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Old 8th July 2011, 12:15 PM   #2101
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Originally Posted by ostripper View Post
Ha ! - PS , under a bright CFL - the external light will affect (modulate) the led exposed to it. (keep your CCS dark )
OS
It will modulate a 1N4148 as well (or any diode with a clear body). (Saw that in Analogue Dialog a few years back.)

In a response to Dennis Colin's low noise measurement amplifier (AX sometime in 2007) it was noted that LED's are better clamp diodes than the switching diodes originally specified -- with the proviso that they be kept dark!
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Old 8th July 2011, 12:48 PM   #2102
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Hi topicreader,

I think you mean :

The ALTMANN SPLIF Amplifier Topology

From my point of view, there is a complete lack of understanding of the global NFB's benefits by the author.
Surely, this chap belongs to anti-GNFB community
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Old 8th July 2011, 12:54 PM   #2103
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The light-dark thing is significant for leakage current when reverse biased. I posted some measurements a few years ago showing that the light modulation of the forward current is absolutely negligible. As far as noise, note that unbypassed LEDs are in series with the signal in the first two stages of my MC phono preamp, and it's among the quietest tube designs around.
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Old 8th July 2011, 01:14 PM   #2104
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SY

Indeed if you can couple (light pipe and LED) to a reversed bias junction (some more than others) leakage modulation can be useful. Assemblies with CTR of ~1% have been used for high voltage regulators and sweeping type deflection biases of electrostatic analyzers. You can even forward bias the device but the CTR is much reduced.
Sorry for the OT application, as I have not yet found and an audio application.

-Antonio
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Old 8th July 2011, 01:48 PM   #2105
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Originally Posted by jackinnj View Post
It will modulate a 1N4148 as well (or any diode with a clear body). (Saw that in Analogue Dialog a few years back.)

In a response to Dennis Colin's low noise measurement amplifier (AX sometime in 2007) it was noted that LED's are better clamp diodes than the switching diodes originally specified -- with the proviso that they be kept dark!
Quote:
By SY - The light-dark thing is significant for leakage current when reverse biased. I posted some measurements a few years ago showing that the light modulation of the forward current is absolutely negligible. As far as noise, note that unbypassed LEDs are in series with the signal in the first two stages of my MC phono preamp, and it's among the quietest tube designs around.
Nothing to worry about , objectively . Most LED's are in the amp case. There are some, (the same ones that put wood blocks under $1000 cables), that suggest using black heat shrink on the LED. For a scientific instruments , this might be prudant.

In the old ARP synthesizers , the RPI professor I knew that repaired them would find old E- waste with the oldest monsanto GaAsP red led's to use for the noise generator. Our newer reds are much quieter , after Fairchild perfected the process.

I've seen (and built- below) a light detector with forward biased LED. It's photo-sensitivity has to be amplified 1000 X for any useful purpose. A pair of glass 1n4148's would work as well.

OS
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Old 8th July 2011, 02:07 PM   #2106
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Try running a milliamp through the diiode, then hit it with a high intensity modulated source. Set the opamp gain high. What you'll see is... nothing. My measurements confirmed the obvious physics.

The circuit as it sits is still looking at leakage currents, not a forward biased diode such as is used for voltage references.
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Old 8th July 2011, 02:52 PM   #2107
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I'd expect the photo current is still there - but 1 uA photocurrent on top of 1 mA would give only 26 uV delta in a Si diode, ~ 87 dB down re 650 mV forward drop

(1 uA is a really large photo current for "accidental" coupling to a packaged signal diode I got about that by sticking 2 smt leds together face to face and driving one with 10 mA)
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Old 8th July 2011, 02:57 PM   #2108
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Originally Posted by Edmond Stuart View Post
Hi Bob,

Cheers,
E.
I guess that most of the people that are interested in Class D design, dont even read this thread and would never buy Bob's book.
In my honest opinion I think that Bob should just skip the whole Class D chapter in his new edition of the book. It is waste of paper.

A Class D amp is not something you are designing in an afternoon with both your arms on your back.
It takes time and a lot of knowledge, unless you choose to use some well known (and not very interesting ) chips and methodes.
ClassD technology is developing and gets better every day, and you will not find the answer in a book.

Cheers
S
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Old 8th July 2011, 03:06 PM   #2109
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Originally Posted by WuYit View Post
Bob,
a lower source impedance increases the bandwidth.
WuYit,

This does not answer the question. Of course I know that a lower source impedance usually increases the bandwidth. You need to stop generalizing and respond with specifics, otherwise your claims will lack credibility.

Cheers,
Bob
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Old 8th July 2011, 03:19 PM   #2110
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Originally Posted by Edmond Stuart View Post
Hi Bob,

More on this subject can be found here: http://dr.ntu.edu.sg/bitstream/handl...pdf?sequence=1
(THD of closed-loop analog PWM class-D amplifiers)

Cheers,
E.
Thanks, Edmond. I appreciate this lead on Class D amplifier distortion sources.

Does anybody know who Ray De Velder is?

Cheers,
Bob
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