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Old 5th January 2008, 01:45 PM   #1011
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The general opinion seems to be that the LEDís in the original design are driven by a voltage source. A good voltage source has an output impedance of say less than 1 Ohm, and a good current source is well above letís say 1 MOhm. Letís take a closer look at what the LEDís are seeing. They are driven by a voltage source with a resistor in series that varies dependant on the volume setting, between 100 Ohm and 100 kOhm. That is, seen from the LED, neither a good voltage source, nor a good current source. It is something in-between. On one side (potentiometer 0 Ohm) it behaves more like a voltage source, on the other side (potentiometer 100 kOhm) itís getting current source specs.
The amount of light a LED emits is primary dependant of the current flowing trough it. If you want to keep the amount of light stable, keep the current stable. That is why semiconductor companies have so many devices that drive LEDís by constant current. And that is why the CCS approach yields better results.

Lostcause and George: The proof of the pudding is in the eating. Try it and you will be happy you did! Let your ears judge and donít be so sceptic without giving it a try.

1543: Your measurements are very interesting, although measured differences seem to be small. With series / parallel LDR in my system it sounded less natural. Instruments and voices became so big spatially that they began to loose focus and natural timbre. I didnít try the series resistor with the original lightspeed but only tried it in the CCS version. I also used a very high quality resistor. Perhaps these two facts (CCS and HQ resistor) make the difference.

Cheers,
Peter
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Old 5th January 2008, 06:06 PM   #1012
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Default They can be expensive...

Arne,

The AD420 current DACs can be very expensive. I bought mine from Rapid in UK who are selling none ROHS devices for 4 UK pounds (~6 euros) each.

http://www.rapidonline.com/productin...moduleno=77339
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Old 5th January 2008, 06:58 PM   #1013
mikelm is offline mikelm  England
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Quote:
Originally posted by pietjers
The proof of the pudding is in the eating. Try it and you will be happy you did! Let your ears judge and donít be so sceptic without giving it a try.
Did you have capacitors across the LED's in either of these modes of experienation ? If you did what size where they ?

I ask because for me putting a 100uF cap directly across the LED's created a big improvement in sound quality and at the time I thought this was probably the most direct & effective way of reducing LED noise. ( perhaps even with current source operation )

I would be interested to here your reaction to this ?

mike
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Old 5th January 2008, 07:20 PM   #1014
1543 is offline 1543  Germany
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Peter: Yes, the measured differences are very small. But even configurations measured the same sound different. Your discription of the Lightspeed sound is correct, have had the same experience. For listening to electronic kind of music it is very good, but when listening to jazz, vocals and acoustic instruments I also miss a little bit focus. Sometimes the interprets and instruments appear to voluminous and loose some of the acoustic aura. This also happens with the series resistor/shunt LDR configuration, but at a very minimal and acceptable level. Perhaps this has to do with inadequate shielding of the experimental setup.

Nordic: Please post pictures of your blasted speakers.

Barry: Yes, indeed. The price is very low. Farnell wants 24 Ä per piece. Thank you very much!
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Old 5th January 2008, 08:22 PM   #1015
Badge is offline Badge  United States
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Peter, how closely do the components in you CCS need to be matched? I am referring to the series/shunt layout.
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Old 6th January 2008, 05:29 AM   #1016
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Mike:
As you can see in the attached circuit diagram I used the capacitors across the LEDís. I remember your posts from some time ago about the positive results with these caps. And I think itís a good idea to add these. I did no additional listening tests to verify this.

I have been looking at digital volume controls such as Maxim DS1669 and many others to control the current sources, but these devices have problems with currents up to 10mA. Perhaps I will jump on that later. For now the DACT is a simple (but expensive) solution.

1543:
I am very happy to hear that you have the same feeling about the sound of series/shunt LDR. Your description is exactly how I experience it. Try the CCS version. You will be amazed!

Badge:
Matching is quite simple. I ordered 10 pieces from Allied. Selected the 4 best of them for a future project and used the 4 next best for this project. You can add an optional potentiometer as shown across one of the LEDís to get them perfectly matched. I measured errors of less than 0.2 dB over the whole range. No conventional potentiometer (except stepped attenuators) can do that.
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Old 6th January 2008, 06:26 AM   #1017
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Quote:
Originally posted by Badge
Peter, how closely do the components in you CCS need to be matched? I am referring to the series/shunt layout.
Badge, the matching of the LDR's (NSL-32SR2S) will be the same chore, regardless of what system is used to power the led section.

Cheers George
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Old 6th January 2008, 08:48 AM   #1018
EUVL is offline EUVL  Europe
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> LEDís. I remember your posts from some time ago about the positive results with these caps.

Maybe my understanding is incorrect, but the cap basically makes the voltage across the LEd constant and supplies any small current fluctuations required to achieve that, so doesn't it destroy the entire purpose of constant current drive at all (i.e. the LEDs are being supplied by the current source AND the caps) ?

> how closely do the components in you CCS need to be matched?

I think he was asking how well matched the TLV431s, BC550s, the 330Rs, the 120Rs and log pots, ..... had to be.

If that was the question, I think they need to be well matched.
But then that is easy enough in comparison to the LDRs.


Patrick
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Old 6th January 2008, 09:07 AM   #1019
Ryssen is offline Ryssen  Sweden
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While discussing parts,is this a good serial resistor?

Resistor...
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Old 6th January 2008, 10:36 AM   #1020
AndrewT is offline AndrewT  Scotland
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I buy my metal film resistors from Rapid online.
They are far better specified than the dale/vishay example and they are 20times cheaper.

If you want quality metal films then look to Holco for a similar price to the Allied retail. The Holco is even better specified than the Rapid. +-0.1% and 25ppm/C
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