Humorous mistake in Douglas Self Active Crossovers book - Page 2 - diyAudio
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Old 9th April 2012, 12:18 AM   #11
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The statement would also be false if they were placed in parallel.

Last edited by Brian Oshman; 9th April 2012 at 12:26 AM.
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Old 9th April 2012, 09:31 AM   #12
tvrgeek is offline tvrgeek  United States
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So, everyone gets it. Now, there have been quite a few books written on crossovers. Is there anything here that is advancing what we need to know that say, Jung, Lancaster, and Linkwitz have not covered?
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Old 9th April 2012, 12:59 PM   #13
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Quote:
Originally Posted by dchisholm View Post
That is true if the value errors are truly uncorrelated from part to part, and uniformly distributed across the tolerance window. In some situations that's probably a good assumption.

But if all 4 parts come from the same production batch . . .
  • Produced at the same time under the same ambient temperature, humidity, atmospheric pressure, etc
  • From the same batch of raw materials, with the same level of impurities
  • On the same machine, with the same degree of wear, misalignment, calibration errors, etc
. . . then I would expect the 4 parts are likely to have correlated tolerance errors, and add all in the same direction. The 4 parts may, in fact, be much more closely matched in value (whether that value is high or low, positive or negative, in the tolerance window) than you would expect from selecting 4 parts "at random" from a group with variations uniformly distributed across the tolerance window.


Dale
Your post is accurate for the most part for parts that are "as fired" or "as processed". Assuming a gaussian distribution for the end value is a reasonable one.

Of course, the manufacturer may post process the devices. Binning at test is one such thing.

If for example, a manufacturer has .1%, .5%, 1%, 2%, 5%, and 10% product, they would cull the closest to value at test, and of course charge more for them.

Given those tolerance ranges I mentioned, if for example you purchase a 5% resistor, you will never get one which is within 2% of the mark. A 10% will never be better than 5%.. And there will not be a gaussian distribution of values in a batch purchased, but rather, each tolerance bin will have a section of the distribution of the bulk product. A batch of 5% resistors at 100 ohms for example, would have values between 95 ohms to 98 ohms, and 102 ohms to 105 ohms. There would be a +/- 2% hole in the distribution.

If the resistors are laser trimed to value, there are two possibilities..

First, they may actively trim to value with a test/zap/test regimen...in that case, they will stop trimming once they have reached the desired tolerance. If they are making a 100 ohm 1% resistor, they will stop when the value of the resistor has risen to just above 99 ohms.

Second, they may do a measure/calculate/zap to theoretical nominal. If they measure a resistor at 90 ohms, they calculate the amount of trim historically required to get exactly to 100 ohms. In that case, they may get very close to nominal, and the distribution will be gaussian and fairly tight.

cheers, jn
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Old 11th April 2012, 03:49 AM   #14
BrianL is offline BrianL  United States
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jn,

Not necessarily true that "you purchase a 5% resistor, you will never get one which is within 2% of the mark..."

If there is not much customer demand for tighter-tolerance units, and they are producing more than needed of tight-tolerance parts, extra tight-tolerance parts will be 'downgraded'. So, the distribution of values in loose-tolerance units will depend on how many tight-tolerance parts are needed and the tightness of the production distribution.

You get what you pay for. If you really want tight-tolerance parts and you don't have time, equipment, interest, etc., to buy and sort lots of loose-tolerance parts, then you should spend more money/ea. on tight-tolerance units if you can get them.
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Old 11th April 2012, 10:36 AM   #15
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Quote:
Given those tolerance ranges I mentioned, if for example you purchase a 5% resistor, you will never get one which is within 2% of the mark. A 10% will never be better than 5%.. And there will not be a gaussian distribution of values in a batch purchased, but rather, each tolerance bin will have a section of the distribution of the bulk product. A batch of 5% resistors at 100 ohms for example, would have values between 95 ohms to 98 ohms, and 102 ohms to 105 ohms. There would be a +/- 2% hole in the distribution.
This is not strictly in accordance with my experience, I often found that quite a lot of a batch are within 1% of the marked value or that many of them can be matched at 0.3%. The dispersion may help to get values not existing in the 1% standard range (for example 98.5 Ohm) . This way, I could build a 6 steps - 6 circuits attenuator (40 to 20 dB) with a 0.2 dB difference between circuits.
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Old 11th April 2012, 10:53 AM   #16
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Quote:
Originally Posted by jneutron View Post
If for example, a manufacturer has .1%, .5%, 1%, 2%, 5%, and 10% product, they would cull the closest to value at test, and of course charge more for them.
This broad a range of tolerances in one resistor type seems to me unlikely in practice - because the 10% parts generally have much worse tempcos than the closer tolerance parts. It might well apply to a smaller subsection (say 0.1 - 0.5% - 1%) but generally its too expensive (time consuming) to test every resistor for tempco so that needs to be guaranteed by design. Closer tolerance resistors use lower tempco materials, in general.

@forr - I think you missed that jn's statement is conditional on them selling a range of tolerances in the same type of resistor. Its not that common in practice (in my experience).
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