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Chip Amps Amplifiers based on integrated circuits

Chinese Mono LM1875 Board
Chinese Mono LM1875 Board
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Old 8th June 2017, 07:02 PM   #1
Twil is offline Twil  United States
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Default Chinese Mono LM1875 Board

Hi, I am new to this forum and I would like to apologize in advance if I cross any lines while posting this. I got two of these mono LM1875 kits on ebay with the intention of making a stereo amplifier. I have attached images of the assembled kit along with the schematic and board layout. This kit is available from multiple sources for around $3 or less and I thought it would probably be popular on the forums. After doing multiple searches using the search feature and google I have found no information about it other than a couple youtube videos made by JohnAudioTech. He seemed to think the board is laid out well and uses properly rated components. He did mention possibly replacing the input capacitor though. What I want to know is if there is any more information on these kits and what components people would recommend replacing if any. I plan to use a known genuine LM1875 chip rather than the one from the kit and powering the two boards with a power supply board from chipamp and an antek AS-1218 - 100VA 18V TRANSFORMER. My apologies if this information is readily available and I'm making a duplicate thread.
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File Type: jpg s-l500 (1).jpg (32.8 KB, 1045 views)
File Type: gif s-l500.gif (66.4 KB, 1031 views)
File Type: jpg s-l500.jpg (20.5 KB, 1095 views)
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Old 9th June 2017, 03:03 PM   #2
AndrewT is offline AndrewT  Scotland
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the sch is a good start.
It has both MF and HF supply rail decoupling.
It has an Output Zobel.
It has both Low Pass and High Pass filtering on the input.
It has AC coupling both at the input and at the NFB.
It has a cap leakage resistor on the input.
It has separated the Audio Ground where the input Return signal references, from the Power Ground where the output Return signal references. This allows multi-channel assemblies.

All good signs that the Designer has at least read the datasheet, or carefully copied a properly executed sch.
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Last edited by AndrewT; 9th June 2017 at 03:06 PM.
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Old 9th June 2017, 06:08 PM   #3
Twil is offline Twil  United States
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Do you think this power supply design would suit the amplifier? C1 and C2 will be 1500 uF capacitors and I'm considering not implementing the snubber network caps and resistors. I don't really know what I'm doing though and was only considering omitting the snubbers because Peter Daniel said he preferred the LM3875 without them.
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File Type: jpg Power Supply Schematic.JPG (32.1 KB, 964 views)
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Old 10th June 2017, 10:10 AM   #4
AndrewT is offline AndrewT  Scotland
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The snubbers as shown are in the wrong place, they should be across the transformer secondaries. You could also fit some at the amplifier end of the PSU cable.

C3 and C4 are also in the wrong place. They are HF supply rail decoupling and should be on the amplifier PCB right next to the chipamp's power pins (see the datasheet).


The smoothing capacitance shown will only suit mid and treble duty. This is what Peter Daniel optimised his chipamp build for and used very high efficiency speakers (maybe around 95dB, or 96dB/W)
Most Builders and Listeners find they require lots more smoothing for better bass response.
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Old 12th June 2017, 04:29 PM   #5
Twil is offline Twil  United States
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The power supply schematic I provided was from the directions for Brian GT's (chipamp dot com) lm1875 kit. I'm not sure how they are in the wrong place but since I don't know how to implement them properly I'll just omit them for now. Seeing as there are already HF supply rail decoupling capacitors on the amplifier boards (C5 and C6?) I'll also omit C3 and C4 on the power supply board. You seem to suggest replacing the smoothing capacitors (C1 and C2?) with higher capacitance pieces. Assuming I would be using this amp with speakers in the low 90's db/w what size capacitors would you recommend? Thank you for all of the information you have provided by the way, it is greatly appreciated.
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Old 12th June 2017, 06:00 PM   #6
AndrewT is offline AndrewT  Scotland
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The MF and HF decoupling are shown on the post1 sch.
The number of capacitors on the sch (8) match those shown on the layout (8). But the sch has no identifiers.

I use +-20mF per 8ohms channel. But nearly everyone else thinks that much less will do the job.
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Last edited by AndrewT; 12th June 2017 at 06:03 PM.
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Old 13th June 2017, 07:15 PM   #7
Twil is offline Twil  United States
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The resistors which came with the kit are light blue and have 1% tolerances according to their color codes so I believe they're metal film type and I think I'll use them. I have Vishay 0.1F polypropylene capacitors that I want to use for the HF supply rail decoupling (C5 & C6 on amplifier board). I want to replace the 10F input cap with a film capacitor of the same rating, do you think that's a good decision? I also want to replace the electrolytic 220F MF supply rail caps and the 47F cap with better quality ones like Panasonic or Nichicon. Should I use the same size for these? And finally I'm considering replacing the small caps that came with the kit as well (C4 220nf & C2 330pf) but I can't tell if they are ceramic or tantalum and don't know if replacing them would be worth it.
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Old 14th June 2017, 09:00 AM   #8
AndrewT is offline AndrewT  Scotland
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0.1uF polypropylene may not be suitable for supply rail decoupling.
Not enough ESR.
Use X7R ceramic for HF decoupling.

Difficult to tell from the partially obscured top view, but I think the yellow/orange caps are probably ceramic. They are very cheap and likely to be what they have used.

Can you label the sch with the capacitors that are actually fitted to the PCB.
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Old 14th June 2017, 10:45 AM   #9
mackeb is offline mackeb
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The schematic is from Silicon Chip magazine in Australia. It was their school project amp meant for total beginners. The accompanying article went in to detail about etching boards and soldering.

Cheeky buggers supplying that drawing in their kits.
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Old 14th June 2017, 11:16 AM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by mackeb View Post
The schematic is from Silicon Chip magazine in Australia. It was their school project amp meant for total beginners. The accompanying article went in to detail about etching boards and soldering.

Cheeky buggers supplying that drawing in their kits.
That explains why they adopted what looks like a good sch.

I wonder what job they made of laying out the PCB?
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