Geophone Amplifier - Page 1
Forum section: electronics / beginners

author: sonicj
author_rank: Frequent Contributor
timestamp: December 31, 2010, 02:14:01 am
content: I'm attempting to construct a geophone amplifier and, as i normally do, neglected to pay attention to the resistor values when placing my parts order as i thought i had a pretty comprehensive collection of standard values on hand... out of the values listed, i think i had only one or two in my bins. this is the first application example i've seen with such uncommon values... is it that this is a highly tuned example circuit and requires this sort of precision to function? or are the values given a best of whats commercially available and can be replaced with closest values on hand? is there a downside to replacing some of the resistors with precision pots?also, in the formula at the bottom, what do the two || represent?cheers!-sj

author: EEVblog
author_rank: Unknown
timestamp: December 31, 2010, 02:59:48 am
content: The values don't look to be critical.The || is a standard symbol meaning parallel (the standard parallel resistance formula)Dave.

author: Zero999
author_rank: Super Contributor
timestamp: December 31, 2010, 08:58:46 am
content: The resistor values are standard E96 values, not as common as E24 but available from most suppliers.You can make any E96 value to within 0.17% accuracy from any two E24 values connected in series or parallel. Attached is a table I use which makes it easier to work out. As you can see there are some duplicates to give yuou a choice between more convenient values and higher accuracy.

author: sonicj
author_rank: Frequent Contributor
timestamp: January 01, 2011, 01:59:06 am
content: Quote from: EEVblog on December 31, 2010, 02:59:48 amthanx dave!anyone happen to know this one? [size=14]?Quote from: Hero999 on December 31, 2010, 08:58:46 amthanx hero! the chart is super helpful! i stumbled onto a few combinations on my own, but the light bulb never clicked that i could make all of the values... good stuff!unfortunately, i have some bad news to report on the geophone endeavor. there will be no earthquake detection at my house tonight as i let the smoke out of one of the resistors. i don't think it had anything to do with the values, more due to mild dyslexia + working with double sided protoboard.i might be better off just starting over with everything on one side as debugging this double sided board gets confusing with the quickness!-sj

author: Bored@Work
author_rank: Super Contributor
timestamp: January 01, 2011, 10:29:03 am
content: Quote from: sonicj on January 01, 2011, 01:59:06 amCongruent toorisomorphic to. Don't you kids learn that in school any more?But ok, datasheet authors also don't learn that in school any more, because?is frequently mistaken for?. The later also meansisomorphic to, but alsoapproximately equal. And it is usuallyapproximately equalwhat they mean when wrongly using the?.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 01, 2011, 12:46:10 pm
content: Linear tech always seem to use stupid resistor values in their appnotes - must come from their 'precision' mindset and no dougt a a huge parts cabinet in their dev lab. I wonder if maybe they bought an E96 kit years ago and nobody ever replenished the E12 values so they base their appnotes around what's left in the cabinet..!It is annoying as it can be hard to tell which are critical. Nothing in this circuit looks like it couldn't be rounded to he nearest E12.

author: Zero999
author_rank: Super Contributor
timestamp: January 01, 2011, 02:43:40 pm
content: Change the components values if you like as long as you're aware of the effect it will have on the current, gain, input impedance etc.At a glance, for minimum impact on the circuit the only values which can be reall changed to E24 values are R6 and R7 which can be replaced with 20k and 100k respectively. You may be able to replace other values without changing it too much but you'll need to do some calculations.

author: FreeThinker
author_rank: Frequent Contributor
timestamp: January 01, 2011, 03:10:37 pm
content: Quote from: BoredAtWork on January 01, 2011, 10:29:03 amIseem to remember we used~as approx = to so there may be different standards around the world

author: Bored@Work
author_rank: Super Contributor
timestamp: January 01, 2011, 09:25:11 pm
content: Quote from: FreeThinker on January 01, 2011, 03:10:37 pm?isasymptotic equal to. There is an international ISO standard about the math symbols, I forgot the number. There is also, however, much sloppiness in their usage. But latest at uni professors should get their act together and use and teach the right ones.Why datasheet authors, especially in the old day American ones, and in the current days Chinese ones, are so sloppy is maybe also an issue with lack of accuracy in their education.