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Old 7th July 2003, 07:13 PM   #1
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Default Problem with Gainclone.

I have built Rob Elliots LM1875 project on his circuit board. I have however subsistuted at the last minuite a OPA544 chip. Guess what, it does't work. I had it powered up through a scope, one channel dragged down the input and clipped off nearly all of the positive side of the signal. the other channel just didnt do anything much apart from click at low voltages, the input signal was completely absent from the output.
I have looked at the datasheet of the OPA544 and the LM1875 and see that on the sujested circuit for the LM1875 the feedback resistor is 20K and on the OPA544 the feedback resistor is 10K, also the +input on the OPA544 is 5K on the input, and the +input on the LM1875 is 22K to ground.
I assume that the feedback resistor is the main source of my problem, and maybe the line reistance is the other problem.

Any clues from those in the know??

Shoog
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Old 7th July 2003, 09:27 PM   #2
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i would follow the example circuit on the datasheet, and chack you are using the correct pins.
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Old 7th July 2003, 10:13 PM   #3
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The circuit on the datasheet does't have a cap to ground off the feedback loop. I dont quite understand what this cap is for, as some opamp circuits have it and some dont.

i dont want to have to alter the board too much as it will inevitably lift the copper.

shoog
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Old 8th July 2003, 07:15 AM   #4
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could you post the circuit from the datasheet, and the one you are using.
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Old 8th July 2003, 07:18 PM   #5
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the circuit can be found at the following link.

http://sound.westhost.com/project72.htm

The datasheet I don't know how to link to at the moment.

However I have got a little bit off a progress report which should shed a bit of light.
I had missed a board link on the channel which was clipping the positive side. With the board link in place everything works fine. The scope shows a little bit of crossover distortion. There is negligable DC offset on the outputs. Shows that the circuit works.

However the other channel has still got no significant output. There is some very high frequency hash which the scope couldnt resolve and there is a 1.5v DC offset on the outputs.
I am going to do a very careful check of all of the components and tracks to make sure everything is as it should be.

Any ideas about what I should be looking for to produce that much of an offset.


Shoog
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Old 8th July 2003, 10:01 PM   #6
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I just checked the soldering and found the resistor to the +input of the opamp was not good. Resoldered it and wonder of wonders it works. Quiet and sounds very promising.
Just got it hooked up to my throw away test speakers at the moment so too early to say for certain.

Will report back when I have both channels in use and can assess the quality of the OPA544.

Thanks.

Shoog
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Old 8th July 2003, 10:35 PM   #7
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good to hear you got it working

the moral of the storry is: always check everything, if you dont fine any problems, have a break, check again.
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