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!
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.
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
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
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
Attachments
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.
The circuit as it sits is still looking at leakage currents, not a forward biased diode such as is used for voltage references.
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)
(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)
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
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
Thanks, Edmond. I appreciate this lead on Class D amplifier distortion sources.
Does anybody know who Ray De Velder is?
Cheers,
Bob
Hi Gyuri,
Thanks for showing that schematic. It appears to be a moderately stone-age amplifier. The modification by the designer appears to be some kind of mis-guided attempt to take negative feedback from the output stage that is "less global" by taking it from an output pair that is not connected to the load. This will do very little to reduce distortion introduced by the output stage and is a waste of power transistor silicon. I think he would do well to read my book
.Cheers,
Bob
I guess diode-connected transistors would not have this problem
.Not that some transistors can be used as Zener diodes by reverse-biasing their BE junctions, but I have not fooled with this very much.
BTW, passing reverse current through a BJT can degrade its beta, so guard against your circuits ever doing that.
Cheers,
Bob
Class-D THD
Hi Bob,
You're welcome. I hope the math is helpful too.
The next step is how to deal with it.
Never heard of that guy. He seems to be a reviewer of several books on audio (Doug Self, Ben Duncan, Randy Slone, and now your book). At least he is not that stupid as he too discovered some serious flaws in Randy's amplifier designs (ill defined VAS Iq).
Cheers,
E.
Hi Bob,
You're welcome. I hope the math is helpful too.
The next step is how to deal with it.
Never heard of that guy. He seems to be a reviewer of several books on audio (Doug Self, Ben Duncan, Randy Slone, and now your book). At least he is not that stupid as he too discovered some serious flaws in Randy's amplifier designs (ill defined VAS Iq).
Cheers,
E.
Nothing to worry about , objectively
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
Hi OS,
Did you go to RPI? I was there from '66-70. Who was the prof?
Cheers,
Bob
Hi stinius,
I'm curious why you think I should skip the Class D chapter in the second edition of the book. Is it because you don't think people here are interested in class D, or because you don't think my coverage of it is very useful.
I did the class D chapter not because it is an easy thing for many DIYers to build from the ground up, but because class D is the wave of the future. It is getting better and better every day, and I have heard some very good-sounding class D amplifiers. This does not mean, of course, that the linear power amps will go away - witness the continuing popularity of vacuum tube amplifiers. Interestingly, I received many questions from people at shows as to whether I had any vacuum tube material in my book. I do plan to put in one or two chapters on vacuum tube amplifiers in the second edition.
Anyway, I'm interested to hear more about your thoughts on the class D material in my book.
Cheers,
Bob
Unfortunately not - I wish ! I am moving back , might go work for AMD.
I don't remember the prof's name (35 years ago - he lived in west sand lake) , but he was the first to show me a curve tracer (he had to match all the semi's for the moog/arp VCO's-VCF's) - got me started at the tender age of 14. There is a lot of "audio brainpower" (R. Dalbeck) in that region , better colleges for my older kids.OS
Hi everyone,
Browsing through some amplifier designs on the net I stumbled into one that was derived from the leach amp. I don't know if this is inherent to the Leach design or if the designer of this mod introduced this, but there were resistors placed on the bases (series) of all IPS transistors (diff-amp, mirror image).
If I am not mistaken I didn't notice this in Bob's book?
What is the meaning of these resistors? Why are they there? Is it usefull or not so much?
Thnx
Olivier
Browsing through some amplifier designs on the net I stumbled into one that was derived from the leach amp. I don't know if this is inherent to the Leach design or if the designer of this mod introduced this, but there were resistors placed on the bases (series) of all IPS transistors (diff-amp, mirror image).
If I am not mistaken I didn't notice this in Bob's book?
What is the meaning of these resistors? Why are they there? Is it usefull or not so much?
Thnx
Olivier
Base stopper resistors. Sometimes, these are included to prevent RF oscillations. Usually seen more often in RF amps, not so much for audio, but still useful if you have stability problems.
Hi Olivier,
While base stopper resistors are most often seen in output stages, they are occasionally useful in small-signal stages. I have often used them in selected places where I may be concerned about parasitic oscillations. I have sometimes used them in input stages as a precaution, and sometimes in the base of an emitter follower. If there is a good chance that there will be inductance in the signal line, I will often use a base stopper. Some op amps also like to have them at their inputs. I seem to recall using them frequently at the positive input of a 5534 operating as a unity-gain non-inverting buffer.
The fact that I did not use them in many places in my book is also more reflective of the fact that many of the circuits were for illustration and that the addition of numerous base stoppers would add clutter or confusion to the diagrams.
They are used normally to counter the effect of negative resistance seen at the input of the stage and/or to damp resonant circuits that could appear there that could lead to parasitic oscillations. Sometimes what we might call "collector stopper" resistors are placed in series with the collector of a stage, often that of an emitter follower, but not always. I sometimes use a collector stopper in the emitter of a cascode transistor.
I think the use of base stoppers is rather layout dependent in many cases, and I'm not sure there is much of an exact science to it. 100 ohms is a favorite number. Of course, in low-noise input stages that may degrade the noise performance a bit.
Cheers,
Bob