Typical analog noise generators make signal by amplifying AC component coming out from zener current of transistors reversely biased between emitter and base (here’s an example circuit).
Any bipolar transistor would work as such a noise source, but noise quality in listening is different from part numbers. 2SC828A is well known as good noise source, but it’s been obsolete for long. So for Analog2.0, I have been recommending 2SC3311 instead. But this part becomes obsolete as well. Now I have to find another one.
However, I do not have a good idea what kind of transistors give quality noise. What I know is a few examples, such as 2SC3311 and 2SC828A as good ones, and 2SC1815 and 2SC2003 as not-so-good ones.
Finding the one randomly would be probably too hard. So I first looked into data sheets of transistors I know to figure out any significance. Among parameters, I’ve find emitter-base voltage (Vebo) interesting.
Here are maximum ratings of them:
Vebo of 2SC1815 is 5V. It’s also 5V for 2SC2003. On the other hand, 2SC8282A and 2SC3311 that produce good noise has higher 7V Vebo. As I looked more, I learned that 7V Vebo is significantly higher than voltages with typical small signal transistors, which is 5V.
This is interesting since we actually use the noise source transistor at the area where this parameter is somewhat related (we bias reversely between emitter and base).
Among other parameters, noise figure drew my attention, too, though significance is not very clear. It is a factor that indicates how much noise the transistor adds in amplifying signal. Smaller value is better in general usage. The value is 1dB for 2SC1815. 2SC828 is significantly different (6dB). There’s no information about 2SC3311.
So I will start trying several transistors that has relatively high Vebo. Follows are the candidates I’ve found so far.
- 2N3904 (Vebo=6V, NF=5.0dB)
- BC547 (Vebo=6V, NF=2.0dB typ, 10dB max)
- 2N4401 (Vebo=5V, NF=1.0dB min) … not really a candidate but for comparison
- 2N5087 (Vebo=3V, NF=2.0dB) … PNP, not really a candidate but for comparison
- MPSA18 (Vebo=6.5V, NF=0.5dB typ, 1.5dB max)
- 2N5210 (Vebo=4.5V, NF=2.0dB) … not really a candidate but for comparison
- KSD5041 (Vebo=7V)