Infrared is lower noise too but has less forward drop so it is kind of pari.
Green has a higher forward drop but a bit more noise so concerning noise there is actually not such a big difference. The real deal with noise is the relationship between forward drop and noise. Plus i do it not like in the Simplistic where the Leds are directly at the bases of the cascode, although shunted with an elcap. I add a resistor of say 200 Ohm between the Leds and base and shunt to ground with a rather big elcap of 2200uF so the base of the cascode sees nearly zero noise impedance from the bias.
Green has a higher forward drop but a bit more noise so concerning noise there is actually not such a big difference. The real deal with noise is the relationship between forward drop and noise. Plus i do it not like in the Simplistic where the Leds are directly at the bases of the cascode, although shunted with an elcap. I add a resistor of say 200 Ohm between the Leds and base and shunt to ground with a rather big elcap of 2200uF so the base of the cascode sees nearly zero noise impedance from the bias.
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I generally use old school dark red GaAsP leds or the yellow-green GaP for reference sources. The red ones I have on hand are old Litronix units, and drop 1.6V with 10ma current, with an incremental impedance of about 1.5 ohms. The green LEDs are good for 2V at 10ma bias, and have an incremental impedance of 13-15 ohms.
I'm starting to use infrared LEDs in some of my projects, but that's a confusing arena, as there are several technologies available, and I'm not sure which one is best, or how to tell the difference between them without known part number and spec sheet. The original infrared devices are GaAs - I haven't characterized them yet, though I have some floating around. One IR LED I'm currently using is the Rohm SIR-311, an obsolete Rohm device floating around in quantity on E-pay. It drops ~1.1V at 10ma bias, and has about 10 ohms incremental impedance. It's a 950 nm emitter. The shorter wavelength devices (especially all the illuminator LEDs flooding the market) are GaAlAs. The shorter wavelength should yield higher voltage drop. Impedance/noise? I'd have to try some to see, though I suspect both would be higher. I'd like to find a diode with similar drop and lower impedance than the Rohm device, as I suspect it would be lower noise, as well as being a stiffer voltage source. I guess I'll just have to start gathering up some likely suspects for characterization.
Edit - I just did some digging, and it looks like the 950 nM LEDs are GaAs. That's probably a good place to confine my search.
I'm starting to use infrared LEDs in some of my projects, but that's a confusing arena, as there are several technologies available, and I'm not sure which one is best, or how to tell the difference between them without known part number and spec sheet. The original infrared devices are GaAs - I haven't characterized them yet, though I have some floating around. One IR LED I'm currently using is the Rohm SIR-311, an obsolete Rohm device floating around in quantity on E-pay. It drops ~1.1V at 10ma bias, and has about 10 ohms incremental impedance. It's a 950 nm emitter. The shorter wavelength devices (especially all the illuminator LEDs flooding the market) are GaAlAs. The shorter wavelength should yield higher voltage drop. Impedance/noise? I'd have to try some to see, though I suspect both would be higher. I'd like to find a diode with similar drop and lower impedance than the Rohm device, as I suspect it would be lower noise, as well as being a stiffer voltage source. I guess I'll just have to start gathering up some likely suspects for characterization.
Edit - I just did some digging, and it looks like the 950 nM LEDs are GaAs. That's probably a good place to confine my search.
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Those look to be GaAlAs devices, I took a look at their V-I curve, and seem to get an incremental impedance of ~15-16 ohms. What sort of voltage drop are you seeing at your typical bias current? On the LEDs I like to use, I find it takes about 10 ma for the impedance to flatten out to a minimum value.
What this says is that I really should get my Quan-Tech noise analyzer up and functioning, and make up some kind of jig for diode testing. That would allow me to sort things out in a hurry. I'll have to add it to the long, long list...
What this says is that I really should get my Quan-Tech noise analyzer up and functioning, and make up some kind of jig for diode testing. That would allow me to sort things out in a hurry. I'll have to add it to the long, long list...
I have done the big RC thing there very long ago. But I reverted to straight elcap. Why? I could not measure any noise floor reduction at 1/f area or hear any difference, maybe because there is an integrated subsonic filter in the Simplistic.
On the other hand because there is a 5mA CCS for the LEDS it took ages for the circuit to come up and bias at nominal voltage when charging the large RC. Long start up black out. So I ditched it.
What I found useful was the CCS impedance to be near the 4 red LEDS impedance. That could reduce the hum spike allowing moderate elcap. So I used the proper impedance JFET source.
The cascoding transistor to have better Early and lower Cob I think I could hear as more natural highs but its anecdotal, could not see any significant change in the steady state distortion measurements.
On the other hand because there is a 5mA CCS for the LEDS it took ages for the circuit to come up and bias at nominal voltage when charging the large RC. Long start up black out. So I ditched it.
What I found useful was the CCS impedance to be near the 4 red LEDS impedance. That could reduce the hum spike allowing moderate elcap. So I used the proper impedance JFET source.
The cascoding transistor to have better Early and lower Cob I think I could hear as more natural highs but its anecdotal, could not see any significant change in the steady state distortion measurements.
Sure, there is a start up time. Some impedance in the base feed of the cascode transistor can also help to suppress parasitics at very high frequencies. The noise difference is minimal too, i agree. We have discussed other variaties. I was particular dissappointed by a mini shunt that someone proposed and claimed to sound much better. I never got it to work properly.
this is not perfect but kind of coool nevertheless :
First 3-D Printed Record is Amazing - Kurt Cobain Nirvana - Wired Magazine - YouTube
First 3-D Printed Record is Amazing - Kurt Cobain Nirvana - Wired Magazine - YouTube