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Old 19th April 2008, 04:01 AM   #1261
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Quote:
Originally posted by ssmith
I'm also biting the bullet and ordering 10 LDRs for one pre. Hopefully that should be enough to get 2 pairs decently matched.

Likewise, however, I can't find on the thread a clear explanation on how to match... would be great if someone could post a little diagram!!
Set up a dc power supply where you can accurately select 5 x dc voltages from 1v to 5v to feed the LED section of each one.
Then measure and plot the LDR section resistances of each one on a graph at those 5 x voltages, and match up 4 with the same resistance graph curve

Cheers George
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Old 19th April 2008, 12:31 PM   #1262
wlowes is offline wlowes  Canada
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I took a similar approach but used the pot to vary the voltage in. Basically put the pot in the circuit. Set it to 9am and measure and record them all. Then 12, 3 and max. In fact I cut a corner, and put a bunch in parallel, set to 9am and measured them all after they stabilized for a minute. You can use IC sockets as a jig. There were very clear pairs that emerged. One matched pair on the series and another pair on the shunt and you are in business. Some have critsized this method, but my lightspeed clone tracks perfectly and sounds great. Out of 10 I got 4 good pairs.
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Old 20th April 2008, 12:52 PM   #1263
ssmith is offline ssmith  France
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thanks -- that's a great help!
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Old 20th April 2008, 11:50 PM   #1264
tubo is offline tubo  Philippines
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Hey guys,

where can i get a variable dc power supply to use for the project?


thanks!
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Old 21st April 2008, 04:04 AM   #1265
mikelm is offline mikelm  England
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Quote:
Originally posted by tubo
where can i get a variable dc power supply to use for the project?thanks!
you do not need a variable power supply for a lightspeed.

Just a fixed supply & a potentiometer

hope that helps
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Old 21st April 2008, 04:16 AM   #1266
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Default powersupply

Hi Tubo.

You just need a fixed 5Volt DC-supply. The best way to do it is using an ordinary 12Volt DC wallwart. Than brin this voltage down in one or preferably two steps to 5VoltDC.
For my lightspeedclone this simple regulator, which is used by Doede Douma (www.dddac.de) is his 1543II Dac project, does his job really well.
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Old 21st April 2008, 03:58 PM   #1267
tubo is offline tubo  Philippines
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Thanks fellas!
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Old 22nd April 2008, 04:30 AM   #1268
agent.5 is offline agent.5  United States
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George,

I read that lightspeed want to feed amps with high impedance. Is high impedance required in the AC realm or DC realm, or both?

The reason that I am asking is that I am designing a tube amp and is interested in using grid choke instead of grid resistor for the first stage.

"
A grid choke in the dc realm looks like a short in theory. But in the case of the BCP16NAGH it is more like about 5000 ohms. In the AC realm because of the inductance it looks an infinite resistence in theory or reality a Z of about 200K.

This is exactly what a tube wants to see. In the dc realm a very low impedance. In the AC realm you want the driving signal to see an infinate impedance and this is why a GC works so well. "


http://db.audioasylum.com/cgi/m.mpl?...nput+impedance
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Old 22nd April 2008, 06:20 AM   #1269
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Quote:
Originally posted by agent.5
George,

I read that lightspeed want to feed amps with high impedance. Is high impedance required in the AC realm or DC realm, or both?

The reason that I am asking is that I am designing a tube amp and is interested in using grid choke instead of grid resistor for the first stage.


http://db.audioasylum.com/cgi/m.mpl?...nput+impedance
It likes to see high AC input impedance (>60k), low DC impedance won't worry the Lightspeed at all, it could be the perfect load for it.
But it may worry the source (CD Player) depending what output stage is used (eg. dc coupled), so tread carefully there, then again you'll have the series resistance of the Lightspeed itself unloading it again depending on where the volume is at.

Cheers George
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Old 22nd April 2008, 07:26 PM   #1270
agent.5 is offline agent.5  United States
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Thank you George
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