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two words... [message #30562 is a reply to message #30561]

Wed, 16 February 2005 05:49

PakProtector
Messages: 935
Registered: May 2009

Illuminati (2nd Degree)

Dynamic impedance. that and the only time I tried one it was noisey. Red is the lowest impedance( and even red ones vary amongst themselves ).

Download the spec sheets and look at the I/V curves. steeper is better. Very small change in V and great change in I means low R.
regards,
Douglas

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but... [message #30563 is a reply to message #30561]

Wed, 16 February 2005 05:52

PakProtector
Messages: 935
Registered: May 2009

Illuminati (2nd Degree)

the blue does look better. maybe they sound fine, I could have gotten two bad ones. it is a fairly cheap experiment when you get right down to it.
regards,
Douglas

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"Valve Amplifiers 3" - again :-) [message #30564 is a reply to message #30563]

Wed, 16 February 2005 06:26

Damir
Messages: 1005
Registered: May 2009

Illuminati (2nd Degree)

Page 47 - typical forward drop @10mA for cheap red LED is 1,7V , for the green 3,6V, for the blue 3,7V.
But, internal resistance is about 4,3 Ohms for the red, green - 30 Ohms, blue - 26 Ohms.
Author advises (pages 177- 178), that LED biasing is best for Ia>10mA, high Rl, and small output signal voltage (distortion `cos of non-linear LED resistance).

P.S. - I can`t resist, I know that you don`t like citations...

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"Valve Amplifiers 3" [message #30565 is a reply to message #30564]

Wed, 16 February 2005 06:43

PakProtector
Messages: 935
Registered: May 2009

Illuminati (2nd Degree)

I got that book for Christmas. It has been useful.

We have a few advantages not taken into account by the author. First and main one is the way the MOSFET CCS is used. The amplifier valve still sees a constant current. The load current is not taken from the regulated amount. If there is no variation in current the LED becomes more of a voltage ref instead of having to deal with varying current.

We are also running with fairly small signal, so that admonition is accounted for.

Besides, this ref agrees with my own research and has little if any internal contradiction. Some of the others( even some authored by folks revered almost to Godhood) have not been so clean.
regards,
Douglas

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