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LDR atennuators
LDR atennuators
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Old 22nd October 2019, 04:00 PM   #1
tonitonitoni is offline tonitonitoni  Croatia
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Default LDR atennuators

do ldr operate like some form of amplitude modulation device on small signal levels, since light is wave as seen by qm?

sorry if dumb question, since lounge...thanks
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Old 22nd October 2019, 04:12 PM   #2
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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They were used with neon or incandescent laps to control amplitude of signals, as ALC's in oscillator (like Wien) or noiseless volume controls. I believe they are still being manufactured with leds.
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Old 22nd October 2019, 05:13 PM   #3
DF96 is offline DF96  England
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The light from any source will contain noise: fluctuations in light level. This is not just a quantum matter but a thermal noise matter. It is unlikely that this will have any significant effect on an LDR volume control, provided that the supply for the LED is not itself unusually noisy. Those who claim otherwise need to back up their claims with some combination of careful calculations, measurements and well-designed listening tests; as far as I know this has not yet happened. Hence the main issue with LDR controls is the small amount of signal voltage nonlinearity, which will add a little low order distortion irrespective of the light source purity.
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Old 22nd October 2019, 06:04 PM   #4
Bigun is offline Bigun  Canada
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LDR atennuators
Quote:
Originally Posted by tonitonitoni View Post
do ldr operate like some form of amplitude modulation device on small signal levels, since light is wave as seen by qm?

sorry if dumb question, since lounge...thanks
The wave nature of light itself cannot modulate the resistance of the LDR. first off the frequency is so high that there are no physical processes for it to interact with other than transferring its energy to the electrons in the material. secondly the energy level of a single light wave is too low, it takes trillions of these waves of light to create meaningful change in the resistance of the LDR and these waves are not in phase with each other unless using a coherent laser source
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Old 22nd October 2019, 07:24 PM   #5
tonitonitoni is offline tonitonitoni  Croatia
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so by using mixture of different laser "colors" or similar to control ldr one could not modulate/switch ldr on very high freq as resistance change frequency is lower than light wavelengths?

it would be nice to at least know what is maximum freq or rise time or whatever it is for ldr for audio use.

i realy dont know how ldr works, sorry. judging by your posts probably something that is got to do with heating resistance element.
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Old 22nd October 2019, 08:08 PM   #6
rayma is online now rayma  United States
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They are rather slow. That's good in this application, since it improves their noise immunity
at higher frequencies.
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Old 24th October 2019, 11:44 AM   #7
DF96 is offline DF96  England
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Yes, LDRs are slow to respond to light changes. The actual speed can sometimes be found in the datasheet. As a resistance element they are good across the audio band, apart from a little nonlinearity.

LDRs are not thermal in operation. The L is for light.
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Old 24th October 2019, 05:52 PM   #8
TheGimp is offline TheGimp  United States
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The majority of their distortion is 2nd order from everything I have read and from the limited testing I did.

A potentiometer is more linear and produces no more noise for the same resistance.
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Old 24th October 2019, 06:33 PM   #9
PRR is offline PRR  United States
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LDR distortion is almost entirely 3rd order.

If you work a common LDR at several volts in and low loss the 3rd is very audible.

As general theory suggests, 3rd falls off quickly with level and 0.1V doesn't sound harsh.

LDRs are quite slow but the specifications must be cleverly corrected to your impedance and light level. It is usually ample to know that they won't respond much to light modulation over 200Hz (certainly to to "light frequency"). In manual control, stray 50/60/harmonics in LED power can be annoying (better when we used dim incandescents). In limiters, signal ripple could be audible, but several popular limiters used no sidechain filtering at all, however with selected slow-release LDRs.
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Old 24th October 2019, 08:09 PM   #10
Blues is offline Blues  United States
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For my application I use D-cells and are voltage regulated -simple, steady and quiet. Four optocouplers; plus 3 LEDs, 2 as indicators and 1 to light up a DC voltage meter and that's about 2+ years of use before battery replacement. I'd say an average of 2 hrs/day of use.

Replaced a Musical Fidelity preamp, now bi-amping using an Aleph 3 and Zen v4j on PSB tower speakers, and have not looked back since ..
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