different rates could show as a channel imbalance.

I recently was sent a DIY implementation of a Lightspeed for a look. The gentleman had issues with series LDRs not changing value and the balance being off to boot. Here he had done a point to point on breadboard using largish gauge wires as signal and power wires. The problem he had was that it took so much heat to get the wires to accept solder that the LDRs were damaged and in fact one gang of the control pot was damaged so that it no longer worked, while the other gang was okay. Heat kills.

I build LDR attenuators from time to time and sometimes I will find that the LDRs match according to my datasheets but past the max value of my datasheets they do NOT match. I never assume that they will but we can always hope.
With this build I have been able to solve the problem. Usually its one LDR of the 4 that is misbehaving at higher values. So lets say that at max resistance its sitting at 14k and its mate is around 12k and lets say this permeates a bit lower so that at 2k they are a great match and up to 6k they are a decent match and beyond they seem to be hundreds of ohms away from each other. I can fix this with a high value resistor in parallel with the misbehaving LDR. Oh LORD! You cant be serious! A real resistor in the signal?! Well, yes its a very high value that usually fixes the problem and its in parallel with a relatively low value LDR so the amount of signal passing the resistor is quite small in comparison to the offending LDR and to my ears and my Audio Precision the sound is not compromised subjectively or objectively. I start with a 249k and work my way to two 249k in parallel then try a 100k or a 75k. This is usually all it takes.. one of these options will make the problem disappear.
This brings us to gootee's suggestion about parallel LDRs. I made a batch of 10 of these boards a few years ago. I still have one of them. I goofed them because I designed it using the default pin sizes in my pcb layout software and never expanded them. SO I had to drill out several of them and then never rebuilt them. One reason only. It was just beginning to be to much for the builders. I was considering 8 LDRs where 4 are dynamic and 4 stay the same value during operation for fine tuning balance and for limiting max resistance. It works and its nice.
Back to matching: I match up to an average of 6k for a few reasons. 1: 6k sounds fantastic to me compared to other values so thats a subjective reason. 2: LDRs are relatively stable at 6k. They start to get a little squirrely around 5k but 6k is not to bad. Squirrely meaning affected to a greater degree by heat. So if I match to an average of 6k I can be sure I am selling the customer a reality up to that point. When I say an average of 6k I mean that I choose a voltage and current that results in most of the LDRs on the test boards to be grouping around 6k. Still I will have many up into 12-15k but most at 6k.
In no way should anyone assume that a set of LDRs that are tested to, lets say 8k, should be a great match at 15k. They might be but I dont know if they are.
So some things can be done here. If its a Lightspeed implementation a trimmer of a few hundred K can be placed in rheostat mode across each of the gangs of the 100k control pot. Max resistance of the LDRs can be brought a bit lower. Lower means more stable. So you can now control your Lightspeed's total resistance.
Lost in all these posts is a way to increase your Lightspeeds total resistance. A 100R multiturn trimmer in series with the 5V fed TO the volume pot. Around 10R will make dramatic changes in max resistance and you will need the multiturn capability as its very dramatic changes with just a touch of the dial on the trimmer.
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Old 18th January 2013, 05:08 PM   #4927
udailey is offline udailey  United States
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I should add that in light of my tests and finding LDRs changing at different rates when subjected to exactly the same power, ambient conditions and having been proven to match at certain resistances I would assume that glueing them together will help but will not guarantee symmetry change. I would never discourage glueing and waxing or boxing. I like these approaches. I just have seen proof that this will not be a fix-all type solution.
I have been discouraged by the quality of these LDRs over the years. I truly love the sound there is no doubt. The manufacturer, Silonex, quite obviously changes the manufacturing plant almost yearly. I have noticed the LDR being built in Canada, Mexico, California and China. I buy in bags of 1k so I usually get original quality control stickers on my bags so I can glean some info that way. How can you make a process better when you consistently change the place of manufacture? The wires have gone from nice and thick, malleable wires to so small and fragile as to cause trouble in the matching jig because I was counting on a little spring action in the wire. Right now they wires have gone back to a respectable size. When I break them open I have noticed the internals have changed. There used to be a sort of lens between the resistive and LED side. I have seen a rubbery tube for the light to shine through and the one I just cracked open has what I suspect is hot glue on top of the LED. These are all different ways to diffuse the light. I can make no guesses as to the better way to do this. It does give differing heat absorbing mass to the LDR.
I just hooked up the cracked open LDR, minus LED, to the DMM and got 1700R. Blowing on it made zero difference. Can we assume its the temp of the LED? I know that we are looking at differences of micro-amps making changes to LDR resistance so I think we can assume that stable voltage and stable current are of importance. Differing currents over the LED cause the Vf of the LED to change which bolsters the argument for a solid supply.
Well, anyway.. That all said and still nothing sounds better than an LDR in replace of resistors and pots.
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Old 18th January 2013, 05:54 PM   #4928
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[QUOTE=mike_mcf;3331175]I have purchased, but yet to build, Uriah's Lighter Note kit. I plan to use it with a DCB1 buffer in hot rod mode (which generates a lot of heat) in the same case. I never gave the heat issue much thought until now, but I see that I need to.

The LDR board is separate from the power supply and small. Would it be good enough to place the LDR board in a small box to keep temps consistent? If so, should it be plastic, metal, or wood? Do I need to fill it with wax, silicone, or epoxy?

If I understand correctly, it is not the actual temperature that is the issue, it is keeping the temperature consistent between the LDRs. A slow gradual rise in case temperature is OK as long as both LDR rise at the same rate.[/QUOTE[/U]]

This was Uriah's response to my email:

Hi Michael
A lot of folks have buit it with the DCB1 and even in the same chassis.
I have reason to believe that could cause a problem but they seem to be doing fine with their implementations so I also see evidence that its often OKAY. Sorry to skirt the issue but I dont have a solid answer for you. They are extremely heat sensitive.
Uriah
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Old 18th January 2013, 06:02 PM   #4929
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Mine looks like this post
Salas hotrodded blue DCB1 build

You can "almost" see the actual lightspeed on bottown right. Mounted it above 2 of the transistors.
My dcb1 does not get hot at all.
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Old 18th January 2013, 07:27 PM   #4930
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You can't get much closer to the heat source. Some of the higher current hot rod implementations require huge heat sinks, so I suspect if you started to crank up the current, stability could be a problem in that config.

On the other hand: if you are that close to the mosfets and there are no stability issues, maybe heat isn't an issue. The mosfets likely generate constant and even heat keeping the temps of the LDRs also constant. So maybe in the end, high heat may be OK as long as it is even and constant.
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