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17th April 2007
| #1 |
| Gear interested Joined: Oct 2005
Posts: 8
Thread Starter |  Polarity Inversion help
A classmate and I are [attempting to] make an 8 channel summing mixer. We've got most of the circuitry down, including the solos (although they technically shouldn't work the way we've got them wired...but they do...), but can't seem to figure out how to invert polarity. This is so we can actually use the balanced TRS in, rather than just grounding R. The plan is to invert R and sum with T to cancel any noise. We've tried using an op-amp as a voltage-follower with inverting input, but feeding it a sine wave outputs a square. First of all, is this just a terrible idea to invert and sum? Secondly, What the crap is going on? Thirdly, any suggestions/schematics? |
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17th April 2007
| #2 |
| Lives for gear Joined: Sep 2004 Location: UK
Posts: 4,822
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Hi Read more textbooks. Walt Jung wrote a couple of excellent 'cookbooks' many years ago. You will then see the errors in your ways! Matt S |
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17th April 2007
| #3 |
| Gear Guru  Joined: Jul 2004 Location: Orygun
Posts: 10,230
| Sounds like you have too much gain.... You can set-up a difference amplifier using a single op-amp. The invert / non-invert and sum uses a lot more parts, but you can do a better job of BW limiting and nulling if you use all of that.... Real men use descrete transistors for this function (or just buy a line-receiver chip...). -tINY |
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18th April 2007
| #4 |
| Gear interested Joined: Oct 2005
Posts: 8
Thread Starter |
There should not be any gain at all. It was just to be used as a voltage follower with the polarity reversed: we even followed the schematic from the data sheet on the opamp! R1 and R2 are equal, therefore unity gain. This may sound really stupid, but I don't understand where you're going with the brick wall limiting + null  .Could you point us in the general direction to some discrete transistors? We appreciate the help! |
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18th April 2007
| #5 |
| Lives for gear Joined: Oct 2004 Location: Burbank, CA, USA
Posts: 1,036
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Possibly the OP left out a feedback resistor and wound up with a comparator instead of an amplifier?
__________________ http://studioelectronics.biz Service & Restoration of UREI dbx Eventide Marshall AMS Tube Gear and more |
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18th April 2007
| #6 | |
| Gear addict  Joined: Sep 2006 Location: SF Bay Area
Posts: 432
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Regards, John | |
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18th April 2007
| #7 |
| Gear Guru Joined: Jul 2004 Location: Orygun
Posts: 10,230
| ...or maybe a 10v p-p signal and +/- 3v rails..... -tINY |
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