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Old 6th February 2014, 01:32 AM   #1
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Default National Semiconductor LM series & Ti LM series differences that affect circuit funct

Hi i"m new to this but want to start by hard wiring an LM1875. Super simple and as I have had jewellery as a hobby for 35 years, fiddly & small is the way I roll. One issue that nags me is this. Most of the circuit designns that interest me (Chip Amp. Audio Sector and 47labs)
were all designed with the Natioall chip. The Texas Instruments specs seem very close but not exactly the same. Are there any differences that affect the above mentioned circuits and so how? The only mention of this I have found is how the change affected design software
like spice. Way over my head .Did TI change anything or "improve " anything?
Can someone please clear this question up for me? Am I creating much ado about nothing and if so prove it?
Cheers theaudiopath.
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Old 6th February 2014, 02:14 AM   #2
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TI has done a really good job with 2 of the acquired companies -- Burr Brown and Nat Semi. I regret that Nat Semi went on a cost-cutting binge before they were acquired to "pretty-up" the P&L.
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Old 6th February 2014, 03:37 AM   #3
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Thanks jackiinnj. That is standard corporate OP in anticipation of takeover. No surprises there.
What I am actually seeking is those who have tried both chips in the same circuit and found an audible difference + or -. I Realise it is AM in your neck of the woods but 3pm here. Also where can I find matched and burnt in chips so I have even a remote chance of getting 2 stereo channels of the amp to match.
Cheers Mark.
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Old 6th February 2014, 04:25 AM   #4
BrianL is online now BrianL  United States
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Mr. Path,

Why do you assume that there is any difference in the National power amp chips before and after the TI acquisition? I'm sure that if you ask them, you'll find that they are made in the same wafer fabs, assembled and tested at the same factories, and all this done with the same equipment and (likely) many of the same people as pre-acquistion. If there is any substantive (i.e., non-cosmetic) change in the datasheet, it's likely a result of normal revision process (find an error and correct it). Semiconductor manufacturing is a highly complex process. You don't just pick it up and move to another 'interchangeable' factory like a clothing manufacturer would do with a pair of blue jeans.

I would expect any two chips from the same wafer lot to perform substantially identically to each other. Even with two random units of the same product, the marvel of negative feedback is that the two will likely perform 'identically' in the final application circuit.
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Old 6th February 2014, 06:11 AM   #5
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Dear Mr L
I can understand that you would assume that I am making that assumption. But you are replying to one assumption with another. YES this is the most likely situation but are you guessing or do actually
KNOW? Assumptons can be dangerous. According to the meticulous measurement of the LM chips by Peter Daniel they of the same batch can vary significantly more than the 10-30 mV DC offset recommended by Chip Amp as optimum in the instruction PDF for the Lm1875 kit.
Cheers Theaudiopath
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Old 6th February 2014, 11:33 AM   #6
AndrewT is offline AndrewT  Scotland
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Peter Daniel's implementations are usually DC coupled.
These implementations are very sensitive to small tolerances resulting in big variations in output offset.

You have choices
AC couple your Power amplifiers
DC couple your Power amplifiers and include set up adjustments to minimise output offset
DC couple and fit all the correction, detection and isolation gear to protect your speakers.
DC couple and take no precautions to protect your speakers.

Note that none of the 4 options blame the manufacturer for the varying offset problem.
regards Andrew T.
Sent from my desktop computer using a keyboard

Last edited by AndrewT; 6th February 2014 at 11:57 AM.
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Old 6th February 2014, 11:56 AM   #7
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Peter generally has a minimal circuit. I would guess that the addition of a feedback cap (between the feedback resistor and ground) would probably reduce that (dc offset) variability between chips to very low numbers.

When I made my LM3886 P2P I had from memory about 3mV dc offset.

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Old 6th February 2014, 02:31 PM   #8
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Cheers AndrewT & Wintermute for your replies. I Think purchasing a few Chips from Element 14 is a good starting point. @ $3 & a bit each makes for cheap as chips (sad pun intended) experimentation.
Have most of the hardware including cases aside from the resistors & some of the caps. A Pair of paralleled 12v mobile phone relay station back up batteries per channel should supply ample clean power. I Think following the popularity of all Mr Daniel's amp incarnations I'll start there. Why mess with success.
What sort of trim pot would you suggest for this project?
Cheers Mark.
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Old 6th February 2014, 04:33 PM   #9
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Originally Posted by theaudiopath View Post
...What sort of trim pot would you suggest for this project?.....
You did say hardwiring, right?
I would suggest this type of trimpot (3006P) rather than the usual miniature square format types which have wobbly leads that fracture easily (or the solder does) if not pulled down flat on a PCB. If you need to screw them about more than a few times, the benefit of a PCB with 0.5 mm mounting wires becomes more obvious. Both types are stocked by au.E14.

The difference with this type, IIRC, is that leads emerge directly from epoxy sealant which makes for a much more rigid and easier to make connection, as I'm sure you're able to do to perfection. This should wind up as good as a board mounted type and of course, you can orient it in the most accessible direction.

Last edited by Ian Finch; 6th February 2014 at 04:51 PM.
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Old 7th February 2014, 12:15 AM   #10
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Thanks Ian Fiinch
I actually intend to use no bread board at all. Just a block of wood with chiseled & pre-drilled holes.
Mount the components upside down & secured with glue & hard wired with spare silver wire from the
top. Your reply for some reason made me think about another issue. I noticed there seemed to be a disprorational no of issues with the LM1875 component. I Find
this quite disturbing.
As I have no idea if it is due to quality control issues, heat stress while soldering 5 points dissipating through such a tiny chip or bad technique by the DIYers fake chipsets or all of the above
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