Lightspeed Attenuator a new passive preamp

[portreathbeach]

Nice little box George. The only reason I spent the time making the test rug is to allow me to test 10 at a time so they will all be subject to the same ambient temp, warm up time etc.

[portreathbeach]

Received the LDRs for Uriah today and started matching them with my rig. The rig holds 10 LDRs at a time and with the jumper on the top of the board, I can select 1K, 2.2K, 10K, 20K, 56K and 82K to be put in series with the LED side of the LDRs.



Here is a graph of the results.



As you can see, there is a big difference between the LDRs. (By the way, F was removed from the graph because it was so different from the rest).

Out of th2 20 LDRs I have 2 good pairs, and maybe a couple other 'not so good' pairs.

Here are the resistance values for my 2 pairs:



They seem pretty close

[AndrewT]

Now compare the ratio of Q/A:C/G
It is the ratio that is important in an attenuator.

[portreathbeach]

Ratio?

I was going to use Q and C for the series and A and G for the shunts. I thought that you only have to a close pair for series and another close pair for the shunts.

[udailey]

Q and C for shunt.
Good job man. Been a long time since someone else matched them.

[portreathbeach]

Q and C for shunt? Is this because A and G are a closer match and that's better for the series resistors?

It was interesting matching the LDRs, don't know how you do hundreds in a day Uriah, would probably drive me crazy.

When people say that they match there LDRs to 1dB, what do they mean, how can I get a dB from 2 resistances? Do I use this equation:

dB = 10 x log10 (resistance1 / resistance2)

or is this equation only for power?

[udailey]

Q and C for shunt because they match best at low resistances and you will be listening, I am assuming, at low to mid volume most of the time where these resistances will be in shunt. Series are the other two because they match best at high resistance where series will be in low and mid volume listening.
Your equation is close but its a log20 equation which I dont recall the rest of. Probably can find it in wikipedia. Either way it doesnt matter because these are your best 2 pair. Who cares if they are 1dB or .1dB, this is what you have.

[portreathbeach]

Thanks Uriah.

I don't necessarily want to work out the dB difference, but I was curious as to how it is worked out. I found a couple of equations for dB power and dB voltage. The dB voltage equation is : dB = 20 x log10 (v1 / v2)

I should hopefully get ordering my parts this week

[udailey]

Yeah, I think thats right.

[udailey]

Mpp
Considering different power supply options? Read this.