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Lightspeed Attenuator a new passive preamp
Lightspeed Attenuator a new passive preamp
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Old 30th July 2013, 01:45 AM   #5101
georgehifi is offline georgehifi  Australia
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To add to this "powering the led's" mix, try a pure DC source such as a 12vdc Li-Ion (Lithium) rechargeable battery. See if you can reliably pick the difference between it and a 12vdc (linear) wall wart. Nobody I know can.
Even Sam Tellig from Stereophile says with his Lightspeed he has to look to see what's powering it at the time, because he switches between the two a bit and cannot tell the difference between the wall wart or battery power by just listening.

Cheers George
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Old 30th July 2013, 03:18 PM   #5102
wapo54001 is offline wapo54001  United States
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Quote:
Originally Posted by wlowes View Post
wapo54001,

Your argument is logical and well thought out. However, build the attenuator using the simple circuit. Then solder 150uf OSCON SEP caps right on the power supply leads of each optocoupler. Listen again. I guarantee you will leave the caps in place.

To your argument however, perhaps it will sound even better if the Oscons are replaced with very low leakage PPE smt film caps. You could never get 150u. But a few 1uf smt caps might be interesting.
I won't question your assertion because I don't have knowledge of the circuit as you have built it (nor the extremely critical power supply), but I would say that if such capacitors do have a beneficial effect you have a power supply issue and the capacitors belong on the power supply side of the volume control pot.
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Old 30th July 2013, 03:26 PM   #5103
wapo54001 is offline wapo54001  United States
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Quote:
Originally Posted by georgehifi View Post
To add to this "powering the led's" mix, try a pure DC source such as a 12vdc Li-Ion (Lithium) rechargeable battery. See if you can reliably pick the difference between it and a 12vdc (linear) wall wart. Nobody I know can.
Even Sam Tellig from Stereophile says with his Lightspeed he has to look to see what's powering it at the time, because he switches between the two a bit and cannot tell the difference between the wall wart or battery power by just listening.

Cheers George
My comment about minimizing the current leaks in the LED path particularly apply to a situation where one is trying to hit a specific target current which is the case with my precision LDR project. In my case, I'm striving for an absolute current value flowing through the LED at a given volume control rotation, not a relative current as is the case with the Lightspeed. But I stand by my statement that eliminating extraneous current paths from the circuit after the control device -- the pot in the case of the Lightspeed, the active control circuitry in a PIC-based system -- would be advantageous in any circumstance.
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Old 30th July 2013, 06:53 PM   #5104
wapo54001 is offline wapo54001  United States
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Originally Posted by georgehifi View Post
To add to this "powering the led's" mix, try a pure DC source such as a 12vdc Li-Ion (Lithium) rechargeable battery. See if you can reliably pick the difference between it and a 12vdc (linear) wall wart. Nobody I know can.
Even Sam Tellig from Stereophile says with his Lightspeed he has to look to see what's powering it at the time, because he switches between the two a bit and cannot tell the difference between the wall wart or battery power by just listening.

Cheers George
That doesn't surprise me.

I simultaneously watch both the control input (current) and the resulting LDR resistance through my computer terminal, with refresh happening several times a second. The control current does vary plus or minus a bit, but the output resistance stays quite steady but not perfect -- when I'm looking at a 50 ohm output, the resistance is steady +/- one one-hundredth of an ohm. At 3000 ohms, it's within about +/- 75 ohms. I believe that the Lightspeed's LDR stability would be similar.

If you think of the resistance of a pot and it's effect on gain, a hundred ohms at 3K ohms of a 10K pot isn't going to be audible -- it's less than .2dB. I have wondered off and on whether the continuous slight variation in resistance characteristic of LDRs might not be the basis for mysteriously exciting quality of the sound it produces . . .
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Old 30th July 2013, 10:08 PM   #5105
georgehifi is offline georgehifi  Australia
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Quote:
Originally Posted by wapo54001 View Post
I have wondered off and on whether the continuous slight variation in resistance characteristic of LDRs might not be the basis for mysteriously exciting quality of the sound it produces . . .
I believe it's a combination of two things:

1: "Diode Effect" Having no touch contacts of disimilar metals in the signal path. Ever done a cleanup with contact cleaner of your rca's and sockets, "hear the difference" or hard wire (solder) eveything and hear an even bigger difference.


2: It could be that cadmium sulphide (CdS) sounds better than metal film, carbon or conductive plastic.

