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Old 14th November 2003, 07:47 PM   #1
matt_uk is offline matt_uk  United Kingdom
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Default Whats a GOOD LM3875 chip amp circuit?

Does anyone have any tried and tested circuits for this amp? I like the look of GregCC's 'Ultimate gain-clone' because of it's simplicity, maybe add some DC protection on the output.

In this type of amplifier application does the pot have to be wired the opposite way around to usual, so when the volume is down there is maximum resistance between the line in and chip in?

Also, is it really worth going for super components (as there aren't many), high audiograde pot, caps? How critical is the PSU design? How much VA rating for the transformer (230V AC)?

Looking at the 'Ultimate Gain-clone' design the max gain is 22 (i think!)? so with an input voltage of 1.5-2V, so should I be looking at around 40V supply voltage?

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Old 14th November 2003, 07:50 PM   #2
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Matt...this topic is explained very well on the forum...take a look

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Old 14th November 2003, 07:57 PM   #3
matt_uk is offline matt_uk  United Kingdom
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forgetting my +/- supply. Can a 2 x 40V transformer be wired into +/- 40V? I'm sure it can but don't know much about PSU design.

Does anyone have a PSU circuit? As I know nothing about bridge rectifiers or anything related to volatages greater than 3.3!

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Old 14th November 2003, 08:11 PM   #4
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Some specific info can be found here http://www.diyaudio.com/forums/showt...921#post261921

Otherwise do search in a Chip Amps forum
“Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC
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Old 14th November 2003, 08:12 PM   #5
GregGC is offline GregGC  Canada
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One of the many good places to start:

Check this on general Power supplies, but please pay attention to the 1000uF in the Gainclone case. It's important.
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Old 14th November 2003, 09:23 PM   #6
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i guess this all depends on how much you want to put into it. i mean in the bare form the chip still works well. some people like the more expensive components and all, but when it comes down to it, they only matter if you can tell a difference. if your putting the speakers in an area with a high noise floor, well...

as for PSU design, remember that your transformer output will be about 50% higher then the ratings, as you are going to get a DC voltage close to the peaks and not close to the RMS value. this means that a +-24V transformer will be closer to +-35V on the output. there are discussions on power supply theory, and there seems to be some indication that a small resistance between the rectifier diodes and the filter caps is beneficail as it forms an RC network and enhances the lowpass filter effect.
if only it could be used for good, not evil...
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