Optical Passive Attenuator

For the three and more legged things
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Ray P
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#1 Optical Passive Attenuator

Post by Ray P »

Just wondering if anyone has tried the Litespeed approach to passive volume control?

There's quite a lot on DIY Audio about building such a device

http://www.diyaudio.com/forums/diyaudio ... uator.html

and I'm thinking it might be an interesting project to build one into a new DAC I'm working on.

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Mike H
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#2

Post by Mike H »

No but only this morning I got a e-mail newsletter from diyaudio (so I must be a member then lol :D ), one item was about all that, the "Serbian poor man's LDR vol control"


Trouble is every time I look at these things it's like "hmmmm...." ~ can't help thinking there must be a better way of doing it

Did have a play some months ago with a simulated device, it's actually quite difficult getting the necessary degree of control. Leaving aside tolerance differences. Anyway the newsletter got me thinking again this morning so I might come up with an idea.


Or not.....


 
 
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Ray P
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#3

Post by Ray P »

Mike, it was the DIY Audio newsletter that prompted my post; I must get up earlier than you!

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Dave the bass
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#4

Post by Dave the bass »

Teenagers get up earlier than Mike.......... :D

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#5

Post by Mike H »

Yep


Speshly if I've been up 'til 2:30 am...


For a week....


 
 
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#6

Post by simon »

I seem to remember Graeme had a thread here about one somewhere? Could be wrong though.
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#7

Post by pre65 »

Mike H wrote:Yep

Speshly if I've been up 'til 2:30 am...

For a week....

2.30 ? See a dentist then. (think about it :wink: )
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Mike H
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#8

Post by Mike H »

Very good Philip :D




line of unrelated context thread being interrupted by on-topic post demarcation
---------------------------------------------------------------------------------------------------------------

Right so, here is my take on the problem, FWIW....

Let's say I had a "cosmic download" :D (divine inspiration)

I had long thought that this needs a servo feedback kind of control system to make it track a potentiometer setting anything like accurately.

Voila....

We have, two LDR type opto couplers, but note U2 is used as a feedback system for a control amplifier. The control amplifier delivers the LED current to both devices. U1 is the variable attenuator part.

Using a 1V DC reference (Vref), a Voltage can be derived by VR1, at point 'A'.

The control amplifier does whatever it has to do with the LED's to make the Voltage at point 'B' equal the Voltage at point 'A'.

This of course means altering the resistance of U2 LDR. Below 0.5V (VR1 position) it is some multiple of R5 (max. 1 megohms). At 0.5 it is 10k; over 0.5 it is some fraction of R5 (min. 2.5k).

U1 LDR must have the same resistance as U2 LDR, within matching tolerances of the two devices U1 & U2.

The output signal level has a linear relationship (well very nearly) with the position of VR1.

Seemples!

For a 1V input the output range is 10mV - 800mV. Or at least the simulation is.


Mind you this is all entirely theoretical. Image
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#9

Post by Mike H »

Quick example plot ~ potentiometer at 50%

Top line(s) is DC control and feedback points 'A' and 'B', although one is obscured by having the other practically right on top of it
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#10

Post by Nick »

Yep, I like that, problem is (sorry) it assumes the two devices work the same, if they did you could just use the two devices from the same voltage and you have a nice balanced system.
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#11

Post by Mike H »

Yes I am aware of its shortcomings :D

I presume you mean two lots for stereo?

Imbalance might be trimmed if VR1 were a dual gang with trimmer presets at top and bottom. :?:

Other possible applications spring to mind, voltage controlled amplifier, compressor....



 
 
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#12

Post by Nick »

Mike H wrote:Yes I am aware of its shortcomings :D

I presume you mean two lots for stereo?

Imbalance might be trimmed if VR1 were a dual gang with trimmer presets at top and bottom. :?:

Other possible applications spring to mind, voltage controlled amplifier, compressor....



I was just not sure what the servo gained you above either just having the pot in series with the LED, or if you wanted, the pot sending a voltage to a unity gain op amp that drives the LDR.

I guess what you are aiming for is reproducing the "law" of the pot. But that would only happen if the two LDR's behaved the same, which is back to the start again.
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#13

Post by Mike H »

Yes, and yes.



And yes.



:D




 
 
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#14

Post by Mike H »

Now here's a radical idea ~


2 x NORPS12 LDR from Rapid, and ~ a bulb!

And how quaintly old-fashioned.


Make a small tube out of rolled up black paper.

Or, probably better, make a small tube out of rolled up white paper, and cover in black insulation tape.

A NORPS12 goes in each end. Like a plug.

Hole in middle of tube, insert 1W wire-ended bulb.

I knew got a NORPS12 floating around somewhere, found it and just did some experiments.

Darkened room, dark resistance, a least 1 Meg (still needs covering with a rag to be sure). Easily goes to 2.5 Meg. Leave it a minute or so, 15.48 Megs....

Light resistance by the light of the little 2.2V 550mW lamp on my soldering gun, 800 Ohms. Distance half an inch.

So I figure my control circuit above will work with that. Hence 1 x bulb & 2 x LDR's in the same tube. Need two lots of everything of course to make a stereo pair of gizmos. Ergo one LDR at one end of the tube is the control feedback element, 'tother end is the variable resistance bit.

I do 'ave 'em sometimes :D



 
 
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#15

Post by Mike H »

Overwhelmed by the stampede of interest, I shall carry on regardless :lol:


Demo sim below to try out the basic principles. I had to modifiy the LDR model to match my experimental findings against the output of the bulb model.
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