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Lightspeed Attenuator a new passive preamp
Lightspeed Attenuator a new passive preamp
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Old 25th November 2011, 10:14 PM   #4331
georgehifi is offline georgehifi  Australia
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Quote:
Originally Posted by Tolu View Post
Hallo Lightspeed fans,

I have a problem with my Lightspeed. One Optocoupler is damaged. The one of the right channel gives a variety of 30 to 19000 Ohm and the damaged one gives with the same voltage a range from 62 to 159 Ohm.

Does anyone of you have one 32SR2S in C or D grade left over? Please send me an email!

Thanks a lot!

Thomas
This is typical of damage from shorted output with higher than normal signal input, esspecially from audio sig gen's.

Cheers George
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Old 8th December 2011, 02:21 PM   #4332
dpac996 is offline dpac996  United States
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Hello all. I have been following this thread / treatise for some time now. Many thanks to George Stantscheff for posting the design. I plan on building one and have purchased 4 matched ldrs as advertised from build an amp website. I was wondering what is typical for turn around/ shipping?

I plan to use the ldrs as the volume controlled followed by a modified buffer based on buf03 (mil spec version). My amplifier presents with a nominal 22k ohm Zin. The buffer currently uses a high quality 25k Noble pot and sounds quite excellent as is but im curious about replacing the noble with the ldr. What is the verdict on placing a bypass cap around the led? One last question: worst case wiper current (trimpot set to minimum resistance, led s fully on) is (5V-0.7V)/ (100//100) = 83mA, for a 100k ohm (control pot).
Im not sure of wiper resistance in conventional carbon pots, but 83 ma seems very high. What are folks using for 100k ohm pots: 2watt? I see quite a few photos of standard quarter watt jobs ...

Thanks
Dave
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Old 8th December 2011, 03:00 PM   #4333
AndrewT is offline AndrewT  Scotland
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Trim pots are rated in W or mW.
But to use the information contained in that tiny bit of data one must convert the power to an equivalent full track current.

eg.
a 400mW 2k pot/trimmer can pass (I=sqrt[P/R]) ~14mA. No part of the track nor wiper can pass more than 14mA if one wants to stay within the manufacturers power rating.

Doing the same for 100k 2W trim pot, Imax = ~4.4mA

If you want to get good reliability from your trim pot then de-rate to either half power (divide Imax by sqrt[2]) or de-rate to half Imax (25% of max power).
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Old 8th December 2011, 03:14 PM   #4334
dpac996 is offline dpac996  United States
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Andrew: you understand then that the circuit as posted will fry conventional 100k ohm carbon pots if one cranks it to full volume or to full closed. I think an option to better handle the led current (just instead of a wiper) would be to use an adjustable voltage regulator; one could easily whip up an adjust circuit with trim for channel balance.
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Old 9th December 2011, 04:53 PM   #4335
dpac996 is offline dpac996  United States
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Uriah: can you please acknowledge the $45 I paypal'd you nov 30?
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Old 9th December 2011, 07:56 PM   #4336
AndrewT is offline AndrewT  Scotland
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Dpac,
yes, that's one of the reasons I suggested a switched control for the LED currents.
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Old 9th December 2011, 10:06 PM   #4337
dpac996 is offline dpac996  United States
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Hi Andrew,
I must have missed your comments its challenging to sift through 10
00's of posts on my DROIDX.
The EE in me cringes when I see a regular old pot used to control voltage ( negates a precision regulator) to any load. There are potentially tens of mA passing through the wipers at various pot positions. I understand the kiss principle is the modus operandi of the lightspeed but sometimes too simple is not the right solution. Its kind of odd that most pot manufacturers don't list the max wiper current.
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Old 10th December 2011, 02:29 AM   #4338
georgehifi is offline georgehifi  Australia
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The ones I use are 24mm pots, are dual, each segment is rated at .5w (500mW). Never had a problem in 6 years.
Except for one time when I tried the smaller di-cast green/blue with the plastic resistive track, it did give out after 6mts of 24/7 use.

24mm http://www.jaycar.com.au/productView...r&form=KEYWORD
Cheers George
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Old 10th December 2011, 05:57 PM   #4339
dpac996 is offline dpac996  United States
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Thanks George. Ill try that
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Old 11th December 2011, 08:28 PM   #4340
udailey is offline udailey  United States
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Lightspeed Attenuator a new passive preamp
dpac I shipped to you a while ago. I dont check this forum every day and its not where I conduct business.
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