what's your fav way to NOISE audit a germanium transistor?

[darron]

i had a search and strangely couldn't find anything on this.


i've been playing more and more with pnp germanium transistors in boosts/fuzzes and have been having great fun finding old stuff to put into use.


the biggest problem is selecting the quieter from the noisier ones. i have a test circuit with a socket but you have to re-bias between checks and try to imagine the noise relative to the gain achieved, and it's hard to A/B. once something is soldered in it's a pain to try to resolder something new in risking damaging the board wires.



so how do you select the best and put aside the rest? do you go mostly by HFE or leakage? i use a Peak DCA55 for testing.



interesting to hear what people do...

thanks!

[theehman]

I used to sort them for noise and gain by putting them in a Rangemaster circuit.

[darron]

that's sort of what i'm am doing. a full-range rangemasterish circuit. it would be nice if there was a way to measure it as a number or more specifically.

maybe i need to have two circuits so i can A/B against something that i know has a similar gain and very low noise.... now it's getting messy though....


any tricks to SEE noise easily on the scope maybe? i'm after some free education here from someone who paid for a EE degree

[Gus]

Plug the circuit with no input signal into your PC sound card and record the noise.  Make sure the circuit is in a metal encloser to record the noise and not 50Hz or 60Hz or100Hz or 120Hz etc.

http://en.wikipedia.org/wiki/List_of_free_software_for_audio

[DDD]

The higher the reverse collector current, the higher the noise level.
It's a rule of thumb.

[R.G.]

Depends. You want numbers with that?

If you want a subjective rating of "OK" versus "too noisy", rig up a circuit where it amplifies a similar amount to what you'll use it for, complete with socket for inserting devices, and listen. This has the problem that biasing is a variable and this will change the subjective noise level. But it's probably good enough for casual use. It might work OK to sort devices into gain bins first so you don't always need to be setting bias.

If you want numbers, you have to concoct something that will set the device up in a circuit and servo the collector current to a consistent level for biasiing, then measure the noise generated. The way the measurement of noise is usually done is to put a somewhat quiet amplifer circuit after the Device Under Test (DUT) and then measure the noise of the amplifier circuit by itself so it can be subtracted from the noise it amplifies. If the gain is high and the amplifier quiet, then the noise it amplifies will dominate the output and its self noise can be neglected. The amplified noise can then be measured any number of ways, including sound card o'scopes, meters, etc.

[darron]

Plug the circuit with no input signal into your PC sound card and record the noise.  Make sure the circuit is in a metal encloser to record the noise and not 50Hz or 60Hz or100Hz or 120Hz etc.

http://en.wikipedia.org/wiki/List_of_free_software_for_audio

would you have the input grounded for example or feed in a signal? i don't like the idea of a computer because any time i try to do ADC the noise level THAT creates will be greater than the noise the device created. but maybe i could see/measure it zoomed right in on the scope. haven't tried.

The higher the reverse collector current, the higher the noise level.
It's a rule of thumb.

that's the sort of general guide i'm looking for (:

do you mean test current I_C?

all of the magnatech one's that i'm reading have the EXACT same readouts except only HFE and leakage vary.. here is something typical:

Current gain H_FE=126

Test current I_C+2.50mA

Base-Emitter V_BE=0.25V

Test current I_B=5.20mA

Leakage current I_C=0.06mA

Depends. You want numbers with that?

If you want a subjective rating of "OK" versus "too noisy", rig up a circuit where it amplifies a similar amount to what you'll use it for, complete with socket for inserting devices, and listen. This has the problem that biasing is a variable and this will change the subjective noise level. But it's probably good enough for casual use. It might work OK to sort devices into gain bins first so you don't always need to be setting bias.

If you want numbers, you have to concoct something that will set the device up in a circuit and servo the collector current to a consistent level for biasiing, then measure the noise generated. The way the measurement of noise is usually done is to put a somewhat quiet amplifer circuit after the Device Under Test (DUT) and then measure the noise of the amplifier circuit by itself so it can be subtracted from the noise it amplifies. If the gain is high and the amplifier quiet, then the noise it amplifies will dominate the output and its self noise can be neglected. The amplified noise can then be measured any number of ways, including sound card o'scopes, meters, etc.

"Too noisy" vs "okay" is exactly what i'm after. here's what i made:



what i'm doing is very similar to what you have suggested. this is turned up all the way (maximum gain and then a voltage divider afterwards) which plugs into a solid state 100watt fender amp in the clean and dirty channel.

pre-sorting into bins for gain might be a good idea. won't leakage also effect bias though? so i'd have to presort into 'actual' gain and hope they work similarly.

DDD's explanation might be what i was thinking of as a 'general rule' before final test.


Edit: I should note that it's not boxed. hum isn't a problem, and it's only hiss and white noise i'm trying to gauge. it's a thin shielded line going in and out with a plug on each end.

one switch is for freq. range. the other is a true bypass which is more important for use with the dirty channel to hear how much gain is really coming out of it.

[darron]

so DDD, what do you say? is that how I would make the measurement?