A precision LED/LDR-based Attenuator

[Wahlis00]

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?

[AndrewT]

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.

[wapo54001]

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.

[cab]

bravo!

[wapo54001]

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.

[gapmedia]

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.

[wapo54001]

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!