Lightspeed Attenuator a new passive preamp - Page 339 - diyAudio
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Old 12th July 2010, 11:30 AM   #3381
drwkng is offline drwkng  Hong Kong
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I think it is your english that is applied incorrectly.
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i thought it was understood that there is a LED within a LDR. so i was a bit lazy when i used LED instead of LED of the LDR.
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Old 12th July 2010, 11:56 AM   #3382
AndrewT is offline AndrewT  Scotland
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I think I will keep my mouth shut for fear of consequences.
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Old 12th July 2010, 03:01 PM   #3383
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ROFL! This is the sort of thing I normally get into, it's great to see someone else do it for a change!

Gentlemen, I believe the technical term for the package is "optocoupler" within said package exists an LED and an LDR.

I think the statement that confused Andrew -- because it confused me, too -- was that you had a CCS fed by a low-voltage regulated supply. Also, I could not see how a CCS could be used in a situation where the whole point was to make the current changeable by introducing variable resistance in the form of a volume control potentiometer.

Upon reflection, it sounds like the CCS limits maximum current to the 10ma, and then the regulated 5V supply prevents the CCS from working to keep current steady when system resistance is increased via the volume control potentiometer. Have I got it right?

BTW, 10ma looks like about 60 ohms minimum resistance on the Silonex chart, is that what you are getting with your devices?
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Old 12th July 2010, 03:12 PM   #3384
udailey is offline udailey  United States
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Andrew, He is applying 10mA to the LDR, yes but the LDR is general speak here for the Silonex LED/LDR package. So the 10mA is to the LED side of the LDR.
Uriah

Edit: I see that was already explained. Sorry for near double post.
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Old 12th July 2010, 09:35 PM   #3385
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Default Voltages and currents

Folks, look at the attached bit of spreadsheet, it will demonstrate something that may not be well understood.

Make the following assumption: you have two optocouplers (LDRs!! ) that you are using as a pair. Power-wise, they are not well matched -- one consumes the target 20ma at the standard 2.5V, but the other is at the outside of tolerances, and consumes 20ma at only 2.0V.

OK, now look at the consequence of using a low voltage like 5.0V to run this circuit, as compared to a more desirable 15 volts. These two LEDs will produce much different power consequences at different power supply voltages. The point is that at the lower supply voltage, the LED voltage is much more critical than it is at a higher voltage and with a higher value current limiting resistor in the circuit.

It appears to me as though the current mismatch problem would tend to go away if you would start out with a higher voltage supply and use a larger value resistor as a current limit.

The reason for this difference is that as a percentage of the total circuit resistance, the LED is a much higher percentage at the lower voltage with lower value limit resistor than it is in the higher voltage circuit. Thus, any minor change in the voltage drop across the LED has a much greater overall percentage effect on the circuit as a whole at lower voltages.

It seems to me that everyone who uses a Silonex optocoupler would be better off with a higher voltage regulated source and a higher value current limit resistor, since this would reduce the current differences between devices.

Is there a flaw in my math somewhere, or have I got it right?
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Last edited by wapo54001; 12th July 2010 at 09:42 PM.
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Old 12th July 2010, 10:21 PM   #3386
tinitus is offline tinitus  Europe
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Optocoupler is an IC
I have been corrected on that issue before

LDR / light dependent resistor is correct term

Why not just stick to that
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Old 13th July 2010, 01:47 AM   #3387
drwkng is offline drwkng  Hong Kong
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was that you had a CCS fed by a low-voltage regulated supply. Also, I could not see how a CCS could be used in a situation where the whole point was to make the current changeable by introducing variable resistance in the form of a volume control potentiometer.
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i must have a serious problem in communication. however i did say i used the LDR for source switching instead of attenuation.
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Old 13th July 2010, 01:51 AM   #3388
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Quote:
Originally Posted by drwkng View Post
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i must have a serious problem in communication. however i did say i used the LDR for source switching instead of attenuation.
Hot Darn!!! So you did . . .

May I ask for you opinion about the "sound" of LDRs -- how it affected your system when you switched to LDR input switching? Did you hear a difference? If so, what was that difference?

Last edited by wapo54001; 13th July 2010 at 01:57 AM.
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Old 13th July 2010, 02:21 AM   #3389
udailey is offline udailey  United States
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Look, I know I sell them and everything so I am supposedly financially biased but they sound better than frankly any preamp I have heard. I just took an LDR preamp to the Lonestar Audio Fest. I got stuck on a floor of the hotel where the conference was at that nobody visits. We show our stuff out of our hotel rooms. So anyway, I said screw it and put the preamp under my arm and went to other guys rooms. Its friendly that way and no one minds. I stuck it in two systems that day. Both guys bought one. Both guys have been speaker manufacturers since the mid 80's. Both said nearly the same exact phrase " I have been professionally building speakers since "86" and I have never heard them sound so good."
They really are that good. They are only a few bucks each. The circuit is easy even on perfboard. You have to try it. Its no use sitting here analyzing nothing. Well, actually it is. Your idea of analyzing them is intriguing, but in the meantime man build the attenuator.
Uriah
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Old 13th July 2010, 03:21 AM   #3390
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Quote:
Originally Posted by udailey View Post
they sound better than frankly any preamp I have heard.
Uriah
Uriah, everything I've read here suggests they sound terrific, and I accept that as a working hypothesis. However, there are vanishingly few posts (there may be some, but I can't remember any, and I've read the whole thread!) that actually describe the phenomenon. I want someone to describe what they sound like!

Asking for someone to describe their experience is not the same as doubting the phenomenon. I'm just asking for something I can wrap my mind around that isn't just "it's great!!"
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