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Old 28th May 2011, 05:48 PM   #121
gootee is offline gootee  United States
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Sorry. I hope I didn't open too many new cans of worms, for you. Just trying to help. As with many things, sometimes they're significant-enough to worry about and sometimes they're not. If I'm doing a new design and the effort or expense would be about the same either way, I try to preclude those types of potential problems as a matter of habit ("best practices", and all that). But now that it would involve making changes, it's a matter of your own tests and measurements, and judgement, or maybe whether or not problems (or even anything noticeable at all) occur in the actual hardware.
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Old 29th May 2011, 12:08 AM   #122
ua100k is offline ua100k  United States
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Gootee,

I did not know all of the items you mentioned. Thanks for sharing. I will use this in my future designs.
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Old 13th June 2011, 03:08 AM   #123
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An improved board, still 2.5" x 3.8":

Separate regulators completely separate the analog (LDR) power supply from the digital power supply. Separate analog and digital grounds meet only at the power input electrolytic capacitor C1, and the ground plane also is connected to power ground only at C1. Signal ground is completely isolated from power ground, and is a simple pass-through of source ground to output ground.

This circuit employs no digital signals outside the control chip -- no PWM and no rapidly switching levels whatsoever, all control is accomplished by analog voltage level. Control voltages to the mosfets will go high or low to reach proper level to drive the LDR accurately, but then change minimally, with only an occasional instantaneous 'bump' of the control voltage to the mosfets to keep voltage to the LDRs stable and accurate.

C16 is a .1uF chip ceramic connected directly across the power pins of IC3,and nests within the IC socket.
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Old 14th June 2011, 04:25 AM   #124
gootee is offline gootee  United States
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Wow. Much better!!
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Old 23rd June 2011, 04:20 AM   #125
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hmmm a modern TB4 attenuator,

how do you implement this? how do you stabilize input impedance in what ever you hook this thing up to?
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Old 3rd July 2011, 04:32 PM   #126
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Finally, the chip I've been waiting for -- it should be perfect for this application.

1. Small (14-pin), cheap and quite fast (32MHz).
2. Performs virtual multi-tasking in software.
3. Externally adjustable ADC Vref+.

This means that I can use a separate chip for each audio channel, and each chip controls only two LDR devices with each device's control occurring simultaneously (multi-tasked) in separate virtual tasks.

An externally settable Vref for the ADC pins will allow the chip's ADC steps to be compressed into just the voltage range required for the application, and there will be many fewer "wasted" steps outside of the required control range (200~250 more control steps into the useful range). This will permit finer control of current through the LEDs. The addition of external Vref low would have been nice, but not as important.

New approach is to design per-channel boards with a separate power supply providing power to multiple boards. First boards are 1" x 3.5"
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Old 3rd July 2011, 08:11 PM   #127
gootee is offline gootee  United States
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I'm too lazy to re-read the whole thread.

But what about the input and output impedances? Have you plotted the input and output impedances versus the volume setting?

What make and model of LDR device are you going to use? If I had their R vs I response curve and the current range you plan to use (or could use), I could simulate them and plot the impedances.

There is a way to use a couple more resistors to make the impedances stay a lot more constant as the volume changes. But I'm wondering if you'd want to consider having optional input and output buffers, possibly on a different board.

I was also thinking that you might want to leave some space and possibly have some pads in place in case you find out you want to have some additional low-pass RC filtering for the LDR diode feeders. But that can wait until you see if it's needed, I guess.
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Old 3rd July 2011, 09:58 PM   #128
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Quote:
Originally Posted by gootee View Post
I'm too lazy to re-read the whole thread.

But what about the input and output impedances? Have you plotted the input and output impedances versus the volume setting?

What make and model of LDR device are you going to use? If I had their R vs I response curve and the current range you plan to use (or could use), I could simulate them and plot the impedances.

There is a way to use a couple more resistors to make the impedances stay a lot more constant as the volume changes. But I'm wondering if you'd want to consider having optional input and output buffers, possibly on a different board.

