Lightspeed Attenuator a new passive preamp

[georgehifi]

Originally posted by jkeny

Or in other words if both series & shunt LDRs were ideal devices how would the table of series & shunt resistances look at various vol settings (0-63dB)?

It depends on the curve of the led controller pot (100kdual pot) itself. In my production Lightspeed Attenuator's case if you use a log all follows close to an audio taper, slow to begin with then increasing faster around the mid position and after. If you use a linear dual pot it starts way too fast for my and others liking.
So in my production Lightspeed Attenuator it is as my circuit I posted up a few times:
"NSL32rs2s all matched, 100ohm resistors on the positive leg fed positive voltage by a 100k dual Logarithmic pot in push/pull arrangement from a 5vdc regulated supply".
If any of the above are changed the ideal audio taper volume curve will be effected.
I tried a single log 100k and the results were not to my liking, as it ended up crackling, I think because the single track was asked to control all 4 led's, where the dual only had to service 2 led's on each track.

Cheers George

[jkeny]

Aha thanks soongsc - should have looked before posting Q! - you mean this: http://www1.silonex.com/audiohm/pdf/levelcontrol.pdf

Edit: Thanks George

[soongsc]

Originally posted by georgehifi


It depends on the curve of the led controller pot (100kdual pot) itself. In my production Lightspeed Attenuator's case if you use a log all follows close to an audio taper, slow to begin with then increasing faster around the mid position and after. If you use a linear dual pot it starts way too fast for my and others liking.
So in my production Lightspeed Attenuator it is as my circuit I posted up a few times:
"NSL32rs2s all matched, 100ohm resistors on the positive leg fed positive voltage by a 100k dual Logarithmic pot in push/pull arrangement from a 5vdc regulated supply".
If any of the above are changed the ideal audio taper volume curve will be effected.
I tried a single log 100k and the results were not to my liking, as it ended up crackling, I think because the single track was asked to control all 4 led's, where the dual only had to service 2 led's on each track.

Cheers George

Taken the fact that log pots have tracking errors, wouldn't you have trouble matching the NSL32rs2s with the pots?
Ooops, looked at your connections. No problem.

[jkeny]

Actually, I was wrong - the control law is here: http://www1.silonex.com/audiohm/constants.html

Control Law

These couplers show a decreasing cell resistance with increased LED current. Figure 4 shows the RON vs. ILED performance for a typical NSL-32SR2 coupler. The transfer characteristic shows three regions at medium currents (3uA<ILED<75uA) the cell resistance is given by:

At low currents, Rb dominates the curve being less steep and becoming asymptotic to ROFF. At high currents saturation of the LED results in the curve becoming less steep tending towards the minimum value of RON achievable. Figure 5 shows the same measurement for the NSL-32SR3. This device has a slightly higher value for Rv and a much higher value of Rb, resulting in no flattening of the transfer characteristic at low currents.

[Tolu]

Originally posted by jkeny
I know this relationship between LEDs & vol control exists:

But is there a formula that defines the ideal relationship/operation? i.e. given the series R the shunt R can be calculated or vice versa?

Or in other words if both series & shunt LDRs were ideal devices how would the table of series & shunt resistances look at various vol settings (0-63dB)?

Would you please take a look here:

Sheet with db - resistor values

[jkeny]

Hi Thomas,
This thread is so long now that gems of info like yours are hard to find - I read forward from your link & saw that you had temp drift issues but you eventually solved them. Also I see that you're using a DAC & remote control. I'll PM you to pick your brains, if that's OK?

Edit: Ah I see that you're not accepting emails - please email me - I'm working on a uC remote controlled multichannel preamp using unmatched LDRs & I wished to confer with you!

[jackinnj]

Maybe this helps:

[jkeny]

Thanks jackkinnj,
Did this come from Silonex web-site?

[jackinnj]

Originally posted by jkeny
Thanks jackkinnj,
Did this come from Silonex web-site?

No, I just measured it myself with a lab current source. It helps if you can maintain a constant temperature.

[troystg]

Originally posted by jackinnj



No, I just measured it myself with a lab current source. It helps if you can maintain a constant temperature.

He's sneaky that way..

"I'm afraid you under estimate my sneakiness..." Mr. Deeds...