Cheers George
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Old 31st July 2013, 02:13 AM   #5106
wlowes is offline wlowes  Canada
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Quote:
Originally Posted by wapo54001 View Post
I won't question your assertion because I don't have knowledge of the circuit as you have built it (nor the extremely critical power supply), but I would say that if such capacitors do have a beneficial effect you have a power supply issue and the capacitors belong on the power supply side of the volume control pot.
I realize it is counter intuitive. After all the LED should isolate the PS. But it does not. It is great just as George builds it. But is can be improved and the difference is not subtle.
The lightspeed circuit I use is more or less per the diagram posted back in spring 2008 when a bunch of people experimented with a variety of caps on the pins of the optocoupler.
I have played around with various power supplies. Walwort, battery and linear. I believe a low noise diy voltage regulator and the caps on the pins are the most important elements. I still prefer linear power supply in front, but with the caps and the low noise reg, not sure if the supply is all that important. For the record I use a 12v toroid, bridge rectifier, CLCLC filter. The last couple of filter caps are Black Gate F. Toroid and first filter element are in a separate chassis.
Sound? Sublime!
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Old 31st July 2013, 03:02 PM   #5107
udailey is offline udailey  United States
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Adding caps and getting better sound does not indicate a problem with the power supply. The caps ARE part of the power supply, as is everything before the LDRs. He now has a better power supply. Caps directly on the pins of an opamp and chipamps are well known ways to get better sound and it works for LDRs as well. Recently experimented with film vs ceramic .1uf's. No difference. I use this parallel with a 10uf-100uf electrolytic. I doubt brand has anything to do with the sound here but I use Nichicon FW, KW and those green bipolars depending on whats in stock. I'm sure any old electrolytic would sound the same but I think people expect a higher quality cap in their kits. I would NOT encourage higher values of caps after the potentiometer as you can hurt the wiper by forcing it to pass higher currents. If you have that 100R current limiting resistor in place you are much more safe using higher values of caps but I dont think you will benefit from higher than 100uf.
The NSL32SR2 used to have larger legs. Maybe 4-5 years ago. Then they started getting thinner. Now they are a little thicker, not as thick as before and have less spring to them. Softer. I am betting that the Sorted are older stock. Nothing wrong with that, just saying they probably sell far fewer of them.
If you have low efficiency speakers I would definitely follow AndrewT's advice. 40 Ohms is pushing them and even a few ohms lower is really a bad thing as you need multiples of the current to get much lower. 50 Ohms as a minimum would be an wonderful break for the LDRs and I run series LDRs in the 250R range. My speakers are 96db efficient and my amp is a gainclone so I really need that 40R low resistance on the shunts. I am willing to replace with a new pair if I have a problem. Been running the same LDRs for 3 years now. If they were to break tomorrow and I was a customer rather than a guy who matches LDRs I might be a bit upset, especially when the datasheets indicate no problem running LDRs at this current. I use LM334's to run my LDRs and they are only capable of 10mA and I still get into the 30's when adjusting a new pair of LDRs in someone's new attenuator.
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Old 31st July 2013, 03:08 PM   #5108
udailey is offline udailey  United States
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By the way I just reread the note above about Oscons. I built a pre using LME49710. Awesome opamp, great sound, bass was so so good. Anyway, I built it and used tantalum caps. Very rock n roll sound. Good stuff but then removed and replaced with Silmic II. After burn in it was just a great sounding pre. Nothing remarkable but really really good nonetheless. Removed Silmics and replaced with Oscons. Now THAT was classy. Extremely smooth and wonderful. Just a few steps above either other cap I used and I liked this a lot. Very expensive caps at 150uf and 20V though. Its off topic I know and I would not use this comparison as a way to determine which caps to use in an LDR circuit. Just something to bounce off of wlowes comment.
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Old 2nd August 2013, 05:36 AM   #5109
soongsc is offline soongsc  Taiwan
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Quote:
Originally Posted by georgehifi View Post
You find a tighter tolerance for the NSL32SR2S (selected), this is why it's a bit more expensive but worth it, it gives typical 40ohm min to 5mohm max.
Where the NSL32SR2 they won't give the typical low figure it can be anywhere with a max to 5mohm.
And the NSL32SR3 is even worse at typical 150ohm min to 25mohm max.

Cheers George
I was talking about signal amplitude into the LDR attenuators. For example, has any tried feeding a 0.5V signal vs a 10V signal and see if the gain is the same. I know with most resistors it is the same. Just have not tested with LDRs.
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Old 2nd August 2013, 05:54 AM   #5110
soongsc is offline soongsc  Taiwan
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Originally Posted by georgehifi View Post
I believe it's a combination of two things:

1: "Diode Effect" Having no touch contacts of disimilar metals in the signal path. Ever done a cleanup with contact cleaner of your rca's and sockets, "hear the difference" or hard wire (solder) eveything and hear an even bigger difference.


2: It could be that cadmium sulphide (CdS) sounds better than metal film, carbon or conductive plastic.

Cheers George
True, these can all make a difference. Higher CdS density might be the cause of it sounding better than other materiel.
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