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Old 4th November 2010, 03:43 AM   #11
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Project 51 is what i was thinking of using. but i need to modify it slightly to provide 6db additional gain boost on top of the 6db added just by balancing.
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Old 6th November 2010, 04:50 AM   #12
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Ok so if i have done this correctly. this modified version of project 51 should give me a gain of 4x

1 volt in should give me 4 volts out. 2volts on the pos side and 2 volts on the neg side. correct??


Does anyone see a reason why these resistor values wouldn't work...or shouldn't be used??


Zc
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Old 6th November 2010, 02:25 PM   #13
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Quote:
Originally Posted by Zero Cool View Post
Ok so if i have done this correctly. this modified version of project 51 should give me a gain of 4x

1 volt in should give me 4 volts out. 2volts on the pos side and 2 volts on the neg side. correct??


Does anyone see a reason why these resistor values wouldn't work...or shouldn't be used??


Zc
You have unity gain on the '+' phase but gain of 2 on the '-' phase.

If the real issue is noise, the previously described diff input with unbalanced drive will achieve the goal with fewer components and less chance overload. Unless you've boosted the power supply voltages, you have less than 12 Vp-p and with 6dB gain your headroom is rapidly disappearing.

G
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Old 6th November 2010, 03:39 PM   #14
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Oh i see what i did wrong. R1 should have connected to the neg side of the opamp and ground and the input directly to the + side. so that then should be a gain if 2 on the positive side as well.

I have a max input of 1.6vrms so i am hoping to get 6.4vrms out at max. with the avg listening level will be with an input around 40-100mv. I need some gain to get the SN up a little bit to keep the audio clean as it travels to the back of the car where the amp is. with balanced lines and a little bit of gain i should be ok. I can't imagine i would ever be at full volume.

with +/-15 rails. I think 3.2vrms from each side should be easily doable? even with a factor of 2 for headroom.

I can adjust the input sensitivity of the amplifier from 200mv at full power to 5v at full power
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Old 6th November 2010, 06:57 PM   #15
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You should even have a couple dB extra. Your gains are now correct. If you have Obsessive Compulsive Disorder you'll match the resistors so that the balance is exact. Or not.

G
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Old 6th November 2010, 09:18 PM   #16
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Thanks for the feedback. I thought about adding a 20 turn trimmer someplace to null the 2 sections but I will build it first and see how off it is. I built up a similar test circuit on the breadboard the other day using scrap bench parts and one side was 2.0v out and the other was 2.12v so even with junk parts it was pretty close. using some 1% resistors should be good enough i think.
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Old 6th November 2010, 11:37 PM   #17
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the gains don't need to be equal, roughly equal would suffice. Which means you can use 22k, which is easier to obtain, instead of 20k. The output resistors should be equal, but the usual 1% metalfilms will do. There is no need for tighter matching than that.
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Old 6th November 2010, 11:43 PM   #18
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How important is the value of the output resistors? What if i used 100 ohm or 10 ohm resistors?
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Old 6th November 2010, 11:55 PM   #19
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I wouldn't hesitate to use 100R resistors. The resistors also provide some basic protection and RF damping to the opamps. Using 10R may be more risky inside such a harsh enviroment like car audio. There is no reason using such a low value. Yet that it is...a small risk...nothing more, nothing less. Why not stick to the 220R?
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