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Old 17th July 2006, 09:26 PM   #111
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Quote:
Originally posted by ErikdeBest


Hi George. What exactly do you mean with the 100k pot increasing the impedance?

This will be long and boring. The pots are not wired up as voltage dividers; input, wiper, ground, but as variable series resistance.
The resistance of the optoisolators goes from infinity, 1 meg, at zero current to say 50 ohms at 20 ma. This decrease is not linear, is very temp dependent, and does drift a little.
With my setup the input voltage is 5 volts, the pot 50K, the series safety resistor 121 ohms. This resistor is there to limit current. If the pot is set to zero ohms, the LED will fry if hit with 5 volts and unlimited current. The LED drops about 2 volts, should be 3, and the 121 ohm resistor drops the other 3, or 25 ma. Actually measures more like 20 ma. This is not an exact science.
With my pot, with the full 50K in series, the measures resistance is 8.5K, with no resistance from the pot in series the resistance across the LDR measures 70 - 80 ohms.
My thinking is that with a 100K in series the resistance of the LDR with increase to 20 - 25K. This may be too much for a passive, but may be perfect. Currently it is a 8K passive. From my graphs the 100K point should be 25K.
Now I just need to locate some 100K dual pots that track together real well.
Right now, the loading my be a little low, all my sources are cap coupled, and the taper is too steep with normal amplifier. The 100K pots will definately increase the impedance of the circuit compared to the 50K used now. And the taper should be better too.
I will try to hunt some pots down this week to see.


George
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Old 17th July 2006, 11:14 PM   #112
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Hi George

Thank you for the answer! I couldn't understand how the value of the pot could influence the impedance of the optocoupler - but with those I limiting resistors the story becomes clear!

Erik
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Old 18th July 2006, 07:03 AM   #113
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Quote:
Originally posted by ErikdeBest
Hi George

Thank you for the answer! I couldn't understand how the value of the pot could influence the impedance of the optocoupler - but with those I limiting resistors the story becomes clear!

Erik

Erik, that's right, in the Lightspeed as I make it, the value of the pot has no reference at all to the impedence value of the LDR's. The only thing that influences the impedence value of LDR's is the intensity of the LED's which are controled by the pot in the first place.

Cheers George
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Old 18th July 2006, 08:15 PM   #114
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I got mine up and running the past weekend. I usually keep the electronics projects for winter. First, let me thank George for sharing his design. I like the results. Considering I threw together most of it from existing parts, the results are outstanding. My power supply might be a bit much since I have low volume with the pot all the way down? No matter. It has plenty of gain driving a 1.5 meter IC into a 100k ohm load. I briefly tried it with the sub amp/electronic crossover. My initial response was the bass went away, but I still need to do more listening before I confirm that. Pot is a 100k alps blue and channel balance was close enough the multi turn pots were sufficient.
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Old 20th July 2006, 04:26 AM   #115
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Im lost on the design i don't even know where to start or how to understand the project..
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Old 20th July 2006, 05:41 AM   #116
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Quote:
Originally posted by jleaman
Im lost on the design i don't even know where to start or how to understand the project..
OK let me try to expain this in the simplest way.
Remember when you said hook up the cd players output directly to the poweramp, and solder a resistor in the interconect to attenuate to a fixed level.
This is BASICALLY what the Lightspeed Attenuator does, except the resistor that is soldered in, is a special Light Dependent Resistor (LDR) which according how much light is shone on it by a LED can vary it's resistance, so you can have control over the level of music by controlling the amount of brigtness of the LED by a pot which varies the voltage going to the LED.
Do you see the light. (Lightspeed) , I hope so.

Cheers George
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Old 20th July 2006, 06:09 AM   #117
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OH CRAP!! NOW I GET IT !! Simular to a photo diode : except a photo diode is either on or off.. So basically i only need a cheap pot and a cheap led then. Where can i get a LDR ? Digikey ? Do you have any to spare ?

Any one have any comments on these yet ? How are people liking them ?
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Old 20th July 2006, 06:30 AM   #118
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Quote:
Originally posted by jleaman
OH CRAP!! NOW I GET IT !! Simular to a photo diode : except a photo diode is either on or off.. So basically i only need a cheap pot and a cheap led then. Where can i get a LDR ? Digikey ? Do you have any to spare ?

Any one have any comments on these yet ? How are people liking them ?

It's all there in previous post to build yourself. Remember that it's a series shunt setup, to keep the input/output impedence constant, a bit more complicated than you think,. (wish you luck!)

As for how people like it, not one negative comment yet from any of my 30 odd world wide customers, in fact extremely positive, to the length that some of them are selling their megabuck pre's because they can not warrent keeping them.

Cheers George
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Old 20th July 2006, 12:30 PM   #119
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Default Easy enough

Diagrams are listed on page four of this thread. The parts are available from Allied Electronics. There are dual pots on ebay.
I like mine a lot also. Still getting it to work better though. Wish I had followed Georgehifi's lead and used a 100K pot to control volume. And I like doing my channels independently, so no nulling circuit is needed.
It works fine in my system, but it does not need any gain. Have used a buffer for many years, first tube, then opamp, then discrete, and back to chip. This is even cleaner than a buffer. Passive.

Good luck,
George


Youmight even consider ordering a real Ligthspeed, delivered looks to run under 400.00.
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Old 20th July 2006, 04:15 PM   #120
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I have had some time to play with this unit. It does everything well except deep bass in my application. Utilizing two outputs, one driving mains amps with 100 K ohm input and Marchand electronic crossover with 25 k ohm input, I still have plenty of gain. The crossover also has gain if needed. For some reason this combination has created a filter and the bass is weak and there is nothing below say 50 hertz IMO. I contacted Phil Marchand about changing the input impedance on the crossover to 100 K ohms. He does not think that will help, but I have ordered the needed resistors to give it a try. Now if I can just restore the last two octaves of bass, I will be one happy DIY'er.
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