Newbie question: LM38xx based amp

[kscharf]

Originally posted by lanchile
I recommend LM3875, it is simple and the sound is excellent.
just fallow some important safety tips. Good luck

The LM3875 is the basic chip amp, no mute function. The LM3876 is the same chip, but has the mute function (requires an extra resistor tied to v- for the chip to function.

The LM3886 chip has the mute function, and is a newer part. The LM3886 can handle 4 ohm speakers better. It has slightly wider bandwidth and higher gain. It might be slightly more instable and sensitive to wiring layout.

Of the three chips the LM3886 is the easiest to find, but the other two are slightly cheaper.
All three will give very good sound.

[AndrewT]

for all that their are followers here that champion the use of the 3886 for impedances around 4ohm I still think that the 3886 is the better part for driving 8ohms.

All the others are too peak current crippled. Even the 3886 is lacking when asked to deliver real current into a reactive 8ohm speaker.

[ChrisA]

The PS circuit is so simple, a PCB simply isn't worth the cost. Guess if you have to cut your teeth on doing p-p wiring, the PS is the place to do it. (Just make sure the big caps and the rectifiers are not soldered in backwards!). [/B]

Simple PCBS are very easy to make. For a power supply, single sided PCB, I will draw outlines of the parts with a sharpie pen, then drill holes then on the copper side connct the holes with black electrical tape, For more detailed work you can draw the traces with a sharpe or buy a speaci pen that work a lttle better. You can use CAD software and photographic printing techniques or "toner transfer" but for these simple PS PCBs black tape works

Also if you make you own PCBs you can use surface mount parts and (1) avoid having to drill holes and (2) get a much better layout because all of your lead lengths are zero.

Some of the surplus places sell small 4x5 inch blank PCB material for 50 cents.

[kscharf]

Sure making your own PC boards is easy and cheap.
For etchant I use a 50-50 solution of Muratic Acid (HCL from pool supply) and
medical grade hydrogenperoxide (H2O2).

For my power supply I use those square bridge rectifiers that bolt to a heatsink
or the chassis with a single screw. The capacitors and resistors in the filter and
snubber are mounted on perfboard and point to point wired. If you are using
those TO-220 diodes instead of a packaged bridge, then the complexity is now
great enough IMHO to bother with a PCB.

[Zoodle]

I opened an old micro hi-fi and found a Sanyo LA4282. The data sheet for this includes a test circuit (screenshot attached). Can I use this circuit to make a power amp? I'm curious about the large caps in the output stage. Are they there for stability?

Now, here's a real greenhorn question: do the component values in the diagram need to be changed if the power supply voltage is changed?

[Zoodle]

...Can't seem to attach files. I tried png and pdf -- no joy.

[kscharf]

I KNOW you can attache .jpg files, if they are under 100K and less than 1000x1000 pixels.

[pacificblue]

Originally posted by Zoodle
I opened an old micro hi-fi and found a Sanyo LA4282. The data sheet for this includes a test circuit (screenshot attached). Can I use this circuit to make a power amp?

Yes, if you don't expect too much of it.

Originally posted by Zoodle
I'm curious about the large caps in the output stage. Are they there for stability?

The large caps are DC blocking capacitors. With single supply amplifiers you need them, because the 0 V reference for the speakers is at 1/2 supply voltage. Without those capacitors you would fry the speaker coils with DC.

Originally posted by Zoodle
Now, here's a real greenhorn question: do the component values in the diagram need to be changed if the power supply voltage is changed?

You don't have to, but it could be an improvement to use bigger values for nearly all the caps, except for the Zobel and maybe the ripple filter. Watch the voltage rating, if you increase the voltage.

[Zoodle]

Originally posted by pacificblue
The large caps are DC blocking capacitors. With single supply amplifiers you need them, because the 0 V reference for the speakers is at 1/2 supply voltage. Without those capacitors you would fry the speaker coils with DC.

Good to know! Thanks.