Lightspeed Attenuator a new passive preamp

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#2,542
put 5V through a large resistor, maybe 50k for example, to the positive lead of one LDR then GND to the other. Measure the resistance of that LDR. Lets see what you get. Should be some decent resistance. If you are in the 100ohm range something is wrong..... OOOOhh. You are using the SR3. Most of us have no experience with these and I dont know what is the proper resistance. Maybe 100k pot works fine, I dont know but they have a huge range compared to SR2 so maybe this has something to do. Not sure.
But try that test. Also make sure GND is actually GND and goes only to the dots like George said. Then check that your signal wiring is correct. I wonder if for some reason your Shunt LDRs are dark and your Series are bright.
Uriah
 
#2,544
100R you need one in front of each LDR positive leg. Just as always. The series one should be between 10 and 30 and you could even go lower. It only adjusts overall impedance of the circuit. There is no protection in the series resistor. You can leave it out. If you will try it then use a trimmer and then replace with the value you settle on. I suggest build it the way George has shown us on page 4 and then tweak with a small value series resistor if you find that to your liking. If its perfect for your system without the series resistor then leave it out.
Uriah
 
#2,547
udailey said:
put 5V through a large resistor, maybe 50k for example, to the positive lead of one LDR then GND to the other. Measure the resistance of that LDR. Lets see what you get. Should be some decent resistance. If you are in the 100ohm range something is wrong..... OOOOhh. You are using the SR3. Most of us have no experience with these and I dont know what is the proper resistance. Maybe 100k pot works fine, I dont know but they have a huge range compared to SR2 so maybe this has something to do. Not sure.
But try that test. Also make sure GND is actually GND and goes only to the dots like George said. Then check that your signal wiring is correct. I wonder if for some reason your Shunt LDRs are dark and your Series are bright.
Uriah
Click to expand...

OK, I soldered a spare LDR to a 47k resistor wired in series with the 5v supply and I am getting a reading of 1K5

LDRTest.jpg
 
#2,549
udailey said:
The test needs to be on the ones in your circuit. One of the 4 looks backward by the way.
Did you test to match these guys before putting them in the circuit so you could know what pot to use? You might possibly have a 4k pot here which might not be best for your system.

Uriah
Click to expand...

I bet you chuckled at that! 🙂

I wanted to check a random LDR out of the circuit to see what value it was.

The LDR isn't backward, this one has SR3 printed on both sides where as the others have SR3 on one side and 309 on the other. The dot is on the right side.

I have just checked the first LDR and its 1.567K, i'll now check the other three
 
#2,550
When you are done with that then turn your Lightspeed off. DMM goes to first gang or second gang of your pot. Turn it to 47k. Now turn on LSA and measure associated LDRs. You wont probably get anywhere near the same reading because when you turn on the LSA the voltage drop across the LDRs will send their resistance higher, but anyway at least make sure their resistance is higher than what you measured them at.
Uriah
 
#2,551
udailey said:
when you are done with that then turn your lightspeed off. Dmm goes to first gang or second gang of your pot. Turn it to 47k. Now turn on lsa and measure associated ldrs. You wont probably get anywhere near the same reading because when you turn on the lsa the voltage drop across the ldrs will send their resistance higher, but anyway at least make sure their resistance is higher than what you measured them at.
Uriah
Click to expand...

First Test

1=1.567K

2=0

3=1.390K

4=0


Second Test (Pot set to 47K)

1=3.69K

2=0

3=3.5K

4=0
 
#2,552
I am going to assume that 2 and 4 are your shunt resistors and that 0 means no reading. See the shunt go to ground and when full signal passes ground some of the signal is tempted to go to ground rather than to your amp. If the resistor is off then there is nothing enticing about going to ground so full signal continues on to your amp.
When series is high and shunt is low then volume is low and the reverse is true.
I dont know why your shunt are either dead or not lighting up.
Uriah
 
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