The 310 Ohm of your microphone deliver alone > 2 nV/rtHz so the 0.3 nV/rtHz
of a transistor doesn't play much of a role. (total noise would be sqrt(2**2 + 0.3**2).
So far for voltage noise. There is also current noise. Let the collector current be 10 mA,
which is too much here. With a beta of 200 the base current will be 50uA.
That creates a base current shot noise of
sqrt( 2 q IB) = sqrt(2 * 1.6e-19 * 50E-6) A/rtHz = 4e-12.
When that current flows through 310 Ohm, you get another 1.2 nV/rt Hz.
q = 1.6 E-19
4KT = 1.6 E-20
So the choice of transistor is not that important.
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I think that assumption of collector current is WAY too high---1.6mV into a typical 1.6KΩ load is only 1µA of current; even with a Beta of 200, collector current would only be 0.2 mA.
I use the ZTX851 in a MC preamp design and have experimented with the ZTX689B. I haven't tried the ZTX690. Flat Balanced Input Moving Coil Phono Preamp Construction Information - Pro Audio Design Forum
The ZTX851 has an rbb' of <2Ω; the ZTX689B is estimated - based on noise measurement in the same circuit - to be around 5Ω. For a mic preamp optimized for mic source impedance the difference in rbb' is deminimis. Low Noise Transistors: ZTX851 versus ZTX689B Noise Performance - Pro Audio Design Forum
The ZTX689B has much higher current gain - it would be my choice for a mic preamp due to lower bias and base noise current.
I will be doing a mic preamp soon and will start with an Ic (quiescent) of 1-2mA versus the 5.5 mA of the MC preamp.
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The main difference I see is that the ZTX689 has a max Vcbo and Vceo of 20 volts whereas the ZTX 690's max is 45 volts. Everything else is the same EXCEPT for the switching OFF time which is faster on the 689 (800nSec) than the 690 (1300nSec). So the 690 might be safer in a circuit with +/- 17 volt rails; but I don't know what difference the switch-off time would make to a microphone signal---???
None, you never run a preamp stage in saturation! Switch-off time is to do with amount of stored charge in saturation mode.
An interesting observation here about the use of ZTX 951 transistors as replacements in a Soundcraft mixing console:
Interesting Soundcraft 1600 mod results post #6:
Interesting Soundcraft 1600 mod results post #6:
I am wondering why this is??
No, but most of the AVAILABLE low-noise transistors these days are chronicled in "The Art of Electronics" by Paul Horowitz and Winfield Hill. It's an excellent reference that also shows the best transistors for different source impedances, which is crucial to actual performance. I actually settled on the ZTX 550/450 used in a complementary feedback differential pair circuit, as this seemed to be the best choice for a 150-250 ohm source impedance as in a mic preamp.
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Well here is an interesting quote:
The MJE13007 is an 8A 400V transistor.
It came from here: Extreme Low Noise Preampifier
The MJE13007 is an 8A 400V transistor.
I did a quick measurement of 3x parallel S8550 generics from the cheap ebay assortment plastic box kits.
I would not be surprised if the original S8550 process and die layout was similar to the National process #63 and die layout I posted here #149 for 2N4403/2N2907.
I have attached the measurements. Divide by 1000 due to the 60 dB gain. I apologize for the 60 Hz harmonics and SB0490 noise spurs.
So of the cheapest available transistors the 2N4403/2N2907 with rbb=17Ohm is a good bet. For more expensive better devices there are the Zetex devices. However the cheap plastic box assortments do seem to have reasonably low noise S8550 devices in them. (Aside: I am having pretty good luck with the cheap generic TO92 devices. And terrible luck with cheap generic power transistors, which I do not buy anymore.)
- I get around 0.9nV/√Hz when my 60 dB LNA uses 3x parallel S8550 generics as the input devices. (First image.)
- I get around 0.75nV/√Hz when my 60 dB LNA uses 3x parallel old Fairchild silver face PN2907 as the input devices. (Second image.)
I would not be surprised if the original S8550 process and die layout was similar to the National process #63 and die layout I posted here #149 for 2N4403/2N2907.
I have attached the measurements. Divide by 1000 due to the 60 dB gain. I apologize for the 60 Hz harmonics and SB0490 noise spurs.
So of the cheapest available transistors the 2N4403/2N2907 with rbb=17Ohm is a good bet. For more expensive better devices there are the Zetex devices. However the cheap plastic box assortments do seem to have reasonably low noise S8550 devices in them. (Aside: I am having pretty good luck with the cheap generic TO92 devices. And terrible luck with cheap generic power transistors, which I do not buy anymore.)
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Same here, many hundreds of A1015, C1815, A733 and C945, and while I can't test noise easily, they do make gain spec and grading nicely.
I miss the ztx107 and 117, though were they really THAT low noise? Lots of BC550/560 fairchild floating around, that are genuine.