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Lightspeed Attenuator a new passive preamp
Lightspeed Attenuator a new passive preamp
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Old 10th September 2014, 07:10 AM   #5231
georgehifi is offline georgehifi  Australia
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Quote:
Originally Posted by dtses View Post
cheers!
a quick question about the wall wart, will this fit? I'll install LM317 after it and set the current to 150-200mA and then U7805 as advised.
That's smp (switch mode) rubbish, better off with one of these.



GPU41090500WD00: JAMECO RELIAPRO: Power Supplies & Wall Adapters


DDU120050F0981: JAMECO VALUEPRO: Power Supplies & Wall Adapters


AD-121-ADT: OEM: Power Supplies & Wall Adapters


DDU120100: JAMECO RELIAPRO: Power Supplies & Wall Adapters


Cheers George
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Old 10th September 2014, 07:59 AM   #5232
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thanks!

I have built mine on the previous week and thought it might be handy for all those guys who don't have time to read all the posts, so here are the main points you need to know:

schematics:
http://www.diyaudio.com/forums/analo...ml#post2713064
http://www.diyaudio.com/forums/analo...ml#post2159480
http://www.diyaudio.com/forums/analo...ml#post1773266

connection dots:
http://www.diyaudio.com/forums/analo...ml#post1931676

Wax everything:

http://www.diyaudio.com/forums/analo...ml#post1708899
http://www.diyaudio.com/forums/analo...ml#post1832203

Adjustment with scope:
http://www.diyaudio.com/forums/analo...ml#post1925602

PS comparison:
http://www.diyaudio.com/forums/analo...ml#post2138093

Salas shunt:
http://www.diyaudio.com/forums/analo...ml#post1918667
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Old 17th September 2014, 09:01 AM   #5233
dtses is offline dtses
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hi George,

could you pls clarify what's wrong with my lightspeed?
Everything works fine and attenuates as it should. When I tried to adjust one channel which is slightly louder I put 1k pot (the one on the second channel is shortened) and started the adjustment process with my scope.

There is 2v 1kHz sine wave of the output on tone generator and I got 3.29V amplitude with my scope (for instance). When I'm trying to adjust the pot, nothing happens, though the LDR changes the resistance (checked with DMM). Am I missing something or doing something wrong?

I use the following schematics.
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Last edited by dtses; 17th September 2014 at 09:06 AM.
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Old 17th September 2014, 09:29 AM   #5234
georgehifi is offline georgehifi  Australia
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Hi DTSEs, double check with this circuit, the 1kohm calibration trimpot goes on the louder of the channels only.

Cheers George
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Old 18th September 2014, 07:01 AM   #5235
dtses is offline dtses
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thanks!
the problem was solved by shorting left and right inputs when tone generator was connected. Both channels are now equal.
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Old 28th September 2014, 07:11 PM   #5236
alexkosha is online now alexkosha  United States
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Lightspeed Attenuator a new passive preamp
Assembled my board with 2 matched pairs of Optos and have an issue when one channel is way higher than the other. It starts from 0.8R difference when pot is in Min and 4K when pot is in Max. Max resistanse is about 14K for R and about 11K for L.
It seems that difference is growing exponentially when I turn potentiometer up. I'm able to adjust R out with trimmer only when Inputs are shorted, but still can get equilibrium. Resistance out (when In is shorted) is growing to 3.2K in mid position of potentiometer and then going down till 20R (same as starts).
I do not know how to match L and R now. Any ideas? Please
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File Type: jpg Potentiomentr.jpg (113.0 KB, 109 views)
File Type: jpg All Lightspeed structure.jpg (261.8 KB, 135 views)

Last edited by alexkosha; 28th September 2014 at 07:18 PM.
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Old 28th September 2014, 08:26 PM   #5237
alexkosha is online now alexkosha  United States
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Lightspeed Attenuator a new passive preamp
Oops,
I measured potentiometer differences between L and R independently from circuit, and its behave the same as whole circuit with Optps.
I have 15k differences in max position from L to R. So, I assume this is my problem.
RadioShack potentiometer is not good for that porpoise.
Trimmer can't help since is linear and constant, and potentiometer is log.
Measure my 20K Alps Blue Velvet and it is very tight. 200R delta in Max position for 23K. Going to buy 100K Blue Velvet Alps.
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Old 28th September 2014, 08:46 PM   #5238
georgehifi is offline georgehifi  Australia
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No the differences in the two segments of the stereo pot don't matter.

You need to test all your ldr's again to see if they are closely matched.

A simple cheap way is to get a $5 Kemo M078 led tester, and measure the resistances at different mA from 1mA to 20mA.

PS You only need a trimpot on the louder of the two channels, just so it can be trimmed back. Heaven forbid if you need this many trimpots, I cringe just looking at it.


Cheers George
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Last edited by georgehifi; 28th September 2014 at 08:56 PM.
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Old 28th September 2014, 09:01 PM   #5239
udailey is offline udailey  United States
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If you use a kemo tester with min current of 1mA thats the same as using your 5v supply through a 5k resistor which seems to be giving you a 240ohm reading on your ldr. Hardly close the the resistance you are having problems with. If you are in fact using a switched circuit like shown in your schematics this is a great tool.
Its important to test in an area with perfect temperature. No variation. No wind/breeze. Make sure each ldr is tested the same amount of time and let them run for a good 10-15minutes before you start testing. It can get frustrating. I dont envy you. Once you get reliable matches it will be awesome.
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Old 28th September 2014, 09:06 PM   #5240
alexkosha is online now alexkosha  United States
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Lightspeed Attenuator a new passive preamp
Can you please send me a link where I can purchase Kemo M078.
Thank you.
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