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Yet another LM1875 amp - review request
Yet another LM1875 amp - review request
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Old 22nd November 2019, 09:16 PM   #21
Sadface is offline Sadface  New Zealand
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I having been having a play with 16mm PSU filter caps and I am struggling to fit them on nicely. I can maybe cram them onto the board with a small change in dimensions but at the expense of the 5mm gab in between the filter caps that allows screw driver access to the chip. This board is somewhat designed for a specific implementation that I am planning.

Perhaps I should make a 2nd more universal board with the fuse rails removed and larger filter caps allowed. Or simply a larger board. I'd be happy to release the Gerber files if people were interested and wanted to order their own.
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Old 22nd November 2019, 11:49 PM   #22
rabbitz is offline rabbitz  Australia
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For the 22uF cap use a good quality BP like the Nichicon ES (green suckers). I've never seen a film cap used in this location. Dump the fuses and if required, add them on the power supply PCB.

Digi-Key is a good supplier of stuff and free shipping over AU$60.
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Old 23rd November 2019, 02:17 AM   #23
Sadface is offline Sadface  New Zealand
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Quote:
Originally Posted by rabbitz View Post
For the 22uF cap use a good quality BP like the Nichicon ES (green suckers). I've never seen a film cap used in this location. Dump the fuses and if required, add them on the power supply PCB.

Digi-Key is a good supplier of stuff and free shipping over AU$60.
I haven't tried digikey before, I typically use Element 14 and RS components. Strangely, they can reliably deliver quicker from Syndey to Auckland than I can reliable deliver across Auckland.

I've read on multiple LM1875 threads that the 22uF feedback cap is the most important part on the whole board.
As far as my limited understanding goes; a film cap is superior to an electrolytic. So it makes sense to me to use a large film cap since the extra cost is only a few $ extra on such a cheap fitout.

Something like $25 to populate a board with genuine and name brand parts makes these hilariously good bang for buck.
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Old 23rd November 2019, 09:08 AM   #24
fdenys is online now fdenys  Netherlands
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Another possibility is to simply shortcut the feedback cap.
Its sole purpose is to block dc amplification (if a dc voltage would be present at the input it would be amplified, resulting in a huge dc offset at the output, which might kill your speaker)
If you're using a quality foil type for the input cap there is no chance of a dc voltage at the input of the LM1875, so it could be ommitted.
When googling for gainclone i noticed many people do that actually without problems.
Of course you should check and recheck the offset voltage in a test setup before actually connecting your speakers.

If you wish to avoid any capacitor in your signal path another option would be to use a professional mini audio transformer 10k:10k.
These little transformers linear from eg 20Hz-80kHz or even 200 kHz.
The advantage is also, that you isolate you amplifier from the input, so it will effectively block "hum" groundloops.
Widely used in studios, but they are somewhat expensive (good quality about 25$ for a pair)
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Old 23rd November 2019, 01:49 PM   #25
rabbitz is offline rabbitz  Australia
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I get deliveries from Digikey (US to AU) in 3 to 4 days. Their prices are generally much better than those in Oz but more important is the range on offer.
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