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Old 6th February 2004, 03:35 PM   #1
amo is offline amo  United States
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Default Balanced Pre AND Bridged Amp

I have read over several threads that deal with this subject, but I am still utterly confused, probably because I have not been able to find a situation that is similar to mine. I need to build a bridged amp using two lm3875 chips, so that I can get higher power output, thanks to the "double voltage swing". However I would love to drive it with a balanced line stage. All the circuits that I have seen so far either deal with driving a singe chip with a balanced line stage, or driving a bridged amp with a single phase.... What if you have both, 2 chips and 2 phases? Is it as simple as having two seperate clones, each wired to one of the phases? Do they need something common? I find it hard to believe that no one has done this. I appreciate any input!
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Old 6th February 2004, 03:48 PM   #2
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Having two clones of the same topology driven by a balanced pre would be fine. However, for this you would be way better off with LM3886 since the current capabilites are better.
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Old 6th February 2004, 03:53 PM   #3
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So I guess the shielded "TF" 3875 chip is out


Can you explain why I would care about better current capability, considering I want to drive an "easy" 8 ohm load consisting of just a driver, with an active filter before the amp? Thanks!
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Old 6th February 2004, 03:54 PM   #4
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". What if you have both, 2 chips and 2 phases? Is it as simple as having two seperate clones, each wired to one of the phases?"
-yes and no, there should be an input resistance that is known and most importantly matched. this means the volume pot if you use a volume pot on the amp. the input impedance on both clones should be identical -- input capacitances and resistances. whereever possible, you should measure the components and match them. the idea behing balenced is in equal noise being picked up. this happens with equal input impedance.
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Old 6th February 2004, 04:07 PM   #5
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I understand the issue with matching both sides as closesly as possible, and I plan to go to great length to do just that. However there will be no pot on the amp - this will be taken care of by the pre. With this in mind, can I use a pair of ultra-matched and stable resistors to set input resistence? Do these go to ground? Thanks!
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Old 6th February 2004, 04:09 PM   #6
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Since you bridge the amp the load as seen by each amp is effectively 4 Ohms. Not to good on the LM3875.
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Old 6th February 2004, 08:33 PM   #7
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Quote:
Originally posted by amo
I understand the issue with matching both sides as closesly as possible, and I plan to go to great length to do just that. However there will be no pot on the amp - this will be taken care of by the pre. With this in mind, can I use a pair of ultra-matched and stable resistors to set input resistence? Do these go to ground? Thanks!

Can someone please confirm the bit about resistors and matching input impedence between the two clones? Thanks!!
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Old 7th February 2004, 10:40 AM   #8
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I think the point about the resistor matching is just try to match the resistors in general, the input resistor, and the two feedback resistors. Meaning go with at least 1% and if available 0.1%. Digikey sells Panasonic 0.1% SMD resistors I believe. There more expensive, I think they were ~0.16 each when I Bought some...just buy like 10ea. I say 10 becuase they are small and you will either a) loose them or b) break the solder pads on a couple. Make sure you get the right size too...even the biggest ones they sell, like 3mmx2mm is pretty damn small to mess with

You just want as-close-as-possible values between each amp, so that one isn't more powerfull and "bullying" the other.

You may also be able to find the blue Vishays that are axial style that are 0.1%.
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Old 8th February 2004, 09:15 AM   #9
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You can use a circuit like the on i've attached. also check out this link: http://headwize2.powerpill.org/proje...=opamp_prj.htm
You can also use an INA137 by Burr Brown. It's basicly the same circuit but in a DIP. this is the datasheet: http://www-s.ti.com/sc/ds/ina137.pdf

You can use either of these as your inputstage whith any existing GC designs.

Hope this helps.
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Old 9th February 2004, 08:42 AM   #10
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not sure i'd call it balenced.

and you can use a suitable multimeter to match values.

a balenced circuit has 2 goals -- 1 is that common-mode noise is canceled, and this is where matching comes in. if you have a little more amplitude of noise on one phase, then it will not cancel as completely. #2 is picking up equal noise. this occurs when the output impances of the output of the pre are equal on each phase, the wire is equal on both condutors, and the input impedances are equal. the differance amp has a fixed input impedance on one phase, and a variable impedance on the other.

since you're brdiging, it makes more sense just to go with 2 identical gainclones. any design, so long as they are identical!
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