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PostPosted: 13 Dec 2011, 04:50 
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Joined: 17 Aug 2011, 06:05
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Location: Australia
Folks

I've not seen anything like this before, but, I am rather intrigued by this. Note the fellow who posted this said the LM3886TF is labelled incorrectly and should be LM1875

Image

There is more information here in this thread over on diyaudio.com (where this came from)
http://www.diyaudio.com/forums/attachme ... lm1875.jpg

Interested in peoples thoughts on this, thinking about etching up a board or two, I rather like the LM1875 if you havent guessed already :)

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PostPosted: 13 Dec 2011, 09:36 
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Joined: 08 May 2009, 08:20
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Location: Duesseldorf, Germany
Hey Grega,

i guess it should work pretty good. What i don't like is the high gain of 33 per IC. The gain always depends on the power supply or internal limits.
Whether you need a FAT power supply or screw down the gain to 20. It is very(!) important to use very close matched resistors for the
voltage feedback and the output resistors (for symmetrizing the output currents). I don't have the input impedance in mind yet, maybe an additional buffer would be a good tweak... Must see..

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Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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PostPosted: 13 Dec 2011, 14:44 
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yeah - (100/3.32) +1 is pretty close to the ragged edge
was going to pull them back to defaults actually - ie: 20k and 1k respectively = 21. i guess so long as we are > 10 we defer any sort of oscillation, plus it doesn't run the chip at the ragged edge. a 25-0-25 will be in order here - suspect 600VA. that will be sufficient and give the required headroom. note this will be for one setup only.
all resistors will be 1/2w metal film .1% tom - no issues there - with the exception of 150 on the output side.
i've drawn the board accordingly - will post pics soon(ish)

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PostPosted: 13 Dec 2011, 15:41 
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Quote:
with the exception of 150 on the output side.
Rather 0.15R than 150R (0.22 should be good too). BTW - i think you're on the way already.
Don't hesitate to use inductive wire wound resistors. Some extra "L", even if it's very low, is good for cutting down HF and to gain some OPS stability.
The trick on these output resistors is that they act like a choke. They balance the current of all ICs to the same level. If one IC throws more current than the others, its resistor jumps in and limits the output current. Thus, the resistors should be matched as close as possible. I often use to order i.e. 5 or 10pcs instead of the needed 2, to pick a pair with the closest values. 50Volt (25-0-25) is too much imo.
You'll get bout 70VDC after the caps.. 18VAC (36VAC CT) to 21VAC (42VAC CT) for 50VDC to 60VDC bout 4A per IC is right in specs ;-).
A triple set running at high voltage will cause a lot of heat too. It might be wiser to run them at a little lower voltage. 50VDC is not on the bad side here.

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Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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PostPosted: 13 Dec 2011, 21:05 
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Proposed PCB layout.
Feedback welcomed.
Image

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PostPosted: 17 Dec 2011, 02:59 
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It looks great :up:.

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Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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PostPosted: 28 Dec 2011, 05:51 
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oh, theres been a fair whack of discussion over on DIYAUDIO also

but theres a small update :

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more stuff over ere : http://www.diyaudio.com/forums/chip-amp ... er-11.html

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PostPosted: 28 Jun 2016, 10:51 
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Parallel or bridge LM1875 amplifier with single power supply circuit is required for us.Can any body post the circuit mentioned with details


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