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LM1877

Posted by djk (M) on January 30, 2011 at 00:18:37

In Reply to: RE: OP AMP driver for Power Amps posted by rcollege on January 29, 2011 at 22:41:40:

"I've really been wondering if I need to look at more powerful supply than 24V 6A per channel. Before spinning my wheels anymore I decided to put together a chip amp I had - LM1877. It is a 14 dip pin dual channel monolithic chip that puts out 2W per channel using a +24V supply. I happen to have a 24V .7A switching (evil I know) supply."

http://www.national.com/ds/LM/LM1877.pdf

Reading the data sheet, we see that it is not safe to run an LM1877 on 24V and drive less than about 16R. Maximum current out is rated at 1A (peak).

"I slapped only one channel together on a breadboard. (0.7A isn't enough for both 2W channels). I hooked it up and got a crisp clear annoying buzzing through the speakers( SPU is to blame here) but proceeded to play music just to see how "loud" 2W would be through my 90db speakers.
I was expecting some weak foggy sounding music. No...I about blew down the house. Turning it up past 1/8 the volume put out a blairing yell of music through the speakers and my reaction caused me to accidentally push the volume pot to full blast. Yanked out the power wire from the bread board as I was saying a few one 1- 2 word sentences. Heck- my kids and wife can't yell that loud!"

Distortion sure does make things sound loud, doesn't it?

Overdriving it like that will put out about 18V P-P (for a very short period of time before something bad happens)into your 4R Boston speakers (measure them and see). 18V P-P is about 20W at 4R, and the distortion will make it seem even louder.

"After I put it back playing it sounded suprisingly nice...and I must say that 2W gives me quite enough volume and dynamic range. Clear sound throughout the townhouse. Still too steril- or just not what I'm looking for. So I figure if I can get 4W-5W of power per channel I should be set for a first design with enough headroom for the ocassional blast of sound.
Start with better sound rather than loud OK sound."

Build the circuit I roughed out with your ±12V supply, it will deliver about 23.5V P-P, or about 17W/4R with less than 0.1% distortion. In the mean time, run the LM1877 on 12V please.

"I just remembered...when I bought these Boston Acoustic A100 speakers from a repair shop, the first thing I did was replace all the internal wires (thin ~20ga) with 14ga speaker wire (pain in the a--). The first speaker made quite enough of a difference where I unfortunately had to do the second one. Each speaker had a total run of 18"-24" of wire. The tweak did result in an increase of volume and clarity....so I may be dealing with speakers that are more efficient than the original posted 90bd."

If the woofers are original, they are 4R. Measure the DCR with your meter.
Replacing the wire only got rid of the corroded push-on speaker connections. Cutting off the original corroded connectors and using the stock wire would have had the same results. I used to do this when they were new, and it makes a big difference in the sound. If yours are older Boston with the electrolytic cap feeding the tweeter, paralleling it with a 0.1µF film cap will make a very large difference in detail and clarity.

Story:

About 25 years ago I modified the then top-of-the-line Boston A400 with 12ga Monster cable, and changed the capacitors to polypropylene. It made a huge, jaw-dropping difference in the sound. Subsequent testing of modifications done on six identical Boston A70 (easy to come by when you're the dealer) showed that cutting off the push-on connectors (using the stock wire) and adding the 0.1µF bypass caps did the same thing.

Edits: 01/31/11

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