Lightspeed Attenuator a new passive preamp

[georgehifi]

Originally Posted by Earfanatic

Paralleling LDR-s could average its characteristics... in this case we would get an LDR (array) that has lower impedance and characteristic that reflects more the average of a lot. But will paralleling resistors mean more distortion? They could use 12 LDRs paralleled and have 4 arrays ar for a SE setup is needed. If it is Balanced then 6 of them / array.

This could be the way, but if one thing the Lightspeed has taught me, it's the more is less principal. To have that many ldr's in the singnal path I don't know, something ring alarm bells to me.
Where you gain in lower labour cost (no matching) you may loose out on extra colourations of too many in the signal path.



Cheers George

[udailey]

The only downside to me is noise. Well, cost, board space, supporting circuitry, MTBF.... guess there could be a lot of downsides. Back to Johnson Noise. A resistor make its own noise the higher its resistance. I dont know if we parallel a bunch of these together and they start acting like one resistor with lower Johnson Noise or a bunch of resistors with a very high cumulative Johnson Noise.

[udailey]

Okay
Got a new batch of 1000 LDRs. The body style is different again. Almost everytime I order there is some sort of chang. This time I am enthused by what I see and feel. I am sure they sound great to of course. The thing I am most hoping for is better matching. The last batch was the second worst matching I ever got. They would match near perfect in the last three measurements but all over the place in the first measurement which means many never made it into customers hands. Lots of lost product there.
So whats different? They still look the same as far as size and being a small black tube with wires coming out of it. This time though the tube seems to be perfectly filled with black epoxy to the point that it seems that it is not a tube but a stick of black plastic. One piece is what it seems like. Of course its still filled with epoxy, but they just did a better manufacture job. The wires are not as thick as they have been but not as thin as last time. A decent thickness and this is encouraging. They are not frail. Writing on the sides is green. Never has been green before and the dot is white while in the past it has often been silver. SO, anyway I am filling the test jig. Will be a week or two before I have results as I dont like to speed through any of it. Would rather spend my time with my young uns!
Hope you guys all had a great summer and are heading into a perfect fall. Its been hot as you know what here. Well over 50 days of 100+ weather. I stopped counting.
Uriah

[portreathbeach]

I am keen to make one of these attenuators, and have already made an Eagle PCB ready for when I order my parts. I have a couple of questions ... (These question may have already been answered already in this thread, but I have only read the first 50 or so pages, there's only so much you can read in an evening.)

1. Why are the resistors used 100 ohm? Surely when the pot is fully in one direction you have 5v/100 = 50mA, the Silonex devise LEDs are only rated at 25mA?

2. Do the devices have a logarithmic or linear response? Anyone got a sample plot of resistance against mA draw I can look at? (Basically I want to know whether I will need a log or a lin pot)

3. I noticed in a few posts, that some people said that their attenuation never reached quiet and was nearly full at about 1/4 turn. Surely, when the pot is turned right down, you would have voltage divider of 40ohms / 1M ohm, this would be silent, wouldn't it? As for max volume from 1/4 turn and up, could this be the 100ohm resistor allowing too much current to the LED?

4. Why are there only 2 trimmers used on 2 of the devices? Why are they not used on the other 2?

Again, sorry if these questions have been answered already in the previous 4233 posts

[portreathbeach]

Another question if someone would be so kind to answer it....

If a 100K linear pot was used and it was set to halfway, that would be 50Kof resistance in series with the top 2 devices and 50K to the bottom 2 devices. 5v / 50K = 0.1mA ! Is 0.1mA enough to light the LED inside?

Is there a point at which the LED doesn't light, and if so, wouldn't this give a band on the pot where the volume doesn't appear to change?

[soongsc]

Most information is in the first quarter of the thread.

[portreathbeach]

Thanks. I'll keep reading on. I'm at post #630.

So far, it looks like a 100K dual log pot with 47uF caps across LEDs seems a good place to start.

[georgehifi]

Originally Posted by portreathbeach

Thanks. I'll keep reading on. I'm at post #630.

So far, it looks like a 100K dual log pot with 47uF caps across LEDs seems a good place to start.

There will be no need for the 47uf across each led, as if you look at the power supply for the production Lightspeed Attenuator, it already has a 470uf cap across the leds.
As far as using the 100k log pot goes with quad matched NSL32SR2S this is the best progressive control curve from min volume to 2 o'clock then it ramps up faster, which is the ideal curve to have.

Cheers George

[portreathbeach]

Hi GeorgeHiFi. Thanks for your reply.

I've only just got to post #757, where I notice you have used only 1 trimmer and single gang pot. I suppose that's why you need quad matched NSL32SR2S now.

I should have read more posts before I spent 3 hours designing a board on Eagle which uses the old design with the 2 trimmers.

Do both these designs give the same quality, as the newer design has less components, but obviously needs quad matching. Also, you mention a 470uF cap across the LEDs....Is there a post with the schematic of this?

Also, where can I buy quad matched devices from?

Thanks again, and great work with this project

[georgehifi]

Originally Posted by portreathbeach

Hi GeorgeHiFi. Thanks for your reply.

I've only just got to post #757, where I notice you have used only 1 trimmer and single gang pot. I suppose that's why you need quad matched NSL32SR2S now.

I should have read more posts before I spent 3 hours designing a board on Eagle which uses the old design with the 2 trimmers.

Do both these designs give the same quality, as the newer design has less components, but obviously needs quad matching. Also, you mention a 470uF cap across the LEDs....Is there a post with the schematic of this?

Also, where can I buy quad matched devices from?

Thanks again, and great work with this project

Yes the trimmer only has to go on the louder of the to channels for final calibration. Quad matching gives you a better log feel to the volume control and more stable i/o impedances. Attached is the schematic it says 100uf but I put 470uf in, what's a few uf's between friends.
Cheers George