Chinese Mono LM1875 Board

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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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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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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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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.
 
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.
 
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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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 0.1µF polypropylene capacitors that I want to use for the HF supply rail decoupling (C5 & C6 on amplifier board). I want to replace the 10µF 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 220µF MF supply rail caps and the 47µF 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.
 
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.
 
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?
 
Interesting to learn where this came from. Here is a schematic I labeled with what I believe are the correct component labels. I also found a picture of the board in plan view and saw that there are versions of this board where the silkscreen labels actually name the value of component to mount.
 

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Here's a few suggestions.
If you want the low bass to be very extended then you need to increase C3 from 47uF to 150uF or 220uF.
This requires the input capacitor to be reduced slightly.
C1 <= C3*R4/sqrt(2)/R2, i.e. C1 <=150uF*1000/1.4/22000 <= 4.8uF use a 4u7F bi-polar, or MKT. Or 6u8F for the 220uF option.
Alternatively if you want less low bass, you can keep C3=47uF but you must reduce C1 a lot. The same formula applies and C1<=47uF*1000/1.4/22000 <=1u5F, use an MKT or MKP

If the amp slews to high DC on the output, then C3 needs some protection. Add two inverse parallel signal diodes across the cap pins on the bottom side. Use 1n4148, or 1n914.

If the source equipment becomes faulty and sends mains down the interconnect, then you need protection across R6 to take the mains Fault Current to your power ground and thence to your protected chassis, to eventually blow the mains fuse in the faulty source equipment. Add a pair of 1A power diodes in inverse parallel across R6. Use 1n4002, or higher.

C2+R5 is your interference filter. It is set a bit too high, although some Members prefer this very high setting because they claim they can hear the effect of any lower attenuation frequency. I recommend you experiment with the C2 value starting at 330pF and going up via 680pF, 1nF to 1n5F until you hear the treble rolling off very slightly and then go back one step. Use MKP

Between the amplifier output and the speaker terminals on the enclosure, add an in line air cored inductor. Use 1.2mm or 1.4mm or 1.6mm diameter enameled copper wire. Wind about 10Turns around an AA battery. Add a 4r7 resistor across this to damp ringing.

The locations of C5, 6, 7 & 8 are good.
Look up the 1875 datasheet and see if R7 is the recommended value.
 
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Thanks for the suggestion phase but those do look a little too beefy for this application and I just received some caps I would like to use instead.

AndrewT I have 2.2uf Panasonic film capacitors (Part #ECQ-E1225KF) I was hoping to use for C1. With this value in your equation the closest capacitor I could get for C3 would be a 100uf. Do you think a 100uf Nichicon ES series cap (bipolar and suited for signal paths) would be a well suited choice for C3 then? And what voltage rating would be required for C3?

Also why would the amp slew to high DC on the output and how likely is this? I want to have safety features included with this amp but I'm new at this and don't want to screw it up by trying to do something more complicated that I don't fully understand. This is why I am also weary of your other two addition suggestions. Not because I don't have confidence in your knowledge but rather because I don't have confidence in my own abilities. So in your opinion could I get away with not adding any of these components?

I planned on including a Velleman K4700U speaker protection kit to protect the speakers from DC current. Would that make two of those diodes you mentioned unnecessary?

According to the datasheet R7 is the recommended value from what I can tell. I have attached the datasheet for your reference.
 

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Thanks for the suggestion phase but those do look a little too beefy for this application and I just received some caps I would like to use instead.

AndrewT I have 2.2uf Panasonic film capacitors (Part #ECQ-E1225KF) I was hoping to use for C1. With this value in your equation the closest capacitor I could get for C3 would be a 100uf. Do you think a 100uf Nichicon ES series cap (bipolar and suited for signal paths) would be a well suited choice for C3 then? And what voltage rating would be required for C3?
OK,
you can use your 2u2F MKT.
Turn the equation around to find C3 instead.
C3>=C1 * sqrt(2) * R2 / R4
giving C3 >= 2.2uF * 1.4 * 22000 / 1000 >= 68.4uF use 100uF
Also why would the amp slew to high DC on the output and how likely is this? I want to have safety features included with this amp but I'm new at this and don't want to screw it up by trying to do something more complicated that I don't fully understand. This is why I am also weary of your other two addition suggestions. Not because I don't have confidence in your knowledge but rather because I don't have confidence in my own abilities. So in your opinion could I get away with not adding any of these components?
The most usual cause of a high DC on the output is a fault, either that DC coupled input letting an error to pass through (not this one it is AC coupled) or the chipamp becoming damaged.
The two signal diodes cost a few pence for insurance to protect the capacitor and also in a discrete amplifier to protect the input device.
Some mains equipment have a fuse but no protected chassis (classII - referred to as Double Insulated). If there were some catastophic mains failure inside that equipment that makes the isolated components live and that equipment is connected to your amplifier via a metal interconnect,
then you are relying on your protected chassis to blow the fuse in the faulty source. That fault current will blow up the resistor and the amplifier's input remains live from the fault. the diodes need to be robust enough to blow the faulty sources mains fuse.
I planned on including a Velleman K4700U speaker protection kit to protect the speakers from DC current. Would that make two of those diodes you mentioned unnecessary?

According to the datasheet R7 is the recommended value from what I can tell. I have attached the datasheet for your reference.
OK to both.
 
I built LM1875 circuit from the manufacturer DS schematic using 22uF in the feedback lag. This kit already uses more - 47uF. I tried it on my friend's Harbeth Super HL5 + loudspeaker, which is high end speaker, and there was no deficit in the low octaves even with 22uF in the feedback lag. LM1875 just played fine.

Of course, you can buy more expensive parts and use them, but this is not some High End circuit in the first place and I think that it would play just fine even with parts supplied with kit.
 
I know that from personal experience. Several months ago I bought 10pcs of LM1875 from Ali Express. When the chips arrived I immediately noticed that metal tab is different from the original chip that I bought earlier. I also received some fake TL084 recently and decided to avoid e-bay and Ali as source of components in the future.

But the chip in the kit is probably some lower grade similar 5 pin TO220 power IC. One can use it for some less important duties and another project and buy original chip from legitimate and certified source for use with the Chinese pcb which is of good quality.

Unfortunately I never had a chance to see LM1875 made in Taiwan by UTC - Unisonic. It is probably good as original. I think that they use licensed metalization. If Chinese kits use these chips than there is no use tobuy "originals". Is there anybody who tested and compared "original" and UTC chips? Is there anybody who at least have photo of Unisonic chip, both avers and revers side?
 
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