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Small desktop amp based on LM-1875 chip

Audiolab 6000A Amplifier

bhrtsingh

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Hi
I needed a small compact amp for desktop mostly. I required it to be compact enough to carried around in the house to be used with the laptop or mobile.
first i thought of buying one ready made class-D, but since I already had booked PeeCeeBee amp I thought why not try on smaller level first before going big.
out goes the plan to buy amp, now will Try to build a class-AB amp.

The amp will be build with LM-1875 chip quite popular for its Sound Quality for its price.
a few pics of what already have been done.

built the first amp board
20180928_170813.jpg

got a Aluminium plate to be the face plate of the amp.
20181005_004229.jpg
20181005_011000.jpg
20181005_011250.jpg

up next build the power supply and get the transformer to check the amp.

Thanks
Bharat Singh
 

bhrtsingh

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This is the heat sink I will be using. I plan to keep it outside on the top. It would give it a vintage look to the amplifier.
would getting it painted have n adverse effect on heat dissipation?
20181005_193557.jpg

Mean while the chassis it also getting shaped up. It would be painted black or dark grey.
20181005_011451.jpg

20181005_011404.jpg

any suggestion and your views are welcome.

thanks
Bharat Singh
 

sadik

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Enjoy he Fun of DIY...

Please don't paint the heat sink, it will downgrade its efficiency + spoil the complete look of Amp. I recommend you to get it anodized. For the Enclosure get it power coated if the material is MS Sheet, and if the material of enclosure is aluminium then get it anodized.

Sadik
 

bhrtsingh

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Enjoy he Fun of DIY...

Please don't paint the heat sink, it will downgrade its efficiency + spoil the complete look of Amp. I recommend you to get it anodized. For the Enclosure get it power coated if the material is MS Sheet, and if the material of enclosure is aluminium then get it anodized.

Sadik
Thanks
Will keep your suggestions in mind.
The chassis is ms will get it power coated.
Will keep the heat sink as it is for now.
 

bhrtsingh

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got the transformer and also made a PSU. done initial testing. sound quality is just amazing and is pretty loud for the wattage.
but facing issue of click sound when i turn on/off the amp. wierd the amp stays on for second or so even when I turn off the power, then makes a click sound. Any suggestion?

20181003_010026.jpg
 

jls001

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The power stored in the power supply capacitor bank will discharge when you switch off mains power and drive audio output for a second or so. This is normal.

For speaker protection, you could buy a ready made speaker protection module. This will help in a few very useful ways: it will have power startup delay so that there will be no thump on the speakers during power on; it will also cut off the power to speakers if it detects the presence of a dangerous level of DC signal in the audio signal. Some modules even have thermal sensor that can help cut off power if there is overheating.

Power off sequence: switch off your 1875 first, followed by preamp, then source.

Heat sink must be anodized, not painted, as already pointed out by Sadik.
 

bhrtsingh

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The power stored in the power supply capacitor bank will discharge when you switch off mains power and drive audio output for a second or so. This is normal.

For speaker protection, you could buy a ready made speaker protection module. This will help in a few very useful ways: it will have power startup delay so that there will be no thump on the speakers during power on; it will also cut off the power to speakers if it detects the presence of a dangerous level of DC signal in the audio signal. Some modules even have thermal sensor that can help cut off power if there is overheating.

Power off sequence: switch off your 1875 first, followed by preamp, then source.

Heat sink must be anodized, not painted, as already pointed out by Sadik.
Thanks alot for your suggestions will definitely look into it .
 

bhrtsingh

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few more updates with chassis and the face plate.
drill the face plate and counter sunk the volume knob.
20181005_192250-jpg.30943


got the body welded up

20181005_192425.jpg

brushed aluminium finish to the faceplate.

20181005_233447.jpg 20181005_233756.jpg

Its a Ugly Frog for now. but getting a feeling will turn out to be a prince.
 

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Aniket

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Good work Bharat,

LM1875 comes with inbuilt short circuit and over current protection. yo might not need additional protection circuit. It has no ON-OFF pop,click sound. Here, in your picture LM1875 looks fake. It was made by national instruments more than 10 years ago. Now it is made by Texas Instruments only. Buy yourself a legitimate LM1875 chip from RS components or Element14, replace it in your kit and you are good to go.
generally these fake ones also work, they are poor clones of TDA2030.

