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Old 6th November 2006, 12:43 AM   #1
BWRX is offline BWRX  United States
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Default Instrumentation Gainclone

Hi all. I don't normally visit this forum since the class d forum is just so much fun but I came over here because I'm thinking about building a gainclone to compare my own little creations to. Anyway, I was reading through the mauro penasa LM3886 thread (holy jeez it's a long thread) and checking out all the great info in there when mauro's ref schematic jumped out at me. It looked like an intrumentation amplifier configuration but it isn't. My next thought was "would it be possible and has anybody tried a configuration like that?"

The instrumentation configuration would have some nice advantages like high common mode rejection, very high input impedance, a differential input, and a good low impedance buffer to drive the 3886 inputs. My question to the chip amp experts is whether or not a chip like the 3886 used as the differential amp in an instrumentation configuration would work or not? Check out the attached schematic to see what I'm talking about. R5 and R4 would have to be chosen so that the 3886 is stable, then you can fiddle with the gain of the buffers to get the overall gain you'd want.
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Old 6th November 2006, 01:24 AM   #2
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I've been searching this whole time and finally found a thread (started earlier this year actually) with almost exactly the same schematic - right down to the use of the 2134 which just happens to be the package I use for dual opamps in a DIP8 package. But there's nothing saying if it was ever implemented. Anyone know if it was?

http://www.diyaudio.com/forums/showt...986#post815986
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Old 9th November 2006, 01:15 AM   #3
BWRX is offline BWRX  United States
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Nothing eh? Either the answer to my inquiry is very obvious and I've missed something in my searches or no one has tried this! Either way, I'll be placing a monster digikey order soon so I might as well add a few LM3886 chips to the heap so I can start experimenting.
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Old 9th November 2006, 02:10 AM   #4
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Default Instrumentation amp

I used a 3875 as a stand alone differential amp, takes a little resistor fiddleling to get frequency and stability correct.
I think you should have good luck, I thought about useing a precision op amp in a feed forward error correction mode also.
Good Luck....
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Old 9th November 2006, 02:21 AM   #5
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I think the reason the mauro penasa LM3886 circuit works well is because the op-amp is inside the feedback loop.

it looks like what you have is a balanced driver, but its right in front of the amp instead of in the preamp.

still it looks like it might be worth trying.
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Old 17th November 2006, 04:53 AM   #6
BWRX is offline BWRX  United States
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I've changed the previous schematic around so that not only the instrumentation configuration could be built, but also many other basic configurations as well. The resulting circuit is fairly simple and lends itself to easy routing with an LM3875 chip, which is the one I have selected to use for these boards. Introducing the highly configurable gainclone...

Would anyone be interested in a surface mount version of this board? The layout is all but finished and it uses 0805 and 1206 size parts and a DIP8 dual opamp package. Very simple, straightforward, and gives you many options as to what to build. If you don't want to do the intstrumentation configuration you can leave R2 open and use the opamp as a buffer. You can configure the opamp to be inverting with unity gain or higher or non-inverting and a gain of 2 or higher. Depending on which input you route the signal to you can have a non-inverting buffer/non-inverting 3875, inverting buffer/non-inverting 3875, or inverting buffer/inverting 3875. You can also choose to bypass the opamp completely and do an non-buffered inverting or non-inverting topology.

Lastly, is there anything anyone would like to see added? As I said, the layout is all but finished and the only thing I'm thinking of adding is an on board regulator for the opamp supply rails.

How necessary is the ~0.1ohm resistor in series with the output of the 3875? I was hoping that the series resistance of speaker cables alone would be enough... If not I'm sure it would be ok to connect a leaded resistor in series from the board output to a binding post (if necessary).
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Old 17th November 2006, 06:45 AM   #7
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Quote:
Originally posted by BWRX
How necessary is the ~0.1ohm resistor in series with the output of the 3875? I was hoping that the series resistance of speaker cables alone would be enough... If not I'm sure it would be ok to connect a leaded resistor in series from the board output to a binding post (if necessary).
It seems that you haven't read the datasheet and the application note AN-1192 carefully. I strongly advise you to do that.

My opinion is that it is wise to have something between the amp and the speaker cable. Don't forget that 5-10 meters of cable is NOT a pure resistance! You have also 50-200 pF per meter also!
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Old 17th November 2006, 08:34 AM   #8
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In my experience, the LM1875/3875 seem to be happy without the output resistor, but the LM3886/4780 and OPA 548/9 need it to run well.
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Old 17th November 2006, 11:07 AM   #9
BWRX is offline BWRX  United States
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Quote:
Originally posted by peranders
It seems that you haven't read the datasheet and the application note AN-1192 carefully. I strongly advise you to do that.

My opinion is that it is wise to have something between the amp and the speaker cable. Don't forget that 5-10 meters of cable is NOT a pure resistance! You have also 50-200 pF per meter also!
Hi Peranders. Thank you for your concern. I've read both very carefully and more than once now, honest!

So they're really that sensitive to what kind of cables are used? If I were to use a single strand of cat5 cable for the conductors and have them spaced an inch or two apart over the length of the cable I could have higher resistance and lower capacitance per meter than what you would normally expect. Do you think a series output resistor would still be needed with cables like that?

If I did build this and the 3875 didn't like whatever amount of capacitance there is on the output I could just as easily put a 5W resistor (and maybe an air coil connected parallel) directly between the output of the board and the binding post (assuming I'd ever get this is in a case!). I don't really see a problem with wiring it like that, do you?

Quote:
Originally posted by pinkmouse
In my experience, the LM1875/3875 seem to be happy without the output resistor, but the LM3886/4780 and OPA 548/9 need it to run well.
Hi Al. Do you remember what kind of speaker cable and load you have used with your 3875 based amps? Thanks for sharing your experiences with the different chips.
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Old 17th November 2006, 06:30 PM   #10
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I now have built 2 LM3886 Guitar amps useing 2 seperate Guitar preamp designs, One Totally discreet useing Fets and NPN Transistors and one useing a OPA2134 (or any dual fet opamp)....
The LM3886 Power amp stage is the same with both amps and is basicly what is in the datasheet....

The discreet one sounds best to me for guitar but it doesn"t have much of a Clean sound , it has a pretty awesome distorted Crunchy sound which sounds great for Guitar and Bass....

If you want me to post the schematics for either preamp design let me know, They are both very simple and both use the same overdrive circuit which is a 4 stage cascadeing NPN Transistor over drive circuit....


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