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Noises caused by resistor (and transistor) on high impedance Amp

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Willen

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Heared that resistors and transistors cause noises in high impedance amplifiers. Here, I can buy few values of resistors in very cheap (comparatively to USA etc) like I can get 1000 pieces at just $3 (for regular customer as a whole sale). I can get 1000 BC547B transistors at just $2. ( But almost no more parts than this available, except few. :) )

Due to its cheap rate, resistors' color bands and transistors case finishing looks little rough (comparatively to USA and etc). These cheap parts from India. So I am thinking that these also have less quality material inside it for sale in low price. Due to the low quality, do any resistor and transistor cause noises? Any engineer studied to choose high quality resistors and transistor? How they make 'low noise' parts like BC549? (sorry for long story)
 
I think a BC547, BC548 and BC549 are made as one transistor then they are tested. Ones that work at high power supply voltages are marked 547 and ones with low noise are marked 549. Most of the transistors are "general purpose" and are marked BC548, but some might work at high power supply voltages and others might have low noise.

I think Philips invented the BC107, BC108 and BC109 in metal cases then they made the plastic cased "54" ones later. I have only used real Philips BC transistors that work very well and I have not purchased cheap copies.
The cheap resistors and transistors available to you might be OK (I doubt it) or might have very poor quality and poor reliability.
 
The article cited above indeed looks like a good read. As has been said, *all* resistors produce noise which is proportional to their resistance & temperature - the Johnson/Nyquist noise referred to in #3. They will also produce "excess" noise of a nature that depends on their construction - carbon composition R's are notoriously bad in this respect.

I guess the question is really, are your cheap resistors going to be what they say they are? For instance, could they be sold as metal film when they're actually carbon film? To be honest I don't know how you'd tell. If you break them open certainly it's be easy to identify certain types, but telling the others apart just by looking at them might be tricky - I've never tried.

Same story for the transistors really, although it should be easier to measure the noise of them. As for what makes them "low noise", as audioguru says... a bit of luck, and some magic I don't pretend to understand...
 
Same story for the transistors really, although it should be easier to measure the noise of them. As for what makes them "low noise", as audioguru says... a bit of luck, and some magic I don't pretend to understand...
Manufacturers call it "yield", not luck. They make a production run then they test and sort every transistor.
Sometimes most transistors of a production run have low noise (BC549), or survive a high voltage (BC547), or have a high current gain (BC54xC).
 
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