Parallel Circuit Question - Page 1
Forum section: electronics / projects

author: jbushnel
author_rank: Contributor
timestamp: December 31, 2014, 02:12:47 pm
content: Hello,I have posted questions on this circuit before but while I was building it, I came across some questions. I just built the three filters but left out the 20K ohm potentiometer since I don't want to vary anything. Since these filters are in parallel, my input resistors are off (and some of the other components of the filters). Is this a problem? I guessing it's not since the schematic doesn't really address the issue...I was thinking maybe when I was done and turned everything on, the circuit wouldn't see it as parallel? I was just wondering what I should do!http://makezineblog.files.wordpress.com/2010/10/ledcolororgan-schem_r2.gif

author: tggzzz
author_rank: Super Contributor
timestamp: December 31, 2014, 02:40:30 pm
content: Quote from: jbushnel on December 31, 2014, 02:12:47 pmWhat do you mean by "off"? Off the board? Open circuit? Not connected? Smelly? Different? If appropriate, numbers please.QuoteWhat do you mean by "the circuit" and "it as parallel"?QuoteTurn it on and see, then determine if you have a problems that needs solving. If so, define what you want/expect, plus what you actually observe.If you have actually done what I hope you have done, then provided the IC1a (what type?) has sufficient drive, then all will probably work.

author: jbushnel
author_rank: Contributor
timestamp: December 31, 2014, 02:51:30 pm
content: By off I meant the resistance of each resistor are not what was intended.And I haven't completely finished it yet so I won't be able to power it up. I was just checking to make sure all of the values were right as I was building.I guess I will just have to wait until it's all done!

author: tggzzz
author_rank: Super Contributor
timestamp: December 31, 2014, 03:30:17 pm
content: Quote from: jbushnel on December 31, 2014, 02:51:30 pmIn that case it certainly won't work as the circuit shown in the circuit diagram.

author: jbushnel
author_rank: Contributor
timestamp: December 31, 2014, 03:49:26 pm
content: Quote from: tggzzz on December 31, 2014, 03:30:17 pmHow can it be a problem for me and not a problem for the schematic if they are exactly the same? Like I said, I think somehow it has to work because jameco would not sell the kit if placing the filters in parallel would throw off center frequency, bandwidth, gain etc...

author: tggzzz
author_rank: Super Contributor
timestamp: December 31, 2014, 04:11:18 pm
content: Quote from: jbushnel on December 31, 2014, 03:49:26 pmOh dear.If you change "each resistors" value so that they are not the "intended value", then the functionwillchange. Whether that's a problem for you is a different issue, and one which we cannot answer because you haven't given sufficient information.

author: jbushnel
author_rank: Contributor
timestamp: December 31, 2014, 05:16:42 pm
content: Let me just phrase it a different way. I calculated all the values (R1, R2, R3, and C) ofeachnarrow bandpass filter so I knew what Q factor and center frequency I was getting (I am not using the component values they gave).. Will putting these filters in parallel change the filter characteristics from what I initially intended?

author: tggzzz
author_rank: Super Contributor
timestamp: December 31, 2014, 06:43:42 pm
content: Quote from: jbushnel on December 31, 2014, 05:16:42 pmI refer you to the last sentence of my first reply.

author: lgbeno
author_rank: Frequent Contributor
timestamp: January 01, 2015, 04:46:44 pm
content: Quote from: jbushnel on December 31, 2014, 05:16:42 pmThink of the circuit in terms of poles and zeros. Having these circuits in parallel means that their input impedance is the sum of all parallel paths so that pole formed by the cap and down stream filters may shift due to the load that it sees. The filters certainly do not have infinite input impedance and 0 output impedance so to that extent, they will interact.Someone could do the math, or you could try something like LTspice to simulate. Bench works too but way more fiddly than a sim. Especially if you don't have proper equipment to measure the filter performance.To directly answer your question, yes it will change BUT the hard work is in finding out how much and if you can compensate.

author: DanielS
author_rank: Frequent Contributor
timestamp: January 02, 2015, 02:37:55 am
content: Lazy but expensive solution: isolate each function from the input and output with buffer opamps. Now you can change whatever you want in any branch without worrying about effects on the others.