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Noise/Filter - Transistor selection for Infra Red
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author Noise/Filter - Transistor selection for Infra Red
eljay
I've just built my Noise/Filter module and am pleased to say it's now all working and sounds good.

I did have a few problems finding a suitable transistor to generate the noise though, I tried 9 different BC547s (as recommended), each one seemed to generate an OK white and pink noise but none of them enabled me to set up the Infra Red. The White and Pink noise signals all seemed to be too high (+/- 12V) and the Level trimmer didn't bring the signal down to the recommended (+/- 6V), the Infra Red signal consequently seemed to be stuck permanently high and adjusting the ADJ trimmer had no perceivable effect. I tried a few other types of transistors and got the same results.

I finally tried an old C3371, this works. The White and Pink noise signal levels are now set at about +/- 6V and the Infra Red is now working as expected and has been adjusted with the ADJ trimmer to give a relatively even spread either side of 0V.

As Tony recommended the BC547s as the most likely to produce the best results I am wondering why they really weren't suitable on my board. I have used high tolerance components elsewhere, resistors are all 1% metal film except the 14k resistors which are 0.4W precision resistors because they are what I could get. Any thoughts?
Synthbuilder
The problem with using transistors for noise is that we're using them as they are not intended. Which means that no device will be specified for their noise performance when used in reverse bias. Most circuits do not heavily reverse bias their transistors because of the noise and possible long term side effects.

The noise we get from a transistor when used like this is essentially an unwanted side effect of the manufacturing process. This means that some devices will end up being better than others. I find I get the best sounding noise from transistors that make the least noise. Clearly those BC547s are noisy little beggars and the noise quality will probably be pants.

Unfortunately, this all means it's the luck of the draw. So just chose the ones you like the best. I had a whole bunch of BC549s that sounded perfect.

Tony
Synthbuilder
eljay wrote:
II have used high tolerance components elsewhere, resistors are all 1% metal film except the 14k resistors which are 0.4W precision resistors because they are what I could get.


Technically 1% resistors are low tolerance. High tolerance would be 5% and 10%. And if you are as old as me you may remember that these used to be the standard ones you could buy. 1% - luxury.

Nowadays we can get 0.1% for next to nothing and even 0.01% aren't that badly priced.

If you do need some odd value - and I don't tend to use odd values in my projects normally - remember you can put resistors in series to get different values. 14K can be made with the more common 13K and 1K resistors joined together one after the other. Just make a little triangle shaped arrangement with the joined ends sticking upwards.

Tony
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