Measuring Transistor Noise

[electrosonic]

I have a bunch of germanium transistors that I want to somehow grade as far as noise level.

I was thinking of making a simple buffer with no signal input that I can socket the transistor into. The transistor would feed a op amp gain stage followed by a opamp rectifier circuit (the  one with the diode in the feedback loop).

Then measure the RMS voltage on my meter. I don't care precisely what the numbers I get are, I just want a consistent way to grade how noisy the transistors are.

Is this a reasonable plan of attack?

Andrew.

[Arcane Analog]

I do not 'measure' germanium devices for noise per se but I do have a little Rangemaster jig set up that is great for selecting 'quiet' devices. Keep in mind that all germanium is probably going to have a little noise.

What I do is simply pop in a device, bias it, have the guitar volume rolled off, max out the gain on the circuit and listen. Note that transistors that measure very low leakage can still be noisy buggers. The really good transistors will still have some small amount of hiss but the bad ones will have lots of hiss, crackle, static, crunch, pop, etc. These noisy devices can still be used of course but for purposes where that noise will not be noticable.

[electrosonic]

The reason I thought to amplify and rectify the noise of a transistor set up as a buffer, was because I wanted to eliminate the transistor gain from the picture. I am guessing that a Ge transistor with more gain will sound noisier with no signal input, because it is amplifying it's own noise more than a lower gain transistor.

Andrew

[LongJohnny]

For the components that I test, I generally use a know circuit that sounds good....

For example, I have some 2n5133's that I have gain tested.  I have an old big muff that is socketed, and will replace the transistor in 2 of the positions, just to make sure noise is not hiding behind a filter of some sort.
If the transistor sounds "good", then I will put it up for sale.

For other items, like the Reticons, I use an old Ross Flanger, and turn up the levels so any noise is going to show up.

Not worth reinventing the wheel, since there are pedals to test out old germaniums readily available, or easily made.

Johnny

[PRR]

A transistor has two noise sources: voltage noise and current noise.

Both change with operating current.

The noise in biasing resistors can be more than transistor noise.

Meter alone is not ideal. Your ear is most annoyed by 1KHz-5KHz hiss but amplifiers have rising noise at low frequency. In clean Silicon the 1/f noise is small, but dirty Ge can rise below 1KHz or even 5KHz. So you need a human's opinion about hiss-timbre.

My opinion is that you may as well build up a "typical guitar stage" including say 100K resistor where the guitar will go. All shielded. Follow it with enough amplification to hear hiss. Find a "medium volume" on the gain and mark it well. Now roll your devices. You may find a few that are quieter, and probably some that are HISSS!!