Regards,
Aniket
 

bhrtsingh

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Good work Bharat,

LM1875 comes with inbuilt short circuit and over current protection. yo might not need additional protection circuit. It has no ON-OFF pop,click sound. Here, in your picture LM1875 looks fake. It was made by national instruments more than 10 years ago. Now it is made by Texas Instruments only. Buy yourself a legitimate LM1875 chip from RS components or Element14, replace it in your kit and you are good to go.
generally these fake ones also work, they are poor clones of TDA2030.

Regards,
Aniket
hi
Thanks Aniket for Advice. In fact I believe the kit came along with a fake chip, never used them I replaced with a locally purchased LM1875 chip.
Here have look what do you think of it?

right one is the one that came along the kit left one is the one I bought.
20181012_133807.jpg
20181012_133751.jpg 20181012_134323.jpg
 

bhrtsingh

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Also I read that fake do not have short circuit protection and accidentally I shorted the output of the amp couple of time with my test speaker have leads too close to each other and the connectors not having any insulation the amp muted as soon as the output leads shorted and resumed playback after correcting the leads.
Can someone tell how else can I check if these are the genuine chips or not?

Thanks
Bharat Singh
 

linuxguru

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hi
Thanks Aniket for Advice. In fact I believe the kit came along with a fake chip, never used them I replaced with a locally purchased LM1875 chip.
Here have look what do you think of it?

right one is the one that came along the kit left one is the one I bought.
View attachment 31028
View attachment 31029 View attachment 31030

The one on the left (presumably JM29AL batch code) is definitely fake, based on the shape of the lead bend. The 68DN is a harder call,
since it matches the original lead bend shape as well as the metal tab - I'll call it a high-quality fake.

BTW, most of the fake Pentawatt chips have excellent sonics and work fine at lower rails like +/- 18V and lower. The original works
fine at +/- 25V.

Related recommendations for amplifier circuits using chipamps (mainly PentaWatt chips):

1) MiniRef (last version 1.04) is my own nested-feedback design with excellent performance up to 20 + 20W, using either LM1875 or TDA2050.
2) Lincor MF-1 is a Russian current-sensed feedback design with excellent tube-like sonics up to about 15 + 15W, and will work even with fake TDA20xx chips, while being relatively bullet-proof with the original TDA2050 (which is no longer manufactured, alas).
3) If you're willing to go with a higher BoM and LM3886TF chipamp, the Mauro Penasa MyRef Rev C is unbeatable on definition and sonics up to 40W per channel. It does require some carefully matched components.
 
Last edited:

bhrtsingh

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Can some one post image or original lm1875.
I am using 25.2vdc rail and it works fine no problem at all. here are some closeup pictures of the Chips.
here is close up of the chip last pic didnt did justice.
20181012_210022.jpg
20181012_204436.jpg
also i read somewhere that original chip has the mark on side of the heatsink on the chip as pointed in the pic below
20181012_201541.jpg
what do you think?
 

bhrtsingh

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The amp is almost finished and sound really amazing for its low power. since I am doubt full of the chip may change it eventually.
here are some more pics took while building the amp.

20181007_140812.jpg 20181007_142855.jpg 20181007_150012.jpg 20181009_152709.jpg

do share you opinion about.
Thanks for Looking
 

linuxguru

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While I am still not sure about the authenticity of the LM1875 chip I found these images on net. the genuine chips do look alike the one I have.
here have a look. A, B & C are genuine and D&E are the Fake once.
View attachment 31140
https://www.diyaudio.rs/topic/7159-lm1875-kopije-ili-ne/

Any thoughts.

B&C - probably genuine.
D&E - definitely fake.
A - difficult to call: the legend/font is wrong, but the leads look OK. Could be old stock.

Your JM29AL may be genuine - leads and font look OK. The previous view wasn't clear and I was mistaken on the lead shape.
 

bhrtsingh

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Amp is finally finished, first of all I would Like to thank every one who helped me with their valuable suggestion and guided me though this whole thing.
The amp looks good to me and sounds sounds amazing. This was my first DIY attempt sure was hard and confusing at times, but learnt a lot of new things during this whole Process. As already mention it was my trial before going into a bigger project. I am glad that I under took this project, it has defiantly boosted my confidence. It sounds so good that at times I feel that do I even need a bigger Amp but since i have already payed for the PeeCeeBee boards and collected most of parts for it already, Also have heard lot of good things about the Amp, I will built that Amp too.

here are a few pics of the amp I built, would like to hear your opinion on it.

20181014_201850.jpg 20181021_180811.jpg 20181021_180853.jpg 20181021_180905.jpg
Here's one besides my hand to compare the size.
20181021_181133.jpg

Thanks
Bharat Singh
 
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