LM1877 ancestors data sheets for 10 Watts stereo - diyAudio
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Old 22nd May 2012, 10:22 PM   #1
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Default LM1877 ancestors data sheets for 10 Watts stereo

OK, futile quest. I just spent a couple of hours over two days looking for old data sheets. Not real complicated, went to Weird Stuff Warehouse and saw a box of LM1877 stereo boards for ~$1.95 so I bought one.

For the bored that want a little background as opposed to just skipping to the question. The LM1877 is a somewhat obsolete 2 Watt/channel stereo amplifier. It replaced the more obsolete LM1377 which replaced the more obsolete LM377.

Tough remembering back 40 years but IIRC National Semi had LM377 @ 2 Watts/channel, LM378 at 4 Watts, LM379 at 6 Watts.

Now is where it gets really tough. Early 70's I had put together a stereo amp with the LM377 and I regarded its 2 Watt/channel as pretty anemic. In the National Semi Data Book the was a circuit modification for the LM379 that just added a complimentary pair of audio power transistors and a few passive parts to boost the power to 12-15 Watts/channel. I modified LM377 circuit to mostly follow the boosted power circuit and there was much rejoicing! The thing put out enough music power to keep me happy. So now I be thinking what I need to get is a couple of that old data book schematic and try the transistor addition to my LM1877 circuit.

Does anyone have a copy of that data book or the circuit? I have googled the hell out of it and all I come up with are single page data sheets. The circuit may have appear on the Radio Shack data sheets that came stapled to their packaging but I'm not sure it did. It isn't too important personally, just a bucket list kind of thing since I can't find the original amp and I would like to see if it made as much noise as I remember.

It would be of general use here. IIRC the design was dirt simple. Just something like a 10 Ohm resistor from the output of the IC to the bases of the matched pair of transistors, call for larger caps on the output to speaker, maybe another change in feedback network. Can't remember if a power supply modification was necessary too.

Rick
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Old 22nd May 2012, 11:30 PM   #2
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Arrrrghhh!!!
Well I finally found it! I did find a couple of broken links and finally just Googled for the file name and it turned up. Memory was a little weak, the circuit is for an LM378, but it makes about enough sense for me to think the original National Semi book was for the same. The ~10 Ohm resistor is actually a 4.7 Ohm tied to emitters/output cap.
http://www.eleccircuit.com/wp-conten...er-otl-15w.jpg
Click the image to open in full size.
Anyway, looks like it would work as a general power booster for that entire family.

Rick
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Old 23rd May 2012, 09:41 AM   #3
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Quote:
Originally Posted by ricortes View Post
Arrrrghhh!!!
Well I finally found it! I did find a couple of broken links and finally just Googled for the file name and it turned up. Memory was a little weak, the circuit is for an LM378, but it makes about enough sense for me to think the original National Semi book was for the same. The ~10 Ohm resistor is actually a 4.7 Ohm tied to emitters/output cap.
Weirdstuff Warehouse - I have fond memories of the place, but it was mostly computer parts and test equipment about 10 years ago. Maybe I didn't search enough for the analog parts then.

Regarding the circuit, the output transistors are working in Class-B, so it isn't going to be terribly great on distortion. I'll take a look at it in simulation later. There's an alternate schematic which uses small resistors from each rail to Vcc, Vee; and the output transistors are in CFP configuration - emitters to each rail; bases to Vcc, Vee; and collectors tied together with the chipamp output to drive the load. That config can be biased to any of Class-A/B/AB.
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Old 23rd May 2012, 02:59 PM   #4
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Kinda of off-topic but, i have tried the higher powered version of it (LM2877, 4w)
this little chip was extremely easy to get oscilated and mine popped within seconds(ouch!)

If i remember correctly, there was also a 9W version of the same chip, along with its reference pcb design in the datasheet. it is discontiuted though.
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Old 23rd May 2012, 07:03 PM   #5
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Looks like the tipical transistor boosted opamp. (in fact it is )

Why not use a split powersupply, and help the little chip elliminate the crossover distorsion of the output transistors with some bias current?
Actualy in similar fashion any opamp can be made quite powerfull.
I might even go as far as using a compound transisto output stage
Sziklai pair - Wikipedia, the free encyclopedia

Something in the range of 10 volts at ouput and ability to drive 4 ohm load (or 2 ohm, or whatever depending on transistors used) is managable .( say... 20 watts to 4 ohm within acceptible THD is realistic).

I'm not a big circuit designer, so my sketch may not be the best out there, but the basic idea is something like this picture(note, i did not draw a darlington or compund output)
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Old 23rd May 2012, 08:08 PM   #6
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Quote:
Originally Posted by linuxguru View Post
Regarding the circuit, the output transistors are working in Class-B, so it isn't going to be terribly great on distortion. I'll take a look at it in simulation later. There's an alternate schematic which uses small resistors from each rail to Vcc, Vee; and the output transistors are in CFP configuration - emitters to each rail; bases to Vcc, Vee; and collectors tied together with the chipamp output to drive the load. That config can be biased to any of Class-A/B/AB.
IIRC I just used the simple schematic for the LM377 that didn't include resistor R5 and cap C4 in the feedback. I haven't checked the Weird Stuff board closely but if you do run a simulation could you please check it w/o those two parts? Subjective but if distortion is under 3% in the audible. Heck, I can't hear anyway.

I think you can see where I am going with this. I must have a 4 or 5 old powered computer speakers floating around the house. It looks like all that would be needed is to pull ~4 pins up from existing ICs [feedback, output] jumper to the $1 worth of transistors/resistor, and get 10-15 Watts, albeit of questionable quality, from just about any of low power speakers.

Rick
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Old 23rd May 2012, 08:39 PM   #7
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Quote:
Originally Posted by Arty View Post
Looks like the tipical transistor boosted opamp. (in fact it is )
<snips>
Why not use a split powersupply, and help the little chip elliminate the crossover distorsion of the output transistors with some bias current?
If I understand you correctly.

I was thinking of keeping it the lowest common denominator. Switching to some form of darlington could make it more desirable IMO. I'd much rather substitute a single darlington like a TIP120<?> for the two transistors if I could, I just don't know how choose it or do it. Likewise if I am still powering the existing IC from say a 6-15V DC supply I don't know how to design a circuit that would allow me to run the output transistors on a split supply. I assume it would require some DC blocking caps for coupling and that would increase the complexity a bit.

Looking at the original design, ~4 Watt chip amp feeding transistors with a hFE of ~15 so switching to darlingtons with hFE in the range of a 1000 would mean you could power it from a whisper.

Rick
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Old 24th May 2012, 12:46 PM   #8
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well to be honest desgin like this is not realy sensitive, infact it is verry forgiving.
The reason behind it simple, the output transistors are whitin the feedback loop of the IC,
so even if no bias current is applied, the IC will do whatever it can to keep the output transistors in class B operation. Actualy the basic concept is a real workhorse of "cheap" watts for ... well hell knows for how long time, i guess since opamp chips exsist.

I try to make a working example, but before building it....
wait for others approval as i'm not the best in designing an amplifier.
On the otherhand someone has to be reaaaly clumsy and unlucky to fail on thisone.
No ected board is needed, plain old fashion point to point soldering should absolute work.
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Old 24th May 2012, 04:06 PM   #9
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hm, seems the schematic You found will work for LM1877, same pinouts
So afterall i do not have to draw anything, just use the schematic You found.

Might be a good idea btw to check out TL072 opamp, roughly 1.5 $ , a better toy to play with.
One can add transistor to thatone too, to get some nice power.
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