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Old 18th February 2008, 11:17 AM   #1
Paswa is offline Paswa  India
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Default LM 1875 questions

I've some questions on LM1875 amps.

1>>>> http://sound.westhost.com/project72.htm , here the link to sound westhost project using the chip. , niw see there has only two 1Amp fuse in both rails, now for my purpose, I'll use it as a 15 watt Centre channel Amp. Accoarding to the datasheet it'll gimme the 15/18W RMS at +/- 20V. So how much current will draw the circuit? can I use the same 1Amp fuse.

And can I use a 150VA trafo for that?

Again, I never made a PCB of the 1875 amp. I made it in a veroboard. now I heard that there is a issue with the amp if some capacitor or resistor not placed near the IC. is ya true? my veroboard layout given below.
Click the image to open in full size.
Click the image to open in full size.

now, go below the website http://sound.westhost.com/project72.htm, you'll see a power supply design. I wonder there is no any regulator, only two 10,ooo mfd caps, will it be sufficient for 1875?
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Old 18th February 2008, 03:48 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
150VA can supply sufficient for between 75W and 150W of output power.

This could power two or even three 1875s if you wish.
You do not need a regulator.

Just ensure the PSU voltage suits the amp and it's speaker.
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Old 19th February 2008, 10:54 AM   #3
Paswa is offline Paswa  India
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many thanks friend andrewTT,

I wish to run TDA2050 and LM1875 amps from 150VA trafo, but 150VA is toooo large sized to fit my amp cabinet, can I use 100VA trafo. I'll use it for my home theater PC. I dont want clipping. again TDA2050 is for subwoofer and LM1875 is for center
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Old 19th February 2008, 04:33 PM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
can I use a 150VA trafo for that?
why ask this question if it is too big to fit?
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Old 20th February 2008, 12:25 AM   #5
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100va (if ei core) is about 3 times larger than I've ever used for LM1875, so one would think its sufficient if it doesn't get warm.
They vary greatly in quality, so you never know if 100va is a peak rating or if it can do that all day long.

When they're really big, its popular to bolt them to the side of the case instead of the bottom of the case.

Looking at the board layout, one would suggest to relocate the NFB (gain loop) resistors to directly onto the chip pins (underneath the board).

It seems to be short a few capacitors. Recommended minimum power caps are on LM1875.pdf from national semiconductor's website. . . but, here's my guess: 220uF parallel with 0.1uF--one set each rail.

Check out the K50 kit at www.electronics123.com and click on the reference documents.
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Old 20th February 2008, 05:27 AM   #6
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Quote:
Originally posted by danielwritesbac
[B]Looking at the board layout, one would suggest to relocate the NFB (gain loop) resistors to directly onto the chip pins (underneath the board). /B]
No, don't do that. Mounting components off the PCB (except for the possible exception of bypass caps) is unnecessary, reduces reliability and looks bad. Unless daniel can provide you with a reason why you should do that, leave the resistor where it is.

This obsession the "gainclone" crowd have with short feedback paths is not based in reality. If you do the sums, you will find that the effects of a little bit of PCB track on your feedback are well below the THD+N of the amp chip (and I mean *well* below). I suspect all this came from the original gaincard, where the NFB loop was made short because it was built extremely cheaply, without using good PCB design techniques.
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Old 20th February 2008, 11:02 AM   #7
Paswa is offline Paswa  India
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thanks all

so you all think that the gainclone on veroboard is all right??
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Old 20th February 2008, 11:12 AM   #8
Paswa is offline Paswa  India
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Quote:
why ask this question if it is too big to fit?


At first i was confused about 1oo or 150 VA, so I asked ya. now i'm going for a single 100 VA for using with LM1875 and TDA2050 . I think there would be no any problem accoarding to andewT, all right. I'll test it after a week. If i success i'll inform, or if it burn , i'll write it heere
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Old 20th February 2008, 01:17 PM   #9
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Originally posted by Paswa
thanks all

so you all think that the gainclone on veroboard is all right??
You probably need better decoupling caps (ceramic or film) closer to the chip itself, or risk oscillation.
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Old 20th February 2008, 04:00 PM   #10
Paswa is offline Paswa  India
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Quote:
decoupling caps (ceramic or film) closer to the chip itself
ok, i understood all the things, but can u tell me why the decoupling caps need to be placed near the ic?
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