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Old 6th January 2008, 10:38 AM   #1021
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Except for the LDRís the components donít have to be closely matched. Standard 1% resistors will de the job. And all the voltage references are already very precise.

Patrick:
The capacitors will help to reduce LED noise. After the caps are initially charged they will take no current from the current sources.

Ryssen:
Dale resistors are okay, but not in the same league as Shinkoh / Audio Note Tantalum resistors.
I bought my resistors here:
http://www.partsconnexion.com/catalog/resistors.html
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Old 6th January 2008, 11:47 AM   #1022
EUVL is offline EUVL  Europe
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Correct me if I am wrong.

I would use capacitors to maintain a costant voltage, or suppress voltage noise, as capacitors has low impedance at high frequencies, and will shunt any varying components of voltage. I would use inductors to maintain constant current, or suppress current noise, because they resist changes in current.

So if I wish to drive my LEDs with constant current, then I would perhaps put an inductor in series with the LED, rather than a cap in parallel ???

Unless of course one wish to drive the LEDs with constant voltage afterall. In that case, the current source in combination with the cap in parallel with the LED merely acts as a very good low pass filter to surpress power supply noise, not too different from a standard LC filter. But that is not really the same as driving the LEDs in pure current mode.


Patrick

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Old 6th January 2008, 12:19 PM   #1023
mikelm is offline mikelm  England
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Patrick,

I see what you are driving at but I think that the caps are a good idea on either version of the LDR volume control.

Even with a very good CCS it will be difficult to stop the LED's generating some noise which will be expressed as tiny fluctuations in the voltage across them.

So even if the CCS maintains perfectly constant current, the tiny voltage fluctuations across the LEDs will cause tiny power fluctuations, which will probably cause light emission fluctuations and noise in the resistor element of the LDRs.
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Old 6th January 2008, 12:35 PM   #1024
mikelm is offline mikelm  England
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Peter,

Thanks for your reply.

Could you please explain exactly what you compare it with when you say this CCS powered circuit sound much better.

Was in a bought lightspeed or a built one ?

Exactly what circuit ?

Did it have caps across the LEDs

Did it have a filtered low noise power supply ?

Until we know all these things it is hard ( without hearing it ) to really understand the significance of the improvement you heard because compared with the original I could say exactly the same about my heavily filtered voltage driven version.

I am not saying that CCS is not better but I would appreciate some more understanding of what you have compared.

( It would be a pity to spend hours building a new cct only to find we had achieved similar subjective results via different methods )

thanks

mike
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Old 6th January 2008, 02:02 PM   #1025
psymo is offline psymo  United Kingdom
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I would like to try capacitors across the LED's of my Lightspeed clone (based on George's original), can somebody recommend what type/make will be most suitable.

Thanks

Pete
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Old 6th January 2008, 02:07 PM   #1026
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Default Voltage vs Current Control

I think the better sound heard from the current control is stemming from a better supply. A better voltage supply will provide the same effect.
Mine has simple tracking regs, and is choke loaded. Chokes are on both sides of the LDR to keep ground plane noise out. And a 10 ufd high quality electrolytic across the LDR as close as possible. Maybe 1 cm total lead length. The chokes make up for the high frequency performance of the three pin regs.
Using the chokes did improve the sonics. Using two was much smaller. The cap across the LDR made it better. The sound was excellent to begin with. Better than any of the pots, or switched attenuators I had tried.
Controlling the current sources looks to be much tougher than sourcing some usable chokes. Mine are 1 mH, 250 MA, 1 OHM surplus. Think they were 1.50 USD each. So three dollars. The caps were 7.5 cents, another 30 cents to the total.
I saw pics of Rainwater's unit, he has some monster foil chokes in it. Now that is overkill. Those chokes cost more than my complete unit.
One area with the LDR that is sometimes missed is matching to get the same impedance. Even with the matched LDR's, measing resistance for the control can vary by 10% or so. I use sloppy matched dual pots. Parallel the tracks to help with taper, and mix and match. By using mismatched pots, the measured impedances can track very closely.
Listening has shown that this impredance matching is less critical than expected. I have used some pots in the past that sounded and appeared to track very well. Measurements showed large variations in taper and overall balance. The ear is very forgiving.

George
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Old 6th January 2008, 02:08 PM   #1027
mikelm is offline mikelm  England
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Pete,

I used 100uF oscons but I think any reasonable electrolytic will have a big effect.

I do not know if esoteric will sound better but I suspect you will get 90% or more of the effect with any type electrolytic.

make sure you get polarity right

mike
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Old 6th January 2008, 02:35 PM   #1028
psymo is offline psymo  United Kingdom
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George/Mike,

Thanks for your replies, I think Ive got some 100mf Panasonic FC's, so will give these a try to start with.

I will also have a look at upgrading the 5V power supply as I am currently using a fixed regulator (cant remember which one) without any other components.

Ive been using the Lightspeed for about 6 months and have been really impressed. It will be interesting to see if it really can sound any better.

Cheers

Pete
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Old 6th January 2008, 03:12 PM   #1029
AndrewT is offline AndrewT  Scotland
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Hi Psymo,
try to get used to the correct multipliers and units before you confuse yourself and us.
In this case it's easy to see you meant 100uF.
f=femto F=Farad
u=micro, m=milli, M=mega
k=kilo, K=Kelvin
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Old 6th January 2008, 05:09 PM   #1030
Badge is offline Badge  United States
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Peter, George, I was referring to the CCS components when asking about matching. I see that has now been answered.
Is it a decent guess that a certain well respected Swiss preamp uses the LDR's with a CCS, and buffer?
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