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Chip Amps Amplifiers based on integrated circuits

LM1875 Stereo Amplifier with integrated PSU
LM1875 Stereo Amplifier with integrated PSU
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Old 3rd October 2017, 03:43 PM   #1
prasi is offline prasi  India
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LM1875 Stereo Amplifier with integrated PSU
Default LM1875 Stereo Amplifier with integrated PSU

Hi all,
I always wanted to create something for a beginner which is simple to build and very convenient and yet cheap.
what better chip than LM1875, 'the beginner's Hi-Fi' or ' beginners gainclone'.

Criteria was just a single cheap Chinese boardhouse pcb, left and right channels on the same pcb + PSU + all though hole components.

so just pcb + heatsink +center tapped transformer + some hardware and a cheap case is all that would be necessary to build your own 'my first hi-fi amplifier'

or it can also be a learning exercise for young ones who are interested.

One thing to note here , THT + all in one (stereo +PSU) PLUS small PCB dont go hand in hand. so one cant expect loads and loads of capacitance in power supply section.

I havent tested it, but layout follows the schematic perfectly. comments welcome.

Cheers to the spirit of DIY.

regards
Prasi
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File Type: png lm1875stereo-R3.png (77.4 KB, 293 views)
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Old 3rd October 2017, 04:10 PM   #2
OlegSh is offline OlegSh
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LM1875 Stereo Amplifier with integrated PSU
I would recommend to remove the fuses from DC rails. If one fuse goes off it will take the woofers of your speakers with it. If you really need fuses then put them on the AC side of power input. Also instead of "spider" ground you can make a big copper pour instead covering all of the PSU filter caps pads. This will significantly reduce ground impedance of the PSU.

Edit: Even better would be to make GND plane over the entire top half of the PCB.

Last edited by OlegSh; 3rd October 2017 at 04:17 PM.
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Old 3rd October 2017, 05:06 PM   #3
prasi is offline prasi  India
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LM1875 Stereo Amplifier with integrated PSU
there are so many conflicting views on "to star" or "to ground plane"...I am not getting involved in another. Some people say making a plane with THT components is creating multitude of loops. + the datasheet mentions it is better to use different return paths for different components.
I have built a bigger SS class AB with such "star" configuration, it works splendidly well. So I am not going to make a "ground plane".

regarding fuse, leave it for users, who can just jumper it, but some prefer fuses on PSU. more advanced users can implement a properly designed speaker protection.
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Old 3rd October 2017, 05:10 PM   #4
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by prasi View Post
there are so many conflicting views on "to star" or "to ground plane"
I use a combination of both.
Ground planes connected to ground around sensitive audio signals.
Star grounding for the likes of zobel networks and smoothing capacitors.
The biggest culprit of hum is the charging impulses into smoothing capacitors. It can modulate the ground line badly.

I recently did a chip amp pcb.
I learned a bit from the first one.
Keep tracks to feedback resistor as short as possible as the tracks act like inductors and phase shift the feedback to the point where the chip amp oscillates.
Keep decoupling as close to chip amp pins as possible.
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Old 3rd October 2017, 05:22 PM   #5
Chris888 is offline Chris888  United Kingdom
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Does C5 actually do anything?
Should C6 and C7 form a tighter circuit around the IC supply pins for best HF performance?

Don't be afraid of SMT, you just need two irons to remove passives.

I agree about the fuses. You could consider smaller fuses, perhaps even resetting PTC types? Or are these too slow?
The general grounding, I'm not sure a full plane will be easy to do, you still need to think about capacitor current causing issues in the signal paths.
But a star for each channel might be an option?
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Old 3rd October 2017, 05:29 PM   #6
Chris888 is offline Chris888  United Kingdom
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Quote:
Originally Posted by prasi View Post
....

regarding fuse, leave it for users, who can just jumper it, but some prefer fuses on PSU. more advanced users can implement a properly designed speaker protection.
I don't dislike fuses on the PCB, it's whether they should be in the AC or DC nets is the question.
If you moved one to each AC net, a blown fuse would result in the amps being fed with half wave rectified supplies, reduced power, probably a bit of hum, but no big DC on the speaker?
Whereas one blown fuse as drawn means the chip amp will probably go to a rail?
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Old 3rd October 2017, 05:40 PM   #7
prasi is offline prasi  India
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C5 DC block cap, as per TI app note, its 22uF, but generally its kept at 100u to 220u, sometimes even larger.

C6/C7 any closer, may require SMD, which I am not willing, as the whole purpose as stated in post #1, this is meant as a first amp for a beginner, a very easy one at that. fuses are ordinary PCB mount fuses (5 x 20mm) and holders are amply available everywhere. may be because the PCB is just 55mm wide, so fuse holders appear very big?


TO Nigelwright,
Yep I have done exactly that, i/p grounds connected with a plane. rest on its own to the star.
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Old 3rd October 2017, 05:49 PM   #8
Mark Whitney is offline Mark Whitney  Netherlands
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prasi, sorry to have to say this but your design will not get near the datasheet specifications for this chip. Here are some of the large loops for HF and LF output current that will seriously impact THD+N.
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Old 3rd October 2017, 05:54 PM   #9
sevenup2278 is offline sevenup2278  Netherlands
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I have made something similar before
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Old 3rd October 2017, 06:01 PM   #10
prasi is offline prasi  India
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what happens when we use an external power supply... those loops must be enormous...
PCB design is a compromise of some sort...
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