so I added an LED power indicator...but...

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tedrick

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Hi Guys - so I am putting together a little lm386 amp using a nice guide at:

Build a Great Sounding Audio Amplifier (with Bass Boost) from the LM386

I wanted to add a power indicator so I found spot on the perma proto board I'm using -

What I'm finding is with the led and resistor I have a big drop in voltage to the circuit -- is this because the led and resistor are drawing power/changing the circuit?

I'm using a 500 ohm resistor - not sure what draw of the led is --

without the resistor/led in the circuit Vs out of the LM386 is about 7V but with the resistor/led the Vs is about 4V --

is the droppage because of my power indicator LED?
 
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tedrick

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Joined 2007
avtech23 said:
Are you connecting the LED in series with the Pin 6 of the LM386 or parallel?
Click to expand...
Thank you - I think that's what I did wrong - I think I wired it in parallel -- so is that causing it to draw more power?

the led is getting + voltage from the rail at a separate point than where the IC is getting +

I'll try adding the LED at the same point as pin 6 of the IC -- as shown here:

http://www.generalguitargadgets.com/pdf/ggg_little_gem_sc.pdf

Thanks guys!
 
Mooly

Mooly

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Connect your LED and resistor in series and then connect the series combination across the LM386 using pins 6 and 4. Make sure to get it the right way around.

If you have allowed the LED to pass to much current then it could be impaired/damaged. Rayma's suggestion of 5k sounds reasonable for your supply voltage.
 
R

radtech

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Joined 2006
With a 9V battery an LED with a 500 ohm series resistor will draw maybe 15 mA. If that is loading down the voltage that much you either have a bad battery or a bad connection. A good 9v should be able to handle 15 mA. For battery power you'll want to use as little current as possible for the LED and still have it bright enough so you can try higher resistances and possibly a high efficiency LED. The first thing I would try though is a fresh battery.
 
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tedrick

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turk 182

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can't say for certain as to the series or parallel thing without seeing what's connected on the under(foil)side of the board...
the led is in series with the current limiting resistor but is in parallel with the ic power pins, don't know if that helps your understanding.
series parallel combination circuits can be confusing if your not familiar.
 
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steveu

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home.pcisys.net/~sungstad/
There are several "problems" with the "little gem".
1. 1N914 is not a power diode and may the reason you are loosing voltage. It's supposed to protect the chip in case you connect the power supply backwards, but even a better diode will waste almost 1 volt. You don't need to protect the LED so I would connect the dropping resistor directly to the supply. 1N4002 (1N400x) would be a better choice, or just skip the diode and be careful connecting the power supply.
2. C1 blocks anything below 318Hz, ie no bass. C3 blocks anything below 90Hz for an 8 Ohm speaker. Larger caps pass lower frequencies. C1 should be about 0.1uF and C3 should be 1000uF.
3. Rheostat R3 allows you to clip the sh1t out of the amp quietly. Ie use this amp as a fuzz box. If that's not what you want to do then skip R3 and maybe put a 5-50 K pot between the input and C1.
4. The power switch is kinda useless since you can unplug the input or adapter instead.
5. The dropping resistor for a 2V, 20mA LED should be (9-2)/.02 = 350, but 500 (470) should be bright enough. 20mA is a serious extra load for a 9V battery so a larger resistor (lower current) is probably a good idea.
Cheers,
Steve
 
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T

tedrick

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turk 182 said:
time to break out the magnifying glass and do an up close inspection of all solder joints and check for bridged pads/connections.
patience is a virtue...
Click to expand...

exactly - I had hurried and soldered in temporary leads for the power switch and the 9v battery connector - they were not soldered in securely - and were loosened even more with handling the device outside an enclosure -

soldered up properly and testing fine right now --

Steveu - thanks for the additional recommendations!

Thanks guys - now onto the enclosure!
Ted
 
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steveu

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home.pcisys.net/~sungstad/
turk 182 said:
question for Steveu with increasing the low frequency bandwith does that not deplete the battery faster? and would paralleling two 9v's offset the current draw?
Click to expand...

Possibly, yes. But 9V batteries are perishables anyway. I used a lot of them when I was a kid until I put a door bell transformer; a ~10K pot and some bits on a piece of peg board and made a simple variable power supply. In those days most transistors were germanium so that's what I used, ie 2EF PNP from 1000uF and a bridge of 1N400x diodes. A smaller cap on the pot output ~10uF.
Today you can buy a 9V (12V etc) wall wart for cheap. I have a pail full of them, left overs.
But a couple followers off a pot is a good learning exercise to learn about transistors. Then you need to look at the LM386 data sheet until you understand the internal circuit, the rather clever input circuit, the voltage gain and the "quasi-complimentary" output. It's important to be able to follow the current flows in a schematic and the "bias drops". The way a schematic is drawn makes it easy or almost impossible if it's a real mess. The way it's drawn tells you if the author understands the circuit or not.

A few years later, I replaced the tape head in a cassette player, and put a couple (stereo) LM380 on a sharpy PCB with a 2x2 transistor tape pre-amp. It worked pretty good in my car but having loose speakers and stuff on the car seat was not a great plan. Note that the LM380 were running at 12 to 14VDC, off the car, which was pushing it a bit. LM380 is the original, such amp, in a bigger package.

cheers,
Steve
 
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tedrick

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