I was also thinking that you might want to leave some space and possibly have some pads in place in case you find out you want to have some additional low-pass RC filtering for the LDR diode feeders. But that can wait until you see if it's needed, I guess.
Input and output impendance:

I have calculated the series and shunt resistances at 1/2db intervals so that the overall system presents a constant 5K system Z from about .5db minimum attenuation to a maximum of 42db. At .5db attenuation it's 280 ohms series and 84K ohms shunt; at 42db it's 4960 ohms series and 40 ohms shunt. I could bring the range to 48db by paralleling two shunt LDR devices for total 20 ohm shunt impedance but that would require matching LDRs at 40 ohms. We could also get 48db of attenuation by allowing the system Z to rise to 10K over the upper one-third of the attenuation range. I can actually control LDR resistances well beyond 84K ohms, but at 150K the control becomes pretty loose due to the extremely small current involved. I plan to experiment a little to see if a resistor in parallel with the LDR to raise the minimum control current through the mosfet won't give the mosfet a better grip on the resistance at that end of the spectrum. I expect control to improve quite a lot with the "recovery" of about 200 steps of ADC using the external Vref+.

Model of LDR device:

All of my experimenting has been with the Silonex device used in the Lightspeed. I've looked at the Fairchild devices, but minimum impedance seems quite a bit higher so will stick with the Silonex for now.

Input/Output buffer:

If someone wants those, it'll be up to them to implement them. I am after an LDR control that uses sophisticated control techniques to yield a simple and totally clean fixed-Z attenuation device.

Low pass LC filtering on LDR diode feeders:

Not sure where you are talking about putting those. The control of the mosfets is effectively DC, with only an occasional tiny 'blip' visible on the most sensitive setting on my HP scope once the voltage on the gate of the mosfets has been servoed to the proper level.

I need to order some of the chips (brand new on the market) to verify, but the features I've been wanting are suddenly all there. This chip will either make the project move faster or, if it doesn't do the job, I'll give up on it.

Last edited by wapo54001; 3rd July 2011 at 10:07 PM.
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Old 4th July 2011, 08:49 AM   #129
AndrewT is online now AndrewT  Scotland
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What is your "system Z"?

The attenuator has an input impedance that is seen by the Source as a Load.
The attenuator has an output impedance that is seen by the Receiver as a source impedance.

At -0.5dB the input impedance is 84k28 (84k + 280r)
and if Rs is 200r then the output impedance is 477r (84k//[280+Rs]).

@-40dB the input impedance is 5k (4k96 + 40r)
and the output impedance is 39r7 (40r//[4k96+Rs]).
Only one of these 4 limiting values gets to 5k. What are you trying to achieve?
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Last edited by AndrewT; 4th July 2011 at 08:52 AM.
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Old 4th July 2011, 10:23 PM   #130
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Andrew, of all the pieces of the puzzle I'm working with, the desired resistance values for the optimal control are the most difficult for me as I'm just getting started in calculating values for audio circuits. I haven't worried about it much because once I gain the kind of control I'm looking for and the practical aspects of controlling multiple LDRs simultaneously are dealt with, then it will be time to design the resistor values and that time hasn't come yet.

If you were king and could have anything you wanted, what would the resistance values of the optimal completely passive (no buffer) control look like given the constraints I list below? What would the input and output impedances look like? For planning the resistor values, I'd like to see resistor values at 1/2 db per step attenuation.

I can drive the LDRs to any value at any slope between values, and I can set the series and shunt LDRs completely independently of each other to any value within the 'doable' control range. The bottom of the range is 40 ohms for each LDR value, and (roughly speaking) anything up to 50K would be great, up to 100K doable, and up to 150K difficult.

Do you have a series of formulas I could plug into a spreadsheet? That would make my life easier, but I still need to find out what input and output impedances would be satisfactory for the greatest number of source/load situations.
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