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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

A precision LED/LDR-based Attenuator
A precision LED/LDR-based Attenuator
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Old 20th June 2012, 09:15 AM   #191
Wahlis00 is offline Wahlis00
diyAudio Member
Join Date: Dec 2010
A very fundamental question. I have tried to find information of the proper values of series and shunt resistor for my LDR attenuator.
For high frequency transmission a common subject is to match source and load impedances, for this matter there are exact laws of how to create a minimum loss attenuator. But for audio volume control, this seems to be a subject where it is more of a “black art” of how to find to optimal resistor values. In many stepped attenuator common series resistor values are 10kOhm or 100kOhm, but there must be some optimum based on exact theories?
Basically what I like to do is to adjust the current to the LDR:s so at normal listening volume I will have the best source and load impedance match.

In my system I have:
A DAC with 100 Ohm output impedance
Monoblock with 60 kOhm input impedance

Can someone guide me with finding the optimal minimum loss attenuator?
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Old 20th June 2012, 10:39 AM   #192
AndrewT is offline AndrewT  Scotland
Join Date: Jul 2004
Location: Scottish Borders
one does not "match" impedances with analogue audio frequencies.

"matching" is used for transmission lines and these only become "matched" at VHF, eg. digital (almost squarewaves).
"matching" is also used for maximal power transmission as in RF aerial to RF input of a receiver.

For analogue audio, each Source Receiver combination must be organised such that the passband includes all the audio frequencies you want to hear.
This is usually done by using Source impedance <1k and Receiver impedance >10k

The volume control pot (or your LED/LDR) must be considered a Receiver at the input end and then must be considered a Source at it's output end.
regards Andrew T.
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Old 24th September 2012, 09:41 PM   #193
wapo54001 is offline wapo54001  United States
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Join Date: Nov 2008
A precision LED/LDR-based Attenuator
I've been on a long break from this project -- I needed to get away, and a photography project had been long beckoning . . .

Attached is a view of my final effort with getting the best possible ground plane for this project. There are two main changes -- one is fewer traces interrupting the ground plane (more traces were moved to the top of the board) and second, the edge where the digital ground plane and the analog ground plane abut was adjusted for a slightly better configuration.

In the next few days I intend to get back to programming the PIC.
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File Type: jpg ScreenShot002.jpg (502.7 KB, 560 views)
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Old 24th September 2012, 10:25 PM   #194
cab is offline cab
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Old 10th April 2013, 05:23 AM   #195
wapo54001 is offline wapo54001  United States
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A precision LED/LDR-based Attenuator
My photography project is finally set up and running, so it's time to get back to the LDR project.

When I thought I'd refined the board design to the nth degree, I kept finding little tweaks that seemed to be a bit of an improvement. But I've run out of ideas and the board has gone to manufacturing.

The ground plane has improved some more, additional traces moved from the bottom of the board to the top, see attached image.

The new board should be here Thursday, and digi-key has already delivered the parts for stuffing. Hopefully by the weekend I'll have a board with one LDR path working so I can use that to do some LDR testing. I'll post a photo when I have the board stuffed and ready to test.

Between previous purchases and a recent purchase I think I have fifty + LDRs to test. My test will be fairly straightforward. Can a given unit achieve 50 ohms or less resistance at 11 milliamps of current or less. If yes, I'll test for resistance at .005 milliamps and if the resistance is 20K ohms or more, that will be a shunt LDR. If it passes the 20K test but not the 50 ohm test, it'll be a series LDR. Once I have all of my LDRs tested and sorted, I'll stuff the board with a full complement of six LDRs and start working on expanding the single channel code that's working now into the full four channels needed for a stereo LDR volume control.
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File Type: jpg ScreenShot2004.jpg (666.7 KB, 411 views)
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Old 10th April 2013, 12:56 PM   #196
gapmedia is offline gapmedia  Australia
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Keep up the excellent work. I really like the sound, or no sound of LDR optocouplers from my short experiments. Sadly, I don't have the electrical knowledge to go down your track. It is such a pity that they vary so widely from unit to unit.
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Old 13th April 2013, 02:50 AM   #197
wapo54001 is offline wapo54001  United States
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A precision LED/LDR-based Attenuator
Here is an image of the partially-stuffed board.

One row of four resistors will not be inserted until I've had a chance to operate one channel to determine the optimal value resistor to operate with the capacitor value I chose. Also I'm holding off on stuffing the row of trimmer resistors until I'm comfortable that the value I chose for the pots will cover all eventualities.

I did not enjoy installing those smt ics and capacitors!
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File Type: jpg Partially Stuffed PCB.jpg (451.0 KB, 395 views)
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Old 28th June 2013, 01:03 PM   #198
wapo54001 is offline wapo54001  United States
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Join Date: Nov 2008
A precision LED/LDR-based Attenuator
I have been working on this board off-and-on (mostly off!) for some time. I really wanted to keep it simple, and for some time I tried to use voltage control of the LDRs. That simply has not worked out.

Three days ago, I was ready to throw the whole project into the trash and simply forget about it, but then I decided to go back to an earlier, more complicated design that used current control. I worked with that a bit using software that I had been improving while working with the voltage control board. That helped, but wasn't perfect. I had one last thing to modify and when I did, Bingo! This board now controls four channels of variable current from 12ma down to around 20 micro amps with excellent precision. It was a combination of software tweaking (keep it simple and fast!) and adjusting the integrator RC. The last key step was to abandon tantalum which was leaky and required constant large corrections in favor of a smaller value polyester in the integrator.

It turns out that this setup will reliably control LDR resistance to about 75K ohms. Above that, if you reduce the current beyond a certain point it appears that the LED completely extinguishes and it is necessary to increase the current substantially before it will light back up. But it seems a 50K pot would be entirely feasible.

Through measuring, I find that the board draws a maximum of 24ma not including the LDRs, so with the LDRs drawing a maximum of 12ma per channel, the board and LDRs together will draw a maximum of about 50ma. Accordingly, I've replaced the 317T with a 317L which is guaranteed to deliver 100ma under all conditions and 200ma if the in-out differential is kept below 13 volts. That is plenty of leeway. Most of the board current is consumed by the voltage-to-current conversion chips.

I'm finally in a position to start work on combining the travel of a linear potentiometer with the odd response curve of the LDRs and using the PIC to output the log response of a volume control.
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Old 28th June 2013, 01:08 PM   #199
kuribo is offline kuribo
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Excellent! Really looking forward to more updates!
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Old 28th June 2013, 04:07 PM   #200
canever is offline canever  Italy
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Location: Venice Italy
A precision LED/LDR-based Attenuator A precision LED/LDR-based Attenuator
will you never set up a buying Group for this project?

If so, I am interested.

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