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resistors in parallel

HiFi, LoFi, Tubes, Transistors, ICs, Commercial, DIY

Postby and_m » Thu Apr 19, 2012 12:00 am

On the latest preamplifier 2012,

R19 / R20 are two resistors in parallel to improve accuracy.

each resistor is 2K ohm,

how would two resistors, likely supplied next to each other on the same reel, be of better accuracy than one resistor of 1 K ohm?

If they were statistically distant, like from two separate manufacturers, then they are likely to be more accurate than the one resistor. As they are unlikely to be statistical distant, then the summation rule does not apply.

same goes for R28 / R29 etc.

all paralleling up these resistors is defiantly doing is adding capacitance, and lowering the inductance.

or do I have this totaly wrong, and this has been discussed to death, in which case sorry.

And as for capacitors in parallel,
yes lowers the ESR, but would it not increase the leackage, and all the extra traccking needed does what ?

if tracking was not a problem, we could do all designs on strip boards no problem.
and_m
 
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Postby rob g » Mon Apr 23, 2012 12:00 am

As I recall, resistor production lines measure every resistor and spit them out into the appropriate tolerance bin. Because measuring systems drift, they use sampling theory to measure the risk of more than a stated quantity falling outside the preset tolerance. Provided that sampling is truly random, their confidence increases as the square root of sample size. I would think that you would need to accurately measure about 70 samples before you were reasonably sure that any two taken from a large population were any closer to the centre of the Bell curve. Easier to simply to select the closest one to nominal by measrement,l An interesting question, I await more comment on this,
rob g
 
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Postby ricks » Wed Jun 06, 2012 12:00 am

Paralleling resistors to me does not make a whole lot of sense, unless you want higher power. If the two resisitors are both on the high or low side of the nominal, you can get a little bit better accuracy. For this to work in your favour, it is best to have one on the high side & one on the low side of nominal. If you need more accuarcy then measure or use higher tolerance resistors. If you need a value that is not available in the E96 series, then it makes sense to parallel two different vlaues to get what you require.
Capacitors are somewhat the same reasoning but as you probably alrady know, high tolerance capacitors are less availble than high tolerance resistors. Capacitors usually come in E12 series of denominations. There are no E24/E96 series readilly available. Paralleling caps makes more sense in this regard.
Paralleling resistors and the additional capacitance is insignificant at audio frequencies. Not an issue until you are dealing with a RF.
Paralleling Caps is a method to achieve a higher quality of capacitor if lower ESR or higher ripple current is your requirement. Mostly used in power supply designs. At audio freuqencies I do not believe that lead inductance is an issue.
ricks
 
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